Sauropod Vertebra Picture of the Week

Murphy and Mitchell (1974: fig. 1)

One thing that I’ve never understood is why some people are skeptical about sauropods using their tails defensively, when lizards do this all the time. I’ve been digging through the literature on this for a current project, and there are some really great accounts out there, and by ‘great’ I mean ‘scary’.

Here’s a key passage from Murphy and Mitchell (1974: p. 95):

V. salvator uses the tail to strike repeatedly in combination with biting for defense…Captive Varanus (varius, spenceri, mertensi, and salvadorii) use the tail for defense, but only salvadorii appears to aim directly for a handler’s eye. An adult male V. salvadorii accurately struck the senior author’s eye with the tip of the tail as he was attempting to maneuver the lizard. On many subsequent occasions, the monitor tried to strike the eye of the handler with accuracy.

Not being a monitor expert, I was initially thrown by the V. salvator/V. salvadorii issue. V. salvator is the water monitor, V. salvadorii is the crocodile monitor. Both get pretty darned big; Wikipedia lists 3.21 m (10.5 ft) for V. salvator and 2.44-3.23 m (8.0-10.6 ft) for V. salvadorii.

Anyway, I’d heard of lots of anecdotal reports of lizards from many clades using their tails to lash at rivals, predators, or handlers, but I’d never read about a lizard aiming directly for the target’s eyes. It immediately made me think about (1) sauropod tails, especially the whip-lash tails of flagellicaudan diplodocoids and at least some titanosaurs (Wilson et al. 1999), and (2) the supraorbital crests and ridges in many theropods, especially big Morrison forms like Allosaurus and Ceratosaurus. Of course, supraorbital crests in theropods could serve many functions, including shading the eyes and social and sexual display, but it’s interesting to speculate that they might have had a defensive function as well. Has anyone ever proposed that in print?

Diplodocus longus USNM 10865, from Gilmore (1932: plate 6)

Most of the papers that pooh-pooh the use of whiplash tails in defense (e.g., Myhrvold and Currie 1997) argue that the tail-tip would be too small to do any serious damage to a multi-ton attacker, and too fragile to survive an impact. This seems wrong-headed to me, like arguing that unless you find putative animal weapons broken and caked in their adversaries’ blood, they aren’t used as weapons. A structure doesn’t have to do lethal damage or any damage at all to serve as a weapon, as long as it dissuades a predator from attacking. I’d think that getting hit in the eye by a 35-foot bullwhip might convince an allosaur to go have a look at Camptosaurus instead.

Now, one could argue that if the whip-lash doesn’t do any serious damage, predators will learn to blow them off as dishonest signals (we’re assuming here that having your eye possibly knocked out doesn’t count as ‘serious damage’ to an allosaur). But it’s not like the whiplash was the only weapon a diplodocid could bring to bear: the proximal tail could probably deliver a respectable clobberin’, and then there’s the zero fun of being stomped on by an adversary massing a dozen tons or more. In that sense, the whip-lash is writing checks the rest of the body can certainly cash. It’s saying, “Getting hit with this will be no fun, and if that isn’t enough, there’s plenty more coming.”

All of this is leaving aside more obvious defensive adaptations of the tail in Shunosaurus, maybe Omeisaurus and Mamenchisaurus, and probably Spinophorosaurus (although I’d feel better about Spinophorosaurus if the association of the spikes and the tail was more secure). I suspect that all sauropod tails were useful in defense, but only some sauropod taxa used that behavior enough for a morphological enhancement (club, spikes, whiplash) to have evolved. Similarly, common snapping turtles, Chelydra serpentina, will wiggle their unspecialized tongues to attract fish (I’ve witnessed this myself in captive specimens) but lack the worm-shaped tongue lure found in the more ambush-specialized alligator snappers, Macrochelys temminckii. On reflection, there are probably few morphological changes in evolution that aren’t preceded by behavior. Not in a Lamarckian sense, just that certain variations aren’t useful unless the organism is already (suboptimally) performing the relevant function.

Bonus observation: Mike noted back when that Shunosaurus and Varanus retain complex caudal vertebrae all the way out to the end. Since in this case ‘complex’ means ‘having processes that muscles can attach to’, maybe that has something to do with keeping up relatively fine motor control in your bad-guy-whomping organ. Would be interesting to compare caudal morphology between tail-whomping lizards and committed caudal pacifists (assuming we can find any of the latter that we’re certain about – maybe tail-whomping just doesn’t get used very often in some taxa, like those that have caudal autotomy). Anyone know anything about that?

References

• Murphy, J. B., & Mitchell, L. A. (1974). Ritualized combat behavior of the pygmy mulga monitor lizard, Varanus gilleni (Sauria: Varanidae). Herpetologica, 90-97.
• Myhrvold, N. P., & Currie, P. J. (1997). Supersonic sauropods? Tail dynamics in the diplodocids. Paleobiology, 23(4), 393-409.
• Wilson, J. A., Martinez, R. N., & Alcober, O. (1999). Distal tail segment of a titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of Mendoza, Argentina. Journal of Vertebrate Paleontology, 19(3), 591-594.