A former Doswellia sp.
(Heckert et al. 2012) has be reexamined and renamed Rugarhynchos sixmilensis by Wynd et al. 2020.
The resemblance is remarkable
(Fig. 1) and the size is similar. Both are from the Late Triassic of North America (Virginia and New Mexico). Wynd et al. did a good job of tracing the bones, but provided no reconstructions (they pictured the premaxilla on a separate page spread). They also misidentified the surangular (SA) as the quadratojugal.
Is this just another species of Doswellia?
We’ve seen more variation in Rhamphorhynchus, and Pteranodon, but naming a new genus is reserved for full professors and their students. In this case, the resemblance is readily apparent, and so are the various enlargements and reductions. The problem lies, as it often does, in the published cladogram (Fig. 2) suffering from taxon exclusion.
Figure 1. Doswellia skull compared to Rugarhynchos, here reduced to a similar length for rapid comparison.
From the abstract:
“Stem archosaurs exhibit substantial cranial disparity, especially by taxa either shortening or elongating the skull. This disparity is exemplified in the North American Late Triassic proterochampsians by the âshort-facedâ Vancleavea and the ong-faced doswelliids.”
When more taxa are added, as in the large reptile tree (LRT, 1695+ taxa; subset Fig. 3), Vancleavea nests with Helveticosaurus in the Thalattosauria, as we learned several years ago. Missing from the Wynd et al. taxon list are any choristoderes. Those are close relatives to doswellids in the LRT.
“To critically investigate skull elongation and character evolution in these proterochampsians, we evaluate Doswellia sixmilensis, known from much of a skull, cervical centra, and osteoderms from the Bluewater Creek Member of the Chinle Formation of New Mexico.” (See Fig. 1).
Figure 2. Cladogram from Wynd et al. 2020 with colors added to show where these taxa nest when more taxa are added, as in the LRT. Avemetatarsalia is invalid because it includes both pterosaurs and dinosaurs, neither of which is related to Vancleavea or Phytosauria in the LRT. Remember to check your results for mismatches like this.
From the abstract:
“Rugarhynchos sixmilensis, gen. et comb. nov., exhibits an elongate snout with characteristics known in stem and crown archosaurs, including a downturned premaxilla and fluted teeth.”
In the LRT, archosaurs include only crocs + dinos (including birds). Due to taxon exclusion (chiefly bipedal basal crocodylomorphs) Wynd et al. expand that list to include many other taxa.
“We included R. sixmilensis in a phylogenetic analysis of archosauromorphs consisting of 677 characters and 109 taxa under both parsimony and Bayesian models.”
Now do you see why increasing the number of taxa is MUCH more important than increasing the number of characters? How one taxon relates to other taxa requires a lot of other taxa… and a sufficient number to traits (150+). The LRT includes 238 multi-state taxa and it nests everything from fish to humans with high resolution.
“We recover R. sixmilensis as a doswelliid, sister to Doswellia kaltenbachi. Our parsimony and Bayesian models differ in the placement of Doswelliidae, either as sister to or within Proterochampsidae, respectively.”
Wynd et al. excluded too many pertinent taxa. Here’s where the LRT (Fig.3) nests Doswellia and the pararchosauriformes.
“We use archosauromorph relationships from the Bayesian model to estimate cranial disparity between stem and crown archosaurs and find a narrow breadth of morphological disparity in the stem. Our results suggest that crown archosaurs evolved disparate crania from a low-disparate archosauriform condition.”
Without a valid phylogenetic context (Fig. 3), the results of Wynd et al. cannot be validated. They need more taxa.
Figure 3. Subset of the large reptile tree focusing on the pararchosauriformes and the Choristodera, still similar since 2015. Euparkeria is basal to the Euarchosauriformes, including Archosauria.
The skull of Rugarhynchos was added to a graphic
(Fig. 4) that included Doswellia and its relatives to scale. Many of these taxa were omitted from Wynd et al. 2020.
Figure 4. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa.
Wynd et al. considered Rugarhynchos a proterochampsid.
With more taxa added (Figs. 3, 4) that’s not confirmed by the LRT. Doswellia is slightly closer to choristoderes, a clade not shown in the Wynd et al. cladogram (Fig. 2). It would have been better if Wynd et al also added a variety of proterosuchids, as in the LRT. They are all as different and distinct as Rugarhynchos is from Doswellia.
Wynd BM, Nesbitt SJ, Stocker MR and Heckert AB 2020. A detailed description of Rugarhynchos sixmilensis, gen. et comb. nov. (Archosauriformes, Proterochampsia), and cranial convergence in snout elongation across stem and crown archosaurs. Journal of Vertebrate Paleontology Article: e1748042