SAUROPOD HEADS, RHABDODON

[darren.naish@port.ac.uk]

Just stumbled across something new, and pertinent to recent 
discussions...

CHRISTIANSEN, P. 1999. On the head size of sauropodomorph dinosaurs: 
implications for ecology and physiology. _Historical Biology_ 13: 
269-297.

Abstract: The heads of prosauropod and sauropod dinosaurs appear to 
be small, compared with those of extant endothermic mammals. This has 
been considered inconsistent with the hypothesis, supported by 
several other anatomical characteristics, that these animals had 
cellular metabolic rates significantly above the usual reptilian 
level. However, prosauropods and sauropods had only moderately 
developed cranial musculature, comparable with that of more typical 
reptiles. This musculature is plesiomorphic, in contrast with the 
speciliazed and powerful muslces of the mammalian cranium. In 
feeding, these dinosaurs used their heads exclusively in cropping 
vegetation, a modest amount of oral processing occurring only in 
certain sauropods. Thus, the heads of prosauropods and sauropods did 
not have to be as long, at any given body mass, as those of mammals, 
for the posterior part of the mammal cranium is expanded to 
accommodate a food processing apparatus as well as the large brain. 
Statistical comparisons show that sauropodomorph skulls are 
significantly shorter, relative to body mass or predicted metabolic 
rate, than those of mammals of comparable mass. In contrast, the 
width of the muzzle is subequal to that of mamals at any given body 
mass and predicted metabolic rate. This suggests that the cropping 
area was not inadequate in mass to enable these animals to ingest 
sufficient food to maintain an elevated metabolic rate.

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Have finally managed to get hold of all of Garcia et al's recent 
paper on the new complete _Rhabdodon_ specimen (_C.R. Acad. Sci. 
Paris_ 328: 415-421). They cite a number of dental featues as well as 
the robust femur, morphology of fourth trochanter and shape of pubis 
as indicating that _Rhabdodon_ is an iguanodontian, and possibly 
related to _Tenontosaurus_ (they follow Sereno, Forster and others in 
treating tenontosaurs as the most basal iguanodontian [judging from 
the _Altirhinus_ paper, David Norman continues to regard 
_Tenontosaurus_ as giant close relatives of _Hypsilophodon_ which 
mimic iguanodontians as a byproduct of giantism]).

Sorry Jim, there is no foot. Upper jaw of the croc _Ischyrochampsa_ 
and some dromaeosaurid teeth were associated with the rhabdodont.

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth, Environmental & Physical Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
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http://www.naish-zoology.com]