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Athena Review Vol. 5, no. 1

Records of Life: Fossils as Original Sources


A        B        C        D          E         F         G         H         I         J         K         L         M         

N         O         P         Q         R         S         T         U         V         W        X         Y         Z


Abrahamskraal Formation:  A Late Permian formation in the Beaufort Group, on the western side of the Karoo Basin in South Africa. It contains part of the Tapinocephalus Faunal Assemblage Zone.

Acanthostega gunnari:   Acanthostega (“spiny jaw”) was an early tetraform lobe-finned fish from the Late Devonian period, found in  East Greenland. The species name gunnari is for Gunnar Save-Soderburgh, who first discovered the fossil evidence of this tetrapod in the 1930s. In 1987 Jennifer Clack of Cambridge University found and published descriptions of more complete specimens. These show that Acanthostega had well-developed limbs, with an unusual array of 8 digits on its hands and feet. Acanthostega was mainly adapted to life in the water. It had a middle ear with a stapes that was used as part of a kind of spiracle for respiration, rather than as an ossicle in hearing as the stapes later became used in reptiles and synapsids. Like Ichthyostega, another tetraform found by Save-Soderburgh in the same Greenland rock formation, Acanthostega also had functional internal gills as adults.

Adelobasileus cromptoni: A small stem mammal found inCrosby County, Texas in strata from the Dockum Formation, dating from the Late Carnian stage of the Triassic (225 mya).  Fossils of a partial skull about 1.7 cm long, and teeth were discovered by Lucas and Hunt (1990). While the lower, rear part of the skull (basicranium) shares  numerous traits with early Jurassic  mammals, other cranial features such as the petrosal region, which has only a small or incipient promontorium housing the ear cochlea, indicate only an intermediate stage of the character transformation from non-mammalian cynodonts to  mammals. If they are mammalian, the Adelobasileus fossils would be at least ten million years older than than those of any previously described mammal (Lucas and Luo 1993). 

Albian:  The second and last stage of the Early Cretaceous, dating from 112 -  99 mya.

Ammonites:  coiled marine cephalopods with a series of whorl-shape chambers, named for the "horns of Ammon" (the Egyptian god Amun) by the Roman geographer Pliny the Elder. Although they have many-chambered shells, they are more closely related to the squid and octopus than to the nautilus. Ammonites, now extinct, were extremely plentiful during the Late Triassic and Jurassic periods

Amnion:  The innermost layer of the amniotic egg found in reptiles and synapsids. It retains a fluid which surrounds the embryo.

Amphibian: A class of tetrapods whose fossil taxa first appeared in the Mississippian period about 340 mya. Extant amphibians include frogs and newts or salamanders, who descended from Lissamphibia. Their reproduction involves larva hatched as tadpoles in open water. Their name amphibia (“two life types” in Greek) stems from the fact that at least the first part of the life cycle must be spent in water. Fossil amphibians  are grouped into several orders, including anthracosaurs (“coal lizards”), temnospondyls (“cut vertebra”), and lepospondyls (“spool vertebra”). Many have a type of tooth with interlayered enamel and dentine, called labyrithodont  (“labyrinthine teeth”). This trait used to be thought to signify early amphibians ancestral to reptiles, but it does not occur consistently in the reptile-like amphibians called Reptilomorphs, which eventually developed into reptiles.

Anapsid:  (“no arches”), referring to reptiles lacking any skull apertures behind the eyes (Romer 1956). Anapsids include both fossil reptiles and present-day turtles.

Anisian:  The first stage of the Middle Triassic, dating from 242 - 234 mya.

Anthracosaurs (“coal lizards”) are a large group of amphibians from the Carboniferous era first discovered in Scotland, named for their typical occurrence in coal measures. They are labyrinthodonts, and include reptilomorphs (reptile-like amphibians).

Antiarchi: A widespread order of Devonian armored fish within the large group known as placoderms. Antiarchs inhabited both freshwater and marine environments, including lagoons, rivers, deltas, and coastal environments, and probably fed on invertebrates such as crustaceans and molluscs. The most diverse genus of antiarchs was Bothriolepis, with over 100 species known in the Middle to Late Devonian (380-359 mya)..


Anticline: In structural geology, a fold which slopes away from a common center or peak. Younger layers are outside of older layers. It is thus the opposite of a syncline. 

 Apatite:  A class of minerals contained in the teeth and/or scales of vertebrates. 

 Apomorphy:  A cladistic term for a character state which is unique to a single taxon. For example: among primates, speech using languages is an apomorphy of the genus Homo. It is diagnostic of humans, but does not help determine their phylogenetic relationships with other primates (such as Gorillas or Chimpanzees), because it is not a shared, derived characteristic of any larger group including both humans and other primates. The converse condition, derived traits shared by two or more groups, are called synapomorphies.

Arandaspida: One of three subclasses of the jawless fish class Pteraspidomorphi.

 Arandaspis prionotolepis: A species of jawless fish that lived in the Early Ordovician period from 480 to 470 mya. One of the oldest known vertebrates, this fossil was found at Alice Springs, Australia in 1959 and named after a local Aboriginal tribe, the Aranda. Arandaspis was about 15 cm (6 in) long, with a streamlined body covered in rows of knobbly armoured scutes. The front of the body and the head were protected by hard plates with openings for the eyes, nostrils and gills. The low position of its mouth suggests Arandaspis foraged the ocean floor. It lacked fins; its only method of propulsion was its horizontally flattened tail.

Artinskian:  The third stage of the Early Permian, dating from 290.1 - 283.5 mya

Asselian:  The first Stage of the Early Permian, dating from 298.9 – 295.0 mya.

Astraspida:  (“star-shields”): One of three subclasses of the jawless fish class Pteraspidomorphi. Astrapida include the armored species Astrapis desiderata from the Ordovician at ca. 450 mya.

 Astraspis desiderata: An Ordovician jawless fish dating from 450 mya. It was found in both Colorado (Sanson et al 1997) and Bolivia (Gagnier, 1993) . The Colorado specimen had relatively large eyes flanked by a series of eight gill openings on each side. The protective bony plates were composed of aspidin (similar to that in modern shark's teeth), covered by tubercles composed of dentine. The subfamily Astrapis is named for these tubercles, which are generally star-shaped. Astraspis had a lateral line system, a sensory structure allowing the fish to detect the direction and distance of movements in the water.

Auditory bulla:   In ear anatomy, the auditory bulla (Latin bulla = “bubble”) is a rounded, osseous covering of the middle ear and the floor of the skull in that region.

Autapomorphy:   a cladistic term for an anatomical trait or character which is unique to a particular taxon.

 Axis:  In skeletal anatomy, the second cervical vertebra of terrestrial vertebrates, from Latin axis (“axle” or “pivot”); referring to the rotary movements of the head, which occur between the atlas and axis.



Balfour Formation:  An Early Triassic formation of the Beaufort Group in the Karoo Basin of South Africa.  This is an exposure in the lowest part of the Lystrosaurus Assemblage Zone, composed of siltstone and mudstone layers deposited by low-sinuosity rivers flowing through a semi- arid region.  Sidor & Smith (2004).

Barremian:  A stage of the Early Cretaceous, about 127-121 mya.

Basal trait:  In cladistic terminology, a basal trait is a "primitive"  trait, the opposite of a “derived” trait. A primitive trait is the five-digit hand found on early tetrapods; a derived trait is a specialized hand, such as the horse's hoof.

Basicranium:  In skull anatomy, the base of the braincase, including the basioccipital and basisphenoid bones, and the otic capsule.

Basioccipital:  In skull anatomy, one of the bones of the occiput, the lower part of the skull which articulates with the spine. The basioccipital is located in front of the foramen magnum and  usually forms most or all  of the occipital condyle(s).

Basisphenoid:  In skull anatomy, the basisphenoid bone forms the floor of the braincase anterior to the basioccipital bone.  The basisphenoid gives rise to the basipterygoid process and other structures of the braincase. 

Bathonian:  The third Stage of the Middle Jurassic, dating from 169 - 164 mya.

Beaufort Group:  A geological region in the Karoo Basin of South Africa, comprising a group of terrestial formations, largely river-deposited shales and sandstones, dating from the Middle Permian through Early Triassic periods. The Beaufort Group includes an important series of Faunal Assemblage Zones containing vertebrate and plant fossils. It overlies a series of Early Permian marine formations called the Ecca Group.

Belemnites:  Extinct, squid-like cephalopods, whose pointed, rodlike "guard" is one of the most commonly found fossils in Jurassic deposits. As the animal grew, it  moved forward in the body chamber and secreted a septum, which became a series of chambers called the phragmocone. The guard was attached to  this many-chambered, cone-shaped midsection. Behind this trailed multiple appendages, as on a squid or octapus.

Berriasian:  The first Stage of the Early Cretaceous, dating from  144 - 137 mya.

Bitter Springs Formation: A Precambrian fossil zone in Australia where fossilized bacteria dating from 850 mya have been discovered. These include fossils named Oscillatoriopsis, Cephalophytarion, and Filiconstrictosusresembling the extant filamentous bacteria Oscillatoria amena

Bothrio-:  A Greek term meaning "pitted". Used in genus names such as Bothriolepis, “pitted scale”.

Bothriolepis canadensis: A successful species of placoderm or armored fish living in the Middle and Late Devonian (387-360 mya). Many fossils of B. canadensis have been found in Escuminac Bay, Québec, Canada, for which country the species is named.  Bothriolepis, part of the antiarch family, were relatively small, freshwater bottom-feeders, with a heavily armoured head attached to the thoracic shield. It also had a long pair of armored pectoral fins, serving as caliper-like, or arthropod-like limbs, used to raise itself out of the mud.  Bothriolepis had both gills and a pair of pouches off the esophagus that may have functioned as lungs. It thus had some functions adaptive to freshwater environments, analagous to those of lobe-finned fish.  Its wide range appeared to have corresponded with the Devonian continental coastlines. 

Branchiostoma: The extant lancelet genus.

Breccia:  In geology, a generic term for conglomerate rock containing pieces of angular gravel.

Buccal:  In skull anatomy, buccal refers to the cheek (Latin  bucca = cheek). In dentition, bucccal refers to the "outside" of the teeth, toward the cheeks. Same as labial and opposite of lingual.



Calcaneum:  In skeletal anatomy of quadrupeds, one of the two tarsals or upper ankle bones. In humans it is the heel bone; Calcaneus means "heel" in Latin. The calcaneum often articulates with the fibula. 

Calcareous:  In geology, a type of sedimentary rock made up of biogenic calcium carbonate (i.e., derived from shells and related materials).  Thus calcareous limestone is limestone of biogenic origin, made up of crushed fragments of shells that have solidified into rock over time and under pressure.

Cambrian:  The geological period from 543 - 490 mya. the first period in the Paleozoic era. The Cambrian is preceded by the Edicarian stage at the end of the Pre-Cambrian, and followed by the Ordovician period.  The Early Cambrian is identified as the Cambrian A epoch (543-520 mya). The Middle Cambrian comprises the Cambrian B and C epochs; in  North America these are named the Montezuman, Dyerian, Delmaran and Marjuman Stages, dating from 520-500 mya. The Late Cambrian is identified as the Cambrian D epoch; in North America these are named the Steptoean and Sunwaptan Stages, dating from 500-490 mya.

Campanian: A Stage of the Late Cretaceous, dating from 83.5 - 71.3 mya.

Canidae: The Latin term for the dog family, from Canis, “dog”. The canidae are divided into two tribes, canini (“dog-like”) and vulpini (“fox-like”). Canini includes all wolves and domesticated dogs, including the Australian dingo,  as well as coyotes (Canis latrans), jackals (Canis aureus, the golden jackal, and two other species), and a number of other genera.

Canine:  In dentition, the term canine typically refers to a single pair of elongated, pointed, recurved teeth in the front part of the jaw. Canines are most frequently found in carnivores and many omnivores. In mammals, the canines are located between the incisors and premolars.

Canis familiaris:  The domestic dog. The Latin terms for the genus (Canis) and species (familiaris) of dogs were first used in Linnean classification. Both archaeological finds and recent genetic studies indicate that dogs were domesticated from grey wolves (Canis lupus) by about 15,000 years ago. In some recent classifications, dogs are listed as a tame subspecies of the Gray Wolf, Canis lupus familiaris (Wang 2009).

 Canis lupus:  The gray wolf (Latin lupus = “wolf ", thus "wolf dog")  evolved during the Pleistocene period by about 1 mya. It is recognized as the direct ancestor of all breeds of domestic dogs, the earliest of whom were simply tamed gray wolves. Based on recent genetic and behavioral studies of dogs, the domestication process and related behavior became genetically inherited in dogs.

Capitanian: The third and last Stage of the Middle Permian, dating from 265.1 – 259.8 mya.

Carboniferous:  The fifth geological period in the Paleozoic era, dating between 354 – 298.9 mya. The Early Carboniferous (354-323 mya) is known as the Mississippian in North America. Elsewhere, it is divided into the Tournasian, Viséan and Serpukhovian Ages. The North American Late Carboniferous (323- 298.9 mya) is the Pennsylvanian. Elsewhere it includes the Bashkirian, Moscovian, Kasimovian and Gzhelian Ages. During the Carboniferous much of the continental areas were near or south of the equator, and warm to tropical vegetation prevailed. It was a time of great development for both amphibians and fishes. In North America, the Mississippian was characterized by shallow seas, the origins of extensive limestone formations, and the Pennsylvanian by extensive rain forests, precursors to today’s coal measures.

Carotid artery: The major artery supplying the brain, whose path may be traced in various skull bones.  The internal  (cerebral) branch of the carotid is of special significance in anatomy because it enters the braincase through various known channels or openings (foramina). In mammals, the internal carotid supplies the anterior part of the brain, the eye and its appendages, and sends branches to the forehead and nose. The external carotid splits into numerous branches which supply external structures.

Carpus:  In skeletal anatomy, the assemblage of wrist bones which connect the radius and ulna with the finger bones.  Humans have eight carpal bones placed in two rows. The basic features of the reptilian carpus show differences with those of mammals. 

Caudal: In skeletal anatomy, pertaining to the tail, such as a tail or caudal vertebra. Caudal is also used to indicate a direction toward the tail.

Cenomanian: The first Stage of the Late Cretaceous, dating from 99.0 - 93.5 mya.

Cenozoic Era:  In the geological timescale, the Cenozoic is the recent era of life (from Latin Ceno-, “near” and Greek -zoic, “life.”), dated 65 – 0 mya.  The Cenozoic is comprised of seven periods: Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene . These are organized into two groups, the Paleogene (Paleocene, Eocene and Oligocene); and the Neogene (Miocene, Pliocene, Pleistocene and Holocene).

Cephalochordata: (“head-chordates”) are a subphyllum of chordates which includes present day lancelets, and a few known Cambrian fossils. These include Pikaia from the Burgess Shale in British Columbia, and Yunnanozoan from the Maotianshan Shale deposits in Yunnan province, China.

Cervical: In skeletal anatomy, pertaining to the neck, as in cervical or neck vertebrae.

Changhsingian: The second and last Stage of the Late Permian, dating from 254.1 – 252.2 mya.

Cheek teeth:  Any teeth located behind the canine teeth, within the area of the cheeks. In mammals, this generic term includes both premolars and molars, which are specializing grinding teeth. In other tetrapods such as reptiles and early synapsids, cheek teeth may have different forms and functions, not being specialized as molars.

Chiro-:  The Greek term for “hand”.

Chondral: In anatomy, pertaining to cartilage (Greek chondros = “cartilage”). Thus the Chondrichthys (“cartilaginous fish”) is an order of fish including sharks and rays, whose skeletons are largely made up of cartilage.

Cisuralian Series:  In the geological timescale, equivalent to the Early Permian (298.9 - 272.3 mya), comprising four Stages: Asselian, Sakmarian, Artinskian, and Kungurian.

Clade: In cladistic terminology, the term for a specific branch of an organism, showing the shared inheritance of physical traits; ie., an organism and all of its descendants. From Greek clados = “branch”, the source of the term “cladistics” .


A system of classification initiated by Willi Hennig, a Swiss entomologist or specialist in insect biology. His 1966 book Phylogenetic Systematics paved the way for the widespread adoption of the cladistic method of classification in biology. The goal of the method is to create phylogenies by arranging taxa by shared traits or characters into clades, or monophyletic groups where all members have the same ancestors. This relies on computer grouping programs to create branching or tree diagrams by nearest-neighbor and maximum parsimony methods. Although not without its critics, cladistics at present is the predominant method of classifying both fossil and extant taxa of organisms.

 CladogramIn cladistic usage, a branching diagram used to illustrate groupings both with and without common ancestors. The points of diversion are called nodes.

 Clastic:  In geology, clastic sediment consists of broken fragments derived from preexisting rocks and transported elsewhere (as by rivers or glaciers) and redeposited before forming another rock. Examples are sandstone, siltstone or mudstone, shale, and conglomerate.

 Clear Fork Group:  A geological formation from the Early Permian in northwest and central Texas and Oklahoma. It mainly consists of thin layers of sandstone, mudstone, and shale, representing flood plains and point bars deposited by rivers with high sinuosity. It also contains thin beds of limestone and dolomite. The Clear Fork exposures have yielded abundant fossil remains of reptiles, lepospondyl amphibians, and pelycosaur synapsids. Taxa include Tetraceratops, Varanops, sphenacodonts, and large “sailback” reptiles which both carnivorous (Dimetrodon) and herbivoran (Edaphosaurus).

Coal Measures:  In geology, coal-bearing formations in which seams of coal, derived from the vegetation of lowland swamps, are separated by sedimentary rock (limestone or shale) resulting from periodic marine incursions. In North America, most coal measures date from the Mississippian and Pennsylvanian periods when such swampy environments were common.  In Great Britain, the Lower, Middle and Upper Coal Measures refer to the Late Carboniferous, Westphalian stages A, B and C.  The Lower and Middle Coal Measures correspond with the second half of the Bashkirian Stage  ( 316 - 311 mya), and the Upper Coal Measures correspond to the first half of the Moscovian Stage (311 - 306 mya).

Cochlea:  In ear anatomy, an elongated process named for a marine shell (cochlea), part of the lagena or container of the fluid-filled inner ear, associated with hearing in mammals.

Columella: In ear anatomy, a slender bone equivalent to the stapes or hyomandibula bone, used as a vibrator or amplifier (i.e., an ossicle) in hearing air-transmitted sounds. In reptiles and, generally, in tetrapods other than mammaliforms, the columella or stapes extends from the tympanic membrane to the inner ear.

Condyle:  In anatomy, a protruding element (from Greek  kondylos = “knuckle”). This forns the protruding element of a hinge, or ball-and-socket joint. Examples are in the knee joint of the femur, or the elbow joint of the humerus; or the occipital condyle, where the base of the skull attaches to the atlas bone.

Coniacian: A Stage of the Late Cretaceous, dating from  89.0 - 85.8 mya.

Conservation of Energy, Principle of:  This law, as formulated by the 19th century German physicist Ferdinand Von Helmholtz,  states that energy cannot be created or destroyed, and the sum total of energy in the universe always remains the same. All living organisms are thus constrained by the Principle of the Conservation of Energy;  there can be no “perpetual motion” machines which create energy.  Photosynthesis, for example, the most widespread activity of life, requires sunlight and water to be converted into energy by plants or algae.

Continental Drift: The theory that the continents were slowly moving around on the surface of the Earth was first accurarely formulated in 1912 by Alfred Wegener (1880-1930), a German meterologist, polar researcher, and geophysicist.  Based on the current outlines of contents such as Africa and South America, Wegerner interpreted that these land masses had moved together by the shifting of continental plates. Wegener's theory of Continental Drift (1912, 1929) was not widely accepted for several decades. Among its most vocal opponents was the American paleontologist George Gaylord Simpson (1943).  The theory began to gain wide acceptance when paleomagnetic samples  from India showed that the country had previously been in the Southern hemisphere, as Wegener had predicted. His theory is now fully confirmed and forms the starting point for current models of plate tectonics. 

Evidence that the continents now detached had once been connected had previously been seen by Austrian geologist Eduard Suess, who between the 1860s and 1900 studied the distribution of the Permian conifer Glossopteris on several now detached landmasses in the southern hemisphere. Suess inferred that these areas, including South America, Africa, India, Australia, and Antarctica, had once been connected by a land bridge. Suess named the interconnected southern land mass Gondwanaland, after the district in India where fossils of the plant Glossopteris were abundantly found. The ancient sea that he postulated to be north of Gondwanaland he named the Tethys SeaSuess's names Gondwanaland and the Tethys Sea have both been retained in mapping the various continental formations. The conjunction of all continents during the Permian and Triassic periods is called Pangaea, or "whole earth" [from pan ("entire"]) and Gaia (the Greek earth goddess)].

Craton: Bedrock that forms an old, stable part of the continental lithosphere (from Greek Kratos, "strength"), in contrast to more unstable, geologically active regions on a continentCratons are made up of ancient crystalline basement rock, which either outcrops as a shield, or is covered by platforms of younger sedimentary rock. The first large cratonic landmasses formed during the Archean eon (4.5-2.0 billion years ago). Much of that rock has been reformed, with only 5 to 40 percent of the present continental crust formed during the Archean. Cratons have a thick crust and deep lithospheric roots, extending up to several hundred km into the Earth's mantle. Cratons are usually found in the interiors of tectonic plates, after surviving various cycles of the merging and splitting of continents. The term craton was originally proposed as kratogen for stable continental platforms, contrasted with orogen for mountain or orogenic belts, by Austrian geologist Lepold Kober (1921).  Examples of cratons are the Slave Craton in Canada, the Kaapvaal Craton in South Africa, and the Gawler Craton in South Australia. Cratons are areas where the earliest fossils of Precambrian microbes have been found, often in the form of stromatolites or mat-like layers of the residues of bacterial colonies.

Cretaceous:  The last period of the Mesozoic Era, dating from 144 - 65 mya. It follows the Jurassic, and precedes the Paleocene. The Early Cretaceous I includes the Berriasian, Valanginian, Hauterivian, and Barremian Stages.  The Early Cretaceous II covers the Aptian and Albian Stages (144-99 mya).  The Late Cretaceous is made up of the Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian Stages (99-65 mya). The symbol on geological maps is K.

Crown Group:  In cladistic terminology, a crown group is a clade defined in terms of living organisms. For example, the crown group archosaurs are the last common ancestor of living birds and crocodiles and all of its descendants.

Cutler Formation: An Early Permian geological formation in Colorado, New Mexico, and Utah. This region, called the Colorado Plateau, is on  the western margin of the North American craton, which in the Early Permian lay between the equator and 10 degrees N latitude. The formation includes Cedar Mesa Sandstone, representing deposits of an extensive alluvial plain with braided streams. The Cutler Formation overlies the Abo Reef, and is partly contemporary with the Clear Fork Group in Texas and Oklahoma, several hundred miles to the east.

Cyanobacteria:  Blue-green algae, one of the most primitive forms of life. In the Precambrian period, these photosynthetic microbes were widespread. They had simple prokaryotic cells, lacking a nucleus and other organelles. Thus the term algae is really a misnomer, since algae are eukaryotes with complex cell structures. Cyanobacteria have survived until today from over 2.1 billion years ago, when they first appear in the fossil record at the Gunflint Formation in Canada.



dactylo-:  Greek root for "finger." An example is Pterydactylus, “winged finger”, a Late Jurassic flying reptile (151-148 mya), first found in  Bavaria, Germany. The name refers to  how the wing is attached to a single greatly extended finger.

Dapingian:  the first stage of the Middle Ordovician, dated at 472 - 468 mya..

Darbasa Formation: A Late Cretaceous (Campanian stage) formation in southern Kazakhstan.

Darriwilian: The second stage of the Middle Ordovician, dated at 468-461 mya.

dens, dentis:  Latin for “tooth”; often used as a suffix for taxa names (especially as “-don” or “-dont”). Examples are conodonts (“cone-teeth”), eel-like vertebrates with teeth living from the Cambrian through Triassic periods; and Cynodonts, “dog-teeth”, the Late Permian and Triassic synapsid group directly ancestral to mammals.

Devonian:  The fourth geological period of the Paleozoic Era, between the Silurian and Carboniferous Periods, and dating from 417 - 354 mya. It was named for exposures in the region of Devon, England. The Early Devonian comprises the Lochkovian, Praghian and Emsian stages (417-391 mya). The Middle Devonian includes the Eifellian and Givetian stages (391-370 mya); and the Late Devonian is divided into the Frasnian and Famennian stages (370-354 mya). The Devonian, often called the "Age of Fishes," was a time of major developments of aquatic groups, and terrestial plants. During the Late Devonian, a group of early tetrapods derived from lobe-finned fish first colonized the shorelines between 380 and 360 mya.

Diapsids: In skull anatomy, a grouping of reptiles based on the presence of two skull apertures behind the eyes on each side, used as jaw muscle attachment points (diapsida is Greek for “two arches”).  Diapsids include a number of both fossil and extant taxa including dinosaurs, crocodiles, most lizards, and birds. Diapsids (excluding birds) and anapsids together make up the large reptilian group called saurapsids.

Dimetrodon limbatusAn early Permian synapsid of the cotylosaur grouping, with a distinctive sail-back ridge composed of elongated neural spines. Dimetrodon, a large carnivore dating from 290-280 mya, has been abundantly found in the Clear Forks Group of northern Texas.

Dorsal: In anatomy, referring to a direction facing the back of the animal (the opposite of ventral, which refers to the front side). Also used in  dorsal vertebra, one of the vertebrae between the neck and the sacrum.



Edaphasaurus boanergiAn early Permian synapsid of the cotylosaur grouping, contemporary with Dimetrodon and also having a sailback of elongated neural spines, but a vegetarian or herbivore. Edaphosaurus is commonly found in the Permain Basin redbeds of Texas.

Edentulous: In anatomy, referring to animals without teeth as a normal condition. Examples are some Anomodonts, a group of Therapsids from the Late Permian and Early Triassic periods, who had hard, beak-like jaws used for eating plants.

Ediacaran: the last stage of the Precambrian, dated 635-542 mya, and named for the Ediacara Hills in Australia where many fossils from the period have been found. The Ediacaran is a synonym for the contemporary Vendian stage, named for a fossil-rich region in Russia. The Ediacaran biota is exceptional in that organisms lacking hard parts were preserved in medium-coarse sandstone deposits. Opinion is not yet resolved on what original body structures and/or taphonomic processes may have caused this (Conway Morris 1998). The Ediacaran fauna, in any, case, respresent the oldest known group of multicellular organisms with tissues. Some of the most common types found at Ediacara, and at contempary sites in Nova Scotia, resemble present-day disk-shaped jellyfish. Such disk forms are marked either with internal lines (e.g., the genera Arkarua, Dickensonia, Marywadea, and Yorgia), or concentric circles (Ediacaria). Other Ediacaran fossils resemble fronds of kelp-like seaweed with holdfasts or root-like anchors (Charnia). There also may be segmented organisms. While one or two of the Ediacaran taxa may have lasted into the Middle Cambrian and were found in the Burroughs shale, there is little overall continuity with the Cambrian, and no evidence of primitive notochords.

Efelian: The first stage of the Middle Devonian, dating from 391 - 380 mya.

Emsian: The third and last stage of the Early Devonian, from 400 - 391 mya.

Entelognathus primordialis:  a placoderm or armored fish from the Late Silurian (419 mya), the first fish to show osteichthyan-like jaw bones, including the dentary, maxilla, and premaxilla. The well preserved fossil of E. primordialis, about 20 cm (8 in.) long, was discovered in Yunnan province of south China by Zhu et al. (2013) in shale deposits of the Kuanti Formation.

Entropy: A concept in physics for how energy runs down or is dispelled. As defined in 1865 by Rudolf Clausius, Entropy is a thermodynamic property related to work performed by engines, machines, or energy conversion devices like refrigerators. During such work, entropy accumulates in the system, then dissipates in the form of waste heat. This is known as the Second Law of Thermodynamics.

Eocene: The second geological period of the Cenozoic, dating from 54.8 - 33.7 mya. The Early Eocene (34.8-49 mya) is equivalent to the Ypressian stage. The Middle Eocene (49.0-37.0 mya) includes the Lutetian and Bartonian stages. The Late Eocene  (37.0-33.7 mya) is the Priabonian stage. The Eocene saw much diversification and radiation of early primates throughout the world.

Ethmoid:  In skull anatomy, the ethmoid bone is the most anterior of the four principal braincase regions. The name  means “resembling a sieve,”  from Greek ethmos , “sieve”, and eidos, “resemblance”. The ethmoid bone is  associated with the nasal capsules and the sense of smell. In mammals, the ethmoid is reduced to a series of turbinals, very thin bones in the nasal passage that help recover respiratory water vapor.

Eukaryote: Organisms with complex cells, which include a nucleus containing the cell's DNA, and organelles such as mitochrondria, ribosomes, and chloropolasts, each of which also has its own DNA. Eukaryotes first appeared at or before 2 billion years ago, when oxygen in the atmosphere had gradually built up.

 Eusthenopteron fordii:  a lobe-finned fish (sarcopterygian) from the Late Devonian (380-360 mya) with a close relationship to tetrapods. While Eusthepteron was a strictly aquatic animal, it is classed as a tetrapodomorph, considered the most direct precursors to tetrapods. At least 2,000 Eusthenopteron fossils have been found at Miguashain in Quebec. Canada.  It grew up to 1.8 m in length, and had labyrinthodont teeth characterized by infolded enamel, as did all of the earliest known tetrapods.


Famennian:  The last stage of the Late Devonian, dated at 364-359 mya

Femur:  In skeletal anatomy, the upper leg bone. The femur articulates with the pelvis, and with the tibia  and, usually, the fibula.

Fenestra: In skull anatomy, the term fenestra (Latin for “window”) is  used for an opening in a structure, usually a paired opening in complemenary (right and left side) bones. The term is used for the skull openings providing jaw muscle attatchments in synapsids and diapsids, as well as in the anatomy of the ear region.

Fenestra ovalis: In ear anatomy, the “oval window” in the inner ear, which communicates with the stapes or columella, and with the operculum if present. (The same meaning as fenestra vestibulae, a synomym).

Fenestra rotunda:  In ear anatomy,  the “round window” in the inner ear, which relieves pressure in the inner ear. (The same meaning as fenestra cochleae, a synomym).

Fibula: In skeletal anatomy, the smaller of the two lower leg bones, which faces outside.

Floian: The second stage of the Early Ordovician, 479-472 mya. 

Foramen: In anatomy, an opening, usually a single opening in a bone for a nerve or blood vessel; from Latin foramen, “opening” (from the verb forare , “to bore” or “drill”).

Foramen magnum:  In anatomy, the large opening in the lower rear of the skull or occiput, through which the spinal nerves enter the brain (from Latin foramen, “opening” and magnum,”big” or “great”).

Fossa: In anatomy, depressed areas, gaps, or grooves in bones (from Latin fossa, “ditch”). An example in mammals is the canine fossa, a vertical groove on opposite sides of the lower jaw to accommodate large upper canine teeth when the jaw is closed.

Fossa trochanterica:  In skeletal anatomy, a gap on the head of the femur or upper leg bone, located between features known as the median tuberosity and the greater trochanter.

Francis Creek Formation:  A  shale formation in northeastern Illinois, dating from the Pennsylvanian Period (324-298.9 mya). The shale contains both fish and notochord fossils, some with excellent preservation, including a fossil hagfish named Myxinikela siroka.

Frasnian: The next-to-last Stage of the Devonian, dated at 377 - 367 mya.

Frontal: In skull anatomy, a paired midline bone of the vertebrate skull. The frontals lie behind the nasal bones and in front of the parietal bones, typically at the level of the eye orbits.


Ganoid: In fish anatomy, a heavy form of enamel characteristic of the scales of various early fish and extant Polypteriformes, such as the gar (Lepisosteus). 

Gauja Formation: A mid-Devonian formation in Latvia of sandstone and siltstone, representing a low energy fresh water environment, such as a meandering river channel. The Gauja Formation contains numerous fish fossils from the Givettian Stage (380-370 mya), including both lobe-finned fish (sarcopterygians) and antiarch placoderms, including psammosteids. Most of the fauna is comparable to that of the  Escuminac Formation in northern Canada.

Geology: The study of the earth and its layers.  Essential concepts include Stratigraphy (recording and interpreting the sequence of rock layers); Superposition (that the lower layers are normally older than the upper layers); and Formation (repeated groupings of rock layer sequences, and the history of their deposition). Fossils found in the different strata and formations are keys to dating and interpreting the ancient environments lived in by the ancient plants and animals.

Glenoid: In anatomy, the shallow cavity of the upper part of the scapula by which the humerus articulates with the pectoral girdle. The term glenoid fossa is also used to refer to the similar cavity in the squamosal bone of mammals, or the quadrate bone of reptiles or non-mammalian synapsids, forming the jaw joint.

Gondwanaland: The name of the southern landmass which existed between the Cambrian and Triassic periods, made up of South America, Africa, Madagascar, India, Australia, and Antartica. The fact that fossils of the Permian gymnosperm Glossopteris were found in all those now separated continents led Austrian geologist Eduard Suess to infer that they once made up an interconnected land mass, which he named Gondwanaland, after the district in India where Glossopteris fossils were common. This view has since been confirmed by conclusive evidence of continental drift.

Gracile: In anatomy, gracile refers to a slender or light-weight form, as opposed to a robust or massive form, of approximately the same length or height. Thus, in early hominids, Australopithecus robustus is contrastive to A. africanus, the latter being regarded as a gracile form.

Guadalupian Series: In the geological timescale, equivalent to the Middle Permian (272.3 – 259.8 mya), comprising three Stages: Roadian, Wordian, and Capitanian.

Givetian: The second and last stage of the Middle Devonian, dating from 380 - 370 mya. 

Guiyu oneiros: ("ghost fish"). The earliest known bony fish or Osteichthyan, whose fossils come from Chinese deposits dating from the Late Silurian, 419 million years ago (Zhu et al 2009).  About 33 cm in length, Guiyu had a combination of both ray-finned and lobe-finned features, although overall analysis has placed it closer to lobe-finned fish.

Gunflint Formation:  A formation of chert deposits around Lake Superior, dating from about 2.1 billion years ago. In the early 1950s, Taylor and Barghoorn (1954) discoved fossils in the chert of early, photosynthetic microbes called cyanobacteria or blue-green algae.  These photosynthetic bacteria were linked with the buildup of oxygen in the earth's atmosphere.

Gzhelian: The fourth and last Stage of the Pennsylvanian or Late Carboniferous, dating from 305.0 – 298.9 mya.

Haeckel's Law: An observation by 19th century anatomist Ernst Haeckl that “ontogeny recapitulates phylogeny.” Thus in the developmental or placental stage of a human infant (i.e., its ontogeny or coming into being), various ancestral forms emerge and then disappear (i.e., are recapitulated), including fish gills, and the reptilian jaw feature called Meckel’s Cartilage, prior to the formation of the mammalian ear,  jaw, and other features fully developed prior to birth.  Considered neither true nor false, but always thought-provoking, Haeckel's Law remains an important tool for studying developmental sequences in animals.

Hagfish: (class Myxini). Hagfish are primitive, eel-shaped marine creatures who live in the continental shelf waters off California. New Zealand, and East Asia. A total of 5 genera with 77 extant species of hagfish are recorded, which range from 4 cm to 1 m in length, lack side fins, and have simple paddle-like tails. They feed mainly on dead fish and polychaetes (sea worms) on the ocean bottom. Their vision is poor, but they have well developed senses of touch and smell, including four pairs of sensing tentacles arranged around their mouth, and two pairs of tooth-like rasps on top of a tongue-like projection  There are two fossil species quite similar to modern hagfish, both from the Pennsyvanian: Myxinikela siroka, and Myxineidus gononorum.

Haikouichthys: ("fish from Haikou"). An early Cambrian notochord fish genus from the Lower Cambrian Maotianshan shales at Ercaicun in Yunnan, southern China, Also found in the same formation were Myllokunmingia and Zhongjianichthys. All three genera haved been placed in the same family, dating from 535-520 mya. 

Hair cell: In ear anatomy,  a hair cell is a sensory cell connected with a nerve cell, and activated by microvilli or hairlike sensors or tendrils. The hair cells of the ear are located within the fluid-filled inner ear,which receives noise vibrations amplified by the ossicles of the middle ear. When these vibrations create mechanical pressure via the inner ear fluid, this deforms the microvilli, causing the cell to increase or decrease the rate at which it sends an electrical signal to the attached nerve cell. Depending upon the direction in which the villi are bent, the rate of signal discharge to the nerve cell increases or decreases. Hair cells are the basic cellular unit involved in both hearing, and in the labyrinth or balance organ. They are also used in the lateral line sensing system of fish.

Hell Creek Formation:  A Late Cretaceous formation in Montana, from the Maastrichtian stage (70-65 mya). Fossil taxa included multituberculates (insectivores), turtles, birds, crocodiles, and champsosaurs. G.G. Simpson and others from the 1930s -1960s also identified the small primate Purgatorius as being from the Hell Creek Formation, making it the earliest known primate. This identification has been much debated, with Purgatorius now thought to be from the early Paleocene, and possibly also not a primate but a multituberculate.

Heterodont:  In anatomy, a vertebrate with teeth of more than one form (Greek hetero, “different”; Latin –dont, “teeth.”) Mammals have heterodont dentitions with four forms of teeth, including incisors, a canine, premolars, and molars. The converse is homodont, such as the teeth of a crocodile which are all similar in form.

Heterostraci: ("Different scales"): One of three subclasses of Pteraspids, the others being Arandospida and Astraspida. They are a long-lived group who arose in the Ordovician, between 488 and 443 mya, and lived through the Silurian and into the Late Devonian (380-370 mya), primarily in marine and estuary environments. Heterostraca is divided into two orders, Pteraspidiformes ("Wing Shields"), and Cyathaspidiformes (“Cup Shields”). Members of the larger,  “wing-shield” order occur first in the Late Silurian, but become much more diverse during the Early Devonian   As with many agnathan groups, heterostracans had no fins besides the tail or caudal fin. 

Hirnantian: The third and last stage of the Late Ordovician, dating from 446-444 mya.

Histology: In biology, histology is the microscopic study of the anatomy of the tissues and cells of plants and animals. Histos is Greek for “tissue.”

Holotype: A single specimen designated as the name-bearing type of a species or subspecies when it was established.

Homodont: In anatomy, a vertebrate with all teeth of the same form, although they may be of slightly different proportions and different sizes. Examples are reptiles such as crocodiles with similar cone-shaped teeth; aquatic mammals such as dolphins and whales with thin, pointed teeth;  and some insectivores with small, pointed teeth. In all cases, the animals swallow their food whole and thus do not require extensive chewing. In the case of reptiles, the homodont dentition is a retention of an original, primitive condition of tetrapods. In the case of aquatic mammals such as whales and dolphins, their mammalian teeth were originally heterodont; the present homodont dentition is a later, derived trait related to changed eating habits in a marine setting.

Homology: Homologies are similar anatomical structures in different but related animals or plants, representing modifications of the same original structure of a common ancestor. Examples include the relatively large brains of humans and dolphins, both descended from a common early mammalian ancestors with relatively large brain.  This contrasts with analogy, where two structures are related by form and function unrelated to common ancestry. Thus dolphins and bony fish both have tail fins,  but have no common ancestor since the evolution of early tetrapods ca 360 mya. 


Icthys: Greek for “fish”, as in the two fish orders Chondricthyes, “cartilagenous fish,” or Osteichthyes, “bony fish.”

Ichthyostega:  ("fish roof").  An early tetrapod genus from the Upper Devonian (374 – 359 mya), up to 1 m. in length, and  one of the first tetrapods known in the fossil record Ichthyostega was first described in 1932 by the Danish paleontologist Gunnar Save-Soderbergh, who named five species from a collection of 14 fossil specimens, found in 1931 in Late Devonian rock strata by the Danish East Greenland Expedition. Ichthyostega was at first seen as transitional between fish and the early Stegocephalians (“roofed heads”), an amphibian group named in 1868 by the American paleontologist Edward Drinker Cope, for the large amounts of dermal armor on their skulls.  Ichthyostega combined  a flat, heavily armoured stegocephalian skull, filled with labyrinthodont teeth, with a fishlike tail bearing fin-rays. It had more than 5 digits on each foot, which was somewhat fin-like, and a vertebral system which was structurally weak.  The earlier Ichthyostega also possessed functional internal gills as adults.  At the close of the Devonian, forms with progressively stronger legs and vertebrae evolved, and the later groups lacked functional gills as adults. The genus became extinct during the Early Mississippian.

Ichthyostegalia: An order of early tetraforms created for Ichthyostega, which until the 1980s contained only three genera: Ichthyostega, Acanthostega, and Tulerpeton. Ichthyostegalia contains the most basal of tetrapods which have toes rather than fins. This includes all taxa more advanced than Tiktaalik, the closest relative of tetrapods known to have retained paired fins rather than feet (Coates and Clack 1990).

Incus: In ear anatomy, the incus (from Latin incus , “anvil”) is an auditory ossicle derived from the quadrate bone, which was part of the lower jaw in fishes, early tetrapods, reptiles, and non-mammalian synapsids.

Induan:  The first Stage of the Early Triassic, dating from 252.2 – 251.0 mya.

Innominate: In skeleton anatomy, the mammalian pelvis or hip bone, consisting of three consolidated bones, the ilium, ischium, and pubis, which are separate in reptiles and other tetrapods.. The name come from Latin innomina, meaning “unnamed” or "nameless".



 Jugal: In skull anatomy, the jugal bone forms the lateral margin of the orbit. It occurs in most reptiles, amphibians, and birds as well as mammals. In mammals, the jugal is often called the malar or Zygomatic.The jugal bone is the only bone that lies entirely in the zygomatic arch. At its anterior margin the jugal articulates with the maxillary bone.

 Jurassic: The second geological period of the Mesozoic era, coming after the Triassic and before the Cretaceous. By the Early Jurassic, the first mammals had evolved from cynodonts.


Kuanti Formation:  Shale and muddy limestone deposits from the Late Silurian (Late Ludlow stage) in Quijing, Yunnan province, China. These have produced significant fish fossils such as Entelognathus primordialis, a placoderm (armored fish) with osteichthyan-like marginal jaw bones, and Guiyu oneiros, the first known Osteichthyan or bony fish, both dating from about 419 mya.

Kungurian:  The fourth and last stage of the Early Permian dating from 283.5 – 272.3 mya.


Labyrinthodont: Pertaining to a type of complex folded-enamel tooth structure ("labyrinth teeth") in both early tetrapods and lobe finned fish (sarcoptygerians). This tooth structure, easily seen in microscopic cross-section, is the basis for identifying many early amphibians as Labyrinthodonts.

Lagerstätten: (from German lager, “storage” and stätte, “place”) Shale deposits with exceptional fossil preservation. Examples from the Cambrian period are the Burgess shale in British Columbia and the Maotianshan shales in Yunnan, China.

Lancelets: Slender, eel-like filter feeders, classified as Cephalochordates. They appear pointed at both ends, giving rise to the  name amphioxus (amphi = “both”, oxy = “sharp”).  Lancelets have a distinct, funnel-like mouth surrounded by oral cirri or tentacles,  somewhat like those found on fossil chordates such as Pikaia. The lancelet has a series of muscle segments called myomeres located both sides of the notochord, also found in the Cambrian chordates.

Linnean classification:  The hierarchical system of taxonomy first developed by the Swedith naturalist Carolus Linneus or Karl Linne (1690-1760), whose Systema Naturae was published in several editions from 1740 to 1760. This contained a comprehensive classification system for all known organisms. The Linnean system, which was universally adopted, was organized in levels ranging from general to specific, with categories including Kingdom, Phylum, Class, Order, Family, Genus, and Species. The system was later revised in the late 19th century by evolutionary phylogenists such as Ernst Haeckl and Thomas Henry Huxley, who added additional categories such as phylum, sub-phylum, superclass, etc.  in order to incorporate new findings in fossil taxa. The system is still standard for zoological and botanical classifications, and is used along with cladistic definitions in paleontology.

Lithosphere: The outermost shell of the Earth and other rocky planets (from Greek lithos, "rock", and spheros, "sphere"). The lithosphere includes the crust and the uppermost mantle, a rigid outer layer which is underlain by the asthenosphere, the weaker, hotter, and deeper part of the upper mantle. Over time the more rigid lithosphere breaks up into tectonic plates, while the underlying aestheosphere is more viscous and plastic. This results in movement of the tectonic plates around the surface of the earth, a process now understood as continental drift, first formulated by Alfred Wegener in 1912. The concept of the lithosphere itself was introduced by Joseph Barrell  (1914) and developed  by R.A. Daly (1940). The two essential concepts of a rigid lithosphere over a plastic underlayer, and of continental drift, were synthesized in the theory of plate tectonics which emerged in the 1960s.and 70s.

Lopingian Series:  In the geological timescale, equivalent to the Late Permian (259.8 – 252.2 mya), comprising two stages: Wuchiapingian and Changsingian.



Maotianshan shales: A Lower Cambrian formation with diverse shallow water fauna preserved in extensive fossil deposits at Ercaicun in Yunnan, southern China, (535-520 mya). Due to its excellent preservation of the soft body parts of numerous fossils of the Cambrian fauna, it is classed as a Lagerstätten deposit.

Meckel's groove: a feature of the dentary or lower jaw bone in reptiles related to Meckel's cartilage, which is retained in early placental stages of current marsupials such as short-tailed opossums (Monodelphis), and in Early Jurassic stem mammals such as Sinoconodon and Morganucodon.

Mississippian:  The first period in the Carboniferous system, equivalant to the Early Carboniferous, dating from 359 – 324  mya.

Morganucodonts: Small, early stem mammals named for a site in Glanmorgan, Wales, dating from the the Late Triassic to Middle Jurassic (~ 220-180 mya). Three main genera are known, Morganucodon, Eozostrodon, and Haldanodon, with widespread distribution in the northern hemisphere (Laurasia), as well as Africa and India in the southern, Gondwanan region (Kielan-Jaworoska et al. 2004). Morganucondonts had skulls 27-38 mm long and estimated body weight ranging from 27-80 grams (Luo, Crompton, and Sun 2001).  Various skull features representing mammalian traits first appear in  morganocodonts. They have a  mammalian jaw joint plus a tiny, retained reptilian joint, a condition which prevents their being classed as fully mammalian. The orbit around the eye is fully enclosed, as in all mammals. The development of the mammallian middle ear is also advanced in morganuconodonts, with both Meckel's groove and the angular bone more reduced than in Sinoconodon or nonmamalian cynodonts.

Morganucodon watsoni:  a stem mammal species discovered and named in 1949-1958 by Walter Georg Kuhne from Duchy Quarry in Glanmorgan, Wales. Fossils include teeth. jaw fragments, and post-cranial bones. The deposits at Glanmorgan are dated from the Sinemurian stage of the Early Jurassic, at about 200-190 mya (Kermack et al. 1981) 

Morganucodon oehleri: a stem mammal species named for a complete skull found in 1941 explorations of rich fossil beds in the Lower Lufang formation in Yunnan, China. This was first identified by E.T. Oehler in 1948, and later described by W. H. Rigney. Additional skulls of M. oehleri were found in Yunnan in the 1970s and 80s by the Beijing Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) (Crompton and Luo 1994).

Myllokunmingia: An early Cambrian notochord proto-fish genus from the Lower Cambrian Maotianshan shales at Ercaicun in Yunnan, southern China, dated at 524 mya. Also found in the same deposits were Haikouichthys and Zhongjianichthys. All three genera have been placed in the same family. 

Myxineidus gononorum: A fossil hagfish (class Myxini) from the Late Carboniferous (Pennsylvanian), found at Montceau-les-Mines, France. This fossil, preserved in a concretion, includes a natural cast of the pharynx and oral cavity, and the impressions of two pairs of symmetrical tooth rows, similar to those in modern hagfish.

Myxinikela sirokaA fossil hagfish (class Myxini) from the Pennsylvanian (324-298.9 mya), recovered from the Francis Creek Shale of northeastern Illinois. This fossil preserves the paired tentacles found in living hagfish, along with internal organs, and detail of the cranium. (Bardack, 1991).

mya:  Abbreviation for “million years ago.”



 Nasal: In skull anatomy, the paired midline bones over the nostrils which attach to the maxilla and frontal bones. The form of the nasal bones is quite variable in different vertebrates. In lobe-finned fish and tetrapods, the nasal bones are the most anterior of a set of four paired bones forming the roof of the skull, being followed in sequence by the frontals, the parietals, and the postparietals.



Ordovician: The second geological period of the Paleozoic, following the Cambrian and preceding the Silurian.

OssicleIn ear anatomy, any of the small bones in the middle ear used to conduct sound vibrations to the fluid-filled inner ear. The term is normally used to refer to the three ossicles in the mammalian inner ear, the stapes,  malleus (articular), and incus (quadrate). It also applies to the columella (hyomandibula) used as a sound amplifier by other tetrapods; and the bones in the Weberian organs of various fishes.

Oste--: A Greek prefix for “bone”, as in Osteichthyes, “bony fish”, or  osteology, the “study of bones.”

 Osteichthyes: The class of fish with bony skeletons, as opposed to fish with cartilagenous skeletons, including sharks and rays. Osteichthyes, defined in 1880 by Thomas Henry Huxley, is the largest class of vertebrates in existence today. They are divided into ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii, or "fleshy fin"). The sarcopterygians are the ancestors of all land vertebrates, while actinopterygians are make up over 95% of all extant fish, comprising about 28.000 species. The earliest known bony fish is Guiyu oneiros ("ghost fish"), who lived in Chinese water during the Late Silurian, 419 million years ago (Zhu et al 2009).

Osteolepis: ("bone scale"). A lobe-finned fish from the Devonian period, covered with large, square scales. Its fossil, found in Lake Orcadie of northern Scotland, show it to have been 20 cm. or 8 in. long. The scales and plates on its head were covered with  a thin layer of spongy, bony material called cosmine. Pores on this surface layer led to tubes or canals connected to sensory cells deeper in the skin, probably used for sensing vibrations in the water. Osteolepis is classed as a tetrapodomorph, with numerous features in common with tetrapods, and may have been fairly closely related to the first tetrapods.



Paleontology: The study of ancient life, based on the discovery and interpretation of fossil evidence. This is closely linked to stratigraphy, or the geological study of successive rock layers, and interpretation of the past environments they represent.

Paleozoic:  In the geological timescale, the era of  “ancient life” [from paleo- "ancient" and -zoic "life") representing living organisms from the start of the Cambrian period to the end of the Permian period (540-252 mya). The period of nearly 300 million years spans the first evidence of notochords and primitive vertebrates during the “Cambrian explosion,” through the rise of large reptiles and synapsids during the Permian, ending with the massive end-of-Permian extinction. 

Panthalassic Ocean: The large body of water, roughly comparable to the Pacific Ocean, which existed on the western side of the northern landmass Laurasia, and the southern landmass Gondwanaland, during part of the Paleozoic era, and most of the Triassic period. Panthalassic means "whole sea", from the Greek words pan ("all") and thalassa ("sea.")

Pennsylvanian: A geological period in the Paleozoic era, preceded  by the Mississippian and followed by the Permian. It is the second period in the Carboniferous system, equivalant to the Late Carboniferous, dating from 324 – 298.9 mya.

Permian: The final geological period in the paleozoic era, preceded by the Late Carboniferous (Pennsylvanian) and followed by the Triassic. 

Phanerozoic Era: In the geologic timescale, the era of "Visible Life" begins at the end of the Late PreCambrian at about 650 mya, when the earliest evidence of macroscopic (i.e., non-microscopic)  life appears.  Fossils resembling sponges or seaweed have been found in Australia at an inland site named Ediacara, giving its name to the Ediacaran stage. Another site with Ediacan fossils is in Novia Scotia.

Photosynthesis: a complex biochemical procedure which transforms sunlight and water into metabolic energy for plants and algae via the molecular exchange of ions. There are two phases of photosynthesis, a light phase which captures light ions from sunlight via pigments in chloroplasts, and converts them into free hydrogen ions, and a dark phase which combines the hydrogen with water. The overall process creates energy-storing molecules called ATP for the plant, and releases oxygen into the atmosphere as a secondary byproduct. This process began  in simple microbes called cyanobacteria by or before 3.5 million years ago, during the Archean era, and continues to be the mainstay of life on Earth.

 Phylogenetic systematics:  The term originally used by entomologist Willi Hennig (1966) to describe his system of biological classification, now called cladistics. Its approach, based on computer groupings of anatomical traits, may be compared to, and contrasted with, that of Evolutionary Phylogeny, which places a strong emphasis on paleontology, geological context, and comparison of fossils.

Pikaia: A notochord proto-fish from the Burroughs shale of western Canada dating from the Middle Cambrian (520 mya). It is classified by Conway Morris as a cephalacordata (Conway Morris and Caron 2012). Pikaia had a flattened body, notochord, paired muscle blocks (myomeres), head tentacles, and a series of short appendages, which may be linked to gill slits, on either side of its head.  In some ways Pikaia resembles the living lancelet Branchiostoma, which has a flattened body and a notochord. The lancelet, however, lacks  head tentacles and gill slits. The former trait is comparable to both extant and fossil hagfish, while gill slits are are also present in the Early Cambrian Chinese notochords Haikouichthys and Myllokunmingia.

Placoderm: Armored fish, whose expansion peaked in the Middle and late Devonian.

Plesiosaur:  A large diapsid sea reptile from the Mesozoic era whose fossils have been found worldwide. The name Pleisiosaurus means "near lizard" They are identified as Sauropterygia, a group of lepidosauromorph reptiles that returned to the sea. Most are extinct; the living Lepidosauria include snakes and lizards. Plesiosaurs first appeared in the Rhaetian stage of the Late Triassic Period, about 205 mya. They flourished through the Jurassic and Cretacous periods, lasting until the Cretaceous–Paleogene extinction event about 65 mya. The first described fossil of the order Plesiosauria was by William Stukely in 1719,  who saw it on a stone slab from a quarry at Fulbeck, England. Other famous examples were discovered in the 1820s by the fossil collector Mary Anning in Lyme Regis, on the south coast of England.  

Prokaryote:  The simplest forms of life, composed of a single cell lacking a nucleus, with the DNA used for reproduction simply floating about within the cell. Prokaryotes were the only life forms on Earth until about 2 billion years ago, when organisms called Eukaryotes with complex cells first appear, in conjunction with increased oxygen in Earth's atmosphere. Examples of  prokaryotes are bacteria; while algae are eukaryotes.

Proterogyrinus:  A fossil amphibian whose name means “early tadpole”, dating from the Late Mississippian period, about 330-320 mya. Proterogyrinus, found in Novia Scotia, was a medium sized reptilomorph over 1 meter long, partly reptilian in its anatomy, including its vertebrae, pelvis, humerus, and digits, but retaining amphibian labyrinthodont teeth, skull, and ankle bones.

Proterozoic: In the geological timescale, the era of  “proto-life" or "preliminary life”. This began about 2 billion years ago during the Precambrian, when Eukaryotic organisms such as algae appear, probably related to an increased oxygen supply The oxygen in Earth's atmosphere had  been gradually created by photosynthetic microbes called cyanobacteria during the preceding Archaan era. The Proterozoic era lasts until about 700-650 mya, when the Phanerozoic or era of "visible life" began.  By then, multi-celled forms of marine life had appeared, ranging from photosynthetic seaweed (algae) to various multi-celled animals resembling sponges, whose fossils have been found in coastal sites in Australia and Nova Scotia.

Pteraspidomorphi: (“wing-shield forms”): A class of early jawless fish who lived from the Ordovician through Devonian periods (495-359 mya). They are characterized by massive dermal head armour composed of median, ventral, and dorsal plates.  Many had hypocercal tails to facilitate swimming. The taxon contains the subclasses Arandaspida, Astraspida, and Heterostraci. Some species may have lived in fresh water (Janvier 1997).

Pteraspidiformes ("Wing Shields") one of the two orders of Heterostraci, the other being Cyathaspidiformes. They occured first in the Late Silurian, but became much more diverse during the Early Devonian. The Pteraspidiformes are divided into five families, Anchipteraspididae, Protopteraspididae, Pteraspididae, Protaspididae, and the Psammosteidae. Four of the five heterostraci families became extinct during the late Devonian (ca. 380-370 mya). The fifth, Psammosteidae, lived until the major Devonian extinction at 359 mya.

Pter- :  The Greek root for “wing”. Used in various descriptive terms such as the pteroid bone, or in taxa names such as Pterodactylus ("winged finger"), the first flying reptile to be discovered.





In the geological timescale, referring to the Glacial era of the Pleistocene, combined with the Recent or Holocene period, from about 2-0 mya. In an early 19th century formation, The Quaternary was preeded by the Tertiary, which includes the first five periods of the Cenezoic (Paleocene, Eocene, Oligocene, Miocene, and Pliocene). While the original system using “Tertiary” and “Quaternary” is largely defunct, these two terms have remained in familar use.



Red Bed:  In geology, red beds are sedimentary layers which are predominantly oxidized or red in color, but also may contain reduced layers which are grey, grey-green, brown or black layers. Red Beds represent non-marine depositional environments which were originally exposed to the air at least periodically. These derive primarily from riverine deposition in deltas, alluvial fans, floodplains, channels, and points bars, as well as lake strata. The oldest known red beds date from the early Proterozoic era, about 2.1 billion years ago, representing rivers from that period.

Regression:  In geology, a period of low sea level when previous marine sediments are exposed. It is the opposite of transgression, when the sea level rises and the coastal areas are flooded.

Reptile: The term reptilia was originally used as as a class of vertebrates in Linnean taxonomy. In the late 19th century, after the theory of evolution became current in paleontology, the term amniote was introduced as a phyllum by Ernst Haeckl to distinguish reptiles from amphibians, on the basis of reptiles being amniotes (i.e., born in protective amniotic eggs), while amphibian tadpoles were released in water. At about the same time, Thomas Henry Huxley (1863) distinguished, on the level of subclass, between “lizard face” (sauropsid) from “beast face” (therapsid) reptiles, the latter constituting “mammal-like reptiles” whose fossils were beginning to be recognized in Late Permian deposits in South Africa. In the early to mid 20th century, Alfred Romer distinguished diapsids and anapsids (both reptiles) from synapsids (which include therapsids), the latter deviating from reptiles and ancestral to mammals. Presently, the term synapsid is used as primarily contrastive with reptile, since the split between the two groups was early (ca. 315 mya), arising not long after reptiles (amniotes) split from amphibians (non-amniotes).

Reptilian jaw:  In skull anatomy, a jaw where the joint between the lower jaw and skull comprises the articular and quadrate bones, as opposed to the mammalian form of jaw articulation, made up of the dentary and squamosal bones. Thereby, the reptilian articular and quadrate bones have been transformed into the malleus and incus of the mammalian middle ear.

Rhaetian:  The last Stage of the Triassic, dating from 210 - 206 mya.

Rhino-: Greek root for "nose", as in Rhinocerous, “Horny nose”; or platyrrhine, "flat nosed" for New World Monkeys.

Roadian: The first stage of the Middle Permian, dating from 268.8 – 265.1 mya.

Romer's Gap: Named for the paleontologist Alfred Romer (1956), who described a discontinuity in the fossil record between vertebrates at the end of the Devonian period, with its high diversity of fishes, and low density of fossil assemblages of both fish and tetrapods in the early Carboniferous. Romer's gap (360-345 mya) corresponds to the first 15 million years of the Carboniferous, the early Mississippian (Tournaisian stage). Several lines of evidence tend to confirm Romer's observations. A major extinction event at the end of the Devonian reduced most surviving marine and freshwater groups to a few lineages. Before the event, oceans and lakes were dominated by lobe-finned fishes and armored fishes called placoderms. After the gap, modern ray finned fish, as well as sharks and their relatives, became the dominant forms.  The period also saw the demise of the Ichthyostegalia, early fish-like tetrapods with more than five digits, named for the type genus Ichthyostega  (Coates et al. 2008). Evidence for a sharp reduction  in marine fishes at the start of Romer's Gap is also seen in an equivalent rise, during the same period,  in hard-shelled crinoid echinoderms which were previously the chief prey of shell-crushing predator fish. When the number of shell-crushing ray-finned fishes and sharks increased later in the Carboniferous, coincident with the end of Romer's gap, the diversity of crinoids with Devonian-type protective shells fell sharply (Kammer and Ausitch 2006)


Sacabambaspis:  A genus of jawless fish of the order Pteraspidomorphi: (“wing-shield forms”) from South America, which lived from the Ordovician through Devonian periods (495-359 mya). Sacabambaspis had a head shield with a large dorsal (upper) plate that rose to a slight ridge in the midline, and a deep curved lower or ventral plate. Linking these along the sides were narrow branchial plates which covered the gill area. The rest of body was covered by long, strap-like scales behind the head shield.  This genus (related to Arandaspis) is also characterized by large, frontally positioned eyes, between which are two small nostrils, and a mouth armed with thin oral plates.

Sacral: In skeletal anatomy, pertaining to the sacrum, the lower region of the vertebral column involved with the pelvic girdle. The sacral vertebrae lie between the dorsal (or, in mammals, the lumbar) and the caudal (tail) vertebrae. They are usually fused to each other and to the ilium.

 Sagittal crest: In skull anatomy, a bony ridge running longitudinally along the top of the cranium or braincase. It is used as a jaw or facial muscle attachment, and is often formed by a coalescence of temporal ridges.  Mammals with prominent sagittal crests include extant gorillas (Gorilla)  and chimpanzees (Pan), particularly the males of either genus; and the early hominid Australopithecus boisei, found in the Olduvai Gorge of Tanzania, and dating about 2.0 mya.

Sakarya Continent: In geology, the southern part of northwest Turkey. From the Jurassic and Early Cretaceous through Eocene periods, this represented a micro-continent composed of a carbonate platform.

Sakmarian: The second stage of the Early Permian, dating from 295.0 – 290.1 mya.

Sandbian: The first Stage of the Late Ordovician, dating from 461-456 mya..

Santonian:  A Stage of the Late Cretaceous, dating from 85.8 - 83.5 mya.

Sauropsid:  Sauropsids (“lizard faces”) were a late 19th century grouping of fossil reptiles, which according to current definitions, may be said to include both anapsids and diapsids. The term sauropsid was coined by Thomas Henry Huxley in 1862 to include most fossil and extant reptiles. It later was contrasted with Therapsids (“Beast faces"), equated with mammal-like reptiles. In present usage Sauropsid is used as a generic term for non- synapsid reptiles. Synapsid, meanwhile, is typically used to descripe the tetrapods who split from reptiles (diapsids and anapsids), and which eventually were ancestral to mammals.

Scala tympani:  In ear anatomy, the compartment of the inner ear adjacent to the fenestra rotunda (“round opening”).  The scala tympani is immediately impacted by vibration of the middle ear ossicles.

Scala vestibuli:  In ear anatomy, the scala vestibuli is the compartment of the inner ear adjacent to the fenestra ovalis (“oval opening”).  It contains the vestibular apparatus.

Scapula: In skeletal anatomy, the scapula or “shoulder blade” is one of the two main bones of the pectoral girdle, the other being the coracoid. The scapula is the principal link between the forelimb and the main or axial skeleton. It normally contains the glenoid process or joint of articulation with the humerus, It also articulates with both the clavicle (via an acromion process) and the coracoid. It also articulates with the ribs and/or spine. The scapula may be fused with the coracoid to form a scapulocoracoid.

Sclera:  In anatomy, a recess or compartment, as that containing the circle of bone(s) supporting the orbit, called the sclerotic ring.

Sclero-: A Greek prefix meaning "hard." The Latin equivalent is dura-.

sclerotic ring: In skull anatomy, a ring of bone or small bones inside the sclera or eye compartment, serving to support the eye. It is found, for example, in Late Silurian and Devonian fish.

 Secondary Palate:  In skull anatomy, the structural separation of the mouth and upper throat from the nasal passages. This functions to separate the the acts of  respiration and feeding - an important adaptation permitting the organism to eat and breathe at the same time, or to breathe without opening the mouth. The secondary palate requires secure attachment of the upper jaw to the braincase,  and is thus inconsistent with extreme jaw kinesis or very wide mouth openings such as in crocodiles.

sensu latu: A Latin term meaning “in the broadest sense”. The opposite is sensu strictu , meaning “in the narrowest sense,” or “strictly speaking.”  .

Shale: In geology, a sedimentary rock primarily composed of clay and other fine-grained and organic particles, normally deposited by water in relatively calm conditions. Given its fine-grained structure, shale is often an ideal medium for fossil preservation.

Silt:  In geology, fine sedimentary particles, defined as between 1/256th and 1/16th mm in diameter.

Siltstone:  In geology, a fine-grained sedimentary rock primarily composed of silt, usually deposited in very quiet water conditions such as on river levees by retreating floods , or by aeolian processes (wind). A frequent term for water-deposited siltstone is mudstone.

Silurian Period:  The third geological period of the Paleozoic Era, from 443-417 mya, between the Ordovician and Devonian Periods. The Early Silurian (443-423 mya) is usually referred to as the Llandovery (443-428 mya).   The Middle Silurian is the Wenlock (428-423 mya). The Late Silurian (423-417 mya) includes both the Ludlow (423-419 mya) and Pridoli (419-417 mya) stages.  Most of the major groups of fishes probably originated by the Early Silurian.

Sinoconodon ("Chinese tooth") A proto-mammal from Lower Lufeng Series in Yunnan province, China, dating from the Sinemurian stage of the Early Jurassic period (208-191 Mya). Sinoconodon is regarded as the most basal of the mammaliaforms, and the sister−taxon to a clade that includes Morganucodon and the living mammals ( Luo et al 2002). Fossils of Sinoconodon rigneyi, discoverd by Patterson & Olson (1961), include a partial skull and post-cranial fragments from several individuals. It shows a unique combination of reptilian and mammalian features, including a double jaw joint seen in other advanced cyndonts. This comprised a mammalian jaw joint between the dentary and the squamosal bones, which had replaced the primitive reptilian joint between the articular and quadrate bones, although a tiny reptilian jaw joint was still present. 

Sister group:  In cladistic terminology, a sister group is a clade thought to be the closest genealogical relative of a second taxon, with which it shares a common ancestor.

South American Land Mammal Ages: A chronology of the South American Cenozoic era (Paleocene through Holocene Periods, 65-0 mya) based on characteristic terrestrial mammal assemblages.

Sphen-: A Greek prefix meaning "wedge" or "wedge-shaped."

Sphenoid: In skull anatomy, the region of the braincase associated with the eye orbit. It lies between the ethmoid and otic regions. The sphenoid is typically made up of several bones, including the orbitosphenoid, the basisphenoid and, in various taxa, the alisphenoid, and mesethmoid.

Spiracle: In skull anatomy, a small respiratory aperture or opening behind the eye of certain fishes, such as sharks, rays, and skates, through which respiratory air or water is admitted and/or expelled. This is thought to be a remnant of the gill slit from the hyoid arch. The term is also used independently for the blowhole of a cetacean (whale).

 Squamosal: In skull anatomy, a bone located on the back or posterior corners of the skull. It is closely associated with the quadrate bone and, in amniotes, with the temporal fenestra(e). 

Stanton Formation: A limestone formation in Kansas, dating from the Late Pennsylvanian and earliest Permian periods.  Primarily a marine and deltaic component noted for crinoids, it also has an important terrestrial or lacustrine component with a Permian-like biota, including fossil conifers, insects, and the edaphosaurid Ianthasaurus. (Formerly called the Admiral Formation or Stanton Limestone).

Stapedius muscle:  In ear anatomy, a small muscle of the middle ear which acts to damp excessively loud sounds by restricting the movement of the stapes.

StapesIn ear anatomy, the stapes (Latin for “stirrup”) is a small bone used as an ossicle or sound amplifier. It evolved from the hyomandibular bone of fish. Originally this may have been the main upper element of a gill arch in jawless fish. It later (by the Late Devonian) appeared in lobe-finned fish as the hyomandibular bone, an accessory jaw element. In early tetrapods from the Late Devonian and early Carboniferous, it became a stout bone bracing the braincase against the quadrate. As the paroccipital process took over this function, the stapes was reduced, eventually (by the Triassic period) becoming specialized for hearing as the columella  in reptiles and amphibians, or the stapes in mammaliforms. The stirrup-shaped stapes in mammals has a footplate which fits over the foramen ovale.

Stem group: In cladistic terminology, the term “stem group”  or simply "stem" as an adjective,  is often used as a mixed, paraphyletic, or otherwise loosely defined grouping. An example is stem tetrapod, which includes Baphetidae, Colosteidae, and other difficult-to-classify amphibian taxa outside the probable crown group of Tetrapods.

Steno's Law: The rule of superposition, namely that in undisturbed locations, the deeper the stratum, the older it is; and conversely, that the topmost strata are the youngest. Nicholas Steno was a 17th century Danish anatomist who first formally defined the rule of superposition with regards to understanding context of fossils within geological strata.

Strata: In geology, a term for layers or levels (from Latin "strata" for "spread layers"; stratum for a single layer), The term was originally used in Roman road building for spread layers of paving stones; paved roads were called strata via. Now the term is used generically for any kind of layers or layering, such as the "upper strata" in society.

Stratigraphy:  In geology, pertaining to the vertical position of any individual rock layer (stratum) within a rock column, and the fossils or other inclusions found within that stratum. In terms of relative geological age of sedimentary rock, in any given sequence, undisturbed columns have older formations in lower positions (see also Steno’s Law of Superposition).

Symphysis: In anatomy, an area where two paired bones meet and articulate, particularly the bones of the lower jaw (i.e., the "mandibular symphysis").

Synapomorphy: In cladistic terminology,  an anatomical trait or character which is shared by all basal members of a clade and is derived from their common ancestor. The synapomorphy is the main criterion used to infer phylogeny or the evolutionary relations of clades. 

Synapsid:  The term synapsid ( “one arch” or "one opening") was originally used by Romer (1956) as a grouping of the class of reptiles, referring to those whose skulls had one aperture on each side, as contrasted to both anapsids (“no arches”) and diapsids (two arches).   The term synapsid is now used to distinguish the reptilian ancestors of mammals from the ancestors of later reptiles, who split from each other quite early. The earliest synapsid presently known, Eocaseia martini from Kansas, dates from the Pennsylvanian period at about 315 mya, not that long after the earliest reptiles, who were amniotes, were derived from non-amniotic amphibian reptiloforms, which occurred during the Mississippian or early Carboniferous period.

Syncline:  In structural geology, a syncline is a fold with younger layers closer to the center of the structure. On a geologic map, synclines are recognized by a sequence of rock layers that grow progressively younger, followed by the youngest layer at the fold's center or hinge.  For example, in the Moscow Syncline in western Russia, the sequence of exposures in a group of concentric formations is 1) Permian  2) Triassic  3) Jurassic., with the oldest formation, the Permian, on  the outside, containing both the Triassic and Jurassic, and the youngest formation, the Jurassic, nested within the Triassic.

Systematics:  The science and methods of taxonomic classification. Examples in biology are the Linnean system, introduced by the Swedith naturalist Carolus Linneus in Systema Natura (1740), and continously refined since then; and Cladistics, developed by Swiss entomologist Willi Hennig in his book Phylogenetic Systematics (1966).


Tethys Sea: The ancient area of ocean between China  and the southern landmass called Gondwanaland, as named by Austrian geologist Eduard Suess in the 1860s. Combined with the larger Panthalassic Ocean in today's Pacific area, the Tethys Sea was one of the two main bodies of water surrounding the mega-continent Pangaea, during the Permian and Triassic periods.

Tibia: In skeletal anatomy, one of the two lower leg bones, along with the fibula. The tibia is the thicker of the two, and constitutes the shin bone, extending from the knee to the ankle.

Triassic:  The geological period dating from 252.2 – 214 mya, following the Permian period, and preceding the Jurassic period. The Triassic is the first of three periods of the Mesozoic era.


Ulna: In skeletal anatomy, one of the two lower arm bones, along with the radius. The ulna is the thicker of the two, and is on the outside, whereas the radius rotates with the interior wrist. The other end of the ulna forms part of the elbow, where it joins with the humerus or upper arm bone.

Ungulate:  mammalian herbivores with hoofs, usually of relatively large size. The grouping contains two main orders, odd-toed ungulates (Perissodactyla) and even-toed ungulates (Artiodactyla). The odd-toed ungulates include three families: equidae (horses, zebras, and asses), tapiridae (tapirs), and rhinoceratidae (rhinoceroses). The even-toed ungulates include camelidae (camels and llamas), suina (pigs and peccaries), cervoidea (pronghorn, giraffes, okapi, deer, musk deer, oxen, cows, and antelopes),  hippopotamidae (hippopotamuses), and Cetacea (whales and dolphins).


Vendian: The last stage of the Precambrian, used for Old World fossil-producing sites such as found along the White Sea in Russia near Archalesk, dating from about 625-540 mya. It is a synonym for, and contemporary with, the Ediacaran stage, named for a fossil site in Australia.

Ventral: In anatomy, this refers to the front side, as opposed to the dorsal which refers to the back. T

Vertebrate: Animals with backbones. In the Linnean system of classification, vertebrates are a sub-phyllum of the phyllum notochord, which is the more primitive condition.  


Whatcheeria deltae: An Early Mississippian tetrapod found in Delta quarry, Iowa, dating from about 340 mya.

Wuchiapingian: The first stage of the Late Permian, dating from 259.8 – 254.1 mya.

Wordian:  The second stage of the Middle Permian period, dating from 268.8 – 265.1 mya.



Yunnanozoon lividum: A primitive chordate from the Lower Cambrian Chenjiang fauna, found in the Maotianshan Shale deposits of Yunnan province, southern China. Yunnanozoon ("Life form from Yunnan") is somewhat comparable to the Burgess shale fossil Pikaia. It was probably a cephalochordate, something like a lancelet. Yunnanozoon was only about an inch long (1.6-2.2 cm). It is represented by about 60 fossil specimens (Chen and Huang 2008).


Zhongjianichthys: An early Cambrian notochord, proto-fish genus from the Lower Cambrian Maotianshan shales at Ercaicun in Yunnan, southern China, Also found in the same formation were Haikouichthys and Myllokunmingia . All three genera have been placed in the same family dating from 535-520 mya.

Zygomatic arch: In skull anatomy, the cheekbone is composed of the zygomatic process of the temporal bone, which extends forward over the side of the skull, and over the ear opening, and attaches to the jugal bone which forms the lateral margin of the orbit. The term zygomatic derives from the Greek word zygoma for "bar" or "yoke"; the same meaning as in Latin juga.






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