Expanding the Known Oviraptoverse

Normally I wait a few days to post something, but as the last post is a technical post that only seems to get the attention of func-morph people, I’ll throw some meat out here. This one comes courtesy Andrea Cau of Theropoda (warning: in Italian, but Andrea provides a translator link to Google). Two oviraptorosaurs are now freshly named, one from southern Alberta, and the other from New Mexico, and they deserve a few comments. Bob Sullivan, Steve Jasinski, and Mark van Tomme describe a newly uncovered partial pair of pubes recovered from the early Maastrichtian Naashoibito Member of the Ojo Alamo Formation, in northern New Mexico. The specimen, SMP VP-1458, measure about half a meter long, and thus come from a fairly large animal. It is distinctive, the authors argue, due to the presence of a unique depression on the dorsal surface of the anterior projection (“foot”) of the fused pubic boot, a slight downward curvature of the distal end of the shaft, a medial fossa of the proximal pubis further separated from the acetabular rim than in comparable taxa, and the shape of the sutural surface of the iliac peduncle of the pubis.

The Ojo Alamo Snatcher … Lizard … Thing

Compared to known oviraptorosaurian pelves (excluding oviraptorids), the new specimen — given the rather uninspiring name of Ojoraptorsaurus boerei[n1], the species epithet honoring Arjan Boere, who discovered the it — is fairly large, around the same size as the otherwise largest known pelvic remains, ROM 43250 (described by Hans Sues in 1997), and 20% larger than a specimen referred to Chirostenotes pergracilis, RTMP 79.20.1, described by Phil Currie and Dale Russell in 1998. In the figure below, I show the relative size and preservation (missing material shown in grey) of specimens compared by Sullivan et al.; SMP VP-1458 is “E.”

Preserved pelvis of several North American oviraptorosaurians. A, RTMP 79.20.1, referred to Chirostenotes pergracilis Gilmore, 1924 (after Currie & Russell, 1988); B, AMNH FR 3041, holotype of Microvenator celer Ostrom, 1970 (after Makovicky & Sues, 1998); C, GI 100/119, holotype of Nomingia gobiensis Barsbold et al., 2000; D, ROM 43250, holotype of Epichirostenotes curriei Sullivan et al., 2011 (after Sues, 2008); SMP VP-1458, holotype of Ojoraptorsaurus boerei Sullivan et al., 2011. Scale bar equals 10cm.

Here, I show the range of pelvis variation in North American oviraptorosaurs, along with Nomingia gobiensis (Barsbold et al., 2000). The ilia are arranged so that the dorsal apex of the acetabular rim (at the supracetabular crest) is aligned horizontally with the m. cuppedicus shelf, which forms the ventral margin of the anterior iliac blade (preacetabular ala). The position of the pubis and ischium here are shown as “best-fit” models.

The new specimen is well-differentiated based on a range of ostensibly unique features, although the range of comparison with other taxa is limited: SMP VP-1458 can only be reasonably compared to a few taxa, some of which are likely not caenagnathids as Ojoraptorsaurus boerei apparently is. These features include a rather curvaceous pubic shaft which, instead of being cranial concave along its length, as apparently posteriorly concave instead. Less significantly, a proximal and medial fossa near the acetabular rim of the pubis is well-separated from that rim in SMP VP-1458 and AMNH FR 3041, holotype of Microvenator celer Ostrom (1970); it differs in this respect from closely-sized and aged specimens from the Late Cretaceous, such as Nomingia gobiensis and ROM 43250.

Finally, a unique apomorphy of a dorsal fossa on the anterior foot of the pubic boot indicates a taxon that differs from other oviraptorosaurs. However, this region in ROM 43250 is crushed , and I am hesitant to use a distinction in this respect as distinctive “enough.” The shape of the proximal articulation of the pubis, which contacts the ilium, is typically triangular in coelurosaurs, but is quadrangular in aspect in SMP VP-1458; both proximal pubes are available, both partially preserved, and missing the contact with the distal fragment (see above), but both proximal articular regions (iliac pedunculae) are missing portions of their margins, and I suspect this alters the shape of the articular region enough to cast doubt on this feature.

Nonetheless, several autapomorphies appear strongly suggestive that we have a new taxon, Ojoraptorsaurus boerei, even though some features are less strongly supportable.

An Appendix For Your Species

Sullivan et al. devote most of their work to description and comparison of SMP VP-1458, but in the text, they refer to another new taxon. This one, named Epichirostenotes curriei [n2], is granted to ROM 43250, and comes after a page of citations in a page devoted to an appendix [n3] whose sole purpose is to provide systematic nomenclature.

Caenagnathid skeletons. NMC 2467 is the holotype of Chirostenotes pergracilis Gilmore, 1924, while NMC 8538 is the holotype of Macrophalangia canadensis Sternberg, 1932, which is considered by most authors along with RTMP 79.20.1, to be Chirostenotes pergracilis.

As far as Hans Sues was concerned, ROM 43250 was a distinctive specimen, but he was cautious as to its precise affinities, eventually settling on treating it as “Chirostenotes pergracilis.” Due to the lack of overlapping material with other caenagnathids, the lack of expansive body fossils (lots of jaws, though), and the differences when it was comparable, Sues (1997) was conservative and placed it wholly within the container that was most readily available to him at the time. This remained for the next decade, even as new caenagnathids were recovered, because there was a general lack of comparable material. Most distinctive was the presence of the sacrum, and partial pelvis. Compared to RTMP 79.20.1, ROM 43250 was merely larger; what variation existed could not be reasonably compared, as individual variation cannot be assessed on a sample per bone basis often having a range value of only two specimens.

Sullivan et al. disagree (pg.428):

We consider Sues’ assignment of ROM 43250 to Chirostenotes pergracilis to be unlikely. We agree with Sues that the differences between TMP 79.20.1 and ROM 43250 are taxonomically significant, but at the generic level.

I am somewhat confused by this passage. It is clear that Sullivan et al. mean that Sues thought that variation between RTMP 79.20.1 and ROM 43250 was taxonomically significant, however Sues (1997) is fairly rigid on the idea that ROM 43250 could be encompassed by the name Chirostenotes pergracilis, and for the reasons I note above. Sues especially drew attention to the form of the sacrum and to the ischium. In speaking on the affinities of the specimen, Sues devoted some attention to species of Chirostenotes (Sues, 1997:pp.707-708):

Currie and Russell (988) distinguished a larger, more robust and a smaller, more gracile “morph” among the postcranial material referable to Chirostenotes. They interpreted the differences between these morphs as sexual dimorphism. However, I adopt a more conservative approach and consider the differences taxonomically significant, based on the structural differences between the two types of caenagnathid mandibular remains[…]. ROM 43250 clearly represents the larger form, along with CMN 2367 (holotype of Chirostenotes pergracilis), CMN 8538 (holotype of Macrophalangia canadensis), and the isolated metatarsal CMN 9570. […] The smaller morph, for which the binomen Chirostenotes elegans (Parks, 1933) is available, is documented by the partial postcranial skeleton RTMP 79.20.1, and the left metatarsus ROM 781 (holotype of “Ornithomimus” elegans[…]).

Sues offers the phrase “taxonomically significant” only to the shape of the mandible, which as I’ve noted before, indicates at least two forms in the Dinosaur Park Formation: Caenagnathus collinsi Sternberg, 1940 and Caenagnathus sternbergi Cracraft, 1970, although Sues does consider RTMP 79.20.1 to belong to the “gracile morph,” represented by “Ornithomimus” elegans (and presumed synonymy with Chirostenotes sternbergi). Two types of dentary are also known, but their relation to these names is unknown. Sues (1997) further applied a distinction between the metatarsal morphs called elegans and pergracilis, and conflated these to the mandibular remains on the basis of ROM 43250. Currie and Russell (1988) distinguished RTMP 79.20.1 and ROM 781 specifically as two aspects of the morphs that Sues mentions (e.g., pg.980):

The type specimen of “Ornithomimus elegans,” ROM 781, is almost 20% shorter than the metatarsus of TMP 79.20.1 and is even more gracile […]. The correlation between size and gracility suggests that the differences may be related to age at time of death, but it seems more likely to us that it is a combination of age and sexual differences.

It should be noted that, following Varicchio (2001), elegans is a species of Elmisaurus separate from Chirostenotes, due to novel features of the metatarsus. These features are absent in RTMP 79.20.1, and are noted by all subsequent authors, who place RTMP 79.20.1 into Chirostenotes pergracilis. Thus the “taxonomically significant” features do not apply to the taxa Sullivan et al. think it does, however based on the conclusion that sternbergi is the same thing as elegans. The lack of distinct overlapping material between the types of sternbergi and elegans prevent these taxa from being synonymized in my view: even the assumption that all “gracile” material should belong to a single taxon can be taken as true and this argument fails due to the lack of assessment of variation, sexual dimorphism, the potential for taxonomic variation among the small specimens, or ontogeny. Beyond this, Sullivan et al. further attempt to distinguish the specimens in Chirostenotes pergacilis by elaborating on the variation between RTMP 79.20.1 and CMN 8538 by noting proportional differences in the pedal phalanges, among other things. This underscores the lack of certainty in splitting specimens based on perceived differences when the variation is unknown.

ROM 43250 possesses a range of morphological differences from other North American oviraptorosaurs, although the proximal pubis is crushed and features including the shape and position of features, and the clear mediolateral flattening of the specimen places several of these variables into question. Sullivan et al. distinguish the taxon largely from Sues (1997), including various braincase characters, however, postcranial features noted include the shape of the ischium, and the proportional “broad”ness of the post-obturator shaft (the death, I think they mean), as well as the shape of the obturator process itself (“hook-like”). Because the specimen is larger, we can expect shape-based differences if scaling is relevant, an aspect not ruled out; and RTMP 79.20.1 also has a “hook-like” obturator process. It is unclear to me which features Sullivan et al. note are actually autapomorphic until CM 78000 and 78001 (the “Hell Creek oviraptorosaur”) is described, and even then the lack of non-mandibular cranial material in the Dinosaur Park Formation prevents such clarity for the key taxon of Chirostenotes pergracilis.

Map of North america, after Zanno & Sampson (2005), showing the relative locations of key localities recovering oviraptorosaurs. A, Dinosaur Provincial Park, Alberta, Canada, producing Caenagnathus collinsi, sternbergi, “Ornithomimus” elegans, Chirostenotes pergracilis and Macrophalangia canadensis; B, northern and eastern Montana, USA, producing remains referred to “Ornithomimus” elegans; C, Wheatland County, Bighorn Basin, Montana, USA, producing Microvenator celer; D, near Township 30, Alberta, Canada, producing newly-minted Epichirostenotes curriei (ROM 43250); E, northwestern South Dakota, USA, producing FMNH PR 2081, CM 78000 and CM 78001; F, Grand Staircase-Escalante National Park, southern Utah, USA, producing Hagryphus giganteus; G, Barrel Springs, New Mexico, producing Ojoraptorsaurus boerei.

We have a new, distinctive taxon from the early Maastrichtian of New Mexico, a period corresponding to the middle Horseshoe Canyon Formation of Alberta, which itself has produced a distinctive taxon. Sullivan et al. apply what I think is a tad shaky reasoning to support their taxonomy, including overlumping certain taxa in order to distinguish the taxonomic distinctiveness of these forms. This is nothing particularly unique to them, and I suspect that the authors could have distinguished their taxonomy without having to stretch the conceptual distinctiveness of their “generometric” device. On that note, I see no difference from naming new species versus placing those species into a cladistic “genus” container which amounts to the same taxon … if only Sullivan et al. agreed with this, which they do not as Sullivan has made frequent use of the technique of species moving, synonymizing and erecting of new “generic” taxonomy.

Update: Mickey Mortimer has a post over at the Theropod Database Blog that discusses the the purpose of coding ROM 43250 either as a separate OTU in a matrix from specimens attributed to Chirostenotes pergracilis, or combined with them. While I agree with the issue of the quality or value of ROM 43250 relative to Chirostenotes pergracilis in general, as he does agree with me on what I’ve stated in expressing skepticism, I question the value he gives to ROM 43250 were it in any way valuable as a separate taxon, i.e., as curriei or Epichirostenotes curriei. In this case, as I’ve said elsewhere, I do not see the difference; we’re still playing games with ranks, even if we don’t call them that. While I suspect that Bob Sullivan’s leaning on biostratigraphy (taxa from lower Maastrichtian being likelier to be new taxa than if they were in the upper Campanian) strongly influenced his decision to do taxonomy here, it matters not that if it were a unique species equivalent to pergracilis, it should stand as equally as other taxa.

[n1] Literally, “Ojo [Alamo Formation] snatcher lizard.” The usage given is for “plunderer” (“raper,” a term equivalent and derivative to raptor, is not mentioned in polite company), but I typically use “to snatch” as a more useful and effective meaning for rapere, the genitive. I must admit it, I do not like this name. I will use it, no doubt, but I still do not like it. This name reminds me too much of two names, Megapnosaurus (“big dead lizard,” coined as a joke from an entomologist to replace Syntarsus Raath, 1969 and contain rhodesiensis Raath, 1969) and Raptorex. I do not hate either name, although I question the method and process the first was named in, while the latter, being very “cool” may be said to have been named as part of Paul Sereno’s kid-oriented outreach program Project Exploration. If that name gets more people interested in Science, I am all for it. But it combines the two most popular “dinosaur words” kids like into one name, and that means it was coined for effect. The same, I think, may be true of Ojoraptorsaurus. There is no reason raptor and saurus should ever be bound together, for effect or otherwise: raptor means “one who snatches” (or “snatcher,” “plunderer,” “thief,” “rapist”) and is pretty final in effect. It’s appearance in the Nomenclator Zoologicus is rooted as the last part of the name construct in all names — with the exception of Rapator, which appears to be a modification of the word raptor, and not raptor + something. [I mean no ill-will towards Bob Sullivan et al. for this name, and understand they might appreciate this name for an unspoken reason; this statement is based solely on an aesthetic.]

[n2] Literally, “after Chirostenotes.” While the specific epithet honors Phil Currie, who is by far worthy of it, the “genus” name is a little tricker. The Greek ἐπί is typically used in anatomy to mean “upon,” and carried a connotation of “around,” where the primary object is surrounded by a periphery, and that periphery and any directions from it bears the term ἐπί. Sullivan et al. use it in the sense of “after,” as in having a temporal connotation, and I think this is a literal reading of the word used, rather than the positional relationship of the word. Perhaps they are influenced by the mythological personae Prometheus (forethought) and Epimetheus (afterthought), and took the terms literally, without understanding that the names were relational: pro-metheas and epi-metheas refer to the concepts of “action” and “thought,” where the former was thought before action, and the other was thought after action, or rather, thinking of action after it occurs (i.e., “hindsight”), which in many respects grants the sense of “memory” for the one and “prophecy” for the other (thinking of the action before it occurs). While the historical use of the term suggests, as does its anatomical meaning, a secondary relationship to the main object, the nomenclature for the taxon subsumes this to refer to the temporal placement: Epichirostenotes curriei literally is found in sediments younger than Chirostenotes pergracilis, such that one came after the other.

[n3] This is not the first time Bob Sullivan has done this — for example, Sullivan, (2006). I am troubled by this, because it suggests that the importance granted the need for taxonomy can be rendered slap-dash into an “appendix,” which which should elaborate basic data, but is adjunct yet not part of the main text. I would have been far further supportive had Sullivan et al. included the taxonomy in the body, as is done in conventional journal articles, but the process of rendering taxonomy in the appendix, with both papers here in the NMMNHS Bulletin, seems to set an odd precedent.

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