Osteology of a Near-Complete Skeleton of Tenontosaurus tilletti (Dinosauria: Ornithopoda) from the Cloverly Formation, Montana, USA
The character diagnosis of Tenontosaurus tilletti has been revised and redefined into a more robust and quantifiable state. Significant emphasis is placed on constructing phylogenetic definition in such a method, as it prevents occlusion of true character states by alleviating potential individual interpretational bias. Previous placement within the Iguanodontia is refuted based on the lack of character affinity with the defining synapomorphies of the clade. The clade Hypsilophodontidae (=Hypsilophodontia), along with Iguanodontia, however is deemed to be in critical need of refinement to account for recent discoveries and re-classifications of certain euornithopods. Several of the synapomorphies are out-dated and deemed redundant in favour of a more quantifiable approach. Re-definition of these clades is critical if the current state of basal euornithopodan relationships is to be resolved. Phylogenetic studies must be approached from a multidisciplinary perspective; integration of tectonostratigraphical, ontogenetic, palaeoecological, and biomechanical data with sets of well-defined primary homologies are essential in increasing phylogenetic resolution and generating stratigraphically feasible ancestor-descendant relationships. Material attributed to Tenontosaurus tilletti is in need of strict re-analysis; the significant quantity of specimens attributed to this species is potentially the result of poor stratigraphic constraints and the vast spatiotemporal span occupied. Future revision of this material is expected to reveal temporal variations on the species-level inherently linked to environmental evolution, as well as possibly provide clues to sexual dimorphism in contemporaneous, yet morphologically distinct tenontosaurs.
OK, it’s not actually that decent. All I did was describe and draw a pretty much complete skeleton of T. tilletti – good experience, but little real value. Bollocking previous work is all well and good too, but when I wrote this, I had little real clue about phylogenetics/taxonomy/systematics. I just liked rocks. Since I wrote this, there has been some tremendous work in increasing the phylogenetic resolution within Ornithopoda. This has largely stemmed from substantial specimen re-analysis, and the description of many new species. A quick search on Google Scholar, restricting articles to the last two years says it all really.
A Geometric Morphometric Analysis of Ruminant (Ungulata: Artiodactyla) and Ornithopod (Dinosauria: Ornithischia) Snouts: Comparative and Functional Ecomorphology
Snout shape is a prominent aspect of herbivore ecology with respect to feeding strategy, affecting both forage selectivity and intake rate. Within ruminants, feeding classes are delimited based on snout shape, with grazing species attributed ‘blunt’ snouts and browsing species ‘pointed’ snouts. These functional varieties have yet to be tested in a robust and quantifiable structure. In this study, using a variety of geometric morphometric techniques, this aspect of functional ecology is analysed in a statistically rigorous geometry-based framework, principally testing to see if feeding strategy is consistent with snout morphology and using a two-dimensional profile of the premaxilla in ventral aspect as a proxy. The secondary objective is to assess this approach using ornithopod dinosaurs, the putative extinct analogues of modern ruminants, to see whether similar patterns in shape variation can reveal their feeding strategies. The results here reveal that when ruminants are classified ecologically based on a priori assigned feeding strategies, they cannot be discriminated on the basis of their premaxilla shape profile, instead forming a shape variation continuum. Moreover, previously used terminology such as ‘pointed’ and ‘blunt’ are deemed inadequate for delimiting snout shape varieties, also lacking the descriptive power to define the morphological disparity demonstrated. These results are statistically significant, and found not to be an artefact of phylogenetic similarity. Conversely, ornithopods are found to exhibit a strong shape variation pattern between ‘blunt’ and ‘pointed’ snout shapes. These patterns exist in both a phylogenetic and temporal context, and may relate to both browsing height and forage selectivity as has been previously proposed for diplodocoid sauropods. The pattern shows a trend from a plesiomorphic selective ‘browsing’ condition in the Jurassic and Lower Cretaceous for many clades, through a transitional experimental phase with iguanodontids in the Lower Cretaceous, culminating in the large-bodied hadrosaurs in the Upper Cretaceous, which have distinctively flat and laterally-widened snouts for non-selective, ground-level ‘grazing’. This transformation is proposed to be related to increased nutritional requirements relating to a systematic increase in body size, and possibly associated with increased angiosperm
diversity during the Upper Cretaceous.
Apologies about the title, it’s horrific. This was my MSc project, undertaken under the supervision of Norm MacLeod. The man is a genius. If you’ve ever read his PalaeoMath series in the Palaeontological Association’s newsletters, you’ll know too. Essentially, this was two projects. The first, involved obtaining two rather large datasets, which actually turned out to be the two biggest of their kind. Odd considering for my ruminants, I got them all from the NHM. I won’t go in to too much detail, but the end result of the first series of analyses (with the mammals), was that the last 50 or so years of work on this topic was crap.
STOP PLACING ARBITRARY THRESHOLDS ON SUBJECTIVE GROUPINGS would probably be the main thing to take away from the project. This was in the context of ecological groupings in ruminants. This was actually an unexpected result. When I completed the analyses, and there was 0 resolution for any of the groupings (pretty much), I went to Norm trying to figure out what I’d done wrong. It wasn’t wrong, and it turns out he’d kinda expected it all along. So, that was good. As a functional ‘trait’, snout shape was redundant in the context of ecological groupings. This actually makes the number of previous citations using this a little worrying.
Right, so it’s not a diagnostic feeding trait. So what next? Using a rather cool bit of software (thanks Norm again), after you have run a principal components analysis with a particular dataset, you can then project, or overlay, another comparable dataset’s PCA scores onto the space defined during the first analysis. So, on the dinos went. Shock number 2. There was structure. I delimited groups based on ‘grades’ that broadly represented the major inclusive clades of Ornithopoda. This is a temporal pattern, kinda, too. Needless to say, great things happened.
RESULT: Don’t send a zoologist to do a paleontologist’s job