Saturday, April 15, 2006

The hands of sauropods: horseshoes, spiky columns, stumps and banana shapes

I’ve stated before on this blog that I do quite a bit of consultancy work for companies that produce prehistoric animal books for children. In advising and assisting artists as often as I do, I find that they consistently screw up on the same things, every time. One of the biggest problem areas seems to be the hands and feet of sauropod dinosaurs – I reckon that every single artist whose work I’ve had to check has screwed up on these. By the way, the artists I’m talking about here lack palaeontological expertise or training: I’m not talking about your Luis Reys, Todd Marshalls and Mark Halletts, but rather about wildlife artists who find themselves being asked to illustrate dinosaurs.

I also want to note that in no way is it the ‘fault’ of the artists concerned, given that (1) they’ve mostly based what they’ve done on the published work of those who have gone before them, and (2) while many of them have a history of working with palaeontologists, none of the experts they’ve been advised by before have bothered to tell them what they’ve been getting wrong. In fact I note that book-writing palaeontologists in general (you know who you are) rarely seem to bother providing their artists with any information, nor correct their mistakes, hence the incredible number of god-awful restorations that clutter the literature. Incidentally, this situation is getting worse as companies increasingly use CG images produced by people who seem to know nothing about animals, let alone fossil ones. But enough of that.

To save myself work in the future it occurred to me that it might be a good idea to post on sauropod hands and feet: that way, I can just direct interested parties to this web page in future, and save myself the usual to-ing and fro-ing of notes, scans, and scribbled on diagrams. Several people have already produced overviews of sauropod hand and foot morphology, but unfortunately they’re somewhat obscure and inaccessible to many. Greg Paul (1987) discussed in detail what sauropod hands and feet probably looked like, based on trackways and morphology, and provided a summarized version of the same information in his Japanese book The Complete Illustrated Guide to Dinosaur Skeletons (Paul 1996). Tracy Ford (1999) also published a guide to restoring sauropod hands and feet. Numerous technical studies describe or review sauropod hands and feet, with the most useful works being Christiansen (1997), Bonnan (2001, 2003, 2005) and Apesteguía (2005).

The basics

Let’s note to begin with that sauropod hands and feet were only very superficially like those of elephants, and in fact in several details were fundamentally different. Sauropod hands, in particular, are unique in that the metacarpals did not spread out from the wrist as they usually do in tetrapods, but were arranged in a vertical column. My biggest peeve concerning restorations of sauropod hands and feet is that people seem unable to resist the temptation to illustrate multiple claws sticking out all over the place, or to depict hooves on all of the digits. As we’ll see, the evidence is against both details. I know that many skeletal mounts give the animals multiple claws on the hands and/or feet, but that’s because the mounts are inaccurate, outdated composites.

I should note here that the following discussion applies only to members of Eusauropoda, given that it now seems that basal sauropods outside of Eusauropoda lacked the distinctive hands and feet of the better known, fully graviportal sauropods.

The metacarpal colonnade

As noted, the eusauropod hand is formed from a columnar arcade of five vertically arranged metacarpals. If you were to look at the animal’s hand from underneath (the plantar surface), the distal metacarpal tips would be seen to be arranged in a semi-circle, and the posterior surface of the hand would be concave. The hand was not backed by a pad (as McGowan (1991) wrongly stated), as it is in elephants. There is no doubt that this configuration was the case in life, as it’s verified by numerous horseshoe-shaped hand tracks. In a new, as-yet-unpublished sauropod from the Lower Cretaceous of the Isle of Wight, it’s been claimed that the first and fifth metacarpals virtually touch on the posterior surface of the hand, but this is unique so far as we know (this animal wouldn’t have left horseshoe-shaped tracks, but subcircular ones… if the proposed interpretation is valid, and it might not be). An even more peculiar claim – in fact it’s downright ridiculous – is that sauropods walked with their fingers curled under the distal ends of the metacarpals (Beaumont & Demathieu 1980). This is totally at odds with the morphological and trackway evidence and can be disregarded.

Not only did the metacarpals form this unique tubular structure, they and their digits seem to have been bound together to form a sort of pseudo-hoof: the digits didn’t splay outwards from the metacarpus, but were bound together with them, nor did digits II to V possess hooves, claws or nails. This is supported by both anatomy and trackways, so distinct digits were almost certainly not visible in life.

Thumb claws, or lack of them

Claws were absent from all digits except for digit I (the thumb). This thumb claw varied in size, shape and orientation among the sauropod groups: it was particularly big in diplodocoids, where it was also laterally compressed and notably deep, and clearly separated from the rest of the metacarpus. In contrast, in brachiosaurids it was small, subtriangular in cross-section, and not separated from the rest of the metacarpus. In forms with large pollex claws (like diplodocoids), the anatomy of the penultimate phalanx and the distal end of the first metacarpal indicates that some flexion and extension of the claw was possible: in other words, these sauropods could both lift and lower their claws (albeit not by much compared to what was possible in other dinosaurs).

What did sauropods do with this lone thumb claw? Pretty much every conceivable function has been proposed, including fighting, digging, ripping foliage or tree trunk gripping. The claw’s anatomy and range of motion led Upchurch (1994) to conclude that a trunk-gripping function was most likely, but if this is right then it seems odd that titanosaurs – the sauropods perhaps best suited for bipedality and rearing – were the ones that lost the claws.

That’s right: even the thumb claw was not present in all eusauropods. During the evolution of titanosauriform macronarians (the group that includes brachiosaurids and titanosaurs) the thumb claw was reduced in size and eventually lost altogether. We know that brachiosaurids such as Brachiosaurus possessed a short, small thumb claw, but Lower Cretaceous trackways apparently produced by other brachiosaurids indicate that the thumb claw was absent in these forms (it’s been suggested that the thumb claw was still present in these forms, but that it was so small that it failed to leave an impression. I find this less likely than the idea that the claw really was absent). With the exception of the controversial Jurassic form Janenschia (and probably a few basal taxa from the Cretaceous), titanosaurs were all devoid of thumb claws.

Pads and spiky tubercles

But there’s more - derived titanosaurs lacked not just claws, but finger bones too, and thus fingers. Their column-shaped hands were bizarre fingerless stumps, and they walked on the distal ends of the metacarpals, which is pretty odd to say the least. As described by Apesteguía (2005), the distal ends of the metacarpals in fingerless titanosaurs were wider and more rectangular than those of other sauropods, and with unusual sculpturing. The latter feature suggests that some kind of cushioning tissue encased the metatarsal ends, and a few titanosaurs preserve what appears to be some kind of soft tissue in this area.

Evidence for unusual soft-tissue structures on the sauropod metacarpus might have been present in other sauropods, according to recently described trackway evidence. A manus impression from the Upper Jurassic of Lourinhã in Portugal – probably produced by a brachiosaurid – preserves vertical score marks on its sides that seem to have been produced by rough tubercles on the metacarpal surface (Milàn et al. 2005). These authors proposed that at least some sauropods had spiky skin covering the distal end of the metacarpus, though how widespread this was among sauropods we don’t know.

Banana-shaped first metacarpals: why?

While all five metacarpals in most eusauropods were more or less straight and parallel, this was not true of some titanosaurs. In these forms the first metacarpal was curved outwards at its distal end, and thus roughly banana-shaped. This is first seen in Janenschia, in which a thumb claw was present, but it’s also the case in various other taxa, including Andesaurus and Argyrosaurus. Apesteguía (2005) made the intriguing suggestion that the bowed first metacarpal may first have evolved in claw-bearing basal titanosaurs in order to help support the claw, that it was then later retained when the claw was lost, and that it was later reversed (back to the straight condition) in derived lithostrotian titanosaurs.

This raises the question as to why a bowed first metacarpal was needed ‘to help support the claw’ however, given that other sauropods with thumb claws had straight, rather than bowed, first metacarpals. In Janenschia, a raised lip around the outer surface of the first metacarpal’s distal end is very nearly in contact with the dorsoproximal part of the claw. This creates the impression that the metacarpal’s distal end had evolved to help conduct stress along the curved ungual’s dorsal margin. If that’s true (let me emphasize that this is just an idea), it could mean that the thumb claw was used in a manner quite different from that of other sauropods. Maybe the claw tip was actually used for piercing something: presumably a substrate, or bark. Any better ideas?

I had planned to cover sauropod feet as well, but that’ll have to wait for a future post. The image used above was borrowed from…

http://www.cc.whecn.edu/tate/VIRTTOUR/BRONTOSAUR_LEG.HTML

Refs - -

Apesteguía, S. 2005. Evolution of the titanosaur metacarpus. In Tidwell, V. & Carpenter, K. (eds) Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press (Bloomington & Indianapolis), pp. 321-345.

Beaumont, G. & Demathieu, G. 1980. Remarques sur les extremités antérieures des sauropodes (reptiles, saurischiens). Compte Rendu des Séances de la Société de Physique et d’Histoire Naturelle de Genève 15, 191-198.

Bonnan, M. F. 2001. The evolution and functional morphology of sauropod dinosaur locomotion. Chapter 3: osteology of the forelimb. Unpublished phd thesis, Northern Illinois University.

- . 2003. The evolution of manus shape in sauropod dinosaurs: implications for functional morphology, forelimb orientation, and phylogeny. Journal of Vertebrate Paleontology 23, 595-613.

- . 2005. Pes anatomy in sauropod dinosaurs: implications for functional morphology, evolution, and phylogeny. In Tidwell, V. & Carpenter, K. (eds) Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press (Bloomington & Indianapolis), pp. 346-380.

Christiansen, P. 1997. Locomotion in sauropod dinosaurs. Gaia 14, 45-75.

Ford, T. L. 1999. How To Draw Dinosaurs, book 1. T. L. Ford (privately published).

McGowan, C. 1991. Dinosaurs, Spitfires, & Sea Dragons. Harvard University Press (Cambridge, Mass. & London).

Milàn, J., Christiansen, P. & Mateus, O. 2005. A three-dimensionally preserved sauropod manus impression from the Upper Jurassic of Portugal: implications for sauropod manus shape and locomotor mechanics. Kaupia 14, 47-52.

Paul, G. S. 1987. The science and art of restoring the life appearance of dinosaurs and their relatives - a rigorous how-to guide. In Czerkas, S. J. & Olson, E. C. (eds) Dinosaurs Past and Present Vol. II. Natural History Museum of Los Angeles County/University of Washington Press (Seattle and London), pp. 4-49.

- . 1996. The Complete Illustrated Guide to Dinosaur Skeletons. Gakken.

Upchurch, P. 1994. Manus claw function in sauropod dinosaurs. Gaia 10, 161-171.

3 Comments:

Blogger Darren Naish said...

The following posted on behalf of Kent Stevens. Thanks indeed for the comment Kent...

---------------

I fell into your delightful darrennaish.blogspot.com and was reading along, and the buttressed first metatarsal in the following passage caught my eye: "This raises the question as to why a bowed first metacarpal was needed 'to help support the claw' however, given that other sauropods with thumb claws had straight, rather than bowed, first
metacarpals. In Janenschia, a raised lip around the outer surface of the first metacarpal's distal end is very nearly in contact with the dorsoproximal part of the claw. This creates the impression that the metacarpal's distal end had evolved to help conduct stress along the curved ungual's dorsal margin. If that's true (let me emphasize that this
is just an idea), it could mean that the thumb claw was used in a manner quite different from that of other sauropods. Maybe the claw tip was actually used for piercing something: presumably a substrate, or bark. Any better ideas?"

If you think of directing the forelimb in a ground-scratching
movement, this osteological stop would prevent hyperdorsiflexion.
Maybe for digging holes for eggs, or making traction in locomotion
where digging in permits an even longer extension with thrust against the ground (foot doesn't slip backwards at end of support phase). Also, given orientation, it might be a stabilizing ground-anchor as well. It's notable that the distal tip of the ungual is so rounded, providing a blunt spike with easy extraction from the substrate. In any case, I'm thinking the substrate would be the ground.

Cheers,

Kent

2:37 PM  
Blogger emanuel said...

One little question about your statement that distinct digits were almost certainly not visible in life:
I agree with that, if you only look at the metacarpal colonnade, but taking the phalanges into account, I'm not that convinced... Is your statement just for the metacarpals or do you think that also the phalanges were not visible?

Greetz

Eme

3:30 PM  
Blogger Darren Naish said...

Thanks for your comment Eme. Trackways and articulated hands show us that the phalanges were bound into the same tubular pseudo-hoof structure as the metacarpus: they did not protrude from the metacarpus as distinct stubby little fingers. At most, they may have formed blunt nubbins in non-titanosaurs. I hope this helps - thanks for reading.

8:09 PM  

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