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Journal of Paleontology 92(4):734–742
(Fraser, 1993). Some of the diagnostic characters of the genus Clevosaurus based on features of the skull could not be observed in ‘C. latidens’ for obvious reasons. However, at least the dentition of ‘C. latidens’ did not match that of Clevosaurus, which consists of larger, blade-like teeth with lateral flanges. It has been suggested that the tooth morphology of Clevosaurus was very specialized for a possible omnivorous or carnivorous diet (Jones 2006b, 2009; Rauhut et al., 2012; Martínez et al., 2013), whereas the dentary and maxillary teeth ‘C. latidens’ were more like those of herbivorous taxa. Fraser (1993) also pointed out that the wear facets on the teeth of ‘C. latidens’ suggested a propalinal movement of the lower jaw, which contrasts with the orthal jaw movement seen in Clevosaurus. Based on dentary, maxillary, and premaxillary tooth
morphology, as well as the suggested propalinal movement of the lower jaw, our review of ‘C. latidens’ specimens confirms that this taxon is not referable to Clevosaurus. Our phylogenetic analyses, including both parsimony and Bayesian approaches, confirm its position outside Clevosaurus. We rename ‘C. latidens’ as Fraserosphenodon latidens n. comb. The parsimony tree (Fig. 3.1) suggests that F. latidens is an early- diverging opisthodontian, but not closely related to Pelecymala as was previously suggested by Fraser (1986, 1993), Martínez et al. (2013), and Hsiou et al. (2015). While reviewing the type specimens of Pelecymala (AUP 11140, 11214–11215), we noticed that the teeth of Pelecymala are not transversely broa- dened as had been described by Fraser (1986); in contrast, their shape is more conical, slightly curved, and labiolingually flat- tened. The tooth morphology of Pelecymala is actually more similar to that of some of the earliest diverging rhynchocepha- lians, which is also confirmed by our phylogenetic analyses (Fig. 3). A complete taxonomic redescription of Pelecymala appears necessary, but is beyond the scope of this study. The Bayesian tree (Fig. 3.2) could not recover the exact relationships of F. latidens, because this taxon is found in a polytomy that includes many other species. Like the parsimony analysis, however, the Bayesian approach recovered F. latidens as a genus distinct from Clevosaurus and not closely related to clevosaurs. Following the parsimony analysis, we consider F. latidens as an early diverging opisthodontian. The parsimony analysis of Rhynchocephalia showed better
resolution than the Bayesian approach. This result is not unex- pected, because studies have shown that Bayesian methods are more accurate but less precise than parsimony-based analyses (O’Reilly et al., 2016). There are some minor differences between the internal branches in both trees, but several higher clades were recognized by both phylogenetic methods (Fig. 3). Some of these higher clades within Rhynchocephalia have been frequently recovered in other recent phylogenetic analyses, and have been informally named as ‘crown-sphenodontians,’ ‘derived-sphenodontians,’ or ‘eupropalinals’ (e.g., Apesteguía et al., 2012, 2014; Apesteguía and Carballido, 2014). We propose formal names for two well-supported clades:
Eusphenodontia and Neosphenodontia (Fig. 3). We define Eusphenodontia as the least inclusive clade containing Poly- sphenodon muelleri Jaekel, 1911, Clevosaurus hudsoni, and Sphenodon punctatus. In the 50% majority rule consensus tree, three unambiguous character transitions were recovered for Eusphenodontia under both ACCTRAN and DELTRAN
optimization: (1) wear facets on marginal teeth of the dentary and/or on marginal teeth of the maxilla are clearly visible (character 46: 0 to 1), (2) the premaxillary teeth are merged into a chisel-like structure (character 49: 0 to 1), and (3) the palatine teeth are reduced to a single tooth row with an additional isolated tooth (character 52: 0 to 1). Neosphenodontia is defined as the most inclusive clade containing S. punctatus but not C. hudsoni. In the 50% majority rule consensus tree, Neosphenodontia is supported by the following six unambig- uous character changes that are recovered under both ACCTRAN and DELTRAN optimization: (1) the relative length of the antorbital region is increased, reaching one-quarter to one-third of the complete skull length (character 1: 2 to 1), (2) the posterior edge of the parietal is only slightly incurved inward (character 18: 0 to 1), (3) the parietal foramen is found at the same level or anterior of the anterior border of the supra- temporal fenestra (character 19: 0 to 1), (4) the palatine teeth are further reduced to a single lateral row (character 52: 1 to 2), (5) the number of pterygoid tooth rows is reduced to one or none (character 55: 1 to 2), and (6) the ischium is characterized by a prominent process on its posterior border (character 60: 1 to 2). The families Homoeosauridae, Pleurosauridae, and Spheno- dontidae form in our analyses, as in others, the content of the clade Neosphenodontia. Levels of homoplasy in Euspheno- dontia and Neosphenodontia are generally high, with individual character consistency indices (CI) often <0.5. For both clades, no individual character has a CI of 1 in the 50% majority rule consensus tree (for the complete list of characters, apomorphies, and other tree statistics, see the Supplemental Data). We con- sider the formal naming of these higher clades necessary to facilitate future discussion about the phylogenetic relationships of rhynchocephalians.
Conclusion
This study confirms previous doubts about the referral of ‘C. latidens’ to Clevosaurus. The recognition of ‘C. latidens’ belonging to a new genus now formally named Fraseros- phenodon emphasizes the high generic diversity of Rhyncho- cephalia in the Mesozoic, especially among herbivorous taxa. Furthermore, our study demonstrates that the use of Bayesian approaches can be useful to contrast and validate phylogenies that were previously based only on parsimony methods. Baye- sian inference exhibits generally lower resolution in some parts of the tree, but a few higher clades are strongly supported and are consistently recovered by both Bayesian and parsimony analyses.
Acknowledgments
We thank A. Hastings (VMNH) for the loan of type specimens of Fraserosphenodon.We also thank T. Colin (AUP), M. Lowe (UMZC), and J. Hanson (BRSUG) for access provided to their collections. We thank N. Fraser (NMS) for his assistance to JAH-F during his visit to Aberdeen, and D. Whiteside (Uni- versity of Bristol) for his helpful comments and discussions on Clevosaurus. This work was funded by a Ph.D. scholarship from CONACYT, Mexico, and Bob Savage Memorial Fund
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