Incredulous Teeth, II: The Many Similar Teeth of Zanabazar

Zanabazar junior is a troodontid, named for famed Mongolian Öndör Geγeen Zanabazar (Өндөр Гэгээн Занабазар), an icon who brought Buddhism to the Mongols from the north from China. The name is translated from Mongolian as “known-vigor” and is derived from the Sanskrit Jnana-vajra, which means “thunderbolt of wisdom.” How peculiar that this is a name given to a “brainy” troodontid, animals known for their legginess and apparent adeptness at running down hatchling Maiasaura (and little else). But what is most peculiar about it is not that it is among the largest of troodontids, but that most if not all of its teeth were essentially homodont, or having the same morphology as one another.

This degree of homogeneity is relatively well-known among a variety of groups, mammalian, crocodilian and lepidosaurian. An homogenous tooth morphology is seen throughout the jaws in Gavialis gangeticus, where the tooth row is long, needle-like, and outwardly pointed; similar teeth are seen in a large number of pterosaurs, from Ctenochamsa roemeri to tiny Anurognathus ammoni, where in the latter they are all very small but spaced well-apart. Such teeth are useful when you’re jaws are not required to handle food in different ways along their margin, and when you generally only eat one type of food.

GI 100/1, left dentary and splenial of holotype of Zanabazar junior (Barsbold, 1974) in lateral view. After Norell et al., 2009.

Troodontid teeth are essentially homodont, and in some cases “isodont,” where the teeth are identical to one another in the tooth row (although the term is decidedly mammalian as it requires a variation between the upper and lower dentition). Virtually all troodontids have this type of dental morphology, which corresponds to a short, triangular and strongly laterally compressed crown, and occasionally a constriction between root and crown, and with large denticles on at least the distal carina.

Troodontid teeth in various views. A, ANSP 9259, holotype of Troodon formosus Leidy, 1856; B-D, GI 100/1, holotype of Zanabazar junior (Barsbold, 1974), premaxillary (B), maxillary (C) and dentary (D) crowns in labial (1), mesial (2) and distal (3) views. ANSP 9259 is in labial, lingual and mesial views. After Barsbold, 1974 (fig.6).

Large denticles are useful for providing extra points of trituration for each tooth as it is pulled through its preferred dietary substrate, or “food.” They even possess wear facets, where the upper dentition occluded the lowers and formed a large, distinct triangular wear facet (which are not actually apparent in Zanabazar junior). When you couple this homogeneity with morphology (very narrow crowns, “ziphodont,” apically-hooked denticles) researchers have been led to the conclusion that the likely food stuffs were flesh (Fiorillo, 2008), and likely did not include vegetation (as supported by Holtz et al. (1998)).

Skulls of Zanabazar junior with jaws open and closed.

Troodontids, with Zanabazar junior especially, appear to have relatively long jaws, but this is something of an artifact based on the particularly large braincase relative to rostrum size. The snout is instead shallow and the teeth particularly low. As noted by Norell et al. (2009), the teeth are small and become progressively larger posteriorly on both upper and lower jaws, where the crowns also become progressively higher in aspect (closer 1 when considering mesiodistal basal length and crown height), but their jaws were also broadly rounded rostrally, forming a distinct U-shape. These jaws functional anatomy have not been clearly resolved, and Holtz et al. (1998) and Fiorillo (2008) have only seemingly scratched the surface when it comes to the potential functional implications of the jaws.

Previously: A monster with a pair of saucer-plates  set on end receives a makeover, shrinks, and becomes the tiny thing from Jurassic Park. The Twin Crests.

Up next: What did it use those teeth for!? The Nitpicker.

A Note on the Name

Barsbold (1974) granted his new troodontid from the Nemegt Formation at Bügiin Tsav the monicker Saurornithoides junior, a reference to the “younger” age of the older type species of Saurornithoides, mongoliensis, from Bayan Zag in the Djadokhta Formation (Osborn, 1924). The name could literally be translated as “Saurornithoides, the younger,” as in classic monickers for philosophers and historians, such as Pliny the Elder, et al. When considering the nomenclature of this rather important taxon, Norell et al. (2009) separated junior from mongoliensis by employing a strategy that has influenced most of the AMNH group’s working and effective taxonomy, placing new taxa into genus-species couplets, and named Zanabazar as the monophyletic and monotypic container for junior. Write Norell et al. (2009) [pg.28]:

We chose to construct the name Zanabazar for Saurornithoides junior based on the morphological differences between the holotype and that of Saurornithoides mongoliensis and the questionable monophyly of Saurornithoides as defined by the sistertaxon relationship of S. mongoliensis and S. junior. This decision also was influenced by the common practice in phylogenetic analyses of assuming monophyly by compositely coding taxa (especially ‘‘genera’’). The differences between Zanabazar junior and Saurornithoides mongoliensis certainly merit coding these taxa separately. The temporal difference separating these specimens was not, and should not, be used to support taxonomic decisions.

This decision generally follows from the general remarks of the paleontologists working with reptiles, mammals and birds at the AMNH to dismiss the value or reality of “genera,” and they have been highly influential in setting the stage for the philosophy that grounds the abandonment of such rank-based taxonomy in general. They are also supporters of PhyloCode, although it is a personal decision for each worker to consider.

I have been highly critical of the argumentation in regards to Mike Taylor’s support for Greg Paul’s Giraffatitan as the container for Brachiosaurus brancai Janensch, 1914, separately from the type species, Brachiosaurus altithorax (Riggs, 1903) — not because I disagree with the choice, but because the argument made was based on a presumption that extreme forms of morphological differences can somehow categorize “genera” and thus distinguish species. Had Taylor used the reasoning employed by the AMNH team, including that of Flynn et al. (1999) coining the bionomen Ambondro mahabo for a small mammal from the Jurassic of Madagascar, who explicitly rejected the argument that the name was a genus-species couplet, even though it functioned as one (indeed, the Wikipedia entry for Ambondro mahabo lacks any mention of this fact, and refers to the taxon only by the “praenomen” throughout the text), this issue would have been more a “taste” issue (resurrection of a name that was not used by sauropod workers in print since it was coined) and than a methodology issue. Taylor deliberately invoked the “generic-o-meter” approach by touting the number of features as the effective reason. It is likely possible that a taxon can be coined as a new “genus” on the basis of a single autapomorphic feature, or on an autapomorphic suite, comprised of a number of features which are themselves not unique to the taxon except in combination. This latter approach is useful, because it is based on a scientific process which can be tested; to date, the argument that the number of features in a taxon is usefully attached to rank has never been proven reliable, merely asserted (e.g., Paul, 1988).

Zanabazar runs into the same “raised for purpose” wall as Giraffatitan; however, the former differs in that Norell et al. clearly emphasize the utility is purely one of taste: Taylor argues the name choice is a reference to some scientific process, which follows Paul on this matter, which is as I hope I’ve pointed out by now, unscientific but pretending to be so, and thus dangerous. This is basically the same as most criticisms of Greg Paul’s typological arguments for taxonomy, which have allowed him to with equal vigor split Iguanodon into and lump Citipati from as many named taxa. It is no more effective a purpose than to claim that you are choosing to regard your species taxonomy with bionomens, and coin new praenomens for all standard and well-described species currently pretending to be “second” or “third string” species within “genera” (the rank, that is). That isn’t how they are used by most systematists, and that isn’t how they should be used, especially for fossils.

I still, however, would have preferred if not considered it mandatory that some phylogenetic purpose be given to split “genera” (or any clade) so that species become more equalized relative to their former containers.  Using phylogenetic taxonomy, we can simply treat each node as a taxon, named or not, with a specific address: if this falls on a “genus” name, then it is not an actual “genus” but a non-least inclusive taxonomic unit, and thus a clade like any other. Logically, retaining some nascent concept of “genus,” with pretense of rank-hood, to be the name that contains species, leads to problems that “genera” (as a rank) currently play with taxonomy. I will get to this specific point very shortly.

Barsbold R. 1974. Saurornithoididae, a new family of small theropod dinosaurs from central Asia and North America. Palaeontologia Polonica 30:5-22.
Flynn, J. J., Parrish, J. M., Rakotosamimanana, B., Simpson, W. F. & Wyss, A. R. 1999. A Middle Jurassic mammal from Madagascar. Nature 401:57-60.
Holtz, T. R., Jr., Brinkman, D. L. & Chandler, C. L. 1998. Denticle morphometrics and a possible omnivorous feeding habit for the theropod dinosaur Troodon. Gaia 15:159-166.
Leidy, J. 1856. Notices of the remains of extinct reptiles and fishes discovered by Dr. F. V. Hayden in the badlands of the Judith River, Nebraska Territory. Proceedings of the Academy of Natural Sciences of Philadelphia 8:72-73.
Norell, M. A., Makovicky, P. J., Bever, G. S., Balanoff, A. M., Clark, J. M., Barsbold R. & Rowe, T. 2009. A review of the Mongolian Cretaceous dinosaur Saurornithoides (Troodontidae: Theropoda). American Museum Novitates 3654:1-63.
Osborn, H. F. 1924. Three new Theropoda, Protoceratops Zone, central Mongolia. American Museum Novitates 144:1-12.
Paul, G. S. 1988. The brachiosaur giants of the Morrison and Tendaguru with a description of a new subgenus, Giraffatitan, and a comparison of the world’s largest dinosaurs. Hunteria 2:1-14.
Taylor, M. P. 2009. A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914). Journal of Vertebrate Paleontology 29(3):787-806.

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One Response to Incredulous Teeth, II: The Many Similar Teeth of Zanabazar

  1. Pingback: Incredulous Teeth, I: With Twin Crests | The Bite Stuff

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