Outgroup Taxa and Phylogenetic Miniaturization
Based on the present set of outgroup taxa (Fig. 1) basal tetrapods (represented by Ichthyostega) gave rise to embolomeres (represented by Proterogyrinus and Eoherpeton), which gave rise to seymouriamorphs (represented by Seymouria, Kotlassia, Utegenia and Silvanerpeton), which ultimately produced basal amniotes (represented by Gephyrostegus bohemicus) and ‘second generation’ amniotes (represented by Westlothiana and Thuringothyris).
A general reduction in overall size is apparent in this lineage.
Proterogyrinus is more than a meter in length (Fig. 1). Eoherpeton is even larger. However, Seymouria and Kotlassia are down to 60 cm long with at least a 50 cm snout/vent length. The basal amniotes, G. bohemicus, Eldeceeon and Bruktererpeton, each have a snout-vent length of 25 cm or less. The ‘second generation’ amniotes, G. watsoni, Westlothiana, Casineria, Brouffia, Thuringothyris and Cephalerpeton, reduce that length to 13 cm or less. Thus, under the present hypothesis of phylogenetic relationships, the evolution of basal amniotes includes phylogenetic miniaturization (Hanken and Wake, 1993). This is convergent with the miniaturization already recognized in the evolution of basal mammals (e.g., Pachygenelus, Megazostrodon, Hadrocodium) from larger cynodonts (Luo, et al. 2001) and in basal birds (e.g., Sinosauropteryx, Archaeopteryx, Sinornis) from larger theropods (Lee, et al. 2014). Based on the few taxa that are known (Fig. 1), basal amniotes apparently remained small to tiny for the first 30 million years, until the advent of Solenodonsaurus and the arrival of pelycosaur-grade synapsids in the Late Carboniferous to Early Permian.
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