|The skull of the type specimen of Teratophoneus curriei in left lateral view, modified from Carr et al. (2011).|
Sunday, July 14, 2013
Q&A I: Teratophoneus curriei ontogeny
If I have sufficient time, I’ll answer questions put to me directly that illuminate aspects of tyrannosauroid paleobiology. Recently I was asked about Teratophoneus curriei, a new genus and species of tyrannosaurine that I named recently (2011) with Thomas Williamson, Brooks Britt, and Ken Stadtman. These questions come from a reader in South Korea.
Q: If the base of the interfenestral strut (see diagram) in T. curriei is concave, and that is considered to be a Stage 3 (adult) adult feature of Albertosaurus libratus (Carr, 1999), then why in T. curriei is it considered to be a subadult feature?
A: In the article our goal was to establish the relative maturity of the holotype specimen based on my earlier work (Carr, 1999) on other tyrannosaurids, particularly A. libratus. Since we were working on the only specimen available to us at the time, we could not establish its maturity with precision. In addition to a high number of immature features, it turned out that the specimen had several that clearly indicated that it was neither a young juvenile nor a full adult. So what about that nagging strut?
The table in Carr (1999) shows that the strut is flat in small stage 1 (i.e., juveniles), whereas it is concave in large stage 1 specimens. Ergo, the presence of a concave strut in T. curriei is consistent with what is seen in relatively immature A. libratus. However, in A. libratus, this is not a perfectly clean pattern, because individuals of greater maturity (Stage 2; subadults) also have the flat condition, whereas the concave condition is seen in the most mature specimens (Stage 3; adults). So where does that leave us?
Keep in mind that in Carr (1999) I established growth stages – large intervals of ontogeny - that would assist in distinguishing ontogenetic variation from phylogenetically informative variation. However, many specimens can be grouped into the categories of small juvenile, large juvenile, subadult and adult, indicating that growth stages have limited resolution because the relative maturity of the specimens within those categories is not specified. This indicates that the growth stage categories themselves are arbitrarily defined. Is there a way to solve these issues of resolution and arbitrariness?
Clearly this is an area where a more rigorous approach to ontogeny is required, which, in my view, is solved by cladistic analysis of ontogenetic characters (Carr and Williamson, 2004; Carr 2010). It is only through this approach, which solves both problems in one stroke, can we distinguish ontogenetically informative variation from individual variation. I have work in progress for all of Tyrannosauridae, which I have presented over the past several years at the annual meeting of the Society of Vertebrate Paleontology.
In the meantime, I suspect that the concave condition does indicate a relatively mature condition (it is not seen in the smallest, presumably least mature, juveniles), but that its timing is individually variable, somewhat like tooth eruption in people (for example, none of my ‘wisdom teeth’ have erupted and I am middle aged!). We’ll have to wait and see what the results of the analyses show. I’ll be able to answer your question with more clarity and depth once that work is published.
Q: Teratophoneus has a low tooth count, a condition that reflects its short snout. What do you expect the tooth count to be at the opposite extremes of its ontogeny?
A: One trend in tyrannosaurids is to increase tooth count, then reduce it (Carr, 1999); in others the tooth count is somewhat stable (Tsuihiji et al., 2009). I expect that T. curriei will show the latter pattern, where it will show little if any variation in tooth count. The short snout imposes a limit on the number of alveoli (tooth sockets) early in ontogeny, and I do not expect that would change as the animals increased in size. However, if T. curriei increased the size of its teeth in the manner of T. rex, then I would expect a reduction in tooth count with increasing maturity. Presently the answer awaits new specimens.
Carr, T. D. 1999. Craniofacial Ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology 19:497-520.
Carr, T. D. 2010. A taxonomic assessment of the type series of Albertosaurus sarcophagus and the identity of Tyrannosauridae (Dinosauria, Coelurosauria) in the Albertosaurus bonebed from the Horseshoe Canyon Formation (Campanian–Maastrichtian, Late Cretaceous). Canadian Journal of Earth Sciences 47:1213-1226.
Carr, T. D. and T. E. Williamson. 2004. Diversity of Late Maastrichtian Tyrannosauridae from western North America. Zoological Journal of the Linnean Society 142:479-523.
Tsuihiji T, M. Watabe, K. Togtbaatar, T. Tsubamoto, R. Barsbold, S. Suzuki, A. H. Lee, R. C. Ridgely, Y. Kawahara, and L. M. Witmer. 2011. Cranial osteology of a juvenile specimen of Tarbosaurus bataar (Theropoda, Tyrannosauridae) from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia. Journal of Vertebrate Paleontology 31: 497–517.
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