Can Vaughnictis attract caseasaurs back to the synapsids again?

Welcome to another taxon challenge from Dr. David Marjanović
Yesterday we looked at Diplovertebron, a taxon Dr. Marjanović suggested (as others have) was just another Gephyrostegus.

Today we’ll reexamine the traditional nesting of caseasaurs with synapsids with a focus on Vaughnictis (Fig. 1), which looks kind of like a caseasaur. Earlier we made the case that the Caseasauria nested better with Millerettidae than with Synapsida when more taxa are included. Since then Vaughnictis was added to the large reptile tree (LRT, 1012 taxa) and it nested as the last known common ancestor to birds and bats (= archosauromorph diapsids and synapsids). 

Despite great resemblance,
the basal prosynapsid Vaughnictis (Fig. 1) does not attract the clade Caseasauria (Fig. 2; Casea, Cotylorynchus and kin including Datheosaurus and Eothyris,Fig. 1) back to the base of the Synapsida (Fig. 3; Varanosaurus, Dimetrodon and kin). Given their phylogenetic distance from one another, the resemblance is indeed extraordinary, especially in the temporal area. Perhaps even more so between Vaughnictis and Milleretta, than with a basal caseasaurid, like Eothyris.

Figure 1. The basal synapsid, Vaughnictis, and the basal caseasaur, Eothyris. For starters, synapsids have a taller than wide skull and caseasaurs have a wider skull. See text for other details.

Figure 1. The basal prosynapsid, Vaughnictis, and the basal caseasaur, Eothyris. For starters, synapsids have a taller than wide skull and caseasaurs have a wider skull. See text for other details.

At present
the LRT nests caseasaurs with Eothyris + Oedaleops + Colobomycter) and slightly further from Feeserpeton + Australothyris + Eocasea and kin. A shift of the Caseasauria (Fig. 2) to the base of the Synapsida (Fig. 3) adds at least 26 steps.  

Figure 2. Milleretta, caseasaurs and kin. The LRT nests these taxa together apart from the Synapsida, with which they share a lateral temporal fenestra.

Figure 2. Milleretta, caseasaurs and kin. The LRT nests these taxa together apart from the Synapsida, with which they share a lateral temporal fenestra. If any taxon resembles Milleretta, Vaughnictis is a better candidate than any caseasaur.

Despite sharing a lateral temporal fenestra
caseasaurs share more traits with millerettids than with synapsids, which retain their predatory teeth and a taller, narrower skull. Vaughnictis retains a short rostrum from ancestors like Protorothyris (Fig. 3). Synapsids never had the rostral overbite found in caesars, nor did they have that arrowhead-shaped set of nasals. Caseids and kin had three premaxillary teeth, not four or more as found in synapsids and Vaughnictis. The surangular in caseids does not extend anterior to the coronoid process. The dentary tip rises in synapsids, but not caseids, among several other distinct traits.

Figure 3. Vaughnictis is basal to the Synapsida and the Prodiapsida, here represented by Mycterosaurus.

Figure 3. Vaughnictis is basal to the Synapsida and the Prodiapsida, here represented by Mycterosaurus.

Can Vaughnictis make Caseasauria a synapsid clade again?
No. Not with the present taxon list. The reason why experts continue to promote caseasaurs as synapsids goes back to a long-standing tradition of taxon exclusion. They exclude members of the Millerettidae. Expand the gamut of your taxon list, and let the taxa nest wherever they want to.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s