DESCRIPTION OF THE CLASS PROAMNIOTA+ [i]

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Proamniota (pro-am-ne-O-ta) is coined from two Greek roots meaning before the membrane [before -pro (προ); and membrane -amen (υμήν)]. In this case, we coined the name to mean before the amnion (αμνίον), the specialized membrane in the cleidoic egg that allows the exchange of gasses (O2 and CO2) without losing water. Benton (2005) united the Lepospondyli and Reptiliomorpha as subclasses under an unnamed class. We have attempted to provide a name that is appropriate and indicate the sister relationship with the Amniota. The name Proamniota means before the amniotes.

INTRODUCTION TO THE PROAMNIOTA Benton (2005) grouped the Lepospondyli and Reptilomorpha together as groups in an unnamed class that we call Proamniota (see the footnote on this; see Figure 1, Clade 1). He defined the clade with the following characters: both premaxillae make up less than 2/3 of the skull width, the vomers are elongate and strip-like, and the tarsus has an L-like process at the proximal end. Evidence is that they were aquatic, semi-aquatic, or even mainly terrestrial, but they all had larval forms, that presumably were aquatic with external gills. They had appeared during the period known as Romer’s Gap (during the lower Mississippian Period, see the Stegocephali) in one of the early tetrapod radiations (Carroll 2001). Technically, all of these animals considered in the Proamniota were amphibians. That is, they developed aquatic larvae and more terrestrial adults. Like living amphibians, their eggs were laid in the water; so their life cycle tied them to water. This was a decided change from the Stegocephali which were entirely aquatic (Clack 2002) and did not have a larval stage. The paradox is that tetrapods became terrestrial, in part, through the development of an aquatic larva. Despite the relatively simple appearance of Figure 1-A, the Proamniota as defined here likely is paraphyletic because it includes also the sister group to all of the amniotes (mammals, birds, reptiles) and the Batrachomorpha (Temnospondyli and Lissamphibia). The relationships between these groups are still in question (Laurin 2002; Marjanovic and Laurin 2007; Carroll 2001; and Ruta et al. 2003). Note the major differences between Figures 1-B and 2. Thus is the problem with stem groups. In a strict cladistic hierarchy the Proamniotes would rank as high as all of the amniotes. We choose to treat them as a separate stem group and acknowledge that the sister to the amniotes lies within the Proamniota.

FIGURE 1A. (left) A simplified view of the relative position of the taxa in the group that we call the Proamniota.

CLADES OF THE PROAMNIOTA 1. THE PROAMNIOTA CLADE 2. THE LEPOSPONDYLI CLADE 3. THE MICROSAURIA CLADE 4. THE NECRIDEA + AISTOPODA CLADE 5. THE REPTILOMORPHA CLADE 6. THE ANTHRACOSAURIA CLADE 7. THE SEYMOURIOMORPHA + DIADECTOMORPHA CLADE

FIGURE 1B. (above) Major clades of the Proamniota. This figure is a simplification of Benton (2005). The Stegocephali (Basal Tetrapod Families) is the outgroup. The Batrachomorpha (Temnospondyli + Lissamphibia) may be a sister group to the Proamniota or emerge from within the Lepospondyli (Clade 2). The Reptilomorpha (Clade 5) is a sister group to the Lepospondyli. The Amniota may be a sister to the Proamniota or emerge from within the Reptilomorpha (likely from within the Diadectomorpha). Extinct taxa are in red.

FIGURE 2. (left) This is an alternative view of relationships between early amphibians as presented by the supertree analysis of early tetrapods by Ruta et al. (2003). Note that the Amniota emerge from within the Reptilomorpha, and the Lissamphibia emerge from within the Lepospondyli. More surprising is the basal nature of the Temnospondyli (a group that Benton 2005 considers sister to the Lissamphibia in a group called the Batrachomorpha).

The Lepospondyl Clade (Figure 1 clade 2) The lepospondyls were amphibious with peculiar types of bony vertebrae that developed directly around the notochord and were spool-shaped (cylindrical pleirocentra, Benton 2005). Multiple taxa lost the terrestrial adult phase and became sexually mature as larvae, a condition called paedomorphy. They appeared in the Carboniferous (Mississippian and Pennsylvanian Periods) and died out at the beginning of the Permian Period.

The Microsaur Clade (Figure 1 clade 3) The microsaurs (a name that means little lizards) were very diverse. They resembled salamanders with long bodies and relatively short legs. Among their ranks were terrestrial, aquatic, and fossorial taxa. Pantylus (Figure 3) was a stout, largely terrestrial microsaur with a short tail. It was about 20 cm long and likely was an insectivore. When discovered, the animal was assumed to be a terrestrial reptile (Laurin and Reisz 1995). Microbrachis (Figure 4) was long with very small appendages. Likely, it was entirely aquatic and may have been paedomorphic, though the occurrence of external gills is not evident (Vallin and Laurin 2004). The animal must have had gills that were not supported by bone. Also, this animal was somewhat unique in being the only lepospondyl that had a lateral line system, a character common among the Stegocephali.

The Necridea + Aistopod Clade (Figure 1 clade 4) The necridea were entirely aquatic animals with long tails that were flattened and adapted for swimming (Carroll 2001). The hind legs were large but the front limbs were small. Diplocaulus (Figure 5) was a common paedomorphic animal that had a large boomerang-shaped head. The broad, flattened head may have provided lift while swimming, but the whole head shield was armored and very heavy. Our suspicion is that such an extravagance may had something to do with territorial or mating displays. The aistopoda were among the oddest of the lepospondyls. They were very much like snakes with a very long trunk and a short tail (Carroll 2001). The appearance was more than superficial (see Ophiderpeton, Figure 6); not only were there no limbs, but there were no remnants of the appendicular girdle. The jaws were hinged such that they could open very wide. If they were associated with water, they likely were not obligate animals because the tail and lower part of the torso was not flattened.

The Reptilomorpha Clade (Figure 1 clade 5) The reptilomorphs were amphibians that looked like reptiles. That is, despite appearances, they had aquatic larval forms in their life cycle. The synapomorphies for this subclass include premaxillae less than 1/2 of the skull width, and the vomers taper forward (Benton, 2005). They dominated terrestrial habitats from the Carboniferous to the Permian. This group also included the first known tetrapod herbivore, Diadectes.

The Anthracosauria Clade (Figure 1 clade 6) The anthracosaurs were fish eating animals with elongate skulls, which seem to have been able to flex when opening the jaws. They had large vertebrae and flat tails that could have been useful for swimming. Some had legs that were well developed and suited to rapid terrestrial movement, but later forms had reverted to a more aquatic existence. Diplovertebron (Figure 7) is a good example of the anthracosaurs. These animals had returned to the water so completely that they superficially resembled the aquatic tetrapods in the Stegocephali.

The Seymouriomorpha + Diadectomorpha Clade (Figure 1 clade 7) The seymouriomorphs were a diverse and powerful group of animals. The terrestrial forms held their bodies high off the ground. Aquatic forms had broad skulls and fed on fish. Seymouria (Figure 8), one of the first reptilomorphs to be studied was assumed to be an early reptile (amniote) until Necturus-like larvae were discovered (Laurin 2000). The diadectomorphs were very close to the origin of the amniotes (Laurin and Reisz 1995). Limnoscelis (Figure 9), a carnivorous diadectomorph, was one of the early members of this advanced amphibian group. Later members of the diadectomorphs tended toward herbivory. Indeed, members of this group were among the first herbivorous tetrapods. One, Diadectes (Figure 10), was adapted to herbivory by having anterior nipping teeth and posterior crushing teeth (Reisz and Sues 2000). These animals were heavily built with very strong legs and limb girdles.

FIGURE 3. A drawing of Pantylus, a microsaur lepospondyl amphibian. It was largely terrestrial and likely an insectivore. Image by: Dimitry Bogdanov- Wikipedia	FIGURE 4. A drawing of Microbrachis, a microsaur lepospondyl amphibian. It was a paedomorphic aquatic animal with a long body and reduced legs. Image by: Arthur Weasley- Wikipedia	FIGURE 5. A drawing of Diplocaulus, a necridean lepospondyl amphibian. It was a paedomorphic aquatic animal with a long body and and a large armored boomerang-shaped head. Image by: Arthur Weasley- Wikipedia	FIGURE 6. A drawing of Ophiderpeton, an aistopod lepospondyl amphibian. It was a snake-like animal with a long trunk and short tail. Likely, the aistopods were terrestrial animals as adults. Image by: Nobu Tamura- Wikipedia

FIGURE 7. A drawing of Diplovertebron, an anthracosaur reptilomorph amphibian. It was a fully aquatic animal resembling the earlier stegocephali.. Image by: Arthur Weasley- Wikipedia	FIGURE 8. A drawing of Seymouria, a seymouriomorph reptilomorph amphibian. Members of this group were quite diverse with representatives in the water and on land. Seymouria when first discovered was assumed to be a reptile and an amniote until later when aquatic larvae were discovered. Image by: Arthur Weasley- Wikipedia	FIGURE 9. A drawing of Limnoscelis, a carnivorous diadactomorph reptilomorph amphibian. Image by: Dimitry Bogdanov- Wikipedia	FIGURE 10. A drawing of Diadectes, a herbivorous diadactomorph reptilomorph amphibian. The didactomorphs like Diadectes was the first bona fide herbivorous tetrapods. Image by: Dimitry Bogdanov- Wikipedia

[i] We have attempted to provide a name to an unnamed class of Benton (2005) that is appropriate and indicate the sister relationship with the Amniota. The name Proamniota means before the amniotes.

LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. [L] Carrol, R. L. 2001. The origin and early radiation of terrestrial vertebrates. Journal of Paleontology. 75(6):1202-1213. Clack, J.A. 2002. An early tetrapod from ‘Romer’s Gap’. Nature. 418: 72-76. Laurin, M. 2002. Tetrapod phylogeny, amphibian origins, and the definition of the name Tetrapoda. Syst. Biol. 51: 364-369. Laurin, M. and R.R. Reisz. 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnaean Society. 113: 165-223. Marjanovic, D. and M. Laurin. 2007. Fossils, molecules, divergence times, and the origin of lissamphibians. Systematic Biology. 56 (3): 369-388. Reisz, R.R. and H.D. Sues. 2000. Herbivory in Late Paleozoic and Triassic Terrestrial Vertebrates. pp 9-41. in: Evolution of Herbivory in Terrestrial Vertebrates, Cambridge Univ. Press., H.D. Sues, ed.. Herbivory in Late Paleozoic and Triassic Terrestrial Vertebrates. in: Sues, H. D., ed. Evolution of Herbivory in Terrestrial Vertebrates, Cambridge Univ. Press. pp 9-41. Ruta, M., J.E. Jeffery, M.I. Coates. 2003. A supertree of early tetrapods. Proceedings of the Royal Society of London. B. 270: 2507-2516.

By Jack R. Holt and Carlos A. Iudica. Last revised: 04/07/2013