Vaughnictis (Brocklehurst et al. 2016): a new last common ancestor of birds and bats

Recently Brocklehurst et al. 2016
renamed ‘Mycterosaurus’ smithae (Lewis and Vaughn 1965; early Permian, MCZ 2985). The new name is Vaughnictis smithae. The specimen was originally considered a varanopid close to the holotype of Mycterosaurus (Fig. 1). Now the Brocklehurst team nest the MCZ 2985 specimen between Eothyris and Oedaleops (Fig. 1) at the base of the Caseasauria, which they consider a clade at the base of the Synapsida.

Unfortunately, 
the large reptile tree nests Vaughnictis at the bases of two major clades: between Protorothyris and the Synapsida (represented here (Fig. 1) by Elliotsmithia) + the Prodiapsida (represented here by Mycterosaurus). The Caseasauria, as noted five years ago here, does not nest with the Synapsida when the taxon list is expanded, and the Prodiapsida (former varanopids) split from the Synapsida at their base when the taxon list is expanded.

Figure 2. Vaughnictis nests between Protorothyris and the Synapsida (Elliotsmithia) + Prodiapsida (Mycterosaurus) in the large reptile tree - not the Caseasauria (Eothyris + Oedaleops).

Figure 1. Vaughnictis nests between Protorothyris and the Synapsida (Elliotsmithia) + Prodiapsida (Mycterosaurus) in the large reptile tree – not the Caseasauria (Eothyris + Oedaleops). Despite the many similarities, the narrow skull with parallel sides, upturned mandible tip and longer rostrum are a few traits that split Vaughnictis from caseasaurs and lump it with prosynapsids.

Lewis and Vaughn got it right.
Vaughnictis is a sister to Mycterosaurus in the LRT. It is not a caseasaur.

Brocklehurst et al. got it right
in that Vaughnictis is distinct enough from Mycterosaurus to warrant its own genus.

Despite their phylogenetic distance
only 22 additional steps are needed when Vaughnictis is force shifted over to Eothyris. This is largely due to convergence. Both clades developed lateral temporal fenestrae in similar patterns, had large eyes and a short rostrum at this stage.

Figure 2. Vaughnictis skull in situ with color tracings. See figure 3 for reconstruction.

Figure 2. Vaughnictis skull in situ with color tracings. See figure 3 for reconstruction. The jaws shifted posteriorly during taphonomy The parietal and its opening are difficult to read.

The phylogenetic importance of Vaughnictis
was overlooked by Brocklehurst et al. It is the most primitive known specimen in the lineage of synapsids and diapsids to have a lateral temporal fenestra. That’s why it is the last common ancestor of bats and birds, an honor formerly earned by Protorothyris, but now superseded by a taxon with lateral temporal fenestrae.

Figure 1. Color tracings of bones moved to their in vivo positions and traced.

Figure 3. Color tracings of bones moved to their in vivo positions and traced. Note the anterior shifting of the jaws to their in vivo positions based on posterior dentary and jugal positions common to most if not all candidate sister taxa. 

I wish that Brocklehurst et al. had 

  1. created a multi-view reconstruction
  2. showed candidate sisters side by side compared to Vaughnictis
  3. not excluded pertinent taxa (diapsids for the former varanopids and millerettids for the caseasaurs)
  4. used colors to identify bones, rather than lines, which helps when bones overlap. They did color the teeth (Fig. 4), but not all the teeth.
Figure 6. Teeth scanned by Brocklehurst fit to dorsal view of skull. Premaxillary and maxillary teeth were not published. Note the scale bar for the teeth appears to be off by a factor of 2.

Figure 4. Teeth scanned by Brocklehurst fit to dorsal view of skull. Premaxillary and maxillary teeth were not published. Note the scale bar for the teeth appears to be off by a factor of 2. Also note the premaxillary teeth appear to be jammed back from their in vivo position. 

The interesting thing about their cladogram
is that the Brocklehurst team nested Captorhinus, Limnoscelis and Tseajaia as outgroup taxa — which is correct for Caseasauria, but not for Synapsida. Protorothyris is also listed as the proximal outgroup to the Caseasauria (incorrect) + Synapsida (correct). It is clear they rely on tradition, rather than testing for their inclusion set.

In Vaughnictis, as opposed to Eothyris, note the 

  1. relatively narrow skull
  2. the rising mandible tip
  3. the lack of maxilla/orbit contact
  4. the shorter temporal length
  5. the lower rostrum

These traits ally Vaughnictis with Elliotsmithia to the exclusion of basal caseasaurs.

Brocklehurst et al. note:

  1. Vaughnictis lacks these mycterosaurine and varanopid traits (but it is not a member of either of these clades in the LRT) :
    a. slender femur
    b. linguo-labially compressed and strongly recurved teeth – I disagree, the teeth are indeed recurved
    c. lateral boss on the postorbital – I don’t see this on candidate taxa
  2. Vaughnictis has these caseasaur traits:
    a. coronoid teeth – plesiomorphic for synapsids, but they have been lost in derived caseids, ophiacodontids, varanopids and sphenacodontians. Most workers do not include these in their tracings of pertinent taxa, so are rarely noted.
    b. large supratemporal – actually they are long, as in synapsids, not large (and wide) as in caseasaurs
    c. large pineal foramen – unable to confirm, but Elliotsmithia and Mycterosaurus also have a large pineal foramen.

Teeth
Broklehurst et al. published synchrotron scans of the palatal, dentary and coronoid teeth (perhaps the scale bar should be 1 cm, not 5mm, Fig. 4), but did not publish scans of the maxillary teeth. All of the palatal teeth form shagreen fields, not single rows. That’s different than all candidate sister taxa, whether caseasaurid or protorothyrid. What they label as “vomerine teeth” may be premaxillary teeth based on the posterior displacement of the jaws. The dentary teeth are recurved and robust. Coronoid and parasphenoid teeth are present.

Figure 6. Subset of the large reptile tree showing the nesting of Vaughnictis at the base of the Synapsida and Prodiapsida.

Figure 5. Subset of the large reptile tree showing the nesting of Vaughnictis at the base of the Synapsida and Prodiapsida. Also note that the Synapsida is NOT the first clade to branch off from the base of the Amniota. Far from it.

This Brocklehurst team was led by
the venerable Robert Reisz, who has made dozens of great discoveries, but has resisted testing candidates suggested by the large reptile tree. And that sort of paleoxenophobia is unfortunate. Outsiders can make valuable contributions.

Finally, kudos and credit to the Brocklehurst team,
for finding the one best specimen closest to the advent of the Synapsida + Prodiapsida. It should be in every textbook from here on out.

References
Brocklehurst N, Reisz RR, Fernandez V and Fröbisch 2016. A Re-Description of ‘Mycterosaurus’ smithae, an Early Permian Eothyridid, and Its Impact on the Phylogeny of Pelycosaurian-Grade Synapsids. PLoS ONE 11(6):e0156810. doi:10.1371/journal.pone.0156810
Lewis GE, Vaughn PP 1965. Early Permian vertebrates from the Cutler Formation of the Placerville Area, Colorado. Geological Survey Professional Paper 500C: 1–50.
Reisz RR, Dilkes DW and Berman DS 1998. Anatomy and relationships of Elliotsmithia longiceps Broom, a small synapsid (Eupelycosauria: Varanopseidae) from the late Permian of South Africa. Journal of Vertebrate Paleontology 18(3):602-611.

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