If I understand how a "trait" is defined, a turtle's shell is a synapomorphy relative to mammals and the common ancestor of living amniotes. The same would go for a snake's limblessness, etc. But are there any synapomorphies all living reptiles have in common relative to an older trait state found in most or all modern mammals? Traits modern mammals share with the common ancestor of living amniotes which reptiles do not also possess?
The following article lists some distinctive skeletal differences in synapsids and sauropsids, and reptilia and non-reptilian amniotes:
Sauropsida is defined as "reptiles plus all other amniotes more closely related to them than they are to mammals" (Gauthier, 1994). The characters supporting Sauropsida include the following:
- Presence of a single coronoid. Synapsids and the diadectomorph Limnoscelis have two coronoid elements. The coronoids are bones on the dorsomedial surface of the lower jaw.
- Supinator process parallel to humeral shaft and separated from it by a groove. In synapsids and diadectomorphs, the supinator process is strongly angled relative to the shaft.
- Presence of a single pedal centrale. Two centralia were present in the tarsus (ankle) of synapsids and diadectomorphs.
Reptilia is defined as "the most recent common ancestor of extant turtles and saurians, and all of its descendants" (Gauthier et al., 1988). Characters supporting Reptilia include:
- Tabular small. The tabular (a bone on the posterolateral corner of the skull table) of early synapsids and diadectomorphs is large, but reptiles have only a small tabular, when it is present.
- Suborbital foramen present (Fig. 1B-D). The suborbital foramen is a small hole near the lateral edge of the palate, between the pterygoid, palatine, and ectopterygoid (or jugal, when the ectopterygoid is absent). This structure was not found in early synapsids and diadectomorphs (Fig. 1A).
- Supraoccipital anterior crista present. The supraoccipital of mesosaurs, synapsids, and diadectomoprhs lacks anterior parasagittal flanges. The supraoccipital of reptiles has a paired anterior parasagittal flange called an anterior crista.
- Supraoccipital plate narrow. The supraoccipital is a bone in the back of the braincase. The supraoccipital plate of mesosaurs, synapsids, and diadectomorphs is broad and extends farther laterally than the postparietal.
Phylogeny and Classification of Amniotes, Michel Laurin and Jacques A. Gauthier