Better Know a [Dinosaur] – Elmisaurus rarus


This post won’t have much to do with teeth. This will eventually occur, when I focus on toothy things like Suminia (favorite non-mammalian synapsid) or toothless (and known-jawed but edentulous) in which case we come to the oviraptorosaurs, which are odder than you think.

And, plus, I had this image lying around…

Skeletons in silhouette of Elmisaurus rarus. On the left (black silhouette) includes the holotype specimen (ZPAL MgD-I/172, pes) and referred manus+pes material (ZPAL MgD-I/98, partial manus and pes); on the right is the same material expanded to the size of a referred manus+pes (ZPAL MgD-I/20), showing the largest known specimen. Both of these represent only Elmisaurus rarus.

Elmisaurus rarus is a fairly ambiguous oviraptorosaur, universally agreed to be a caenagnathid closely related to Chirostenotes pergracilis (sometimes confused with Caenagnathus collinsi [we’ll get to this some other time, but for the meantime, bear with me and assume that I separate the two species for a good reason]), and that’s because very little has been said about it despite a decently complicated literature that have referred to it specifically. Described in 1981 [1], it represents one of the earliest-named members of Oviraptorosauria, and one of the least well-known.

Its involvement in phylogenies is so limited as to be nearly nonexistent, despite its rather important position in the geography of oviraptorosaurs: It is the only caenagnathid known from the Nemegt Formation of Mongolia, and thus one of the youngest-known oviraptorosaurs from Asia (along with oviraptorid Rinchenia barsboldi). Why it tends to get glossed over is largely due to the extreme similarity to Chirostenotes pergracilis [2,3,4], leading some researchers to synonymize them (or leave rarus standing as a species independent: Chirostenotes (or Caenagnathus) rarus) [2,3,4]. This gets more confusing when you consider that there is another species involved (or two), but we will get to that at another time.

Elmisaurus rarus, referred right manus and right pes of ZPAL MgD-I/98; not shown, fragmentary and undescribed partial long bones.

Metatarsals of Elmisaurus rarus. A, holotype, ZPAL MgD-I/172, left tarsometatarsus lacking MTI. B, referred large specimen, ZPAL MgD-I/20, right partial tarsometatarsus. Top row distinguishes the proximal aspect of each tarsometatarsal "cap;" A is flipped horizontally.

The most remarkable aspect about Elmisaurus rarus, and the one that normally gets mentioned, is the fusion of the distal tarsals (two definitively, three probably) to the proximal tarsals, resulting in a tarsometatarsus. In dinosaurs, this generally only otherwise occurs in birds (or an enclosing clade) or some ceratosaurians (especially Coelophysidae). This fusion, indicated above, occurs without the metatarsals being fused to one another; as an added bonus, the larger specimen above (ZPAL MgD-I/20) shows less fusion between the distal tarsals and the metatarsals, and less fusion among distal tarsals directly,  than the holotype. Ordinarily, this might imply that the latter was a younger animal … except it’s larger. We can suggest, or imply, that this means the larger specimen is a different species than the former; if this were the case, then we’d have two caenagnathids in the Nemegt Formation, and fun times would be had by all.

Other remarkable things include the “spur” of the metatarsals, a feature which originally linked Elmisaurus rarus to another Mongolian Cretaceous (and eventual oviraptorosaur) dinosaur, Avimimus portentosus, and is possibly homologous to the attachment of the m. tibialis cranialis. This feature is also found in the American species referred to Elmisaurus, Elmisaurus elegans [3,5; but see also 6]. The fusion of the tarsometatarsus (it’s very existence, rather) and the formation of the spur are found in all three taxa, but few other features link Avimimus portentosus to Elmisaurus rarus (partly due to the lack of more material in the latter) [1].

Finally, the diaphysis (shafts) of the metacarpals and phalanges of the manus are particularly narrow compared to the proximal and distal ends (epiphyses), a feature not found in other taxa. This is largely restricted to a single specimen, so it is unclear what the distribution of this feature is within Elmisaurus rarus. Among other oviraptorosaurs, however, the distinction of the narrow-shaft/broad-ends is largest in this species alone, without clear intermediate forms (even among other caenagnathids), is fairly unique, which renders this feature a plausible autapomorphy for the species (that’s Elmisaurus rarus, not Elmisaurus alone).

Elmisaurus rarus has enjoyed less of a stormy relationship than some of its relatives. To date, Elmisaurus rarus has never been synonymized with another species, but has been placed in Chirostenotes (as Caenagnathus) on several occasions [2,4]. Generally, its provenance (Asia) has permitted division between this taxon and the other widely acknowledged American (technically, Canadian) taxon, Chirostenotes pergracilis. The reasoning behind why mere provenance should displace taxa at the generic level has yet to be answered — real morphological variance between the two has been limited, leaving Currie [5] and Varricchio [3] to assume material between the two continents would share the same generic label [n1].

Mickey (at the Theropod Database; blog here) indicates two works [7,8] which imply further caenagnathid material from the Nemegt that may belong to Elmisaurus rarus, but this is undescribed.

Such limited material leaves us wanting for more data, and as such, a nice short article to discuss a taxon described nearly 30 years ago seems fitting. An additional specimen, ZPAL MgD-I/85, was described by Osmólska [1] (although not as Elmisaurus rarus) but it is fairly small and has been linked to Avimimus [5].

[n1] We’ll leave off the issue of “What the hell is a genus, anyway?” That tends to be both involving, contentious, and is to date largely unresolved, although a good number of researchers seem to like displacing others’ concepts for their own on the basis of one reason (modern use in extant taxa) for others’ (genetic distance “metrics”). Suffice it to say, my general ongoing principle is to use the original taxonomy provided unless it conflicts with phylogeny; new taxa named stand on their own, unless shown to be directly synonymous (indistinguishable) with another taxon given a substantial amount of data used to indicate this.

[1] Osmólska, H. 1981. Coossified tarsometatarsi in theropod dinosaurs and their bearing on the problem of bird origins. Palaeontologica Polonica 42:79-95.
[2] Paul, G. S. 1988. Predatory Dinosaurs of the World. Simon & Schuster Co. (New York City). 464 pp.
[3] Varricchio, D. J. 2001. Late Cretaceous oviraptorosaur (Theropoda) dinosaurs from Montana. pg.42-57 in Tanke and Carpenter (eds.) Mesozoic Vertebrate Life. NRC Research Press and Indiana University Press (Bloomington and Indianapolis).
[4] Maryańska, T., Osmólska, H. & Wołsan, M. 2002. Avialan status for Oviraptorosauria. Acta Palaeontologica Polonica 47(1):97-116.
[5] Currie, P. J. 1989. The first records of Elmisaurus (Saurischia, Theropoda) from North America. Canadian Journal of Earth Sciences — Revue Canadienne des Sciences de la Terre 26:1319-1324.
[6] Parks, W. A. 1933. New species of dinosaurs and turtles from the Upper Cretaceous Formations of Alberta. University of Toronto Studies, Geological Series 34:1-33.
[7] Currie, P. J. 2001. Nomadic expeditions, Inc. report on fieldwork in Mongolia, September 2000. pg.12-16 in Alberta Palaeontological Society, fifth annual symposium. Presented by Alberta Palaeontological Society and Department of Earth Sciences, Mount Royal College. 12-16.
[8] Currie, P. J. 2002. Report on fieldwork in Mongolia, September 2001. pg.8-12 in Alberta Palaeontological Society, sixth annual symposium, “Fossils 2002.” Presented by Alberta Paleontological Society, in conjunction with Canadian Society of Petroleum Geologists, Paleontological Division and Department of Earth Sciences, Mount Royal College.

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One Response to Better Know a [Dinosaur] – Elmisaurus rarus

  1. Pingback: An Oviraptorosaur Worth the Name? | The Bite Stuff

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