News at the base of the Amniota, part 2: miniaturization

Yesterday we opened this topic (the origin of the Amniota) with an introduction to Gephyrostegus bohemicus at the base of this major clade.

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).

Figure 2. Miniaturization led to the origin of the Amniota.

Figure 1. Miniaturization led to the origin of the Amniota.

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.

Figure 1. Basal amniotes to scale. Click to enlarge.

Figure 2. Basal amniotes to scale. Click to enlarge. Only Solenodonsaurus gets large early.

More later.

References
Hanken J and DB Wake 1993. Miniaturization of body size: organismal consequences and evolutionary significance. Annual Review of Ecology and Systematics 24:501–519.
Lee, MSY, A Cau, D Naish, and GJ Dyke 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds. Science 345:562-566.
Luo Z-X, A.W. Crompton and A-L Sun 2001. A new mammaliaform from the Early Jurassic and evolution of mammalian characteristics. Science 292: 1535–1540.

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