The Caenagnathidae are a group of somewhat rare and enigmatic theropod dinosaurs within the larger group known as oviraptorosaurs. They are currently known from Asia and North America, although other members of the Oviraptorosauria (not members of the Caenagnathidae) have been found elsewhere, including one recently published that is considered an oviraptorosaur or oviraptorosaur-like from Europe (read about it at http://scienceblogs.com/tetrapodzoology/2011/05/tiny_cretaceous_theropod.php). The caenagnathids are some of the least studied members, which is partly from a lack of material in general and nearly complete specimens. I am not going to get too in depth right now on all of this and the background to caenangthids because part of it was covered quite well by Jaime Headden on his blog The Bite Stuff (http://qilong.wordpress.com/2011/01/28/splitting-chirostenotes/).
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| An overview of members of the Oviraptorosauria, with the two newly-named dinosaurs being new members of. |
Needless to say, there is a lot of confusion surrounding this group of dinosaurs, and much work that can be done. Having said this, several years ago, Dr. Robert M. Sullivan of the State Museum of Pennsylvania led a very small field party to the San Juan Basin in the northwestern New Mexico. One of his field assistants, Arjan Boere, came across a peculiar bone. After field prep, extraction, further prep and study, it was discovered that it was an incomplete pubis from a caenagnathid.
The specimen (SMP VP-1458) came from the Naashoibito Member of the Ojo Alamo Formation, which recently went through a thorough study and revision [2] and was confirmed to be early Maastrichtian in age, or roughly 68-72 Mya. The layer is not very especially fossiliferous, and often the fossils recovered from this layer are quite fragmentary or broken. Recently several new taxa have come from the layer, including the ceratopsid Ojoceratops [3]. Nevertheless, with the K-T boundary layer missing from this region, the Naashoibito Member holds the keys to the last remaining taxa present before the very end of the Cretaceous.
The pubis was determined to be distinct from all other caenagnathid pubes and was given the name Ojoraptorsaurus boerei [1]. Its genus was named for the strata in came from, and species for its original discoverer. It includes the autapomorphies: "1) a “spoonshaped” depression on the anterior dorsal surface of the pubic boot; 2) enclosed pubic fossa recessed at least one cm from the acetabular rim, positioned on the medial surface of the pubic shaft; 3) the distal portion of the pubic shaft above the pubic boot is slightly convex anteriorly; and 4) the iliac peduncle articular surface of the pubes is sub-trapezoidal in shape." While some people will feel that naming a dinosaur based on the pubis is not a good idea, there are reasons for it that I and my co-authors felt warranted this. Caenagnathids are commonly known from very fragmentary material anyway. This includes a number named only on lower jaws, and the most complete specimen (CM 78001) is currently under study and has not yet been named. The pubis itself seems like a very diagnostic element in terms of caenagnathids. Through all the trips to look at the other caenagnathid specimens, and many other oviraptorosaur specimens, SMP VP-1458 was still distinct and showed the four mentioned autapomorphies. Therefore, along with its distinct geographic provenance and interesting temporal position, it deserved to be named. This way, if others think we are completely full of crap, they can write something and tell us how and why we are. Still, science and the knowledge of this group of dinosaurs will move forward because of it.
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| Part of the pubes of SMP VP-1458, the holotype of Ojoraptorsaurus boerei [1] |
Ojoraptorsaurus boerei would be quite similar in many aspects to other caenagnathids, including the most well-known member of the group, Chirostenotes. Since we are working off only post-crania, we must extrapolate that the rest of the animal would have been similar to other members, although we can always let our minds wander and come up with something looking completely different and amazing, which is still a possibility until more of the animal is recovered.
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| Illustration of Chirostenotes, close relative of both Ojoraptorsaurus and Epichirostenotes, and very similar to what both taxa would have looked like. |
While doing this research, the authors came across differences between a specimen (TMP 79.20.1) of Chirostenotes that is confidently referred to the genus based on similarities with the holotype (CMN 2367), which is an incomplete set of right and lefts hands. Another specimen that was referred to Chirostenotes though (ROM 43250) was found to have some key differences with TMP 79.20.1, and therefore with Chirostenotes. My co-authors and I, therefore, decided that the differences warranted the erection of a new taxon, Epichirostenotes curriei [1]. The major differences seen firsthand dealt with the ischium, but several more were extracted from the description by Sues [4]. The entire specimen, and the new taxon that goes with it, needs more thorough study and revision. But that discussion, and the potential research surrounding it, is something that will be discussed a bit later.
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| Illustration of Chirostenotes, close relative of both Ojoraptorsaurus and Epichirostenotes, and very similar to what both taxa would have looked like. |
NOTE: I am looking for someone to potentially illustrate both these new dinosaur taxa, so if anyone would be interested or know of someone else who may be, please let me know.
REFERENCES
[1]Sullivan, R.M., Jasinski, S.E. and Van Tomme, M., 2011, A new caenagnathid Ojoraptorsaurus boerei, n, gen., n. sp. (Dinosauria, Oviraptorosauria), from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico: New Mexico Museum of Natural History and Science, Bulletin 53: 418-428.
[2] Jasinski, Steven E., Robert M. Sullivan, and Spencer G. Lucas. 2011. Taxonomic composition of the Alamo Wash local fauna from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico. New Mexico Museum of Natural History and Science Bulletin 53: 216-271.
[3]Sullivan, R.M. and Lucas, S.G., 2010, Ojoceratops fowleri gen. nov., sp. nov., a chasmosaurine (Ceratopsidae, Dinosauria) from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico; in Ryan, M.J., Chinnery-Allgeier, J. and Eberth, D.A. eds., New perspectives on horned dinosaurs: Bloomington, Indiana University Press, p. 169-180.
Steve, thanks for the link back! I look forward to seeing what else may come from these specimens. SMP VP-1458 is certainly interesting, and deserves a fair amount of attention, especially for the southerly extreme to which it extends caenagnathids in North America and the relative large size of it in comparison to other oviraptorosaurs.
ReplyDeleteI en-skeletal-ated most oviraptorosars known, here, and clearly distinguish for technical reasons ROM 43250 from other caenagnathids. Ojoraptorsaurus would certainly fit in there neatly, and it needs to be relabeled with the new taxonomy. I hope this helps!