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Re: Size of *Neoceratodus africanus* and/or *N. tuberculatus*



On Sun, Mar 29, 2009 at 04:20:42PM -0700, Tim Williams scripsit:
> Graydon <oak@uniserve.com> wrote:
> > There's a big difference between "dedicated piscivore" and "has
> > adaptations for piscivory".
> > 
> > Spinosaurids do seem to have some adaptations for piscivory. (At
> > least, the big thumb claws and the jaw morphology would seem to
> > argue for this.)
> 
> Yes, but I think that the piscivorous adaptations of spinosaurids have
> been over-emphasized.  The comparative biomechanical analysis by
> Rayfield et al. (2007) found the gharial (gavial) to be the closest
> modern analog for _Baryonyx_, but they held off from arguing that
> spinosaurids were dedicated/specialized piscivores.  Sues et al.
> (2002) argue that, while fish probably formed part of the diet in
> _Baryonyx_, "there is nothing to suggest that spinosaurids were
> exclusively or even predominantly piscivorous". 

That's not something one could properly derive from fossil evidence,
though, and gharial _are_ at least facultive piscivores.

> I'm not trying to dispute the hypothesis that spinosaurids fed on
> fish.  (After all, the gut contents of _B. walkeri_ is prima facie
> evidence that they did.)  I'm only arguing against the hypothesis that
> spinosaurids were specialized piscivores, or even that fish (including
> giant lungfish) were even a major part of their diet.

I think the jaw morphology is an indication that spinosaurids were
eating something generally small -- rather than typically between half
and one times their own body weight, say a tenth or less -- and that
there are no other known candidates *but* fish that would be
sufficiently common.

> As for the "big thumb claws" being an adaptation for catching
> ('gaffing') fish...  http://dml.cmnh.org/1998Apr/msg00254.html

Has the promised publication occurred?

> Besides, an enlarged ungual of the first finger (pollex, or thumb) is
> also seen in torvosaurids.  Torvosaurids show no piscivorous
> adaptations in the skull.  Also, in spinosaurids (as in torvsoaurids)
> the forearm is quite short, which I would interpret as poorly adapted
> for catching fish - especially if the animal was not submerged.
> Speaking of which...

Poorly adapted for catching, well adapted for seizing and positioning
large fish already caught by the jaws.  The heron-flip won't work with a
200+ kilo fish and any plausible strength of neck, and the catch would
also present issues as square-cube and sectional density start to make
your life unpleasant.

Possibly also adapted for ripping or piercing deeply until large fishy
stops wriggling.

> > Does it _need_ wading adaptations?  Multi-ton animals have a lot of
> > trouble walking on top of the mud no matter what their feet look
> > like.
> 
> I take your point; but I was talking more about aquatic (or
> semi-aquatic) adaptations in general.  If fish were a major part of
> their diet, and spinosaurids entered the water to catch them, wouldn't
> we expect to see at least some aquatic adaptations in the postcranium?

Bone ones?  I'm not sure.

> Maybe aquatic reptiles and mammals?  Hippopotamids and indohyids (both
> suggested as possible sister taxa to whales) show thicker limb
> elements as a result of osteosclerosis, to improve bottom walking.

But that's primarily to make the feet inclined to point down in a
barrel-bodied, short-legged mammal that feeds or travels submerged.

In a proportionally long legged cursorial bipedal theropod with an
air-sack lung system that means its laterally-compressed torso is
already a float and its legs are already comparatively dense, and where
the distribution of buoyancy would already tend to put it down by the
hips (legs and tail much less buoyant than torso) and up by the head
(something a lunge-feeder would want), and where there's no reason to
suspect that it could submerge (or would need to), I'm not sure what
post-cranial adaptations we'd expect to see.

Especially if the habitat was primarily sandy mangroves, rather than
muddy swamps; submerged wet sand as a substrate isn't going to be very
different from dry packed sand.

I *could* see that kind of environment driving both proportional leg
length and sheer size; the bigger you are, the more surf you can wade
through.  But most of what there is to eat in an environment like that
when you weigh more than a ton is fish.

-- Graydon