A New Skull Based on Tracings
Silvanerpeton miripedes (Clack 1994) Visean, Early Carboniferous ~335 mya, ~40 cm in length was originally illustrated with the skull shown in figure 1 (in white) and on this data Silvanerpeton was entered into the large reptile family tree. The addition of a contemporary, Eldeceeon (Smithson 1994) introduced certain problems (autapomorphies) into the dataset. Those problems sparked a review of the specimen that included tracing the skull from photographs provided in Ruta and Clack (2006). A new reconstruction based on specimen NMS G.1998.51.2 (in yellow, Fig. 1) had distinctly different traits. There are several specimens known. Perhaps the original was based largely on another specimen.
A New Reconstrucion of Gephyrostegus (specimen 1)
As part of the process of weeding out invalid autapomorphies, I also retraced and reconstructed a sister of Silvanerpeton, specimen 1 of Gephyrostegus (Brough and Brough 1967) (CGH IIIB 21 c. 587) and counterpart (MP451) to produce a revised skull for that specimen (Fig. 2). Note the virtual loss of the otic notch, the development of the transverse pterygoid flange and those long reptilian toes, more robust than in the larger Silvanerpeton.
A New Nesting
Taxa shifted slightly from the prior tree due to the addition of Eldeceeon and the reinterpretation of traced traits. Both Silvanerpeton and specimen 1 moved away from Utegenia and toward Gephyrostegus. This clarifies the lineage of pre-reptiles. Needless to say at this point, diadectids and limnoscelids do not nest as pre-reptiles, but well within the new Lepidosauromorpha.
A Serial Size Reduction
Silvanerpeton was a 40 cm tetrapod. Specimen 1 of Gephyrostegus and G. watsoni were a quarter of its size. Silvanerpeton appeared just 25 million years after Ichthyostega, and the first amniotes may have been contemporaries, just getting started. These data fall in line with Carroll’s (1970) hypothesis on tiny tetrapods without a tadpole stage were the first to lay tiny amniotic eggs, as blogged just a few days ago.
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.
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.
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.
Smithson TR 1994. Eldeceeon rolfei, a new reptiliomorph from the Viséan of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (3-4): 377–382.