December 19, 2011 | 63
Sauropod dinosaurs are – in my somewhat biased opinion – among the most fascinating tetrapods that ever evolved. Exceeding all other terrestrial animals by an order of magnitude and famous for their extreme and often ridiculous necks, they were also remarkable in possessing an often elaborate degree of skeletal pneumatisation. Remember that sauropods were incredibly successful and were a persistent and obvious group of animals over a huge span of time. Sauropods pose a huge number of questions concerning behaviour, physiology, soft-tissue anatomy, reproduction, ecology and so on. Naturally, many of the questions we’d like to have answered just can’t be, given the limitations of the fossil record.
But in recent years a large number of scientists from diverse backgrounds have worked together in an concerted effort to better understand sauropod biology and determine the causes of their gigantism.
This research consortium, titled Research Unit 533 ‘Biology of the Sauropod Dinosaurs: the Evolution of Gigantism’ and funded by the German Research Foundation, is truly multidisciplinary, involving botanists, physiologists, biomechanists and other experts on living organisms – not just palaeontologists. A large number of technical papers have been produced by this research group, as has a very handsome multi-authored volume (Klein et al. 2011) [cover shown here]. Note that the group’s most influential publication – the major review of sauropod biology and evolution that is Sander et al. (2011) – is open-access and available free to all.
Palaeontologists near-universally agree on several aspects of sauropod biology. Sauropods were herbivores; they didn’t masticate their food or use gastroliths but almost certainly relied on rapid cropping and swallowing and hindgut fermentation; they possessed heterogenous, ‘bird-like’ lungs and a bird-like pneumatic system; and they laid clutches of hard-shelled eggs. There are some other ideas that are not exactly universally agreed on, but look increasingly well supported. One is that sauropods grew quickly (on par with precocial birds) and reached sexual maturity in their second decade. In modern animals, high growth rates like those seen in sauropods are only present in endothermic mammals and birds, and recent reviews of sauropod biology argue that – like mammals and birds – sauropods were most likely tachymetabolic endotherms (Sander & Clauss 2008, Sander et al. 2011). Another mostly accepted idea is that sauropods did not practise post-hatching parental care and that juvenile mortality was high.
One aim of the sauropod research group has been to determine how sauropods were able to grow so large. Clearly, no single factor led to gigantism in sauropods: numerous features were added piecemeal to produce the ultimate recipe for gigantism. These include a long neck, lack of reliance on mastication, avian-style lung, and production of numerous small young (Rauhut et al. 2011, Sander et al. 2011). The presence of the dinosaur bauplan contributed in some way to sauropod gigantism, since traits common to all dinosaurs (like parasagittal gaits and high growth rates) obviously allowed members of all dinosaurian lineages to evolve large body size with comparative ease.
With both our evolving understanding of sauropods, and the significant progress of the sauropod research group as required background knowledge, we now turn to the fact that the group recently hosted the second of its international workshops. Held at the University of Bonn, Germany, the meeting featured over 40 presentations on sauropods, given by researchers from all over the world. I wasn’t able to make the previous meeting (held in 2008), but am very pleased to report that I attended, and spoke at, this second one.
So – - three entire days devoted specifically to sauropod biology, physiology, reproduction, biomechanics and so on. As I said above, we can’t presently answer questions about such things as sauropod swallowing mechanics, pigmentation patterns, vocal abilities or mating postures, but – by the end of the meeting – we had certainly discussed, speculated about, and commented on, the majority of such issues.
Because the meeting was billed as a workshop – not just as a conference or seminar – discussion sessions followed each and every talk. This worked really well and it was typically all too easy to use up the 20 or so minutes of discussion allotted to each talk. In the text that follows, I haven’t discussed or even mentioned all the talks, but (as per usual) have mostly covered the ones that I found the most interesting, most enjoyable, or most memorable.
During the very first talk of the meeting, Robert Reisz discussed the large amount of new information he and his colleagues now have on the ontogeny and nesting behaviour of Massospondylus (note: not a sauropod, but a close relative of Sauropoda within the more inclusive clade Sauropodomorpha. A ‘prosauropod’). Much of this is unpublished so I won’t share it, but we’ve known for a while that juvenile Massospondylus are toothless, quadrupedal little animals, very different from the toothed, bipedal adults (Reisz et al. 2005). Does the association of those tiny babies with adults imply parental care? And what were the babies eating – were they provisioned by their parents or were they just eating something completely different?
Even at this very early stage of the conference, comments and questions were arising about neck posture, in particular because Robert said that Massospondylus most likely had a horizontal neck posture (a conclusion he reached following direct articulation of the fossil neck vertebrae). However, when you plug cervical vertebrae together in living animals, you never get the normal alert posture. I seem to remember a 2009 paper that took the trouble to point this out… we’ll be coming back to this issue later.
There followed a number of talks on sauropod growth rates, thermophysiology, digestion and feeding strategies. Chris Carbone spoke about his work on the ecology of Tyrannosaurus: a peculiar topic for a sauropod conference perhaps, but one with implications for several Mesozoic environments and foodwebs (Chris comes from a background in modelling energetic constraints in terrestrial predators and other animals: see Carbone et al. (1999)). You might think that the whole silly idea of the ‘scavenger rex’ is either dead or a non-issue, but it seems not to be so: one member of the audience implied strongly in the discussion section after the talk that T. rex’s morphology is inconsistent with hunting and only consistent with obligate scavenging. Err, what? The bottom line of Carbone et al. (2011) is that a hypothetical Maastrichtian habitat, realistically scattered with the carcasses of contemporaneous herbivores, would not provide enough available prey mass for a foraging T. rex, given physiological constraints. As I said to Chris after his talk, even this model is a best-case scenario, since there’s no one place in western North America where all the species listed in the analysis actually lived together. Sure, T. rex occurs from New Mexico all the way north to Saskatchewan, but Alamosaurus and various of the other dinosaurs included in the study don’t.
Two talks focused on the incredible Chinese mamenchisaurids, famous for their ridiculous necks (consisting of 16-17 vertebrae and, in cases, being four times as long as the body). In recent years a large number of new mamenchisaurid taxa have been named, and even the genus Mamenchisaurus itself now contains eight species. While these Mamenchisaurus species are superficially similar, it’s generally agreed that they likely aren’t close relatives. So it wasn’t really a surprise when a new cladistic analysis presented by Toru Sekiya scattered Mamenchisaurus to the four winds, though this was preliminary (one Mamenchisaurus species was recovered in a very counter-intuitive/startling position). It’s not as appreciated as it should be that some mamenchisaurids were truly enormous – there are mass estimates for some of the species that exceed 70 tons.
José Carbadillo discussed the growth changes that occur in the vertebrae of the dwarf European titanosauriform Europasaurus [adjacent photo by Ghedoghedo]. For those who don’t know, this was an island-endemic dwarf sauropod, at most 6 m long and less than 1.5 m tall at the shoulder. The Europasaurus situation has become more complicated now we know that some animals reached skeletal maturity at much smaller body size than others – this seems to show that there are actually two europasaur taxa in the assemblage, both of which are dwarfs. Also on axial morphology, Francisco Gascó discussed the vertebral anatomy of the Spanish sauropods Losillasaurus and Turiasaurus, both of which seem to be part of the recently recognised non-neosauropod clade Turiasauria. You probably know that Francisco (aka Paco) blogs at El Pakozoico.
Three talks used computer modelling techniques (primarily finite element techniques) to analyse sauropod (and Plateosaurus) skull or pelvic structure. Phil Manning discussed synchrotron-based imaging work done on the skin of the famous Auca Mahuevo titanosaur embryo. I’ll hold off on saying anything further about this work, since the results are (understandably) embargoed.
Day two included sessions on the constraints of gigantism, feeding and digestion, and on reproduction and life history. Andreas Christian discussed something dear to my heart – the allometry of intraspecific fighting behaviour in animals and what it might mean for sauropods. Small animals can roll around and literally throw each other into the air [fighting cats from here], but this doesn’t work for big ones. They have to rely on body-barging or pushing. If sauropods did fight, it’s likely that this is what they did. Andreas has been a strong proponent of elevated neck postures in sauropods (his papers include useful work on neck posture and neck anatomy in ostriches, camels and other living animals) and his talks always include numerous amusing cartoons about the Necks Wars (an area we’ll return to in part II).
Heinrich Mallison discussed his new ideas about limb kinematics in long-tailed dinosaurs. Heinrich argues that we need to think anew about the importance of femoral retraction and protraction in dinosaurs and what it might mean for locomotion. Basically, he argues for speed-walking/race-walking dinosaurs where it’s the heavily muscled thigh and tail that provides the main power during fast movement, with the ankle being less important than traditionally thought. It all sounds pretty compelling but we know that some biomechanists who specialise on dinosaurs see problems with the idea. You can read all the details on Heinrich’s blog (dinosaurpalaeo), starting here. Heinrich also discussed possible resting postures for sauropods.
Bill Sellers also looked at the locomotor capabilities of sauropods, discussing evolutionary robotics and the construction of computer-generated models derived from laser scanning of mounted skeletons. John Hutchinson reviewed recent work on biomechanics in elephants, dinosaurs and other animals to see how they cope with the challenges of gigantism. Various preconceptions and assumptions are false and unreliable (such as that limb joints are always ‘more columnar’ in big animals than smaller ones). New work on elephant foot dynamics and the stresses transmitted through limb bone shafts paint an increasingly complex picture as goes the biomechanics and abilities of giant quadrupeds [adjacent running elephant image from RVC page here; used with permission]. And John was also interested in testing that age-old question of how big a land animal can become. His answer? Hmm, vaguer than you might like (unless you’re a big fan of Godzilla).
A session on food uptake and digestion included several talks by botanists working on the diversity and growth physiology of Mesozoic plants. Controversies over the composition of the Mesozoic atmospheric mean that radically different models need to be explored (some workers argue for high oxygen levels, others for much lower ones). Jennifer McElwain spoke about her fascinating work on plant-atmosphere interactions. Specially constructed greenhouses allow plants to be grown in simulated ‘prehistoric’ atmospheres.
Carole Gee discussed the plant foods available to sauropods and the nutritional values of those plants (see Gee 2011). Among the many plants available to sauropods, Araucaria species are particularly interesting in that they release a large amount of energy when retained for a long period in the hindgut. They are also particularly good at regenerating broken branches and tree-tops.
Equisetum is also of special interest in the context of sauropod biology given that it was widespread across the Mesozoic world and is also energy-rich. Indeed, juvenile geese can fuel their growth demands on an Equisetum diet. I mentioned earlier that we think that sauropods grew at rates comparable to those of precocial birds. For some time now, those of us particularly interested in sauropod biology have regarded the apparently rapid growth of sauropods as a bit of an enigma. Surely, we mused, low-quality vegetation (ferns and the like) just can’t provide enough energy to fuel this sort of thing? So, we wondered, is it plausible that sauropods were somehow provisioned by their parents? Were babies provided with some sort of secretion produced by the mother, or did they feed from regurgitated, pre-digested plant slop or something? Such ideas are not ridiculous, since parental provisioning of babies is hardly unique to mammals: members of several bird groups produce ‘milk’ for their young, and there are frogs and caecilians that feed their young with special skin, eggs, or cloacal secretions. Yum. Regardless, it seems that these speculations (note: unpublished and technically off-the-record) are now unwarranted. Baby sauropods could apparently fuel their growth just fine on a diet of Mesozoic plants. And, yes, of course there is always the possibility that juveniles were omnivorous, snacking on insects and such on occasion.
On that note, time to call it quits. The remainder of my thoughts to appear in part II. You can read further thoughts on the meeting here at SV-POW! and note also that both Heinrich Mallison and John Hutchinson tweeted continually throughout the meeting under #SauroBonn. For previous Tet Zoo articles on sauropod biology and behaviour see…
Refs – -
Carbone, C., Mace, G. M., Roberts, S. C. & Macdonald, D. W. 1999. Energetic constraints on the diet of terrestrial carnivores. Nature 402, 286-288.
Carbone C, Turvey ST, & Bielby J (2011). Intra-guild competition and its implications for one of the biggest terrestrial predators, Tyrannosaurus rex. Proceedings. Biological sciences / The Royal Society, 278 (1718), 2682-90 PMID: 21270037
Gee, C. T. 2011. Dietary options for the sauropod dinosaurs from an integrated botanical and paleobotanical perspective. In Klein, N., Remes, K., Gee, C. T. & Sander, P. M. (eds). Biology of Sauropod Dinosaurs: Understanding the Life of Giants. Indiana University Press (Bloomington & Indianapolis), pp. 34-56.
Klein, N., Remes, K., Gee, C. T. & Sander, P. M. 2011. Biology of Sauropod Dinosaurs: Understanding the Life of Giants. Indiana University Press (Bloomington & Indianapolis).
Rauhut, O. W. M., Fechner, R., Remes, K. & Reis, K. 2011. How to get big in the Mesozoic: the evolution of the sauropodomorph body plan. In Klein, N., Remes, K., Gee, C. T. & Sander, P. M. (eds). Biology of Sauropod Dinosaurs: Understanding the Life of Giants. Indiana University Press (Bloomington & Indianapolis), pp. 119-149.
Reisz, R. R., Scott, D., Sues, H.-D., Evans, D. C. & Raath, M. A. 2005. Embryos of an Early Jurassic prosauropod dinosaur and their evolutionary significance. Science 309, 761-764.
Sander, P. M., Christian, A., Clauss, M., Fechner, R., Gee, C. T., Griebeler, E.-M., Gunga, H.-C., Hummel, J., Heinrich, M., Perry, S. F., Preuschoft, H., Rauhut, O. W. M., Remes, K., Tütken, T., Wings, O. & Witzel, U. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews of the Cambridge Philosophical Society 86, 117-155.
- . & Clauss, M. 2008. Sauropod gigantism. Science 322, 200-201.