On becoming a reptile: a new list of traits

With the nesting
of Gephyrostegus bohemicus as the last common ancestor to all other reptiles in the large reptile tree, it is worthwhile to list the traits that developed at this node versus the outgroup taxon, Silvanerpeton (Fig. 1). This new list becomes important because Gephyrostegus has no traditional amniote traits.

Traditional amniote traits include:

  1. loss/fusion of the intertemporal
  2. absence of the otic notch
  3. loss/reduction of palatal fangs
  4. appearance/expansion of the transverse flange of the pterygoid
  5. loss of labyrinthine infolding of the marginal teeth
  6. reduction of the intercentra
  7. addition of a second sacral vertebra
  8. narrowing and elongation of the humeral shaft
  9. appearance of the astragalus from fused tarsal elements.

Ironically,
many of the above traits are also found in microsaurs and seymouriamorphs, but not in basalmost amniotes. So there is an odd sort of homoplasy at play here.

Of course,
the chief and key trait of amniotes (= reptiles) is the development of the amniotic membrane,surrounding the embryo. The amnion is only the first of several membranes (later including the egg shell) that reduce egg fluid desiccation. This fragile layer of protection permits eggs to be laid on land, but at first only in moist environments.   Klembara et al. (2014) did not recognize Gephyrostegus as a basal amniote because they employed too few amniotes in their matrix. This was probably due to a mindset biased toward thinking about Gephyrostegus as a pre-amniote, in line with all other traditional paleontologists.

A new list of amniote/reptile traits
(Fig. 1) sets Gephyrostegus apart from its more primitive sister, Silvanerpeton. Yes, this is heretical thinking, but results from letting the matrix scores determine all taxon nestings.

Figure 1. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Figure 1. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Overall
Gephyrostegus bohemicus was more robust and athletic when compared to its phylogenetic predecessor, Silvanerpetion miripedes (Fig. 1). In G. bohemicus the skull, girdles and limbs were all larger relative to the torso. The carpus and tarsus were ossified. The ribs were longer, but fewer in number with a larger lumbar area. Thus the torso was capable of carrying more eggs more rapidly over terrestrial obstacles. The deeper pelvis could expel larger eggs. In summary, the evidence shows that basal reptiles were more fecund and agile than pre-reptiles and those traits were the keys to our success at that node. You can see a video highlighting the origin of humans, including the amniote transition, here.

Large reptile tree traits that appear in the basal amniote, G. bohemicus, 
not present in Silvanerpeton: 

  1. prefrontal (barely) separate from postfrontal
  2. premaxilla not transverse
  3. major axis of naris less than 30º above jawline
  4. naris lateral
  5. nasals and frontals subequal
  6. maxilla ventrally straight
  7. longest metatarsal is number four

Phylogenetic miniaturization
often occurs at the base of novel tetrapod clades. As a pattern, size reduction continued with the advent of amniotic eggs in reptiles, as we learned earlier here, despite the slightly larger size of Gephyrostegus, which may have been substantially larger than its thirty million years older Viséan sister. Certainly tiny reptiles were present in the Viséan in the form of Westlothiana and Casineri on the archosauromorph branch and later with Thuringothryis and Cephalerpeton on the lepidosauromorph branch. Phylogenetic miniaturization has also been overlooked by the latest studies, which generally disregard ‘size’ as a character trait.

Those who had access to the fossils themselves
(Klembara et al. 2014) were not able to make these conclusions because they did not have, nor did they choose to access online, a large gamut cladogram of amniotes. In this case, and many others, the large reptile tree proves again to solve problems despite lacking firsthand access to pertinent fossils. This is heresy, contra to traditional thinking.

On a side note, 
PterosaurHeresies wishes all those vertebrate paleontologists attending in Dallas, Texas, a grand convention filled with good cheer and camaraderie. Wish I could be there with y’all. We’ll review about two dozen published abstracts following the closing ceremonies.

References
Clack JA 1994. Silvanerpeton miripedes, a new anthracosauroid from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (for 1993), 369–76.
Jaeckel O 1902. Über Gephyrostegus bohemicus n.g. n.sp. Zeitschrift der Deutschen Geologischen Gesellschaft 54:127–132.
Klembara J, Clack J, Milner AR and Ruta M 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology 34:774–792.
Ruta M and Clack, JA 2006 A review of Silvanerpeton miripedes, a stem amniote from the Lower Carboniferous of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 97, 31-63.

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