DESCRIPTION OF THE CLASS STEGOCEPHALI+ (COPE 1868)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>TETRAPODA>STEGOCEPHALI |
CLASS STEGOCEPHALI LINKS
| Stegocephali (ste-go-SE-fa-le) is derived from two Greek roots that mean roofed or covered head [roof -stege (στέγη); and head -kephale (κεφαλή)]. This is a reference to the heavy, bony, flattened skulls of these animals. Cope (1868) coined the term Stegocephali to include the basal tetrapods, the way that it is used in this system to include all of the basal tetrapod families. In a strict phylogenetic interpretation, however, Stegocephali would include all of the gnathostome animals with digits. |
| INTRODUCTION TO THE STEGOCEPHALI This is a very problematic collection of taxa. They are the oldest tetrapods [see Laurin (2002) for a discussion of this term] and can be identified by the presence of toes, a character shared with all other tetrapods. In addition, they retain a suite of sarcopterygian characters like the retention of a lateral line, broad fish-like tails (in some), the retention of gills in adult forms (in most), and the retention of fish-like teeth on the palate (a trait seen also in other tetrapods). Thus, there is no single synapomorphy that defines the group. |
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| FIGURE 1. A cladogram taken from Clack (2009). The analysis was based on characters of the skull, axial and appendicular skeletons. The structure of the tree is consistent with other analyses (e.g. Ruta et al. 2003 and Laurin 2002). The presence of digits on the manus (hand) and pes (foot) marks the boundary between the sarcopterygian fishes and tetrapods. That boundary was crossed between Tiktaalik and Ventastega. |
| Figure 1 (from Clack 2009) is a cladistic analysis of the Stegocephali and some of the lobe-fin fishes that were antecedent to them, most of which were from the upper Devonian. Digits seem to have appeared between Tiktaalik and Elginerpeton, a taxon for which fragmentary evidence exists. Most of the Stegocephali were obligate aquatic animals, and if they were alive today, would likely be considered fish. The earliest tetrapods for which significant fossil evidence exists are Acanthostega (Clack 1994; Coates and Clack 1991; and Daeschler and Shubin 1995; Figure 2) and Ventastega (Ahlberg et al. 2008; Figure 3). They would have appeared much like Tiktaalik (see Osteichthyes, Figure 27) with toes. Both Acanthostega (Figure 2), and Ventastega (Figure 3) had autopods (paired limbs) that had the standard skeleton of a tetrapod, but the radius was so short that it could not have supported the animal’s weight on land. Furthermore, both taxa had gills as adults (Coates and Clack 1991). Ichthyostega (Figures 1 and 4) appeared to have been more completely adapted to an amphibian existence. The bones of the appendages were more massive than those of Acanthostega, and the radius and ulna were more approximate in size so some motion on land could be accomplished. Although early reconstructions showed the animal in a salamander-like walking posture (see Figure 4), Ahlberg et al. (2005) showed that the hind legs were articulated with the pelvic girdle so that they would be directed backward like a seal and, therefore must have spent most of its time in the water (Figure 5). Although Ichthyostega could have gone onto land, it would have lumbered along in a seal-like fashion. Furthermore, Ichthyostega had branchial arches, which likely supported gills like those of Acathostega. Though more able to support itself on land, Callier et al. (2009) indicate that a study of the upper bones in the appendicular skeleton indicates that Ichthyostega could have been more primitive than Acanthostega, which may have returned to a complete aquatic existence from a partially terrestrial ancestor. The selective pressures at the end of the Devonian Period on these early tetrapods were not simple or singular. First, they showed a global distribution with many variables in climate, etc. Given the opportunities, the surprise is that the early tetrapods did not simply march from the water to inherit the land. Indeed, they did not seem to be in any great hurry to leave the water at all. Clack (2009) argues that an important lure to a terrestrial existence was an increasing reliance on atmospheric oxygen through the Devonian. She lists the following land-water interactions and selective pressures to become air-breathers in the upper Devonian. They are: – increasing diversity and mass of plants along water margins, including the earth’s first forests – that gave rise to increased nutrient runoff and organic input from fallen plants – that produced periods of oxygen-depletion in marginal waters – at the same time, atmospheric oxygen levels were dropping However, these general trends could have been reversed in local environments. Thus, the upper Devonian had a relatively high diversity of aquatic tetrapods. Clearly, the tetrapod limb had evolved before the animals sought to venture onto dry land. Perhaps, the limb with toes was useful in swimming and pulling itself through vegetation-laden delta environments where they occurred. A stout, muscular fin with toes was then co-opted as a limb that allowed it to exploit an environment in which there was much vegetation and many different invertebrates. In addition, the land could have become a refuge from predation. Tetrapod fossils disappeared during a time called Romer’s Gap (about 360 – 345 MYA). After that, more recognizable amphibians became common in the fossil record in the lower Mississippian Period. However, a number of fossil tetrapods began to be recovered from Scotland in lower Mississippian strata, well within Romer’s Gap (Clack 2002 and 2009, Smithson 1986, and Wood et al. 1985; i). One of the first to be found was Pederpes (Figures 1 and 6). First thought to be a fish, the near complete fossils showed that it had digits. Furthermore, it had feet that were pointed forward like weight-bearing structures rather that the sideways turn of the aquatic tetrapods. Thus, the terrestrial tetrapods had appeared. Although adapted to land, the animals likely still were tied to water where they spent most of their time. Niedzwiedzki et al. (2010) report the recent discovery of tetrapod fossil tracks in marine tidal deposits of Poland suggest that an earlier gap in the fossil record may hide much of the evolutionary history and transition from lobe-fins to tetrapods. This discovery is important in that it shows tracks made by terrestrial or semi-terrestrial animals 18 million years before any skeletal remains of tetrapods and, more importantly, 10 million years before the advent of the presumed group of lobefins that included Panderichthyes and Tiktaalik. Furthermore, because the trackways have been found in marine mudflat deposits, tetrapods may have appeared first in intertidal environments, strata from which have yielded no early tetrapod remains. Thus, tetrapods are far older than suggested by the fossil record and animals such as Acanthostega and Ichthyostega must have been their descendants. The closure of this gap in the fossil record will lead to a major restructuring of our thinking of what is derived and primitive among tetrapods. |
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| FIGURE 2. A drawing of Acanthostega (in the foreground) and Ichthyostega (background). Acanthostega had legs that could not support its weight out of water and likely was a fully aquatic “fish with legs”. Image from: http://www.bertsgeschiedenissite.nl/geschiedenis%20aarde/devoon2.htm | FIGURE 3. A drawing of Ventastega in a recent reconstruction. Like Acanthostega, this animal could not move on land and was relegated to an aquatic existence. Image by: Nobu Tamura | FIGURE 4. A model of Ichthyostega in an earlier reconstruction. Note the salamander-like stance. Image from: the Taiwan Museum of Natural History | FIGURE 5. A drawing of Ichthyostega in a recent reconstruction. Note the hind legs splayed to the sides. Such orientation likely gave it good propulsion in the water, but made movement on land laborious like that of a seal. Image by: ArthurWealsey -Wikipedia Commons | FIGURE 6. A drawing of Pederpes, one of the first tetrapods to be found in Romer’s Gap. Its hind legs were situated so that the feet rested fully on the ground and pointed forward. It was one of the first ‘terrestrial tetrapods”, though it likely spent most of its time in the water. Image by: DiDgd- Wikipedia |
FIGURE 7. Cladogram based on Benton (2005) showing the basal position of the Stegocephali in the tetrapod vertebrates.
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| FIGURE 8. This is Figure 3 from Hinchliffe (2002). He shows the skeletal structures of a pectoral limb from four animals (A-Zebrafish, a teleost; B-Neocerdotus, a lungfish; C-Eusthenopteron, a sarcopterygian; D-Panderichthyes, a member of a sister clade to the tetrapods; and E-a generalized pentadactyl structure after Shubin and Alberch (1986). |
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| FIGURE 9. Changes in fin skeletons of upper Devonian fishes and tetrapods. The first change (marked by the bar is an asymmetry in the skeleton followed by the formation of radius-ulna, and development of digits. Note that Tiktaalik, a fish, has both multiple short terminal bones and fin rays (From Shubin et al. 2006). |
| [i] Romer’s Gap is a period of time in the lower Mississippian Period when there were relatively few known tetrapods in the fossil record. This is a true gap in the fossil record named by Alfred Romer, an important vertebrate paleontologist of the first half of the 20th Century. Reasons for the gap are not known. Ward et al. (2006) suggest that Romer’s Gap corresponds to a period of unusually low atmospheric oxygen further reducing the chances of fossils forming or persisting. Clack (2002), Smithson (1986), and Wood (1985) have been able to add many tetrapods to the period of the gap, but it still remains a period of time that is poorly known given its obvious importance in the understanding of vertebrate evolution. |
| LITERATURE CITED Ahlberg, P.E., J.A. Clack, and H. Blom. 2005. The axial skeleton of the Devonian tetrapod Ichthyostega. Nature. 437(1): 137-140. Ahlberg, P.E., J.A. Clack, E. Luksevics, H. Blom, and I. Zupins. 2008. Ventastega curonica and the origin of tetrapod morphology. Nature. 453: 1199-1204. Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Callier, V., J.A. Clack, and P.E. Ahlberg. 2009. Developmental trajectories in the earliest known tetrapod forelimbs. Science. 324: 364-367. Clack, J.A. 1994. Acanthostega gunnari, a Devonian tetrapod from Greenland; the snout, palate and ventral parts of the braincase, with a discussion of their significance. Meddelelser om Grønland: Geoscience. 31: 1-24. Clack, J.A. 1997. Devonian tetrapod trackways and trackmakers: a review of the fossils and footprints. Paleogeography, Paleoclimatology, Paleoecology. 130: 227-250. Clack, J. A. 2002. An early tetrapod from ‘Romer’s Gap’. Nature 418: 72–76. Clack, J.A. 2009. The fish-tetrapod transition: new fossils and interpretations. Evo Edu Outreach. 2: 213-223. Coates, M.I. and J.A. Clack. 1990. Polydactly in the earliest known tetrapod limbs. Nature. 347: 66-69. Coates, M.I. and J.A. Clack. 1991. Fish-like gills and breathing in the earliest known tetrapod. Nature. 352: 234-236. Coates, M.I., M. Ruta, and A. R. Milner. 2000. Early tetrapod evolution. Trans. Ecol. Evol. 15: 327-328. Cope, E. D. 1868. Synopsis of the extinct Batrachia of North America. Proceedings of the Academy of Natural Sciences of Philadelphia. 1868: 208-221. Daeschler, E.B. 2000. Early tetrapod jaws from the Late Devonian of Pennsylvania, USA. J. Paleont. 74(2): 301-308. Daeschler, E.B. and N. Shubin. 1995. Tetrapod Origins. Paleobiology. 21(4): 404-409. Daeschler, E.B., N.H. Shubin, K.S. Thomson and W.W. Amaral. 1994. A Devonian tetrapod from North America. Science. 265: 639-642. Hinchliffe, R.J. 2002. Developmental basis of limb evolution. Int. J. Biol. 46: 835-845. Janvier, P. 1996. Early Vertebrates. Claredon Press. Oxford. Jarvik, E. 1952. On the fish-like tail in the ichthyostegid stegocephalians. Meddelelser øm Grönland. 114(2): 1-90. Laurin, M. 2002. Tetrapod phylogeny, amphibian origins, and the definition of the name Tetrapoda. Syst. Biol. 51: 364-369. Lebedev, O.A. 1984. The first find of a Devonian tetrapod in USSR. Doklady Akad. Navk. SSSR. 278: 1407-1413. (in Russian) Lebedev, O.A. and J.A. Clack. 1993. Upper Devonian tetrapods from Andreyeva, Tula Region, Russia. Paleontology. 36(3): 721-734. Lebedev, O.A. and M.I. Coates. 1995. “The postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev”. Zoological Journal of the Linnean Society 114: 307-348. Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York. Niedzwiedski, G., P. Szrek, K. Narkiewicz, M. Narkiewicz, and P.E. Ahlberg. 2010. Tetrapod trackways from the early Middle Devonian period of Poland. Nature. 463: 43-48. Ruta, M., M. I. Coates, and D. L. J. Quicke. 2003. Early tetrapod relationships revisited. Biol. Rev. 78:251-345. Ruta, M., J. E. Jeffery, and M. I. Coates. 2003. A supertree of early tetrapods. Proc. R. Soc. Lond. B. 270: 2507-2516. Shubin, N.H. and P. Alberch. 1986. A morphological approach to the origin and basic organization of the tetrapod limb. In: Hecht, M.K., B. Wallace, and G. Prance. Evolutionary Biology. Plenum Press. New York pp. 319-387. Shubin, N.H., E.B. Daeschler, and F.A. Jenkins. 2006. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature. 440: 764-771. Smithson, T.R. 1986. A new anthracosaur amphibian from the Carboniferous of Scotland. Paleontology. 49(3): 603-628. Wood, S.P., A.L. Panchen, and T.R. Smithson. 1985. A terrestrial fauna from the Scottish Lower Carboniferous. Nature, London. 314:355-356. |
| By Jack R. Holt. Last revised: 01/16/2014 |
