Abstract
Fossil specimens illuminating the origin of amniotes are rare, and the known fossils are from a limited paleogeographic area. Taxa from the known localities combined with trace fossils indicate that amniotes had already started to diverge more than 315 Mya. A problem is the patchy fossil record in the primitive reptiliomorph part of the phylogeny, as the taxa occur close to the fossil poor ‘Romer’s Gap’ in the Carbonieferous, and new information has only recently started to fill this void. The currently available evidence suggests that the sister taxon to crown amniotes is Diadectomorpha. Within Amniota proper, the ‘protorothyrid’ Hylonomus , which is the earliest amniote that is known in some detail, may be basal part of a clade that includes both captorhinids and diapsids, despite some analyses linking it to the latter group only. Specimens of roughly contemporary age also indicate the early presence synapsids, the clade leading to modern mammals, and there is strong evidence for an early split between Synapsida and the rest of crown amniotes. Hylonomus is somewhat more robust in the limbs than some previous comparisons to diapsids would make it, and the available evidence indicate that it may indeed be similar to the last common ancestor of Amniota. This ancestor was probably lizard-like in appearance, albeit with somewhat more robust limb bones than most modern lizards, and with a relatively more massive (anapsid-type) skull.
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