Southern Raptors II: Grooves, Cones, and Carinae

When dealing with incompletely preserved or incompletely prepared material, superficial statements are often used to describe a feature so that it can be “assessed” by the readers in some fashion. Take, for example, the following image:

"Conical" teeth.

Here we have the teeth of a small animal, and just look at them! These are distal dentary teeth, and despite their size, they seem neat, conical, recurved to very slightly recurved, and have no traces of serrations on them. But things are not always as they seem. If we move further mesial, the teeth look a little different.

Not so "conical" teeth.

The main difference here is a groove on the side of the crown, a lateral sulcus, which breaks up the seeming perfect “conical” appearance, but at this point we become more confused about the shapes of the teeth. Are they flattened into blades anteriorly, but become conical posteriorly?

The answer is not that odd, as recently described by Frederico Gianechini, Peter Makovicky, and Sebastiá Apesteguía in the latest issue of Acta Palaeontologica Polonica. The teeth above belong to the unenlagiid Buitreraptor gonzalenzorum, and depsite the disparity in form, they come from a region of the dentary separated by only about 10 tooth positions. It is also notable that the difference in shape between the teeth in the two images above is an illustion, formed by the lighting and the lack of examination, and it plagues non-material-handling in description: both sets of teeth are laterally compressed, and neither is particularly conical. Optical illusions like that above lead some researchers to project an apparent image of shape into their work when they haven’t assessed this feature directly, and this happens frequently when the photograph or the lighting is taken at a particular or odd angle. A thourough understanding for the view of the angle or plane of the photographic lens and that of the subject, as well as that of the lighting, will greatly aid in understanding the problems here, but even this does not resolve the shapes of things, merely removes the influences of the photo from the illusion. The observer can see all of these in person, and still argue that the teeth at top are conical, but the ones below are compressed.

The teeth shown above are all demonstrative of two other things, however, first of scale and the second of morphology. In the first, the posteriormost teeth are much smaller, and often they preserve what might be the “tip” of the crown of anterior teeth. Features in posterior teeth tend to be much more muted, reducing many aspects of the more anterior teeth. They often have a lower aspect ration of height to basal mesiodistal length, and the curvature of the crown is more extreme on the mesial margin. If you look up at the images of the in situ crowns, you will notice this directly.

An isolated crown, likely a maxillary, is very large and shows some additional features:

MPCA 245 A1, maxillary? crown of Buitreraptor gonzalezorum, after Gianechini et al. (2011).

Here there are ridges on both sides of the crown, and a longitudinal sulcus travels from the base, giving it an “hourglass” shape, to the tip. These ridges and grooves, not unlike some of the teeth termed “paronychodont” from North America, give the crowns a somewhat unusual appearance. These are not unique to Buitreraptor gonzalezorum, however, as they are present in Austroraptor cabazi as well (Gianechini et al., 2011), which is substantively much larger. As noted by the authors, such grooves are present in the teeth of other theropods, although in those taxa they are much less labiolingually compressed. This aspect, with the crown shape and the grooves, is featured only in a subset of paravian theropods, Dromaeosauridae and Unenlagiidae. In the abelisauroid Masiakasaurus knopfleri, for example, the mesial teeth have a prominent “arcade” of labial ridges forming little grooves that nearly run the length of the crown, but the teeth are not labiolingually compressed even though, like unenlagiids, the crown has a basal constriction between root and crown; it lacks the longitudinal sulcus, and the crown is circular in cross-section.

FMNH PR 2180, mesial tooth (either premaxillar or dentary) of Masiakasaurus knopfleri Sampson et al. (2001) (modified after Carrano et al., 2002). Scale bar equals 2 mm.

The function of such teeth hasn’t been clear, although resistance to higher levels of force without becoming incrassate (as in tyrannosaurs) is possible, as well as resisting torsional effects; this latter aspect is certainly suggested in Masiakasaurus knopfleri, although the procumbency and large size is shared with some crocodilians (recently blogged here) while the form of the teeth is not.

The function of blade-like, “ziphodont” dentition is quite clear, in that they are typically designed to act as knives slicing through various choice meats and delicacies. This is, however, harmed by the lack of serrations in Buitreraptor gonzalezorum (or, in fact, any unenlagiid). Such teeth are then, as in most avialaeans, little unserrated paring knives. Is it no wonder then that such teteh are either well-spaced apart in small jaws, or highly numerous and compacted in larger jaws? The functional effect of the former is one of small puncturing devices, and such teeth are uncompressed and not strongly recurved, while in the latter they are, and I would infer this to be a matter of immitating a sawblade, rather like a reverse rip saw (where the teeth are pointed forward, to match the forward driving force of the saw’s action). This would mean that the teeth are designed to puncture along a measurably even line, then separate the substrate in a single channel, then deepen and wide this channel by applied force of the jaw. This is fairly unlike the action of teeth of serrated dentitions, in which the action of the tooth is to drive in, then cut backwards, where the serrations themselves act to tear the tissues along the action of the jaw, collecting various tissue between them and macerating as they go (Abler, 1992). Thus it would seem that unenlagiids were a bit more delicate in their eating habits, while dromaeosaurids, which have the second type of teeth, not so much.

Abler, W. L.. 1992. The serrated teeth of tyrannosaurid dinosaurs, and biting structures in other animals. Paleobiology 18(2):161-183.
Agnolin, F. L. & Novas, F. E. 2011. Unenlagiid theropods: Are they members of the Dromaeosauridae (Theropoda, Maniraptora)? Anais da Academia Brasileira de Ciências 83(1):117-162.
Carrano, M. T., Sampson, S. D. & Forster, C. A. 2002. The osteology of Masiakasaurus knopfleri, a small abelisauroid (Dinosauria: Theropoda) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology 22(3):510-534.
Gianechini, F. A., Makovicky, P. J. & Apesteguía, S. 2011. The teeth of the unenlagiine theropod Buitreraptor from the Cretaceous of Patagonia, Argentina, and the unusual dentition of the Gondwanan dromaeosaurids. Acta Palaeontologica Polonica 56(2):279–290.
Novas, F. E., Pol, D., Canale, J. I., Porfiri, J. D. & Calvo, J. O. 2009. A bizarre Cretaceous theropod dinosaur fromPatagonia and the ecolution of Gondwanan dromaeosaurids. Proceedings of the Royal Society of London, B 126:1101-1107.
Sampson, S. D., Carrano, M. T. & Forster, C. A. 2001. A bizarre new predatory dinosaur from Madagascar. Nature 409:504–506.

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