Re: CURSORIAL STEGOSAURS?

["Matthew Bonnan" <mbonnan@hotmail.com>]

Dinogeorge writes:
"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)."

Loss of digits is not necessarily correlated with use/disuse.  Current 
embryological studies of digit formation indicate that "vestigal" digits can 
form just because they are part of the inherited genetic program.

"But I was talking about digits, not metatarsals."

Digit is generally used to denote both the metapodials and the phalanges.  
For example: "... there are several digits, each beginning proximally with a 
metapodial element (metacarpals on the forelimb, metatarsals on the 
hindlimb) followed by a chain of phalanges.  The digits rest upon several 
separate bones, collectively known as the carpals in the wrist and the 
tarsals in the ankle ..." from Kardong (1995), Vertebrates: Comparative 
Anatomy, Function, Evolution.

As far as a bipedal ancestry for stegosaurs, likely so but we have no good 
fossil evidence for this.  All the quadrupedal dinosaurs appear to have 
evolved from a bipedal ancestor, including sauropods.  I know that George 
does not think so for sauropods (neither did Charig et al., 1965), but, as a 
single example which does no justice, the forelimbs of sauropods are a bit 
shorter than the hindlimbs.  If you look at quadrupedal mammals who have 
evolved from quadrupedal ancestors, the limb lengths are generally very 
similar, and the manus and pes shape is very similar.  In contrast, all 
quadrupedal dinosaurs have a manus that has a different shape than the pes.  
Bipedal ancestry, where the manus served a different function than the pes, 
would be a viable explanation/hypothesis for this discrepency.  This, and 
many other pieces of evidence I have seen in sauropods, suggest to me that 
sauropods, too, had a bipedal ancestor.

Matt Bonnan
Dept. Biological Sciences
Western Illinois University
Macomb, IL 61455*
*as of August 14, 2001.

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