Wednesday, January 21, 2009

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.

Monday, January 19, 2009

Muscle reconstructions: doing the homework

I know I have been lazy with the postings here, and I really need to finish my Pachyrhinosaurus reconstruction. But I have been faced with a bit of a problem since I've decided to reconstruct this dinosaur one layer at a time, i.e. I wanted to be as accurate in the muscle reconstructions as possible so that the fleshed out version would look realistically three-dimensional even over the skin. Basically, I don't know enough about the postcranial musculature. Previously, I would have just roughly fleshed the whole creature out, from the outside in - basically imagining what a dinosaur would have looked like in life and just sketching it. This way, I didn't really need to be accurate in the myology just as long as the animal looked good enough. However, reconstruction from the skeleton up requires a bit more accuracy on the muscle reconstruction, or at least I would want to be as accurate.

If you have been following my blog for any period of time, you would know by now that I am quite obsessed with jaw muscles. I've dissected numerous specimens of birds (and a couple of crocs) and thus am quite familiar with the attachment sites, general architecture, and relative sizes of the jaw muscles. But dinosaurs (or indeed any animal) is not just the jaws or the head. There is something annoying (ha!) called the postcrania. While the spatial organization cranial/mandibular muscles can be three-dimensionally complex, they are fairly simple, as far as identification goes, in that there are only about a dozen well-defined muscle groups, about half of which are the large adductor (jaw closing) muscles. And when drawing dinosaur jaws, even at a wide open gape, only a couple of these muscles are visible, because most of them are tucked away inside the cranial adductor cavity! So thats all the contribution I get from my thesis when doing full-body reconstructions. But the rest of the body is covered in powerful postcranial musculature, most prominent of which are the appendicular (or arms/wings and legs) musculature. And I don't have much knowledge on the detailed myology of these systems - of course, I've glanced over some papers on reconstructing these muscles in dinosaurs but I don't know exactly which muscle originates where and inserts where...

Unfortunately, as I don't have time or any plans in the immediate future of actually dissecting and documenting first hand the postcranial musculature in birds or crocs (unless of course I suddenly switch to locomotor biomechanics for my postdoc), I have to resort to the next best option: read the existing literature on postcranial musculature, or doing the homework. There are plenty of literature out there on postcranial musculature, primarily appendicular musculature. And there are also an increasing number of papers dealing with the cervical (neck) musculature, most notably by Tsuihiji but also by Snively and Russell. The ribcage and tail musculature is something that is a bit more rare, I think, as I can't think of any publications off the top of my head - I'll have to go through my references to make sure. But I do know that one of my colleagues has been working on dinosaur ribcage reconstructions based on a myological/biomechanical model.

Anyway, some good places to start in reading on archosaur appendicular myology are:

Vanden Berge, J. C. & Zweers, G. A. 1993. Myologia, in Baumel, J. J. (ed.) Handbook of avian anatomy: nomina anatomica avium, pp. 189-247, Nuttall Ornithological Club.

McGowan, C. 1979. Hind-limb musculature of the Brown Kiwi, Apteryx australis mantelli. Journal of Morphology, 160: 33-73.

McGowan, C. 1982. The wing musculature of the Brown Kiwi Apteryx australis mantelli and its bearing on ratite affinities. Journal of Zoology, 197: 173-219

(these are one of the first studies to attempt to identify and correlate bone surface features with specific muscles)

and of course, some good papers on reconstructing appendicular musculature in dinosaurs, for instance:

Jasinoski, S. C.; Russell, A. P. & Currie, P. J. 2006. An integrative phylogenetic and extrapolatory approach to the reconstruction of dromaeosaur (Theropoda: Eumaniraptora) shoulder musculature. Zoological Journal of the Linnean Society, 146: 301-344.

(Sandra Jasinoski is a good friend of mine and so I know that the work that went into this paper is extremely thorough and of very high quality)