Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
13. Amphibians 1: Temnospondyls
Amphibians evolved from Tetrapods during the Mississippian and Pennsylvanian periods, together comprising the Carboniferous era. The first part of the Mississippian period, after a mass extinction at the end of the Devonian period, is scanty with fossils and identified with the interval known as "Romer's Gap" (fig.1).
Fig.1: Chronological Chart of the Carboniferous era, showing major groupings of amphibians, and placement of Romer's Gap at lower right.
Labyrinthodonts
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.2). This primitive trait was originally inherited from the Devonian ancestors of Tetrapods, the Sarcoptygerians or lobe-finned fish.
During the early Carboniferous (Mississippian) period, lasting from 358.9 to 323.2 mya, three primary orders of labyrinthodonts arose in the class of Amphibians: the Temnospondyls, the Reptilomorphs, and the Lepospondyls. The Temnospondyls, covered in this section, were a highly successful order of amphibians who eventually became extinct during the Jurassic period. Reptilomorphs (also called Anthrocosaurs, covered in the next section), a centrally important order, 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.
Fig.2: Labrinthodont teeth, shown in cross-section.
The Mississippian period, when the class of 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. Duri
ng 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.3)Fig.3: The globe during the Early Mississippian period (ca.354 mya).
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 order 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, 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 Henry 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). Fig.4: The early Temnospondyl, Philidogaster pisciformes (after Huxley 1862).
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 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. Others 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
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).
Fig.5: Underside of the skull of Dendrepeton arcadianum (after 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
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. Fig.6: Eryops megacephalus (Yale Peabody Museum; photo: Athena Review ).
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
Cacops aspideformus was a relatively 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.
Fig.7: Cacops aspideformus (Yale Peabody Museum; photo: Athena Review).
Later stages of Temnospondyls.
.
Dring 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).
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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.
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". Palaios.org website.
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.
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
Glossary