Re: CURSORIAL STEGOSAURS?

[Dinogeorge@aol.com]

In a message dated 7/30/01 8:13:00 AM EST, darren.naish@port.ac.uk writes:

<< One _Stegosaurus_ specimen retains a vestige of a fourth metatarsal.  
George has argued that reappearance of a lost digit is effectively impossible 
so..  does this indicate that metatarsal I was retained within Stegosauria 
right up to the ancestor of _Stegosaurus_? I suppose the alternative is that 
this individual was teratological. >>

The robust metatarsals of stegosaurs are II, III, and IV, so you must be 
talking about the fifth rather than the fourth (cf. also Galton's article on 
stegosaurs in The Dinosauria). Metatarsal V was likely present in all known 
stegosaurs as the usual dinosaurian splint or vestige, in which form it was 
undoubtedly a functional part of the tarsal/metatarsal assembly, likely a 
site of muscle or tendon attachment and leverage (otherwise it would 
eventually have vanished, as it apparently did in ornithomimids). This small, 
loosely articulated bone is easily lost before fossilization, which is why we 
see it only rarely, in the best-preserved dinosaur feet.

But I was talking about digits, not metatarsals. In stegosaurs digit V is 
long gone (an ornithischian plesiomorphy, but a synapomorphy shared with 
certain prosauropods and all theropods). Digit I is also gone, along with its 
metatarsal, in all known stegosaurs (a stegosaur apomorphy shared, in 
Dinosauria, only among iguanodontians and hadrosaurs, in which group it 
clearly developed independently, perhaps more than once). Mounted stegosaur 
skeletons (esp. in China, e.g., Tuojiangosaurus) that show four digits in the 
foot are all incorrect, generally having taken digit I of the manus and 
placed it on the pes. The pattern of five manual digits and three pedal 
digits occurs in all known stegosaurs.

The more or less symmetric pattern of pedal digital loss (digits I and V) is 
generally considered an adaptation to cursoriality, and if you can think of a 
reason why a >graviportal< quadruped would lose pedal digits I and V in the 
hind foot but not in the forefoot, I'd like to hear it. The more digits the 
better, for distributing a heavy animal's weight on the ground, if it isn't a 
runner at least part of the time.

Loss of pedal digit I suggests that remote ancestral stegosaurs were small, 
swift bipeds (probably with elongated pedal phalanges) until they acquired 
some of their dermal armor, becoming simultaneously larger, less cursorial, 
and finally quadrupedal (with shortened manual and pedal phalanges). The 
forms of the stegosaur pelvic bones suggest affinities with marginocephalians 
rather than with ankylosaurs, but for the most part the shapes of the pelvic 
bones are unique to stegosaurs and are thus not much help in systematics. The 
fact that all known stegosaurs strongly resemble one another, but no other 
dinosaurs, in their bauplan suggests that they are members of a single, 
slowly evolving lineage that descended from a small, singular family of 
(bipedal) Triassic ornithischians, most of which had become extinct by the 
time the Jurassic began.

PS: Emausaurus is a juvenile huayangosaurid, not an ankylosaur or 
"thyreophoran," as I noted in my 1994 article with Tracy Ford on ankylosaurs 
back in Dino-Frontline.

PPS: The Isle of Wight Dinosaurs book is TERRIFIC. Highly recommended, must 
have it.