Pegomastax and the “Echinodonts”

In my last post, I mentioned Sereno’s monster — a 225-page monograph on Heterodontosauridae — and some of its variant findings. I am especially interested in the features discussed about the “echinodonts,” a small grouping of probably plesiomorphically small, “simple-toothed” heterodontosaurids including Tianyulong confuciusi, Echinodon becklessii, Fruitadens haagarorum, and an undescribed “heterodontosaur” from the Kayenta Formation. Sereno also named a small, “dwarf” taxon that nested with the larger forms as Pegomastax africanus (sic: efer to my endnote in the last post on the problems of its name). A previous “dwarf” form, Manidens condorensis (Pol et al., 2011), is rooted within the “heterodontosaurine” clade along with Pegomastax africanus (sic).

Skulls of Heterodontosauridae, scaled to the same maxillary length (G is adjusted to similar dentary length). Drawings from Sereno, 2012. A, Echinodon becklesii (Owen, 1861); B, Tianyulong confuciusi (Zheng et al., 2009); C, Manidens condorensis (Pol et al., 2011); D, Heterodontosaurus tucki (Crompton & Charig, 1962); E, Abrictosaurus consors (Thulborn, 1974); F, Lycorhinus angustidens (Haughton, 1924); G, Pegomastax africanus (sic; Sereno, 2012).

(Relative size of maxillae to skulls is related additionally to length of the dental row to the jaw, and thus also to rostral limitation of the effective triturating region, maximized at Manidens condorensis.)

As I mentioned as one of its issues, the paper includes a systematic analysis of heterodontosaurids among Ornithischia, and this included the most comprehensive taxonomic listing yet to date for the little critters — but it’s not the most comprehensive paper at all. Sereno included only 30 characters and 23 OTUs (operational taxonomic units), less than the cladistic analyses of Butler et al. (2008: 221 characters, 46 OTUs), Norman et al. (2011: 221 characters, 14 OYUs — pruned from Butler’s analysis), and Pol et al. (2011: 230 characters, 51 OTUs — expanded from Butler’s analysis). Problematically, none of these analyses managed to avoid use of the supraspecific taxon in lieu of species-level OTUs.

A graphic representation of this shows the inadequacy of such a small phylogeny, especially the quality of taxon selection for non-heterodontosaurids, or testing the consistency of the monophyly of Heterodontosauridae. Pol et al., 2011, remains the best analysis to date to focus on heterodontosaurids, given its lack of supraspecific taxa, its sheer number of characters with respect to other analysis, and an increase in selected taxa despite breaking supraspecific taxa like Stegosauria into pieces.

Number of characters for referent cladistic analyses in blue, number of taxa in red; shown to the same scale (bottom), and arrayed by chronology (oldest at top, most recent at bottom).

It should be noted that despite the use of these supraspecific taxa, Sereno argues that his clades are coded directly from species but are generally invariable; this is due to the highly specialized look for the analysis: Serneo is testing only the inclusion or exclusion of heterodontosaurids and for Pisanosaurus mertii, assuming the results of the position of Heterodontosauridae within Ornithischia are generally irrelevant. Pisanosaurus mertii itself is interesting in that it has been occasionally assumed to be a heterodontosaur, what with the en echelon arrangement of teeth, heavy tooth wear, and proximate relationship to heterodontosaurs in recent phylogenies. Indeed, for a spot analysis, it seems fine; but Sereno also modified previous analyses to conform to observations of his own work, especially characters of Echinodon becklesii (originally described by Richard Owen as a lizard) which he argued in this analysis bore various features relating to “heterodonty” and the presence of a large mesial caniniform on the dentary, which opposes the autapomorphic maxillary “canine.”

NHMUK 48215b, a paralectotype dentary referred to Echinodon becklesii (Owen, 1861). After Sereno, 2012.

Reanalysis of Butler et al. (2010, an expanded version of Butler et al., 2008, and of which Norman et al., 2011, is also a further expansion) and Pol et al. (2011) produced topologies similar to that of the original (see below), but in the former Echinodon becklesii moved within a monophyletic Heterodontosauridae. These reanalyses, largely through recoding and ordering several characters, and removal of “phylogenetically uninformative” characters — characters in which no phylogenetic resolution arises, such as autapomorphies, of which a few were present in each analysis — resulted in similar, if not identical results.

A, Butler et al., 2008; B, Pol et al., 2011; C, Norman et al., 2011; D, Sereno, 2012. All trees modified from the original, either throup cropping of alternate schemes (A), or by flipping (B,C), and all with colored bars representing Heterodontosauridae in red, and “fanged” pachycephalosaurs in blue. The closeness of red and blue in D is a result of Sereno’s lumping of neornithischian clades into a representative unit.

There’s room to develop in these analyses, of course: Butler et al. (2008, 2010) and Norman et al. (2011), as well as Pol et al. (2011) coded in their analyses a specimen NHMUK RU A100, a partial skull referred to by Norman et al. as an additional specimen of Lanasaurus scalpridens, a taxon otherwise referred to questioningly. This specimen Butler and Norman considered representative of the Lanasaurus scalpridens morphology, but it has also been comparable to Lycorhinus angustidens (e.g., Thulborn, 1970); while the specimen has also been considered referrable to Abrictosaurus consors (Thulborn, 1970; Hopson, 1975). Sereno follows Norman et al. in referring NHMUK UR A100 to scalpridens, and also Gow (1990) in referring Lanasaurus scalpridens to Lycorhinus angustidens. But I wonder at the efficacy, or its lack of use rather, at specimen-based rather than species- or supraspecific-based analyses, and thus each of the specimens may be assessed relative to one another: analysts may then attempt to constrain their specimen groupings as they might also their species when testing for consistency of their characters and the relationships in the results.

Finally, Sereno argues that Geranosaurus atavus, a purported heterodontosaurid described by Richard Broom in 1911, and thus the chronologically oldest heterodontosaurid, is distinct from, yet indeterminate with respect to, Heterodontosaurus tucki. I am sure, generally, that Sereno’s argument is based on qualification of material that is suitable for taxonomy, but at the same time still wishes to compare what he can. Limited comparison is made to other taxa, but much of the argument about Geranosaurus atavus surrounds possible synonymy with Heterodontosaurus tucki, with which it would be a senior synonym if they represented the same taxon — hence the concern. I disagree, however, with arguments about its “nomen dubium” status: it may be differentiated, and while it may not have autapomorphies, that degree of distinction results in some level of differentiation from other taxa.

I’ve made no secret of my love for Heterodontosauridae, or how the tucki-meister itself is my favorite ornithischian:

My only copy of my skeleton of Heterodontosaurus tucki, based mostly on SAM PK K1332.

I’ve also made no secret of an hypothesis in which I argued that heterodontosaurids exhibit some interesting similarities to pachycephalosaurs, especially Goyocephale lattimorei, despite most analyses placing heterodontosaurids towards the base of Ornithischia and Goyocephale lattimorei … a bit further away. Below is an illustration of the results of analysis by Butler et al. (2008), Pol et al. (2011), Norman et al. (2012), and Sereno (2012), and their similarities (there’s a lot, rather).

Sereno’s analysis, despite being the smallest, is also the only one that has firmly locked in the hypothesis that Pisanosaurus mertii and Echinodon becklesii are basal heterodontosaurs, along with Tianyulong confuciusi and Manidens condorensis. Indeed, the last three appear to form a new, unnamed clade — a sort of “echinodont” clade — despite the recovery in more intensive, larger analyses of these taxa as arrayed in a step-wise fashion outside of a “Heterodontosaurinae.” This latter clade name has been used first by Sereno, despite effective coinage by Kuhn who, due to an aspect of the ICZN’s principle of coordination, coined Heterodontosauridae and Heterodontosaurinae at the same time. This clade includes Heterodontosaurus tucki, and by phylogeny Abrictosaurus consor, Lycorhinus angustidens, and Sereno’s new taxon Pegomastax africanus. Sereno diagnosis the heterodontosaurines by the presence of high “cheek” teeth — taller than wide, by which I presume be means mesiodistal — and by a jaw joint depressed beneath the effective jaw margin (along the tops of the mandibular dental row).

Pegomastax africanus (sic: or “africana”) is a small taxon, and like Abrictosaurus consors (although represented by a purported juvenile) may have probably  been a “dwarf” form, in that its small size occurs within a grouping of larger-bodied taxa, under the presumption that larger, or increasing, size was primary to the clade. This is in direct comparison to the “echinodonts” in which all forms are very small (see discussion in Sereno, 2012, but also Pol et al., 2011 and Zheng et al., 2009).

SAM PK K10488, holotype of Pegomastax africanus (Sereno, 2012). A, as preserved, paired jaws preserved slightly askew and more damaged than well-preserved. B, the same, but viewing the plane of the lateral surface of the jaws more evenly, and exposing the predentary. C, illustration of the jaw as in life, by Carol Abraczinskas.

As a primer, Pegomastax africanus (sic) is diagnosed, despite the meager remains, by the presence of a deep, rostrocaudally-short predentary, a deeply-angled dorsal margin of the predentary (presuming a longitudinal axis of the dorsal margin of the dentary), and dentary post-caniniform teeth with a curved primary ridge, and where the mesial and distal keels are concave along their margins. A further interesting aspect, and one that often characterizes juvenile ornithischians, is that the teeth show very little wear, and so the crowns are very strongly triangular in aspect. Yet, despite this, the crowns are higher than mesiodistally long and relatively clustered together.

The “echinodonts” are themselves represented by an informal clade, but this clade is weakly supported, and indeed support internally for the tree in general often requires their removal. Despite casual utility in testing topological consistency by removal of taxa, inclusion of even partial taxa may be problematic: One should always strive for inclusion, especially when specimens preserve different codings from one another: As long as no specimen is perfectly identical in coding to another, it is useful to include, although it reduces the internal Retention Index. Nonetheless, two characters variably support an “echinodont” clade: a “cingular ectoloph,” and a vascular channel extending rostrally from the dentary foramina (although not present in Tianyulong confuciusi). This is certainly not strong support for cladistic diagnosis, so I am appreciative of Sereno not taking the opportunity to name the clade.

While also being small, Pegomastax africanus (sic) and Manidens condorensis are also grouped together, based on distal curvature of their maxillary teeth in lateral view. I am a little wary of Sereno’s placement of Manidens condorensis, as it seems to be more intermediate in form in several respects to the “typical” heterodontosaurid morphology, but that is something to investigate down the road. The phylogeny also enforces these groupings due to Laurasian “echinodonts” and Gondwanan “heterodonotosaurines.” Despite the relative lack of noncranial described material, I am hopeful that, when the Kayenta “heterodontosaurid” is named, and some more cranial material described through CT, as well as postcranial material (Sereno, for example, suggests that the “columnar-shaped fourth trochanter cited for these taxa is based on incomplete preparation of the SAM PK K1332 femur) are described in greater detail, resolution of the actual affinities of these taxa, a possible destabilization of the “echinodont” clade, and even basal polarity of the heterodontosaurids among ornithischians will be better resolved. It might (he says, crossing his thumbs) more specifically test his hypothesis on heterodontosaur-“goyocephaline” affinitied [I’m not crossing them very strongly].

My next post on this subject will involve me talking about jaw mechanics and muscle reconstruction. This one may take a while.

Broom, R. 1911. On the dinosaurs of the Stormberg, South Africa. Annals of the South African Museum 7:291–307.
Butler, R. J., Upchurch, P. & Norman, D. B. 2008. The phylogeny of the ornithischian dinosaurs. Journal of Systematic Palaeontology 6:1-40.
Butler, R. J., Galton, P. M., Porro, L. B., Chiappe, L. M., Henderson, D. M. & Erickson, G. M. 2010. Lower limits of ornithischian dinosaur body size inferred from a new Upper Jurassic heterodontosaurid from North America. Proceedings of the Royal Society, B 277:375-381.
Crompton, A. W. & Charig, A. J. 1962. A new ornithischian from the Upper Triassic of South Africa. Nature 196:1074-1077.
Gow, C. E. 1990. A tooth-bearing maxilla referable to Lycorhinus angustidens Haughton, 1924 (Dinosauria, Ornithischia). Annals of the South African Museum 99:367–380.
Haughton, S. H. 1924. The fauna and stratigraphy of the Stormberg series. Annals of the South African Museum 12:323-497.
Hopson, J. A. 1975. On the generic separation of the ornithischian Lycorhinus and Heterodontosaurus from the Stormberg Series (Upper Triassic) of South Africa. South African Journal of Science 71:302–305.
Norman, D. B., Crompton, A. W., Butler, R. J., Porro, L. B. & Charig, A. J. 2011. The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962: Cranial anatomy, functional morphology, taxonomy, and relationships. Zoological Journal of the Linnaean Society 163(1):182-276.
Owen, R. 1861. Monograph on the fossil Reptilia of the Wealden and Purbeck Formations. Part V. Lacertilia. Palaeontographical Society Monographs 12:31-39.
Pol, D., Rauhut, O. W. M. & Becerra, M. 2011. A Middle Jurassic heterodontosaurid dinosaur from Patagonia and the evolution of heterodontosaurids. Naturwissenschaften 98:369-379.
Sereno, P. C. 2012. Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs. Zookeys 226:1-225.
Thulborn, R. A. 1970. The systematic position of the Triassic ornithischian dinosaur Lycorhinus angustidens. Zoological Journal of the Linnean Society 49:235–245.
Thulborn, R. A. 1974. A new heterodontosaurid dinosaur (Reptilia: Ornithischia) from the Upper Triassic Red Beds of Lesotho. Zoological Journal of the Linnean Society 55:151–175.
Zheng X.-t., You H.-l., Xu X. & Dong Z.-m. 2009. An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature 458:333-336.