On Saturday, I’ll be giving a short presentation about (very) preliminary results from the ODP, for the 1st Annual Southwest Regional Joint DVM&DCB (Division of Vertebrate Morphology and Division of Comparative Biology) meeting of SICB (Society of Integrative and Comparative Biology). This conference is a one-day event held on the campus of Cal State San Bernardino, and targets functional morphologists and their kin (including paleontologists).
The presentation is entitled “Morphological disparity, locomotion and limb proportions in ornithischian dinosaurs,” and it’s set up as a 5 minute talk. One of the cool things about DVM is the option of a 5 minute format – perfect for work that is in its nascent stages or “crazy” ideas that you just want to throw out there. Given the very preliminary nature of the analysis (and the fact that I’m co-author on three posters at the Society of Vertebrate Paleontology meetings next week), I jumped at the chance for this format. Plus, the talk was a good opportunity to kick my butt in gear and do some real analysis.
I don’t have a lot of time at this second to detail every aspect of the methods, but here is a sketch:
- Data were trimmed down to one entry per taxon, choosing the largest and most complete specimen possible. In a handful of cases, missing data were interpolated via regression (to estimate tibia length from fibula length) or other specimens of the same taxon.
- Taxa were binned into five time categories, each spanning roughly 34 million years. Any finer bins, and there just weren’t enough taxa.
- I ran principal coordinates analyses on the data, for forelimb, hindlimb, and all limbs together. Within each temporal bin from the results, I calculated the sum of variance and nth root of variance. This gives a measure of morphological disparity in each bin – high variance, high disparity. The analyses were run with the raw data, as well as data that were standardized within each taxon by the geometric mean. This was to attempt to remove the effects of body size.
- I plotted the data in each bin. In order to compare the raw results vs. geometric mean results, I normalized the data to the largest value in each category.
- A few notes of caution – I did not perform any statistical tests on the data (bootstrapping, confidence intervals, etc.). So, the results should not be considered to have particular statistical significance at any level. Also, no attempt was made to accommodate for sampling effects or phylogeny. Caveat emptor.
- In any case, there are some cool results. Looking at hind limbs, there is a big jump in disparity after the first 60 million years (or so) of evolution – not unexpected, given the explosion of forms in the mid-Jurassic. What was more interesting was the fact that the disparity stayed constant when looking at raw values, but when values were corrected for size using a geometric mean, there was a big drop in disparity during the last 60 million years or so. On first consideration, this suggests to me that body size is driving some of the disparity values. Body size stayed big after the Middle Jurassic, but overall morphological disparity (in what those large body forms looked like) decreased. I wonder if some of this is due to the extinction of stegosaurs (with their bizarro limbs) at the end of the Jurassic / early Cretaceous. Forelimb disparity (when correcting with a geometric mean) by contrast takes a big jump in the late Cretaceous – I wonder if this is due to hadrosaurs, with their conventional hind limbs but really, really weird forelimbs. Food for thought.
Tonight I put together a first draft of the slides for my presentation. Supporting data are here, and a PDF of the slides is here. I’m going to do some more editing tomorrow, so any suggestions are welcome. Keep in mind that the slides are pretty rough right now, so forgive any ugliness there. Also, remember that I’m dealing with a 5 minute format, so there’s only so much more I can add (and I think I’ll have to trim some stuff – we’ll see how much time is in the mix after I run through it once out loud).
The final version, after presentation on Saturday, will be archived at figshare.