The Hell Creek Oviraptorosaur — What Does It Mean For Chirostenotes pergracilis?

Since the 1990’s, a few specimens have been kicking around of a particularly large oviraptorosaur from the Late Cretaceous of North America. Unlike the specimens that form the backbone of the Chirostenotes/Caenagnathus complex, these specimens come from the Maastrichtian, and unlike the similar material recovered from the Horseshoe Canyon Formation and elsewhere, this material comes from the late Maastrichtian. The Hell Creek, to be precise.

Update: The Hell Creek oviraptorosaur has been described and named, taking much of the wind out of the debate that occurs below. The new taxon, Anzu wyliei, sees print in PLoS ONE by Matt Lamanna, Hans Sues, Emma Schachner and Tyler Lawson. Reference is here.

n 1994, Phil Currie, Steven Godfrey and Lev Nessov described a large surangular/articular compound bone up to 25% larger than the same element in CMN 8776, holotype of Caenagnathus collinsi (Sternberg, 1940). This specimen (FMNH PR 2081), recovered with and catalogued with the skeleton of Tyrannosaurus rex called “Sue,” is possibly the largest North American oviraptorosaur, and it was for a while the largest known oviraptorosaur prior to the discovery of both Gigantoraptor erlianensis (Xu et al., 2011; see here) and a pair of specimens also from the Hell Creek (Russell & Triebold, 1995). The immediate implication is that the surangular/articular complex of FMNH PR2081 and the two “Triebold” specimens, CM 78000 and CM 78001, represent similar forms, which can be affirmed through similarities of the surangular. Together, CM 78000, CM 78001, and FMNH PR2081 represent most of the skeleton of an animal, there are suggestions among the paleos and apparent exhibits that demonstrate there might be more specimens; the jaw fragment that is oviraptorosaurian in the FMNH PR2081 complex of bones probably also includes several of the scraps and other unidentified elements collected with that specimen.

The “Triebold Oviraptorosaurs”, based on CM 78000 and CM 78001. Known material presumes a nearfly complete vertebral column across both specimens, with large sections of vertebrae from each individual represented. The pes is reconstructed far more completely than is preserved, which is limited to phalanges and a metatarsal — they are shown complete here for artistic purposes.

Limited information is available for these specimens, despite their ubiquity: The two skeletons were prepared and cast by Triebold Enterprises, and copies of this were mounted and distributed around the world. Thus, this material is broadly available, although exact preservation has been limited. The latter point is hardly relevant at the moment, however. What is more interesting is that this giant oviraptorosaur represents what appears to be an interesting player in the long game of what, exactly, constitutes Chirostenotes. I covered this subject here and here, where the latter discusses the naming of two new taxa.

In what is certainly a dominant theme of this blog, I argue that the issue of genera and species is subjective, that discovering new taxa and recognizing them through taxonomy are two different things. Recognizing a new species is fine, but where the problem comes into play is when you take this new form and name two new taxa for it. In my case, I think the problem is exactly the same as when someone names a new “genus,” a “family”-rank taxon to contain it, and/or a new “order”-rank taxon to contain that. There are workarounds: A systematist may think that a new species deserves a binomen, where the “species name” is the two-word epithet as a whole; this is a phrase that becomes inseparable from the idea of a species, and no part of it is a “genus.” There is also the argument that a “genus” is a special category of clade that can contain species, but I’ve yet to read a definition qualifying this treatment as anything other than trying to validate use of clades and the conventional system of “genus-species couplets” in the same system. Foundational bases for use of “genera” containing only a single species in complexes with other “genera” also only containing single species are rarely qualified other than implications that the authors feel the number of autapomorphies can justify this, or because of the number of other taxa already named in this manner that are included. “Go with the flow,” as it were, and we are done being objective about the subject.

The Abstract

Nonetheless, the Hell Creek Oviraptorosaur has interesting implications for this situation. Detailed most recently in an abstract and talk presented before the Society of Vertebrate Paleontology in Las Vegas (Nevada, USA) on November 5, 2011, Matt Lamanna, Hans Sues, Emma Schachner and Tyler Lyson presented the following:

A NEW CAENAGNATHID OVIRAPTOROSAUR (THEROPODA: MANIRAPTORA) FROM THE UPPER CRETACEOUS (MAASTRICHTIAN) HELL CREEK FORMATION OF THE WESTERN UNITED STATES
The oviraptorosaurian theropod clade Caenagnathidae has long been enigmatic due to the incomplete nature of nearly all described fossils. Here we report on a new, large-bodied caenagnathid taxon represented by three well-preserved partial skeletons and several isolated bones. The specimens were recovered from the uppermost Cretaceous (Maastrichtian) Hell Creek Formation of Montana and the Dakotas, and are therefore among the stratigraphically youngest oviraptorosaurian remains yet known. Collectively, the fossils include elements from most regions of the skeleton, providing a wealth of information on the osteology and phylogenetic affinities of Caenagnathidae. The new taxon confirms the referral of the mandibular material previously named Caenagnathus to the genus Chirostenotes. The skull of the Hell Creek caenagnathid is characterized by a very tall, crescentic median crest comprised by the elongate posterodorsal processes of the premaxillae. The maxilla has a prominent dorsal process. The mandible closely resembles that of “Caenagnathus collinsi” but differs in the presence of a lateral flange on the dentary. The pterygoids are fused medially to form an X-shaped structure. The extensively pneumatized caudal series terminates in a pygostyle-like conformation consisting of three greatly modified but unfused vertebrae. Manual unguals exhibit a conspicuous proximodorsal “lip.” The distal surface of the pubic “boot” is deeply concave, comprising a “cup-like” morphology. The shaft of the ischium is strongly curved posterodistally, and the triangular obturator process is positioned at its approximate proximodistal midpoint. A well-developed “accessory trochanteric crest” rises from the lateral edge of the proximal end of the femur. The astragalus is coossified with the calcaneum and bears a rugose, ovoid tuberosity on the anterior surface of the base of the ascending process. The Hell Creek caenagnathid provides insight into the paleobiology of these unusual dinosaurs. For example, the two partial skeletons from South Dakota differ from one another in the robustness of certain appendicular bones, and selected elements of all three skeletons show pathological changes.
[Lamanna et al., 2011:pg.140]

I typically don’t include abstract references (as “grey literature”) because, for the most part, this is presented pro tempore. It is not designed to be reference work, and can change drastically from prepared material or eventual publication. Despite that, it’s useful to show here because it qualifies a statement I made over at Dave Hone’s blog Archosaur Musings:

Best course of action, regardless of the work of Steve Jasinski and Bob Sullivan last year to establish larger-bodied caenagnathids as new “genera,” is to refer the material to Chirostenotes pergracilis. This follows Hans Sues’ otherwise published (and presented, at 2011 SVP) views on the material. It may merely allow someone else to come along and attempt a thourough[sic] review of Oviraptorosauria and attempt a clear systematic treatment of the range of Campano-Maastrichtian NA material. Hopefully, in the next few years.

Mickey Mortimer replied there (and followed up on my last post on Acamptonectes densus, where I called out the tendency of naming any new taxon as a whole genus-species couplet, especially when that form occurred within a topology that can be considered monophyletic were the species merely referred to another established “genus”):

Contra Jaime, I would not recommend merely calling this Chirostenotes pergracilis. The situation is different than that for ROM 43250 (“Epichirostenotes curriei” of Sullivan et al.) because the latter doesn’t differ from pergracilis in ways more important than individual variation. Note that Lamanna et al. (2011) DON’T follow Jaime’s views and actually call the CM material a “new … caenagnathid taxon” and state its mandible differs from pergracilis (=collinsi). While ROM 4320 doesn’t preserve the pes which could be used to compare it to Elmisaurus or the mandible that could compare it to elegans (=sternbergi), the CM material does. So we should be able to tell if it’s closer to pergracilis or Elmisaurus and thus make an informed decision once it is described.

Mickey is correct in that the Lamanna et al. abstract does not simply regard the material as Chirostenotes pergracilis (Sternberg, 1924), and make a note of differentiating a feature of it (a flange on the dentary, which I’ve seen and is particularly interesting in reconstructing the animal) from the jaw of Caenagnathus collinsi, which they treat as synonymous with Chirostenotes pergracilis (following Sues, 1997). However, the statement “a new caenagnathid oviraptorosaur” does not mean “a new caenagnathid genus” nor even “a new caenagnathid species.” It can mean the latter, and differentiation of the jaw may substantiate species identification, were it not for the slight problem that the range of morphology in caengnathid dentaries in the Campanian Dinosaur Park Formations may or may not be able to encompass the Hell Creek form, and the species identifications of the material from the Dinosaur Park Formation (Currie et al., 1994) may be less or more substantial than notable.

Variation

This becomes interesting when a jaw (RTMP 2001.12.12) with apparent features similar to that of Caenagnathus sternbergi (Cracraft, 1971) is found to have a similar morphology to that of Caenagnathus collinsi (see Currie, 2005). The statement may simply refer to the specimen, as a potential new animal. The lower jaw also preserves a lateral flange on the posterior half of the dentary, which is visible in the figure above (from Currie, 2005), and is similar to the form of the jaw in the Hell Creek oviraptorosaur (personal observation). It is not identical, but this raises the specter of doubt; without a substantial amount of dentary material, qualifying the variation among three or four jaws as significant enough to warrant species identification, or even “generic” may be reaching. The material in CMN 8776 is also somewhat distorted, although this is not always apparent in images as most are based on Currie et al. (1994), who “perfected” the known material. The original is missing sections of the sub-fenestral bar on one side, and is not symmetrical with one side further bowed outward than the other, while the dorsal margins not preserved in exactly the manner depicted above.

The title is titillating, exciting, meant to draw attention rather than represent a summation of the end of a long research project; so I don’t much think the phrase “a new caenagnathid genus” in the title means much at all. I leave it to Hans Sues and Matt Lamanna to qualify what they write about and present at SVP. But I have discussed the species issue with Hans Sues following Sullivan et al. (2011)’s recent naming of ROM 43250 as a new “genus-species couplet,” Epichirostenotes curriei. The impression is that Sues maintains (1997) the previous published opinion on the specimen, and that’s that there is a limited amount of material to qualify separation of this taxon from Chirostenotes pergracilis, and I agree with this — the two taxa are based essentially on material that cannot be compared, while referral of other material (RTMP 79.20.1 and CMN 8776 to the holotype, CMN 2367) is established on a minimalistic basis.

But this represents one view, one voice; Phil Currie views the material as insubstantive for referring CMN 8776 to Chirostenotes pergracilis without material from the Campanian (at least) showing the hands of CMN 2367, the foot of CMN 8538 (holotype of Macrophalangia canadensis Sternberg, 1932), the vertebrae and hips of RTMP 79.20.1, and the jaw of CMN 8776 all belong to the same animal. Sues (1997) even questioned the consistency of referring RTMP 79.20.1 (Russell & Currie, 1988) to Chirostenotes pergracilis, because the pes differs from CMN 8538: it is far more gracile, and at the time, it was speculated that the name Caenagnathus sternbergi (as Chirostenotes elegans, conflating Parks’ (1933) Ornithomimus elegans into the mix) was a better container for the gracile Chirostenotes/Caenagnathus material. Only when this arrangement of material is sorted out do I think it possible to assess the argument that CM 78000 and CM 78001 do or do not belong to Chirostenotes, pergracilis or not. Despite this, the alternative would seem to name a new taxon, and sort it out later, and that seems incredibly short-sighted and, above all, selfish of the authors to do. Mickey Mortimer further remarked:

You are VERY unique in apparently not caring about nomenclatorial credit, probably in part because you don’t have a career where having new genera attached to your name is job and grant security.

Both of these are true, especially the first: I’m rather iconoclastic in that, I feel, the scientific function of communication does not benefit necessarily from excessive or purposeless taxonomy when it is made to substantiate one’s career. Mickey seems to imply that, if I had a career in paleontology, I’d care more about naming taxa and would want to name more forms if possible. I’m not sure this is the case. There are several disciplines within paleontology that do not qualify alpha taxonomy as relevant, and in some, such as biomechanics, it may just get in the way of the discussion. Some paleontologists may never name a taxon, and go decades in their career without doing so, supported by their work.

Is it difficult to think that the ideal of naming taxa when one can justify doing so is selfish, and doesn’t serve the greater scientific purpose? I can make the argument that naming new taxa over-qualifies the actual degree of relatedness among taxa, allows people to stress that “genera” mean something more than just a taxonomic label equal to any others, or that it gets in the way of us talking about the forms. In ecology, it allows scientists to then claim a nae taxon is more limited or rarer, and thus deserving of protection or conservation status, and this tends (by the arcane redux of the conservation system) to involve “genera,” not species. But is that justifiable to continue to name new “genera”? Yes, naming a new taxon in a paper allows a scientist to “get his foot in the door,” bring attention and thus backing and funding for his research, and promote interest in his work. He can later publish the longer, cleaner work at leisure, while chugging out new taxa every so often to keep the money flowing, and this is ultimately more selfish than scientific. Don’t get me wrong: The taxonomy still promotes the scientific purpose of the scientist, and the work in general creates interest, but taxonomy for its own sake does not (it serves a function, communication, but any nomenclature can do that, even specimen numbers).

Summary

My argument is two-fold, although I admit not always simple:

Until further material substantiates what we know of Chirostenotes pergracilis (restricting ourselves to the ideal of just the three specimens seen in the skeleton collage above), we should be careful what we’re willing to dump into it as a container. Barring this material, all referral not clearly demonstrated with overlapping bones with autapomorphies shouldn’t be referred. But … we also cannot assess the  variation in caenagnathids, and until such a time, should avoid excessive nomenclature when it doesn’t serve a function of expressing a strongly developed conclusion after careful and substantial testing. If we were to name it, I would also prefer a binomial taxon, a “species” with two names but not a “genus-species couplet” and text would clearly demonstrate the distinction in the paper. But that’s me.

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