Of dragons, marsupial lions and the sixth digits of elephants: functional anatomy part II

More recollections from the CEE Functional Anatomy meeting: part I is here. We looked in the previous article at Robin Crompton's overview of primate locomotor ecology and evolution, Renate Weller's overview of new technologies, John Hutchinson's work on dinosaur biomechanics, and Jenny Clack's new look at Ichthyostega [adjacent image is Jaime Chirinos's Thylacoleo restoration]. Still loads more to get through...

Paul O'Higgins took us back to primates. His key message was that two distinct computational techniques used to study anatomy - geometric morphometrics and finite element analysis (FEA) - can now be combined, spurring the anatomical revolution. By using geometric morphometrics, we can model such things as the ontogeny of change and make extrapolations on incomplete fossils, and FEA can simultaneously be applied to such models to test their function under loading. As an example, a macaque skull was morphed to resemble that of a fossil species known from Sardinia, and was then subjected to FEA. Its large, flaring zygomatic processes (different from those of extant species) appeared to reduce stresses acting through the skull when the model was subjected to different bite forces. This made me wonder if anyone has applied FEA to models of fossil Theropithecus species (giant geladas), some of which had massive flaring zygomatic processes and are thought to have been dedicated graminivores (come to think of it, what about Estemmenosuchus and anything else with flaring jugal flanges, bosses and hornlets). FEA can also be applied to long-standing questions like the evolution of brow ridges. Do these structures have a mechanical function: are they load-bearing structures? Some data indicates that brow ridges are subjected to low strains across ontogeny, a discovery which would suggest that they were not important in mechanical loading. If we can now warp skulls and other structures to get them to match different ontogenetic stages, while all the while testing - via FEA - the stresses and strains that occur across these models then, well, the sky's the limit.

The second plenary lecture of the meeting was given by none other than R. McNeill Alexander, the great pioneer of modern biomechanics [shown here; photo courtesy of John Hutchinson]. Robert focused on optimisation theory: if we work on the assumption that evolution directs anatomy towards a given optimum that maximises fitness, organisms should match a theoretical optimum form. Among the several examples that Robert discussed were turtle shell shapes: in order to right themselves after falling over, turtles should have evolved to resemble weighted ping-pong balls. And indeed some do, but only a very few (Domokos & Várkonyi 2008). However, if organisms do not match 'good' theoretical designs, have we fundamentally misunderstood the issues at hand? Phylogenetic constraints may mean that theoretical optima are unobtainable in some groups: we're surrounded by bad design, where organisms have evolved best-fit solutions to problems. However, studies of long bones, mammal canines, the guts of mammalian herbivores, and the snouts of pipefishes show that organisms do generally approach or match theoretical maxima.

Colin McHenry provided an outstanding overview of his journey through the world of biomechanics, starting with his early work on the pliosaur Kronosaurus [adjacent image - courtesy Colin and used with permission - shows Pete Edwards posing with spectacular assortment of extant croc skulls. Can you guess the identity of the biggest skull?]. By comparing data on Kronosaurus with that collected by Busbey (1995) on extant and fossil crocodilians, Colin found that Kronosaurus falls into the same part of morphospace as that occupied by the metriorhynchids (the Mesozoic marine crocodilians covered on Tet Zoo ver 1 here, and since briefly discussed here). In order to better understand crocodilians, Colin then worked on applying FEA to extant crocodilians, and - with Steve Wroe and others - he later applied these techniques to the skulls of big cats, dogs, thylacines, and the marsupial lion Thylacoleo. This work has shown that thylacines and thylacoleonids out-performed their placental equivalents in bite strength (Wroe et al. 2005, 2007a). Smilodon functions poorly as a powerful biter, building up dangerously high stresses in many regions of the skull when subjected to the same bite forces as those experienced by a lion (McHenry et al. 2007). Presumably, its specialised upper canines required less bite force than that exerted by shorter-toothed felids and its bite was 'neck driven' rather than 'skull driven'. This agrees closely with work by others (and based on other lines of evidence) showing that big sabretooth cats used phenomenal forelimb and pectoral musculature, and a flexible and strong neck, to pin ungulate prey down on the ground, keep it still, and then administer a 'conventional' felid throat bite (Antón & Galobart 1999). Steve White will be pleased to hear that his Smilodon vs Bison painting was used in this part of the talk.

New work on Komodo dragons Varanus komodoensis has shown that dragons have (to quote Steve Wroe) a piss weak bite, but that they combine their multiple serrated teeth, cranial kinesis and full body weight to pull backwards on flesh while biting, thereby sawing it to bits. Awesome visual proof for this came from a video Colin showed where a dragon walked in to shot and proceeded to destroy a big chunk of meat in just a few seconds. Together with Steve Wroe, Philip Clausen and Karen Moreno, Colin is also working on FE models of phorusrhacids, hominids and various sharks (the varanid stuff is in in press for Journal of Anatomy; the hominid model has recently been published: Wroe et al. 2007b). You can see that, basically, they pick the coolest and most awesome animals to work with.

You can't have functional anatomy without elephants, and two presentations looked at these animals in detail. As both speakers (Charlotte Miller and Victoria Herridge) said, as the largest living land animals it's natural that elephants have a special significance within the field. Charlotte Miller's talk on elephant foot mechanics was full of stuff that was new to me, but then I think that much of it might have been new to everyone outside the Royal Veterinary College team. The sheer size and compact foot anatomy of elephant limb portions makes them difficult to cut up and analyse in the conventional manner (as partially demonstrated by a series of comedy photos that Charlotte put up during her talk), but new imaging technology has come to the rescue. The massive fatty pads that back elephant hands and feet mean that the manual and pedal skeletons appear strangely tilted up when shown in their in-life postures. How the hands and feet react to vertical loading has been studied by using both live running elephants (dotted with spaced markers) and detached feet. If you didn't know that elephants can run, the news is that they probably can: see Hutchinson et al. (2003). Counter-intuitively perhaps, the digits of the manus and pes become more gracile during ontogeny, although the metacarpals and metatarsals do become more robust [image above, from RVC site, shows running elephant].

CT imaging shows that, in the manus, bending occurs at the wrist and between the metacarpals and phalanges. In the pes, there is movement at the ankle, but no phalangeal movement at all and the foot effectively functions as a single unit. Evidently little known is that elephants possess cartilaginous 'pre-digits' (the prepollex and prehallux) that project downwards through the fat pads and are hypothesised to help provide ventral support for the hands and feet (Weissengruber et al. 2006). Charlotte's new data shows that the prepollex and prehallux behave differently during loading, with the prepollex moving little while the prehallux flexes dorsally relative to the fat pad. Very neat stuff.

Victoria Herridge has been studying dwarf Mediterranean elephants and what they can tell us about limb bone allometry [adjacent image - provided by Victoria, used with permission - shows composite 'Elephas' falconeri skeleton from Palermo, Sicily]. The dwarfs aren't more gracile than large elephants (like Palaeoloxodon antiquus, the probable ancestor of the Mediterranean dwarfs), but are they more robust? They are (increasingly so the smaller they become), with ontogenetic data indicating negative allometry during growth. One problem afflicting work on bone scaling in elephants is that the data is strongly influenced by the age profile of the sample, given that male elephants can still be growing in their fourth and fifth decades and seem not to ever reach a growth asymptote (females do, with their growth trailing off in their 30s). What are the advantages of being so robust when you are a dwarf? Is this because of the gait used by these elephants, or for some other reason? Work is ongoing. At the end of her talk, Victoria referred to the 'elephant in the room'. I suppose this was a reference to the Homo floresiensis controversy: for all the papers that have been written on this animal (I see that a new one has just appeared in PNAS), relatively little is known about dwarfism and the changes associated with it, so the more data we have on dwarf elephants and other mammals the better. A manuscript on this very subject is currently in review actually: more when it gets published.

And that's still not all. Part III tomorrow...

Refs - -

Antón, M. & Galobart, À. 1999. Neck function and predatory behavior in the scimitar toothed cat Homotherium latidens (Owen). Journal of Vertebrate Paleontology 19, 771-784.

Busbey, A. B. 1995. The structural consequences of skull flattening in crocodilians. In Thomason, J. J. (ed) Functional Morphology in Vertebrate Paleontology. Cambridge University Press (Cambridge), pp. 173-192.

Domokos, G. & Várkonyi, P. L. 2008. Geometry and self-righting in turtles. Proceedings of the Royal Society B 275, 11-17.

Hutchinson, J. R., Famini, D., Lair, R. & Kram, R. 2003. Are fast-moving elephants really running? Nature 422, 493-494.

McHenry, C., Wroe, S., Clausen, P., Moreno, K. & Cunningham, E. 2007. Super-modeled sabercat, predatory behaviour in Smilodon fatalis revealed by high-resolution 3-D computer simulation. Proceedings of the National Academy of Sciences 104, 16010-16015.

Weissengruber, G. E., Egger, G. F., Hutchinson, J. R., Gorenewald, H. B., Famini, D. & Forstenpointner, G. 2006. The structure of the cushions in the feet of African elephants (Loxodonta africana). Journal of Anatomy 209, 781-792.

Wroe, S., Clausen, P., McHenry, C., Moreno, K. & Cunningham, E. 2007a. Computer simulation of feeding behaviour in the thylacine and dingo as a novel test for convergence and niche overlap. Proceedings of the Royal Society of London B 274, 2819-2828.

- ., McHenry, C. & Thomason, J. 2005. Bite club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa. Proceedings of the Royal Society of London B 272, 619-625.

- ., Moreno, K., Clausen, P., McHenry, C. & Curnoe, D. 2007b. High-resolution computer simulation of hominid cranial mechanics. The Anatomical Record 290, 1248-1255.