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Athena Review Vol. 5, no. 1


Records of Life: Fossils as Original Sources


Amphibians 

           The development of land-based amphibians derived from tetrapods, all of whom were labyrinthodonts.  The term labyrinthodont ("labyrinth-tooth") refers to the folded, interlocking layers of enamel in their teeth (fig.1). This primitive trait was originally inherited from the Devonian ancestors of tetrapods, the sarcoptygerians or lobe-finned fish.

Fig.1: Labyrinthodont tooth, shown in cross section.

             During  the early Carboniferous (Mississippian) period, lasting from 358.9 to 323.2 mya, three primary lines of labyrinthodont amphibians arose: the temnospondyls, the reptilomorphs, and the lepospondyls.  The temnospondyls were a highly successful branch of amphibians who eventually became extinct during the Jurassic period.   Reptilomorphs led both to the Sauropsid reptiles, ancestors of all later reptiles, and to the synapsids, who led eventually to early mammals.  Lepospondyls were mainly eel-like and lizard-like forms inhabiting rivers or wetlands. Some are considered to have led to present-day amphibians.  The largest genus, Diplocaulus, reached 1 m in length, but most were much smaller.

             The Mississippian period, when amphibians developed, is named for exposed rock formations from this period in the Mississippi River valley, primarily limestones from marine deposits.
             This was a time of high sea levels. During the Mississippian there was marine ingression in the Northern Hemisphere, with the ocean so so high only the Fennoscandian Shield and the Laurentian Shield stood above sea level. The higher areas, called cratons, were surrounded by extensive delta systems and lagoons, and carbonate sedimentation on the surrounding continental platforms, covered by shallow seas, which eventually became limestone. The largest continuous exposed landmass in the Early Mississippian was the vast southern area called Gondwanaland, stretching from today's South America to Australia, partly bounded on the south by a polar cap (fig.2)
Fig.2: Global formation of continents during the Early Mississippian (354 mya).

 Whatcheria

        One of the earliest tetrapods known from the Early Mississippian, Whatcheeria deltae, was found in a limestone quarry in Delta, Iowa, which exposed strata from what was once part of an inland sea. This animal, dating from the Visean stage of the Mississippian at about 340 mya, had a number of primitive traits retained from lobe-finned fish forebears, such as retention of preopercular bones, and other fish-like skull traits found on the slightly earlier Ichthostega and Acanthostega. It had more fully developed limbs, however, than either of the two latter species, and may have had five digits, which became the standard tetrapod formation. It was about 1 m in length, and had a robust skull with large, recurved teeth (fig.3). Whatcheeria shows some similarities with slightly later Reptilomorphs of the suborder Emblomeri, including Crassigyrinus, described below (Lombard and Bolt 1995; Clack 2012).

Fig.3: Skull of Whatcheeria deltae, with bones labelled (after Lombard and Bolt 1995, fig.3).

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Temnospondyls

.           
   
The early temnospondyls arose out of the original tetrapod radiation during the Tournaisian and Visean epochs of the Mississippian period. The Temnospondyls were highly successful in expanding into both aquatic and terrestial habitats. This large branch of amphibians lasted nearly 100 million years, well into the Permian.  Most Paleozoic forms of Temnospondyls became extinct by the Lopingian phase of the Late Permian (265-253 mya), although a few aquatic forms surved longer, into the Jurassic period.

           The first proto-temnospondyl - like species were the Colosteids, who were lizard-like aquatic predators, partly covered with scales. One of the first discoveries of a colosteid was Pholidogaster pisciformes, ("scaly stomach [with a] fish form"), identified by Thomas H. Huxley in 1862 from a nearly complete skeleton found in a coal deposit in Gilmerton, Scotland. These deposits date from the Visean and early Serpukhovian stages of of the Middle Carboniferous (ca. 338-320 mya). The Philodigaster fossil had been originally grouped with fish fossils. Huxley (1862) identified it as a thin, elongated aquatic amphibian about 44 cm. long, with with weak, undeveloped limbs. It also had hard stomach scales, which allowed it to crawl on land, and fang-like canines indicating it was a carnivore. P. pisciformes provided an early illustration of the tetrapod features shown in amphibians, representing the transition to terrestial habits.

            From the Colosteids arose medium to large-sized swamp dwellers known as Baphetids; and from them, in turn, the first true temnospondyls, perhaps arising from aquatic predators such as Eucritta. Early temnospondyls had elongated, somewhat crocodile-like forms  up to 1.5 m in length. During the Late Visean epoch (340-330 mya), these early temnospondyls expanded into various other forms, with many living in tropical swamps and rivers. Other had become semi-terrestial amphibians, including the large Eryops, and the smaller Dendrerpeton.  
              The Paleozoic temnospondyls saw their greatest expansion during the late Carboniferous and early Permian periods.  Some, such as Cacops, who had huge, armor-plated heads and shoulders, led a terrestrial existence, returning to water only to lay their eggs. Others, including Trematops and Eryops, who had strong, thick limbs and large, somewhat frog-like heads, maintained a semi-aquatic existence. Still others, like Trimerorhachis, were fully aquatic, witth long, crocodile-like bodies armoured with protective coatings of scales. Others, like Branchiosaurus, retained external gills even as adults and were unable to leave the water.
              Most temnospondyls had relatively large heads of varying forms. Some, such as Laidleria, had flat, triangular skulls which  are distinctive among vertebrates in general. Others, like Gerrothorax, had bizarre, wide, parabolic heads.   Still others, such as Archegosaurus, had very elongated mouths in a form something like that of gharial (i.e. Nile) crocodiles, and which is associated with reptiles which hunt small fish. 
              The Temnospondyl post-cranial skeleton, meanwhile, varied in proportions related to swimming or walking specializations. The vertebral structure was essentially that of the primitive, late Devonian  tetrapods and osteolepiform lobe-finned fish, with intercentra and pleurocentra. Regarding the limbs and digits, temnospondyls did not develop limb specializations, such as occurred  in the reptilomorph lineage. While the temnospondyl hind foot had five digits or toes, their forefoot had only four.



Dendrepeton       

Class:     Amphibia       Order:   Temnospondyli        Family:    Dendrepetontidae

              Dendrepeton arcadianum was a land-based temnospondyl found in deposits from the late Carboniferous (Pennsylvanian) period. They were first discovered in fossil stump deposits in the Joggins Formation in Nova Scotia by Dawson (1861, 1862; Owen 1862; Carroll 1967).  These deposits date from the Westphalian B stage of the Pennsylvanian period (314-311 mya; Milner 1980).
              The skulls of Dendrepeton arcadianum adult specimens are generally about 95-100 cm in length, and their bodies are estimated to have been about 1 meter long. A number of immature skulls were also found in the Joggins formation, measuring about 70 cm.  Among the diagnostic anatomical traits for D. arcadianum are a short snout; and large, deep, ovoid otic notches in the back of the skull. The large, deeply concave otic notches indicate the presence of a large, circular tympanic membranes or ear drums
(Milner 1980). The position of the suture between the lachrymal and jugal bone (located beside and behind the eyes) is also considered a diagnostic feature for this species. A pineal "eye" foramen is also found in the suture of the parietal bones (Milner 1980).
              
A related species, Dendrepeton rugosum, has also been found in coal deposits in Jarrow, Ireland (Huxley and Wright 1867). These date from the slightly earlier Westphalian A stage (316-314 mya) of the Upper Carboniferous period (equivalent to the Pennsylvanian Period in North America). D. rugosum has a shorter snout, and a larger otic notch, than D. arcadianum (Milner 1980)Dendrepeton fossils in the Jarrow deposits are often found associated with the Carboniferous tree genera Lepidodendron and Sigillaria, within which the Dendrerpeton fossils may be contained. In this condition, their remains are often disarticulated and flattened. 


          
Eryops

 Class:   Amphibia     Order:   Temnospondyli        Family: Europidae

              
                Eryops megacephalus  ("drawn-out face, large head") , one of the more developed forms of temnospondyls, dates from the Early Permian, (295-280 mya). It was first found in the Admiral Formation in Archer County, Texas by Cope (1877). Many specimens have been found in the Permian Basin of northern Texas and Oklamoma (Case 1906).  Its body length of 1.5-2.0 meters (5–6 ft) and weight (90 kg or 200 lb) represented one of the largest land animals of its time. The shoulder girdle was disconnected from the skull, resulting in improved walking. Sturdy limbs and a strong spine supported the body in walking posture while out of water.  
               The broad, flat skull of Eryops had large eye sockets which were directed upward like those of a crocodile. Eryops had a large mouth, with strong jaws containing many sharp teeth of labyrinthodont composition. The fang-like palatal teeth, combined with an ability to  open its jaws widely, suggest crocodile-like feeding.

Cacops

 Class:     Amphibia       Order:   Temnospondyli    Family: Dissorophidae

                Cacops aspideformus  was a relativedly small amphibian, about 40 cm in length, that lived in North America during the Early Permian Kungurian phase (290-280 mya). Its fossils were found initially in the Clear Fork Permian strata in north Texas (Williston 1911). Recently the taxon has also been identified in Oklahoma (Reisz et al. 2009). It was adapted to a terrestrial lifestyle, with a heavily built skull, strong legs, a short tail, and a row of armor plates along its back.
          
Later stages of temnospondyls
.
             During the Late Permian there was an increasingly arid climate brought about by the formation of the single mega-continental landmass called Pangea, with disruption of rainfall and drainage patterns greatly affecting rivers and wetlands. This caused the ranges of amphibians to contract and concentrate into smaller zones of swamps and river courses.  Late Permian temnospondyls such as  Actinodon, Archegosaurus, and Platyoposaurus had an elongated, crocodile-like appearance, while retaining limbs for walking. Their descendents, such as the typically large stereospondyls, became completely aquatic. One stereospondyl branch, the Trematosauridae, reverted to the oceans, uniquely among early amphibians  Other Stereospondyl lines who  lived permanently in fresh water had skeletons which grew increasingly cartiliginous. Their heads developed into very large dimensions, especially among the Capitosaurids and Mastodonsaurids (Palaios 2014c).

            Cyclotosaurus
, a capitosaur, lived in rivers of Europe during the Triassic (Carnian) These were large, flat-headed amphbians 2 to 4 meters long. Until the end of the Triassic they shared these rivers with predators such as crocodile-like phytosaurid reptiles. Also flourishing at this time were the Metoposaurs of Laurasia (the area of Europe joined with North America), which evolved from completely different ancestors to look like Capitosaurs, except that the Metoposaurs had eyes further forward on the skull.
            A large extinction process at the end of the  Triassic killed off both the big temnospondyls, and their rivals the phytosaurs.  Only the short-headed  Brachyopoids survived during the Jurassic  in China and Australia (located in the easternmost parts of north and south Pangea respectively), These  last temnospondyls grew to 2-3 m in length and survived until later in the Cretaceous in the rift valleys of south-east Gondwana, where the climate was too cold to support crocodiles, their main predators. (Palaios 2014c). 
    
.     
 

Reptile-like  amphibians (Reptilomorphs or Anthracosaurs)

     

            Reptiles developed from amphibians in the later part of the Carboniferous era (Late Mississippian and Pennsylvanian periods, about 335-295 mya). The first transitional forms, called Reptilomorphs (Säve-Söderbergh 1934; Panchen 1970) have also been called Anthracosaurs. This alternative name for reptile-like amphibians, used by the influential paleontologist Alfred Sherwood Romer  (1956),  derived from the coal deposits in which some amphibian fossils were first discovered. 

            These two (largely synonomous) major groupings of amphibians (reptilomorph and anthracosaurs) are used inconsistently in the literature partly because  when originally defined, they  included groups now considered less closely related. There is currently no real consensus on which term is preferable. The name Reptiliomorpha was first used by Gunnar Säve-Söderbergh in 1934 to designate various types of late Paleozoic reptile-like amphibians with labyrinthodont teeth.  In 1956 Friedrich von Huene assigned both amphibians and anapsid reptiles to the Reptiliomorpha. This included eight orders, now considered to belong to distinct lineages, whose similarities are the result of convergences or shared primitive features (Palaios 2014d)

            Romer (1956), in his widely-read textbooks on vertebrate paleontology, used the name Anthracosauria instead. In 1970, meanwhile, the German paleontologist Panchen (1970) reverted to Säve-Söderbergh's use of the term Reptilomorphia. Romer's term Anthracosauria, however, is still in use, both by evolutionary taxonomists (e.g. Carroll 1988 and 2002, and Hildebrand & Goslow 2001), and some cladistic taxonomists  (Gauthier et al 1988).  Another likely reason for confusion of the terms is the tendency for cladistic analyses to make improvisational rearrangements of Linnean ranks of classification ( Palaios 2014d). Michael Benton (2000, 2005) made Reptilomorpha the sister-clade to Batrachomorpha, ancestors of all current amphibians. In this solution, Reptilomorpha is made into a superorder, and Anthrocosauria into an order (Palaios 2014d). This inclusive approach, in an attempt at clarity, will be followed here.

           

Embolomeri

Superorder:  Reptiliomorpha   Order:  Anthracosauria    suborder:  Embolomeri    Family:           Proterogyrinidae

   Proterogyrinus ("early tadpole") was a large anthracosaur amphibian from  the Serpukhovian stage of the Pennsylvanian period (328-318 mya).  Two species are known, P. scheelei and P. pancheni.  Fossils have been found in Scotland, and West Virginia. 

Fig.1: Proterogyrinus   

             Proterogyrinus was similar in shape to other reptiliomorphs, such as Crassigyrinus. It was about two and a half meters (7–8 feet) long, and showed a mixture of primitive (tetrapod-like) and derived traits.  Like other reptiliomorphs, such as Seymouria, Proterogyrinus could move further away from water than most amphibians. Like tetrapods and early amphibians, Proterogyrinus  had labyrinthodont teeth with folded enamel. Its rounded skull was amphibian, still retaining a fish-like hinged skull, with a hinge at the neck. In its postcranial anatomy, Proterogyrinus retained amphibian ankle bones, and had small shoulder girdle bones. Yet it also had reptilian vertebrae, pelvis, humerus, and digits, with both five toes and five fingers,  and a 2-3-4-5-3 phalangeal count, which is more reptilian than amphibian.

Diadectomorpha

Class Amphiba; order Reptilomorpha; suborder Diadectomorpha

            More developed reptilomorph amphibians with additional reptilian features arose in the late Pennsylvanian through Early Permian periods. A suborder of these more advanced forms,, called the Diadectomorphs, include the type species Diadectes, Limnoscelis and Tseajaia. 

            Diadectes sideropelicus  ("cross-biter of iron clay"), was a reptilomorph dating from the Early Permian Cisuralian stage (290–272 mya). It was first found in the Permian red beds of north Texas, in the Wichita and Clear Fork group near Wichita Falls. The initial identification was made in 1878 by Copes, who collected extensively there.

            The anatomy of Diadectes shows a mixture of reptilian and amphibians traits, combining a robust, reptile-like skeleton with a more primitive, thick-boned skull resembling that of a Seymourian amphibian.  Its heavy vertebrae and ribs, massive limb girdles and short, robust limbs indicate that D. sideropelicus was a fully terrestrial animal.  Diadectes was one of the very earliest herbivorous tetrapods. Its teeth show advanced specializations for herbivorous diet. Eight front teeth are spatulate and peg-like,serving as incisors to clip off mouthfuls of vegetation. The broad, blunt cheek teeth, used for grinding, show extensive wear associated with occlusion.


  

Limnoscelis (fig.2) was a relatively large reptilomorph, about two meters in length. Its pectoral and pelvic bones were large and robust, showing a well-developed mode of walking. Other reptilian traits include a larger braincase structure, reptilian jaw muscles, and expanded neural arches.
Fig.3: Limnoscelis.


            A major difference between these ancient, large amphibians and the first reptiles, who were small in stature, is the amniotic egg which developed in reptiles. Here the embryo is contained in an egg with nutrient fluid and a protective sheath, as opposed to amphibians and fish whose larvae are hatched in open water.

             Additional differences developed by reptiles from their amphibian ancestors include stronger legs and girdles, different vertebrae, and stronger jaw muscles (cf. Carroll 1988; Gauthier et al. in Benton, 1988). The reptilian skull was also much stronger. The ancestral amphibians had a relatively weak skull inherited from the fish-like tetrapods, and paired aortas or systemic arches.

.

Chronosuchia .

Class: Amphibia  Order:  Reptilomorphs   Suborder: Chroniosuchia 

             Chroniosuchians, first defined by Kuhn (1970) were a suborder of reptiliomorphs that lived from the Middle Permian to Late Triassic in what is now Eastern Europe, Kyrgyzstan, China and Germany. They were all short limbed, with a strong tail and elongated snout somewhat resembling that of modern crocodiles. The group is traditionally considered to be a suborder of labyrinthodonts. Chroniosuchians likely existed as riverine predators. 

             Most forms bore a heavy armour of scutes along the back, possibly for protection against land predators. This is illustrated by a species,  Chroniosaurus dongusensis  from the  upper Tatarian to Early Wuchiapingian levels from the Late Permian period in the Orenburg Region of the South Cis-Urals. C. dongsensis had diagostic rows of interlocking bony plates called osteoderms along their backs, reaching from head to tail (Tverdokhlebova, 1972). These osteoderms constitute the most frequently found fossilized remain of Chroniosaurus. The flat osteoderm plates are connected to the neural arches of vertebra by an extension of bone on their undersurfaces. Each osteoderm thus is paired with a single vertebra.  The front margin of each osteoderm also has a pair of "anterior wings" that slip into a notch in the posterior margin of the osteoderm in front of it.

             Chroniosuchians are also distinguished from other early reptiliomorphs by the lack of intertemporal bones in the skull, as well as the presence of holes in front of the eye sockets, called antorbital fenestrae. Like many early tetrapods, chroniosuchians have vertebrae that are divided into three parts: a pleurocentrum and an intercentrum on the bottom, and a neural arch on top. Chroniosuchians have vertebrae whose pleurocentrum makes up most of the body, while the intercentrum is small and wedge-like (called shizomerous vertebrae).

 

Gephrostegus

            Gephyrostegus bohemicus was a Late Carboniferous, lizard-like reptilomorph from the Pennsylvanian period (about 310 mya), found at Nyran in the Czech Republic (Jaeckel 1902). It is thought to have been derived from a form similar to Silvanerpeton. Gephyrostregus is considered directly ancestral to early reptiles, including Casineria and Cephalerpeton.  

              Gephyrostegus, with an overall length of about 22 cm., was smaller than other basal tetrapods and slightly smaller than its more primitive sister, Silvanerpeton. The top and sides of the body of Geprostegus was covered by tiny circular scales,  while on its underside, there were large V-shaped scales. Juveniles had a relatively larger head and orbit (eye socket). In the rear of its skull, the otic notch was reduced. Some postcranial bones of Geprostegus were relatively shorter than those of  Silvanerpeton, including the pelvis, and the four and fifth hand digits. In its ankles, some bones had fused together to create a larger bone, the astragalus, otherwise found only on reptiles. Further, an elbow or olecranon process had developed on the ulna, one of the two lower arm bones. The humerus (upper arm bone) was more also more slender or gracile.

            While most early tetrapods lived their lives in water, Gephyrostegus preferred land. Its eggs would have been laid in or near water to prevent dessication because, presumeably, no amniotic membrane or shell surrounded the embryo. 



Glossary 
.
References:

Benton, M.J. 2005. Vertebrate Paleontology. Blackwell Publishers. xii–452

Carroll, R.L. 1967, "Labyrinthodonts from the Joggins Formation." J. Paleontology 41, pp.111-142.

Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. W. H. Freeman & Co.

Carroll, R. L., 2002 Early land vertebrates. Nature, 418, pp 35-36

Case, E.C. 1911: A revision of the Cotylosauria of North America. Carnegie Institution of Washington Publication vol. 145, Washington, D.C., 122 pp.

Clack, J.A. 2002. "An early tetrapod from Romer's Gap." Nature 418, pp.72-76.

Clack, J.A, 2012. Gaining Ground. University of Indiana Press.

Cope, E.D. 1877. "On a carnivorous dinosaurian from the Dakota beds of Colorado." Bull. U.S. Geol. Surv. Territories 3, pp.: 805-806.

Dawson, J.W.  1861. “Notice of the Discovery of Additional Remains of Land Animals in the Coal Measures of South Joggins, Novia Scotia.” Proccedings of the Geological Society of London (Nov.6, 1861), pp. 5-7.

Gauthier, J., A.G. Kluge, and T. Rowe 1988. "The early evolution of the Amniota." In The phylogeny and classification of the tetrapods, no 1: amphibians, reptiles, birds. Edited by M.J. Benton. Clarendon Press, Oxford, pp. 103-155.

Hildebrand, M. and G.E. Goslow 2001. Analysis of Vertebrate Structure. New York, John Wiley.

Huxley, T. H., 1862. “On new Labyrinthodonts from the Edinburgh Coalfield.” Quarterly Journal of the Geological Society of London, v. 18, p. 291-196.

Huxley, T.H. and E.P. Wright, 1867. "On a collection of fossil vertebra from the Jarrow Colliery, County of Kilkenny, Ireland." Transactions of the Royal Irish Academy 24, pp. 51-368. 

Jaeckel, O. 1902. “Über Gephyrostegus bohemicus, n.g. n.sp.” Zeitschrift der Deutschen Geologischen Gesellschaft no 54, pp. 127–132.

Kuhn, O. 1970, ed. Handbuch der Palaoherpetologie, Teil 5A. Fischer, Stuttgart.

Lombard, R.E. and J.R. Bolt, 1995. “A New Primitive Tetrapod, Whatcheeria Deltae, from the Carboniferous of Iowa.” Paleontology 38 (3), pp.471-494.

Milner, A.R. 1980.  “The Tetrapod Assemblage in Nytany, Czechoslovakia.” In Panchen, A.L. (ed.), The Territorial Environment and the Origin of Land Vertebrates. Systematics Assoc. Special Volume 15, Academic Press London, pp. 439-496.

Owen 1862, “Description of Specimens of Fossil Reptilia Discovered in the Coal Measures of the South Joggins, Novia Scotia, by Dr. J.W. Dawkins, F.G.S.”. Proccedings of the Geological Society of London, vol.18 (April 2, 1862), pp. 238-244.

Palaios 2014c, "Temnospondyls".
Palaois 2014d, "Reptilomorphs".

Panchen, A.L. 1970, "Anthracosauria," in
Kuhn, O. 1970, ed. Handbuch der Palaoherpetologie, Teil 5A. Fischer, Stuttgart.

Pawly, K. 2006. "Walking with early tetrapods: evolution of the postcranial skeleton and the phylogenetic affinities of the Temnospondyli (Vertebrata: Tetrapoda)."  Chapter 6:  in The postcranial skeleton of temnospondyls (Tetrapoda: temnospondyli). PhD Thesis. La Trobe University, Melbourne.

Säve-Söderbergh, G. 1932. "Preliminary note on Devonian stegocephalians from East Greenland." Meddelelser om Gr¢nland, 94(7), pp.1-107.

Säve-Söderbergh, G. 1934. "Some points of view concerning the evolution of the Vertebrates and the classification of this group." Arkiv för Zoologi 26A(17), pp. 1-20

Tatarinov, L. P., 1972, “Paleontology and the phylogenetic laws of Lower Terrestrial Vertebrates.” Palaeontological Journal, v.6, n.3, pp. 389-398.

Tverdokhlebova, G.I. 1972. "A New Batrachosaur genus from the Upper Permian of the South Urals." Palaeontological Journal, v.6, n.1, pp. 95-103.

Tverdokhlebov, V.P.,  G. I. Tverdokhlebova, A. V. Minikh, M. V. Surkov, and M. J. Benton. 2005. "Upper Permian vertebrates and their sedimentological context in the South Urals, Russia." Earth-Science Reviews 69, pp. 27-77

von Huene, F. (1956): Paläontologie und Phylogenie der niederen Tetrapoden, G. Fischer, Jena

Williston, S.W. 1911. American Permian Vertebrates. University of Chicago Press, Chicago.

Wright, E.P. and T.H. Huxley, 1866. "On a collection of fossils from the Jarrow Colliery, County of Kilkenny, Ireland." Geol. Mag. (4), 3, pp. 165-171









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