Sereno’s Monster

Dr. Paul Sereno hasn’t published much in the last few years. Apparently the reason is because he just finished a monograph on Heterodontosauridae … a real monster at 225 pages. I haven’t the time to review this yet, but there’s a lot of meat in it. I will tackle his Psittacosaurus gobiensis paper as well as the same time, as that involves some things that are important to me, like jaw muscle reconstruction, jaw mechanics; this paper involves heterodontosaurs, which I’ve covered before on another large review of the group (Norman et al., 2011).

Jaw muscle reconstructions in Pionites leucogaster (a), Psittacosaurus gobiensis (b), and Heterodontosaurus tucki (c), modified from Sereno, 2012. Rhamphothecae in green, muscles in red.

Something interesting is going on in this reconstruction, about muscle placement — such as the m. adductor mandibular externalis ventralis attachcing to the lateral surface of the jugal, or the presence of one of the parrot’s unique muscles, m. pseudomasseter, on an ornithischian — which will eventually come back to this blog. ‘Til then, I will be spending some time digging through this monstrosity and working on the projects that feed into the interesting parts.

Oh, and I should mention that Sereno coins new taxonomy in this paper, a new taxon Pegomastax africanus [n1], as a dwarf heterodontosaurid. Sereno adds this piece as a secondary aspect of the paper, a description of material that is interesting; but in a break with typical work of this form, it doesn’t form the main impetus of the paper. That, instead, if the review of all other heterodontosaurid taxa, including an assertion that Lycorhinus angustidens is a valid taxa, Lanasaurus scalpridens probably isn’t, a well-preserved specimen referred by Norman et al. to the latter belongs to the former, and that the type material of Geranosaurus atavus is “not adequate to justify taxonomic recognition,” (Sereno, 2012:pg.10) despite having spent the previous paragraph describing features that “clearly suggest there is a second heterodontosaurid taxon in the Clarens Formation in addition to H. tucki[]” (Sereno, 2012:pg.10) — this latter point made by Mickey Mortimer: Geranosaurus atavus is distinct from Heterodontosaurus tucki … but not deserving of taxonomic recognition? If you can distinguish it, clearly, is it not distinctive? This type of argument only annoys me, given my dislike of “nomen dubium” arguments. And, of course, that Pisanosaurus mertii is a heterodontosaurid.

The review is lengthy, and most of the 225 pages comprises a detailed description of the material of each taxon; the remainder comprises an analysis of the masticatory mechanics of heterodontosaurids, a review of tooth morphology and terminology (which introduces the term “denticule” for the small denticle-like structures that appear on denticles; as well as “paracingular fossa” for the depressions formed on each side of a primary ridge of a “phyllodont” crown) …

ornithischian tooth terminology… and a phylogeny of taxa — which is comprised of only 30 characters and 13 taxa, far less than that of Pol et al. (2011), the most comprehensive analysis including heterodontosaurid taxa to date.

phylogeny of Heterodontosauridae

The presence of a basal “simple-toothed” clade is in direct opposition to Norman et al., and Butler et al. (2012), who argued that the condition in Fruitadens haagarorum and such taxa were the “derived” state, developing from the more complicated jaw anatomy in “basal” taxa like Heterodontosaurus tucki. [Note: The preceding sentence is based on a generalization from various statements made in the paper, but itself does not appear in the paper: Butler et al. do not, in fact argue this although I've implied they did. This is incorrect, and I'm redacting the statement; additionally, Butler et al., as in Pol. et al. and Sereno, consistently find a grouping of these taxa at the base of the heterodontosaur radiation, and none of them suggest the "simple-toothed" forms were derived from the more "complex-toothed" "heterodontosaurines". I am at this point wondering why I even put the statement in there.]

Sereno’s phylogeny places these “simple” jawed taxa basally, despite their temporal range, occurring in much younger strata (Upper Jurassic, as opposed to the Lower Jurassic). There is a concern, of course, that the “simple” jaws of these taxa cause them to inordinately conjoin together, forming a sort of “echinodont” clade (which Sereno left unnamed), but it does raise questions about the reliant on interesting phylogenetic arrangements, which led Butler et al. to assume in the first place the morphology of “simple-jawed” taxa like Tianyulong confuciusi were derived from that of “complex-jawed” Heterodontosaurinae. Sereno, however, notes that biogeographic disparsal is largely handled by this arrangement, where all Upper Jurassic taxa are also Laurasian, while Lower Jurassic taxa are Gondwanan. Crossing the continents as they split may have occurred pretty early, however, due to the presence in the Lower Jurassic Kayenta Formation of a purported heterodontosaurid (MCZ 9092, Attride et al., 1985, Sereno, 1986, 1997). This taxon is gracile, lacks high crowns, has broadly triangular and unworn teeth and looks for all intents and purposes like a longer, lower version of Fruitadens haagarorum (personal observation, here).

Sereno also presents CT scans of a specimen referred to Heterodontosaurus tucki (AMNH 24000), with detailed examination of the teeth, which is essentially a first. This makes a portion of my job (looking at orientation of jaws in section, and coronal-sectional diagrams of various, disparate taxa with odd jaws to look at possible soft-tissue analogs) that much easier. [Note: added a few hours after this article was initially published, for clarification] Sereno is not the only person to present heterodontosaurid CT scans: Porro et al. (2010) presented NM QR 1788, a pair of rostral mandibles, while Norman et al. (2011) presented SAM PK K1334, and of course LACM 115747 (holotype of Fruitadens haagarorum) was treated to CT analysis by Butler et al. (2010, 2012), but the most complete skull (SAM PK K1332, largely the basis of what we expect when we think “Heterodontosaurus tucki“) has yet to be presented in this manner, much less the holotype. Sereno is also the first to present on more extensive sectioning of the upper jaw, including non-maxilla portions of the cranial anatomy, which would only be improved by extensive examination of the cranial anatomy of the best skull.

[n1] Pegomastax is a compound from the Greek words πέγος (pegos, “thick”) and μαστάκος (mastákos, “a thing which chews” from which “masticate” [to chew] derives, and is used tangentially to refer to the action jaws make), but formed as “pegomastax”, where mástax is a feminine form of the verb and may be translated as “thick[-jawed] chewer.” Sereno gives the etymology as “thick jaw.” The species name is given as africanus, a Latin word (“of or deriving from Africa”) which is masculine in form. The ICZN mandates that nomenclatural gender must agree, and unfortunately, a masculine adjectival epithet in apposition to a feminine noun just rubs the wrong way on this. Art. 31.2 mandates that “[a] species-group name, if it is or ends in a Latin or latinized adjective or participle in the nominative singular, must agree in gender with the generic name with which it is at any time combined.” David Marjanović says — and I agree –that this must be “africana;” while the good doctor is correct on this point, he argues that this change is “automatic,” that is that it does not require any publication making the change, but in this we disagree, and I sent a note off to Paul Sereno to suggest he seek a corrigendum in Zookeys to fix the nomenclature. The citation, incidentally, will stay the same (Art. 34.2).

Attridge, J., Crompton, A. W. & Jenkins, F. A., Jr. 1985. The southern African Liassic prosauropod Massospondylus discovered in North America. Journal of Vertebrate Paleontology 5(1):128–132.
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 diminutive new Upper Jurassic heterodontosaurid from North America. Proceedings of the Royal Society of London, B: Biological Sciences 277:375–381.
Butler, R. J., Porro, L. B., Galton, P. M. & Chiappe, L. M. 2012. Anatomy and cranial functional morphology of the small-bodied dinosaur Fruitadens haagarorum from the Upper Jurassic of the USA. PLoS One 7:e31556.
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.
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.
Porro, L. B., Butler, R. J., Barrett, P. M., Moore-Fay, S. & Abel, R. L. 2011. New heterodontosaurid specimens from the Lower Jurassic of southern Africa and the early ornithischian dinosaur radiation. Transactions of the Royal Society of Edinburgh: Earth and Environmental Science 101:351–356.
Sereno, P. C. 1986. Phylogeny of the bird-hipped dinosaurs (Order Ornithischia). National Geographic Research 2(2):234–256.
Sereno, P. C. 1997. The origin and evolution of dinosaurs. Annual Review of Earth and Planetary Sciences 25:435–489.
Sereno, P. C. 2012. Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs. Zookeys 226:1-225.
Sereno, P. C., Zhao X.-j. & Tan L. 2010. A new psittacosaur from Inner Mongolia and the parrot-like structure and function of the psittacosaur skull. Proceedings of the Royal Society of London, B: Biological Sciences 277:199-209.

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4 Responses to Sereno’s Monster

  1. Richard Butler says:

    Can you point out exactly where Butler et al. (2012) or Norman et al. (2011) “argued that the condition in Fruitadens haagarorum and such taxa were the ‘derived’ state, developing from the more complicated jaw anatomy in ‘basal’ taxa like Heterodontosaurus tucki”?

    Here is a direct quote from Butler et al. (2012:27) “Although these temporal patterns do not equate into evolutionary trends when viewed in a phylogenetic context, they do suggest that those heterodontosaurid lineages that persisted through the Jurassic and into the earliest Cretaceous were small-bodied ecological generalists, and that moderately larger and ecologically more specialized taxa such as Lycorhinus and Heterodontosaurus were temporally limited to the Early Jurassic (and possibly the Late Triassic, based upon the Laguna Colorada heterodontosaurid).”

    In other words, we said it was unusual that the temporally later taxa had simpler dentitions, but we did not say that the “simple-toothed” dentition was derived from the more complicated dentition, nor did we say that Heterodontosaurus was basal (look at the phylogeny in the original Butler et al. description of Fruitadens). Our phylogenetic work recovered Echinodon and Fruitadens basal within Heterodontosauridae, and thus “simple teeth” are likely plesiomorphic for the clade according to our results. There’s no real conflict with Sereno’s results with regard to these aspects of dental evolution – merely that Sereno found evidence for a Laurasian clade that we did not.

    • Dear sir, thanks for replying, and telling me what I flubbed.

      Sir, I understand that you claim that the basal forms are plesiomorphic and relatively basal; I also understand you broadly point at this as a generalization in and of itself. I drew this caricature from generalizing a few comments in the paper itself, especially the argument for retention of plesiomorphies in the teeth (which are nonetheless derived from all other ornithischians), the lack of extensive dental wear (especially but not exclusively when considering Pisanosaurus mertii as a basal[-most] member), and the statement that “[n]o evidence for trends towards larger body sizes (i.e. Cope’s rule) exists when the entirety of heterodontosaurid evolution is examined.”

      I broadly take them to assume that, despite the phylogenetic analysis, absolutely larger body size in “heterodontosaurines” (or non-Tianyulong confuciusi, Echinodon becklessii, Manidens condorensis, etc. heterodontosaurids) is an evolutionary trend. Perhaps you ARE being relatively cautious on this point, but the generalization I read — and forgot to double check on what is generally me discussing SERENO’s paper — is that you are intimating the development of the Tianyulong confuciusi dental model from that of “heterodontosaurines”. It is incorrect for me to state that you “argued that the condition … was the ‘derived’ state,” and will change the text to accord with this.

      I read the evolution of “heterodontosaurids” as more complex than I think either group here has, and would not settle on an exclusive grouping of plesiomorphic or homoplasy-ridden forms at the base (Butler et al., Pol et al.), or clustered with one another at the base (Sereno), and eagerly await more substantive phylogenetic analysis. But I also do not take phylogenetic analysis seriously, as if its conclusions are really all that useful without extensive testing of its results with various mechanics. So, I put more stock into what was said outside of the context of phylogeny, which was that the “simple” forms were generally developed from more complex forms.

  2. Rob Gay says:

    An aside, since I haven’t read the paper yet (finished downloading it overnight), but are there any published photos of MCZ 9092 out there?

    • Sereno does include an illustration, but it appears to be an unpublished figure from Sereno’s unprepared work on the specimen, so I presume he will eventually get it published, especially as he’s been working on it longer than he has Eoraptor lunensis.

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