Amniotes vs. Anamniotes
As currently understood, the first reptiles evolved from reptilomorph amphibians during the Paleozoic era about 335-320 million years ago (mya), in the Late Mississippian or Early Pennsylvanian (Carboniferous) period. An early, and definitive reptile innovation was the amniotic membrane, which surrounds and protects the developing embryo in all reptiles, birds, and mammals (fig.1). This provided a major advantage for reptiles over amphibians, in that the reptiles could move away from water into inland and upland environments, and exploit more food resources.
As discussed in the section on Classification, the term Amniota was introduced by Ernst Haeckel in the 1860s and 70s as a superclass for all reptiles, birds and mammals. This remains in standard usage. Amphibians, tetrapods, and fish, on the other hand, who lack this trait, are classified as Anamniotes. The first eggs provided with an amniotic membrane were probably small, lacked a shell or outer leather membrane, and were laid by small adult females in a transitional environment such as moist leaf litter, that stayed humid and protected both the adult and the egg from desiccation.
The reptile egg (fig.1), besides having the amnion or protective membrane over the embryo, has two attached sacs, one called the allantois which exchanges fluids with the embryo, and the other the yolk sac which contains food. Around these is an inner wall called a chlorion, with all being encompassed by a shell.
Fig.1: Diagram of the amniotic egg of a reptile.
Primitive reptiles have long been recognized to have split into two major branches, sauropsids and therapsids (now usally called synapsids)  The first branch, the Sauropsids ("lizard faces") include the forerunners of most reptiles, from dinosaurs to crocodiles and turtles. The term Sauropsid for ancestral reptiles was coined by Thomas Henry Huxley in 1863, when he grouped the vertebrate classes informally into mammals, sauroids, and ichthyoids (the latter containing the amphibians and fish). He subsequently proposed the names of Sauropsida and Ichthyopsida for reptiles (amniotes) vs. amphibians and fish (anamniotes).
term Sauropsida was reused later by E.S. Goodrich in 1916,
in the context of much greater fossil information by then available on
mammal-like reptiles (synapsids) from
Romer (1956) later subdivided the sauropsids into three basic groups, recognizable by the number of openings or fenestrae (Latin for "windows") in the side of the skull, used for jaw muscle attachment (fig.1). This anatomical criteria for all reptiles remains in widespread use (Benton 2005). The anapsids, with no fenestrae, include many basal reptiles or eureptiles, some lizards, and perhaps also the turtles (molecular studies, however, indicate the placement of turtles within diapsids). The diapsids, with two fenestrae, produced the dinosaurs, and more recently, the birds. Huxley (1876) had correctly inferred that birds, who are diapsids, evolved from lizards including dinosaurs, which are also diapsids. An offshoot group of the diapsids, called the eurapsids, with two fenestrae fused into one, produced the large marine reptiles called ichthyosaurs, including Pleisiosaurs and Mosasaurs who were common during the Cretaceous.
Since the advent of cladistics in the late 1960s and its strict focus on phylogeny or evolutionary links, the term Reptilia has fallen out of favor with taxonomists as too imprecise, with the term Sauropsida used in its place to include a monophyletic group containing reptiles and birds. It seems very likely, however, that common usage will long retain traditional distinctions between Reptiles, Amphibians, Fish, and Mammals (all shown as Classes in the Linnean scheme).
Synapsids and "beast faces"
The other major branch of reptiles, the Synapsids (fig.2), have a single large fenestra on each side of the skull, located behind the eyes. The synapsids developed an aorta or arch on the left side only, and strengthened the skull by moving the quadrate bone up and back, eliminating the otic notch. This modification of the jaw (effectively, eliminating the location of the reptilian ear drum, in the otic notch) proved essential in the later development of more complex anatomy related to mammalian hearing (Hunt 1994-7).
Synapsid development led to a larger brain, more developed hearing, heterodont teeth, and homeothermy or regulated body temperature. A group of synapsids known as the Therapsids ("beast faces; the term coined by Goodrich ), equivalent to the term "mammal-like reptiles," are considered to be the ancestors of mammals, by way of the Cynodonts, mammal-like synapsids in the Early and Middle Triassic. Evidence on transitional fossils bears out the development of these late, advanced synapsids into early mammals by the Late Triassic and Early Jurassic. Synapsids and their descendants are covered in later sections.
Fig. 2: Skull fenestrae (after Benton 2005).
The following discussion presents a summary of
some of the
earliest types of Saurapsid reptiles.
The following discussion presents a summary of some of the earliest types of Saurapsid reptiles.
Romer (1956) defined the order Cotylosaura (“socket-lizards”) as a diverse assemblage of primitive anapsid reptiles containing, among other groups, the Captorhimomorphs, Diadectics, and Pareisours. Of these, only the captorhinomorphs are recognized as being related to more modern reptilians (Heaton 1979). Since they have been defined as being at the base of reptilian phylogeny, a number of detailed descriptions of their anatomy have been made by Carroll (1970), Carroll and Baird (1972), Clark and Carroll (1973), Heaton (1979), and others, who probably would be classified as evolutionary systematists (see section on Classification). More recently, cladistic analyses of Captorhinomorphs have been compiled by Reisz et al (2011), shown below in fig.2..Eureptiles and Captorhinomorphs
In an alternative scheme attributed to evolutionary systematists (Palaios 2014b), the most basal reptile group are the Captorhinomorpha, thought to be the single ancestral stem group from which all other reptiles evolved; and from them, birds and mammals (Carroll 1988 ). The name Captorhinomorpha is derived from that of the type genus Captorhinus, meaning "nose capturer.".
In the scheme of Benton (2005), the diapsid reptiles, who were later to flourished in the age of reptiles (Triassic through Cretaceous), are classified as descending from Romeriids, one of the Eureptile groups. They and anapsids are thus distinguished from synapsids who separated from all other early Sauropsids at some time in the Pennsylvanian. Sauropsids then developed during the Permian and Early Triassic into Therapsids and Cynodonts, the forefunners of early mammals (Benton 2005; Palaios 2014).
The Eureptiles were terrestrial forms who were still partly lizard-like, characterized by very primitive features, showing that they had only just diverged from their reptiliomorph ancestors (Palaios 2014b). They had rounded, amphibian-like skulls, with most taxa aretaining from fish and tetrapods an opening in the frontal bone for the pineal gland (a “third eye”). They also had amphibian-like shoulders and hips (small, relatively weak pectoral and and pelvic girdles), and limbs (i.e., fused lower leg bones). The rest of their skeleton was mainly reptilian, with spool-shaped vertebral centra.
on early reptile familes from both the
Eureptile and Anapsid branches of Saurapsids. In the Eureptiles, this
focus on early reptile familes from both the Eureptile and Anapsid branches of Saurapsids. In the Eureptiles, this includes
theProtothyrididae family (within the Romeriida clade), plus one very early genus named Casineria, still unclassified but which is probably a Eureptile. Within the Anapsid branch, descriptions are given of a number of genera within the Captorhinidae family, also considered as very early reptiles.
Casineria was a small insectivore with five fingers, and with claws on each hand, representing the earliest evidence of claws. The development of claws is linked to the formation of keratinous scales in reptiles; thus Casineria very likely had scaly, reptilian skin. The skull of Casineria had no otic notch, as is also the case with another early Eureptile, Brouffia, described below (thus, it probably had no tympanic membrane or eardrum, and consequently had poor hearing of high frequency sounds). Casineria had a mixture of anatomical features, some from the amphibian reptiliomorph groups such as Seymouriamorpha and Diadectomorpha, but others linking it with early reptiles, such as lighter leg bones, unfused ankles, and toes terminating in claws.
All this would have enabled the animal to walk more efficiently, and indicates a primarily terrestrial lifestyle. The small size of Casineria is also seen as relevant to a speeded-up evolutionary path, which has probably occurred often in vertebrate branches. Smaller animals have relatively short lifespans, but generally mature faster and thus reproduce more often, which likely had evolutionary significance in the case of Casineria (cf. Carroll 1970).
Several anatomical traits characterize the romeriids. Skull changes include the separation of the tabular bone from the opisthotic bone. In the post cranial skeletal, there are vertebrae changes to stronger, more reptilian forms, and a trend to longer and more slender wrist and akle bones (carpals and tarsals).
Protorothyrididae is a family within the Romeriida clade, representing small, short legged, lizard-like insectivores. They lived from the Late Carboniferous to Early Permian periods (Pennsylvanian-Asselian stage, dated at 307.1–294.6 mya), in the midwestern region of North America. In the cladistic analysis of Müller and Reisz (2006), the Protothyrididae aree defined as the stem lineage from which the Diapsids evolved. Among the basal eureptiles in this family are Brouffia, Coelostegus, Paleothyris, and Hylonomus.
Protorothyris archeri was first named by Price (1937). Carroll & others place this as early amniote stem group with Hylonomus.
. Cephalerpeton is a protorothyridid found at the Mazon Creek site in Illinois, in the Francis Creek Shale Member of the Carbondale Formation. These levels date from the late Westphalian stage of the Pennsylvanian (Late Carboniferous, 306 mya). Its holotype at the Yale Peabody Museum (YPM 796) is a partial skeleton. The type species, Cephalerpeton ventriarmatum, was first named by R. L. Moodie in 1912 as an amphibamid amphibian . The genus was later assigned to the basal reptilian Protorothyrididae family by Robert L. Carroll and Donald Baird in 1972, and this placement has been widely accepted.
Its upper (maxillary) teeth were very large. The mandible was concave on its upper (dorsal) surface, in order to accommodate the upper teeth. The maxilla was slightly raised to just above the lower rim of the orbit. the premaxilla was bent downwaard, with the longest premaxillary teeth located toward the center. The palate was relatively shorter. The pterygoid bone in the skull also had a single transverse row of teeth.
orientalis is a species of basal reptile dating
from the Late Carboniferous,
and found at
Nyr'any in the Czech Republic.
Müller & Reisz (2006), they are even more basal than Paleothyris, in the
Class: Sauropsida : Clade: Anapsida : family: Captorhinidae
Captorhinidae is a family of Anapsids dating from the Late Carboniferous (Pennsylvanian) and Early Permian. It was first defined as a family by Case (1911), based on examples of Captorhinus aguti found by Cope (1882) from the Arroyo Formation of the Leonardian phase of the Early Permian (280-270 mya) in northern Texas. The name, Captorhini or "capture-nose", refers to a view that the hook-like premaxilla was used to capture prey (Cope 1882; Palaois 2014b). The family was further defined in numerous studies, including those of Romer (1933, 1956), Fox and Bowman (1957), Clark and Caroll (1973), Hatton (1979), and Kissel et al. (2002). Its taxa have been recently studied by Reisz et al. (2011), whose analysis of their phylogeny or ancestral relations is presented in a cladogram (fig.3).
Captorhinids inhabited an area of Pangea called the Wichita uplift, now part of the Permian Basin in Oklahoma and north Texas. They have broad, robust skulls that are generally triangular or heart- shaped when seen from above (i.e., in dorsal view), caused by a swollen cheek region. The premaxillae are characteristically hooked or downturned. Early, smaller forms including Protocaptorhinus and Romeria possessed single rows of teeth, while larger, more derived forms such as Captorhinus aguti have two or three rows of teeth on both the maxillary and dentary bones. The teeth are short, with chisel-like tips.
The stapes or columella, a hearing ossicle, is short. The jugal bone has a sharply pointed anterior process, and lacks a medial process, with distinct anterior and posterior ventral margins. (Müller & Reisz 2005)
Fig.3: Cladogram of Captorhinidae (after Reisz et al. 2011).
Paleothyris acadiana was found in Middle Pennsylvanian deposits in Nova Scotia (Carroll 1969). It had a number of features that were probably typical for both protorothyrids and basal eureptiles as a whole. These included an inncrease in the strength of the jaws, when compared with Hylonomus. A muscle attached to the pterygoid bone of the skull (the Pterygoideus), supplemented the adductor chewing muscles in pulling the jaw up and forward.
The rear upper (palatial) teeth were small, possibly designed to hold food rather than chew it. Its stapes was relatively heavy as in basal tetrapods, so its hearing probably limited to low frequency vibrations. There was no otic notch, and hence no tympanium or ear drum (Benton, 2000, pp.104-5)Romeria texana dates from the Asselian stage of the Cisuralian epoch of the Early Permian (299-294 mya). It was first found by Llewellyn Ivor Price (1937) at the Cottonwood Creek site, in the Archer City Formation in northern Texas (holotype MCZ 1480). A second species, Romeria prima, from the same location, was found by Clark and Carroll in 1973 (holotype MCZ 1963; Heaton 1979) Both species were represented by skulls .
Captorhinus laticeps ("broad headed Captorhinus") was first discovered in north Texas as a poorly preserved sample by Cope (1882), who named it Ectocynadon. It was later renamed Patriatus laticeps by Williston (1909). After a number of subequent name permutations, on the basis of well-preserved samples from Oklahoma, it was renamed Eocaptorhinus laticeps ("dawn Captorhinus") by Heaton (1979, p.11). Eocaptorhinus, in Heaton;s words (1979, p.5) showed "an astonishing correlation of similar anatomical characters with Sphenadon and primitive iguana lizards, leading to the use of these modern forms as the major source of information." Liike the more primitive forms, Romeria and Protocaptorhinus, C. laticeps has only a single rows of teeth in its dentary and maxilla. The skull of C. laticeps, from 6.5-8.0 cm in length, is larger and wider than those of either Romeria or Proocaptorhinus. Like them, it also had a pineal foramen (third eye) in the top of the skull, a primitive trait.
Heaton (1979) considered Eocaptorhinus as an intermediate form between Protocaptorhinus and Captorhinus. On the basis of stratigraphic finds of Captorhinidae with single rows of teeth (i.e., lower down and considered earlier) vs. those whith multiple rows (i.e., higher up and considered as later), Heaton proposed the evolutionary sequence of 1) Romeria to 2) Protocaptorhinus to 3) Eocaptorhinus to 4) Captorhinus.
More recently, E. laticeps has been assigned to Captorhinus by several authors who, on the basis of cladistic analysis, perceived a general similarity to C. aguti, the type species of Captorhinida (Kissel, Dilkes, and Reisz 2002; Reisz et al. 2011). The former E. laticeps is now "officially" named Captorhinus laticeps (as reflected on the chart by Reisz et al. 2011 [fig.2 in this article], which does not list Eocaptorhinus.)
Captorhinus magnus , the most recently discovered species in the captorhinidae family, and by far the largest, was found in Permian levels at Fort Sill, Oklahoma (Kissel, Dilkes, and Reisz 2002). C. magnus has only single rows of teeth, and thus (according Heaton's evolutionary scheme) would lie somewhere between Protocaptorhinus and Captorhinus in the scale of advanced or derived traits of this family. According to cladistic analysis, however, it has been placed as a sister clade to C. aguti , and both are seen as further removed from C. laticeps (Palaios 2014b).
attribution does not seem
perfectly clear. C. magnus
distinct from both C. aguti
laticeps in being twice as
large as either. It also contrasts
with both, in having ogival-shaped teeth in the posterior portion of
both maxillary and dentary (the upper and lower jaw bones). Another
difference is in the distal articulation surface of the femur of C.
magnus, which is concave, whereas in both C. aguti
laticeps it is
convex. Finally, some collections at the Ft. Sill site in
Oklahoma showed that specimens of C.
magnus are more abundant in
the lower parts of the
section, whereas C. aguti
is more abundant in the higher part. All of this would seem
fit into the evolutionary, stratigraphically-based view of Heaton
(1979) that C. magnus may be earlier and more primitive than C.
Class: Sauropsida : Clade: Anapsida family: Captorhinidae subfamily: Moradisaurinae
The Moradisaurinae, a subfamily of the Captorhinidae, had a widespread distribution in China, Morocco, Niger, Russia, Texas and Oklahoma. The Moradisaurines were insectivores and herbivores.
Lepidosauromorpha vs Archosauromorpha.
This is another pair of classes that, for those considering modern reptiles, are of basic significance, but here, where the concern is primarily with the emergence and development of Synapsids (the fossil taxa leading to the evolution of mammals), these two groups are necessarily relegated to a lower level of priority. Lepidosauromorpha include all the reptilian taxa sharing a more recent common ancestor with living lizards (called the Lepidosauria), than with Archosauria (crocodiles, and birds). Conversely, Archosauromorpha includes all reptilian taxa sharing a more recent common ancestor with Archosauria than with Lepidosauria (Gauthier 1986). Since Synapsids appear by the end of the Pennsylvanian and early Permian (about 310-280 mya), these more recent reptilian taxa (including dinosaurs) will be largely bypassed in this discussion.
Notes: Therapsids: now the term Synapsids is used as an overall counterpart of Sauropsids.
Benton, M., 2005, pp.104-5
Brough, M.C. and J. Brough, 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): pp. 147–165.
Carroll R.L. 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society B 257: pp. 267–308.
Carroll R.L. 1988
Carroll R.L. and Baird D. 1972. Carboniferous Stem-Reptiles of the Family Romeriidae. Bulletin of the Museum of Comparative Zoology 143(5): pp. 321-363.
Case, E.C. (1911): A revision of the Cotylosauria of North America. Carnegie Institution of Washington Publication vol. 145, Washington, D.C., 122 pp.
Clark, J. and R. L. Carroll 1973. Romeriid Reptiles from the Lower Permian . Bulletin of the Museum of Comparative Zoology 144 (5): pp. 353–407.
Cope, E. D. 1882 Third contribution to the history of the Vertebrata of the Permian formation of Texas. Proceedings of the American Philosophical Society 20: pp. 447-461.
Gauthier J.A. 1986. Saurischian monophyly and the origin of birds, . In K. Padian (ed.) The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Science 8, Berkeley, California, pp. 1-55.
Goodrich, E.S. 1916 . "On the classification of the Reptilia". Proceedings of the Royal Society of London 89B: pp. 261–276.
Gregory J. T. 1948. The structure of Cephalerpeton and affinities of the Microsauria. American Journal of Science, 246: pp. 550-568
Heaton, M.J. 1979. Cranial Anatomy of Primitive Captorhinid Reptiles from the Late Pennsylvanian and Early Permian, Oklahoma and Texas. Oklahoma Geological Survey, Bulletin vol. 127. The University of Oklahoma, Norman, 83 p.
Heaton, M.J. and R.R. Reisz. A skeletal reconstruction of the early Permian captorhinid reptile Eocaptorhinus laticeps (Williston). Journal of Paleontology 54: 136-143.
Hunt, K. 1994-1997
Haeckel, E. 1866 Generelle Morphologie der Organismen : allgemeine Grundzüge der organischen Formen-Wissenschaft, mechanisch begründet durch die von C. Darwin reformirte Decendenz-Theorie. ) Berlin
Huxley, T.H. 1863. The Structure and Classification of the Mammalia. Hunterian lectures, presented in Medical Times and Gazette.
Huxley, T.H. 1876. Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138
Kissel, R.A., D.W. Dilkes, and R.R. Reisz, 2002. "Captorhinus magnus, a new captorhinid (Amniota: Eureptilia) from the Lower Permian of Oklahoma, with new evidence on the homology of the astragalus." Canadian Journal of Earth Science, 39 (9), pp. 1363-1372
Laurin, M.; and R. Reisz, 1995 . "A reevaluation of early amniote phylogeny". Zoological Journal of the Linnean Society 113 (2):pp. 165–223.
Moodie, R.L. 1912. "The Pennsylvanic Amphibia of the Mazon Creek, Illinois, Shales". Kansas University Science Bulletin 6 (2): pp. 232–259.
Muller, J. and Reisz, R.R. 2006. . "The phylogeny of early eureptiles: Comparing parsimony and Bayesian approaches in the investigation of a basal fossil clade." Systematic Biology, 55(3): pp. 503-511.
Palaios.org 2014b, "Captorhinidae".
The Paleobiology Database: "Moradisaurinae".
Reisz, R. L., Jun Liu, Jin-Ling Li and J. Müller (2011). "A new captorhinid reptile, Gansurhinus qingtoushanensis, gen. et sp. nov., from the Permian of China". Naturwissenschaften 98 (5): pp. 435–441. .
Reptileevolution.com, "Basal reptiles "
Romer, A.S. 1952.. "Late Pennsylvanian and Early Permian Vertebrates of the Pittsburgh-West Virginia Region". Annals of Carnegie Museum 33: pp. 47–113.
Romer, A.S. 1966. Vertebrate Paleontology. University of Chicago Press., 3rd ed. (orig. 1933).
Sumida, S.S.; J. Dodick, A. Metcalf, , and G. Albright, 2010. "Reiszorhinus olsoni, a new single-tooth-rowed captorhinid reptile of the Lower Permian of Texas". Journal of Vertebrate Paleontology 30 (3): pp. 704–714.
Watson, D.M.S. 1914. "Eunotosaurus africanus Seeley and the ancestors of the Chelonia". Proceedings of the Zoological Society of London 11, pp. 1011–1020.
Williston, S.W. 1909. New or
Little Known Permian Vertebrates. Pariotichus.
17(3), pp. 241-255.
Athena Review Image Archive™ | Guide to Archaeology on the Internet | free trial issue | subscribe | back issues
index of Athena Review
| Research Pages |
Copyright © 1996-2014 Athena Publications, Inc. (All Rights Reserved).