Olfactory bulb in theropods

Ever since I started having suspicions of the role of olfaction in scavenging behaviours of Tyrannosaurus rex, I have been interested in the olfactory capabilities in predators and how they correlate with their behaviours. So much that I bought a nice book called Predator-prey dynamics: the role of olfaction. However, I shan't write about that book today, even though I quite like it. Instead I shall comment on a paper that became available in Proceedings of the Royal Society B.

I refer to the new study by Darla Zelenitsky and colleagues on the olfactory bulbs in theropod dinosaurs and alligator. Zelenitsky et al. (2009) provide the "first quantitative evaluation of the olfactory acuity in extinct theropod dinosaurs". They calculate relative olfactory bulb sizes (olfactory ratio: the size of the olfactory bulb relative to the size of the cerebral hemisphere) in 21 species of theropods (including Archaeopteryx) and 1 species of crocodilian (Alligator mississippiensis) using the greatest linear dimensions measured from endocasts, CT-scan slices, or from the impressions left on the ventral surfaces of the frontals and parietals. This last bit of information, I thought is very interesting. Although the authors themselves state that there are uncertainties associated with olfactory ratios calculated from the fronto-parietal measurements and thus exclude them from independent contrasts analysis, it demonstrates that we don't necessarily need complete endocasts to extract information regarding that dinosaur's brain, however partial the information may be; it still provides some information regardless. In calculating the olfactory ratios, Zelenitsky et al. (2009) use the greatest dimensions, meaning that their ratios are not restricted to homologous measurements, i.e. depth/depth or height/height, but attempt to capture a purely geometric ratio between maximum measurements; height/depth or depth/height ratios, whichever ones happened to represent the greatest diameter - so that's kind of neat.

Their total data of 29 points are replotted here (see above) as log olfactory ratios against log body mass. A least squares regression (LSR) line is fitted onto 19 of these data points. LSR is conducted on all theropod taxa, so excluding Alligator but also Gorgosaurus and Albertosaurus because the data for these two taxa are derived from the less reliable fronto-parietal measurements. Each species is represented by a single datum, i.e. redundant points are excluded. The LSR line with 95% confidence intervals on this reduced data set is shown superimposed onto a bivariate plot of the total 29 data points.

As you can see, I followed Zelenitsky et al.'s (2009) procedure in data reduction but conducted LSR directly on log transformed data while they performed a LSR on independent contrasts. In other words, their regression analysis is based on data with any phylogenetic signals inherent in the dataset removed. However, my plot and LSR best-fit line above looks identical to their Figure 2, which is labeled "independent contrast least-squares regression", but their actual independent contrasts LSR presented in their supplementary materials show a line forced through the origin so is clearly not the same line as presented in Fig. 2. But the stats presented (slope and r value) in the same fig are for the independent contrast LSR. A minor point, I know...

So regardless of what LSR best-fit line is presented, it is quite clear that the relationship between olfatory ratios and body mass for theropod dinosaurs is probably different from that of Alligator; although as crocodilians are only represented by three members of a single species A. mississippiensis there is no knowing what the crocodilian relationship truly is. However, it is obvious that Alligator has a much higher olfactory ratio compared to those in theropods of similar sizes. Perhaps, the role of olfaction is different in alligators from those in theropods.

Another thing to note is that all theropods including Archaeopteryx seem to fall along the tragectory of the best-fit line. The notable exceptions are tyrannosaurids (excluding Dilong), dromaeosaurids, ornithomimosaurs and Citipati, the former two groups above and the latter two below the line. Zelenitsky et al. (2009) determine if these groups are higher or lower than expected based on the 95% confidence intervals, but I'm not entirely sure if that is a good criterion as these confidence intervals are surely for the slope and intercept and nothing to do with the quantile of data distribution...? Analyses on residuals and other regression diagnostics are used to identify outliers so why not use that? - well, I don't know, who cares...

Anyway, the point of this paper was that, it was the first attempt at quantifying olfaction in extinct theropods, and it shows that aside from Tyrannosaurus rex, which always gets the spotlight for having high olfactory ratios, other large tyrannosaurids but also dromaeosaurs also have relatively high olfactory ratios, and that this is not unique to T. rex. On the other hand, ornithomimosaurs and an oviraptorosaur seem to possess relatively low olfactory ratios. Now, the physiological or ecological interpretations from these results are still unknown, but we could infer that there was something different in the physiology/ecology of these groups of theropods from an "average" or primitive theropod condition.

Reference:
Zelenitsky, D. K., Therrien, F., and Kobayashi, Y. 2009. Olfactory acuity in theropods: palaeobiological and evolutionary implications. Proc. R. Soc. B. 276: 667-673.