You may recall that all living and extinct reptiles can be divided into two clades, the new Archosauromorpha and the new Lepidosauromorpha. Yesterday we looked at lepidosauromorph ears. Today we’ll examine archosauromorph ears.
Living members of the new Archosauromorpha (Fig. 1, crocs, birds and mammals) all have well-developed ears, but only higher mammals have the erect external ears we typically think of.
At the base of the Archosauromorpha we don’t see evidence for ears. The stapes, the first and typically (except in mammals) the only ear bone, tends to be robust, helping to support the jaws. That evidence for an eardrum frame first appears in basal therapsids as a creation of an angular flange that thereafter thins to become a gracile encircling eardrum frame (Fig. 2). In higher mammals the
There is likewise not much of a clue in basal diapsids with regard to their hearing. We skip the enaliosaurs, which were underwater creatures and don’t see a lepidosauromorph-like eardrum frame on any taxa before protorosaurs like Czatkowiella (Fig. 3) and perhaps some younginids, but data is sparse on them at present. Prehistoric crocs and dinosaurs probably developed like living crocs and birds.
The more birdy or croc-like a taxon gets, the easier it seems to be able to imagine an eardrum framed at the back of the skull and deeper than at the surface.
Here, if anyone has additional data, I will gladly add it later.