For some years now, Mary Schweitzer and her team have been researching the idea that organic molecules can be preserved for millions of years, specifically within dinosaurs. They have used a plethora of chemical and biotechnological techniques to demonstrate that, within animals like Tyrannosaurus rex, it is possible to find the residue of structures such as blood vessels and even proteins. Naturally, her research has been met with a whole wad of stiff resistance from the scientific community, seemingly for no other reason than “We don’t like the sound of that..”. Scientific rigour ftw!
This is a post about pachycephalosaurs. It’s not a post about feathered dinosaurs, huge dinosaurs, or any of the ones which you may be more familiar with from popular media. Pachycephalosaurs were the dome-headed little scrappers of the Cretaceous, around 85 to 66 million years ago. Their name means ‘thick-skulled lizard’ (pachy: thick, cephalon: skull, saurus: lizard), and they were a small group within the larger herbivorous group of dinosaurs called ornithischians.
It’s probably fair to say that these dinosaurs are one of the least popular groups; they didn’t have razor sharp teeth and sickle-switchblade claws, they didn’t grow to the size of houses, and they didn’t have rows of armoured shields and spikes along their backs. What they did have, however, is an unusual behaviour that signifies them as unique, and pretty amazing, beasties.
Fig.1 – A pachycephalosaur suffering an ‘ouchie’, or cranial lesion (PLoS)
Childhood memories of dinosaurs have received another shattering blow today. The latest culprit is Yutyrannus huali, a large basal tyrannosauroid from the lower Cretaceous of China, complete with elongate integumental filament structures, or ‘protofeathers’. The etymology is quite special, says lead author Xing Xu, translating into a blend of Mandarin and Latin as ‘beautiful feathered tyrant’. This species is the latest wonder to be exhumed from the fossil treasure trove known as the Yixian Formation, a series of volcanogenic and lacustrine deposits that are currently guiding and re-working our understanding of dinosaur evolution. Of course, being such prominent study, Nature saw fit to pop it behind a $32 paywall.
The time of digital technology is upon us. No scientific domain is embracing it’s fast-paced and dynamic progression more so than Palaeontology. One such realm that is exploding with new studies and enrapturing the minds of people and the global media is the increasing possibility to digitise and manipulate three-dimensional fossils. Surface laser-scanning, C-T scanning and mechanical digitizers are all commonplace now in palaeontological studies. The implications of such techniques are far-reaching, from reconstructing robotic dinosaurs (see video), to understanding vertebrate biomechanics at an intricate level. Other palaeontologists digitally reconstruct the internal anatomy of various organisms; for example, in the Herefordshire deposits in the UK, digital models are recreated from exquisitely preserved fossils within nodules to look at the evolution of the internal structures that were pivotal in the evolution of extant hyperdiverse invertebrate groups, such as arthropods.
It is pretty well established that the fossil record is fraught with completeness issues. I covered the problem of this in a previous post in terms of understanding biodiversity patterns in deep geological time, in the context of lineage completeness. Another problem however is individual specimen completeness. Several authors have attempted to compensate for this secondary level of ‘bias’, using various quantitative metrics, and use these to guide assessments of biodiversity through time in specific lineages (e.g., sauropod dinosaurs). Another problem is that often, fossils have been ‘squished’ and distorted by the weight of successive layers of rock over the thousands or millions of years they have been buried for. This is a problem which is typically found in dinosaur skulls, making them somewhat resemble Imhotep in The Mummy (this may be fictional).
Ugly beyond all reason, possibly as a result of post-mortem decay. "You talking to me?!"
Geometric morphometrics is something that I’ve mentioned in previous posts. It sounds awful, the very mention of it usually enough to put people off or smash a keyboard upside your head. But thanks to several review papers, the basic concepts are now much easier to grasp and apply to a variety of scientific hypotheses. Statistics are quantitative, easy to record, less subjective than qualitative statements, and available for repeated manipulation through a wide variety of methods. The integration of geometry-based analysis is now commonplace in almost every aspect of Palaeontology, intimately coupled with an increase in the availability of digital techniques. The fact that you don’t have to damage unique specimens during the processes (usually) is a bonus too!
The latest analysis, and a critical study for palaeontologists and museum curators around the world, uses geometry-based reconstruction of a poorly-preserved fossil to digitally reconstruct missing or distorted parts. And the best part about it, is that it’s fully open access (including all supplementary videos); the comment that “this method does not require specialised software or artistic expertise” is perhaps a bit misleading, as you firstly need a fossil and a CT scanner (or a previous scan), a pretty beasty PC, and the software mentioned is hardly cheap (Rhinoceros is €195 for a student license, and for Geomagic the cheapest price I could find was $8000). The actual software used (Mimics) appears to be free, but I’m still awaiting confirmation for downloading. Additional software, such as MeshLab and Autodesk Maya are freeware, at least for trial versions.
Clack et al. set out to build a method of digital reconstruction that builds upon previous methods, giving greater geometric accuracy. The methods revolve around using a digital mesh obtained through laser or C-T scanning as a model for a landmark-based geometric reconstruction. The sample specimen is a vertebra from the infamous tetrapod fossil Acanthostega. Only one half of the vertebra is actually preserved, therefore this was digitally reconstructed and attached to its mirror image, creating a bilaterally symmetrical three-dimensional element.
Landmark selection involved a mixture of Type 1 and Type 2 landmarks; that is topographically homologous points, mixed with sites of geometric significance, such as local maxima or minima of curvature. These were used as the basis for constructing a surficial grid of contour lines describing the medial and lateral geometry of the neural spine. Videos of the processes involved are actually available online, embedded within the article, a really awesome and useful addition, making the whole methodology more transparent and easier to replicate, should you wish. There’s not really much else to say about the methodology; the processes, such as modelling and surface extrapolation are laid out systematically and reasonably easy to understand for anyone with an understanding of the concepts of geometry and fossils.
The resultant reconstructions are high quality, smooth and geometrically faithful in representing the original vertebra in three dimensions, free of any taphonomic deformation or distortion, and with missing parts accurately reproduced. The groups of models created are validated using Procrustes superimposition and principal components analysis, two standard statistical techniques. The first two principal components do appear to have a low explanatory power however (PC-1 = 24.3%), which may be an issue relating to the complexity in the form of the vertebra. The authors are right to discount the use of the thin-plate spline technique, as this is known to be misleading in that the deformation patterns it produces are homogeneous with respect to the landmark configuration, leading to potentially false morphological variation in areas of no data, something which is largely overlooked.
Acanthostega model reconstruction, half-fish half-muppet; Copyright - Eliot Goldfinger
The advantages of the techniques explored here are in the handling style of the models, and their statistical power and accuracy. Furthermore, anyone can conduct or replicate these methods, providing they have access to an initial CT scan. The potential applications are numerous too: digital models of reconstructed elements can give more accurate parameters for biomechanics where data may have been previously extrapolated in a subjective or qualitative manner; it may yield hitherto unknown data for character construction, which may in turn increase the validity of phylogenetic analysis. The landmark mapping procedure may need refinement in terms of increasing the number of points, such as by using semi-landmarks, which will more accurately reconstruct the surface geometry and open the way for other statistical procedures.
The study represents a great step forward though in accurate specimen reconstruction, and reveals another field in which the power of geometric morphometric techniques is unparalleled. A limitation could be that to reconstruct missing parts, you have to have an idea of what the gross geometry is, meaning at least one half of a bilaterally symmetrical element must be present. This means that if you wanted to reconstruct the neural spine for example, it would be impossible if the whole part was absent, even if the entire centrum was preserved. This is something that could be integrated in future using close relatives of the species that are being reconstructed.
Not surprisingly, the latest Archaeopteryx study has kicked up quite a stir within the media and scientific realms, considering the iconic status it has attained since discovery some 150 years past. This latest paper by Carney et al. and published in the journal Nature Communications claims to have resolved the plumage colour of a feather possibly from Archaeopteryx, a pretty neat little addition to the reconstruction of this critical species. They use methods employed during previous studies, namely the morphological or structural analysis of melanin-bearing organelles within feathers called melanosomes to infer that Archaeopteryx possessed an entirely black plumage.
The notorious black feather (Image Copyright: WitmerLab at Ohio University)
There is no doubt that the structures are in fact melanosomes, and no doubt that melanosomes contribute to plumage colour. But the question is, how much do they contribute to the plumage colour..? Well, I don’t know. In fact, no-one knows, at least in extinct avian theropods. The authors don’t discuss this either. It has however been discussed elsewhere in similar studies, albeit only qualitatively and in passing.
One comment is from Zhang et al. 2010: “Melanosomes are lyosome-related organelles of pigment cells in which melanins are stored and are responsible in part for the colours exhibited by modern birds.”
And Li et al. 2010: “Other molecular pigments such as carotenoids and porphyrins also produce plumage colours but are not preserved morphologically, thus we cannot address their possible effects here.”
That seems like a pretty big caveat. It’s like saying if you mix green, yellow and red paint in unknown quantities, you get red every time. Or something similar, I suck at analogies. The point is, if in modern birds, there are other significant structures that dictate or contribute towards plumage colour, does it make sense to try and predict colour when these are absent?
UPDATE: Ryan has been kind enough to clarify this point in a comment below.
Furthermore, the 95% confidence intervals the authors use are practically useless. Look at the ordination provided in Figure 4 (not sure if I can copy it here, so won’t..). These 95% confidence ellipses mean nothing in the slightest, or at least nothing meaningful. What they should have shown is an envelope includes 95% of all points within a sample, so that when you insert data ‘blind’, if it falls within a completely discriminated envelope, you can be 95% certain that it belongs to that group (i.e., 95 times out of 100, a blind data point will be correct). The envelopes shown in figure 4 clearly do not show this (if you don’t have access to the paper, ask me for a copy, or take my word for it). As a result, the points calculated for the particular Archaeopteryx feather analysed could really be grey or black, or maybe brown at a push (the number of colour choices is simply overwhelming..).
So was Archaeopteryx lithographica black?
Probably, probably not.
Carney et al. (2012) New evidence on the colour and nature of the isolated Archaeopteryx feather, Nature, DOI: 10.1038/ncomms1642
Li et al. (2010) Plumage patterns of an extinct dinosaur, Science, 327, 1369-1372
Negro et al. (2009) Porphyrins and pheomelanins contribute to the reddish juvenal plumage of black-shouldered kites, Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 153(3), 296-299
Zhang et al. (2010) Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds, Nature, 463, 1075-1078
Green Tea and Velociraptors 