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As blogged earlier, Dr. Robert L. Carroll (1970) figured out that the earliest of all reptiles, the ones that biologically invented the amniotic egg, would be small specimens similar to Gephyrostegus. Carroll (2008) wrote: “On the basis of the present fossil record, all adequately known Palaeozoic reptiles appear to have had a common ancestry among the predecessors of the known gephyrostegids.” Carroll (2008) considered the possibility of a long ghost-lineage for gephyrostegids, unfortunately he did not undertake a cladistic analysis like the one here. Later Carroll (2009) dismissed his earlier hypothesis because, “[…gephyrostegids] lack many definitive features [of reptiles] and the best-known genus only occurs long after the appearance of unquestioned amniotes.”
The Importance of Small Body Size
Carroll (2008) wrote, “Study of the earliest known reptiles and their closest relatives among contemporary amphibians indicates that the initial adaptation leading to the emergence of the class was assumption of a terrestrial habit, with accompanying small body size. The small body size of the immediate ancestors of reptiles would have made it possible for them to produce sufficiently small eggs that they could develop in damp places on land without initially being supported and protected by extraembryonic membranes.”
Carroll (1991) presumed the hatchling of tiny early reptiles would have been very large relative to the adult, skipping the tadpole stage and adapting to life immediately as an adult. He noted living salamanders that lay eggs on land have the largest hatchling to adult ratio. Carroll (1991) estimated the size of such an egg at 1 cm with an adult of 10 cm in snout-vent length. Cephalerpeton was exactly that size. Gephyrostegus watsoni (Fig. 2) was less than half that size.
The Origin of the Reptilia
Here Gephyrostegus is the closest outgroup taxon to the Reptilia and Cephalerpeton is the most primitive reptile (despite being 30 million years younger than the oldest known fossil reptiles, Westlothiana and Casineria). Another species, Gephyrostegus watsoni, was smaller than both. Whether G. watsoni was a juvenile or not (it is not a tadpole but certain tiny bones remain unossified), the fact remains that the earliest of reptiles were all smaller than Gephyrostegus bohemicus and basal reptile stayed small for several succeeding phylogenetic nodes.
Distinguished from basal tetrapods, basal reptiles lose their palatal fangs, the intertemporal (a tiny skull roof bone) and the occipital notch at the back of the skull. The occiput includes a large supraoccipital bone linking the skull roof bones to the back of the braincase. The pterygoid developed a transverse flange. The intercentra were reduced to crescents. The proximal tarsals integrated to form an astragalus and calcaneum, which was present in Gephyrostegus and reversed in Weslothiana.
Rapid Size Increase?
Carroll (2008) wrote: “The rapid increase in body size in all lineages of Pennsylvanian reptiles indicates the prior development of an amniotic egg.” Perhaps not with reptiles first appearing some 340 million years ago (e.g. Casineria to Hylonomus) and not getting much bigger for the next 40 million years (e. g. Cephalerpeton to Milleretta).
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Carroll RL 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf.
Carroll RL 1991. The Origin of Reptiles in Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Schultze H-P and Trueb L (eds). Cornell University Press.
Carroll RL 2008. Problems of the Origin of Reptiles. Biological Reviews 44(3):393-431.
Carroll RL 2009. The Rise of the Amphibians 365 Million Years of Evolution. The Johns Hopkins University Press. 360 pp.