RE: Regarding Spinosaurus

["Jaime A. Headden" <qilongia@yahoo.com>]

Christopher Lavers (Chris.Lavers@nottingham.ac.uk) wrote:

<If they were 'high metabolic' it is even more difficult (at least for me)
to imagine them as piscivores - how many big-bodied, freshwater,
warm-blooded, obligate fisheaters does anyone know of? (Some probably
incorrect speculations on why freshwaters tend to be dominated by big
coldies can be found in ch7 of my book, for those of you sad enough to own
a copy.)>

  I doubt there's any extant analogues that could support or disprove the
near-exclusive piscivory theory for spinosaurs. This is merely the
best-suited explanation for diet to an animal whose jaws are perfectly
suited for feeding on fish. No other explanation seems to fit. Animals
with such _severe_ adaptations can be given similar grounds, such as the
gavial, for which the jaws of spinosaurids tend to resemble a good deal.
Gavials do not eat anything but fish.

  But one has to contend with multi-ton fisheaters. For this, we shift
environments, which I am aware most people are aware, is in the sea. Fish
are abundant to the absence of larger terrestrial carcasses to support any
other type of feeding style as well as possessing a group of vertebrates
that some would say specialize in dealing with the few carcasses that are
received. Orca are the only other vertebrates of a size that would
compare, and they are generalist feeders, going for occasional shore-prey,
but mainly tend to subsist on aquatic prey. Dolphins and porpoises are
indeed specialized on fish and feed on nothing else. But they are smaller,
generally less than a ton by even less than half.

  So some comparison can be drawn, but the environment is entirely
different. As for the terrestrial piscivore hypothesis, spinosaurids are
distinctly predaceous, but the idea that they scavenged carcasses is a
case for behavior that cannot be supported by anatomical tests. There are
no anatomical specializations in carrionovores that would be able to test
for this. It is all behavior. What specializations are present appear to
be in snatch-and-grab (in or out of water is irrelevant). Teeth are
recurved in baryonychines, but not spinosaurines, so in the latter they
would most likely be almost true piscivores, whereas the "more" croc-like
head but recurved teeth would be somewhat as suited to non-piscine flesh.
They may have taken live prey, or even scavenged for terrestrial
carcasses. This is something that would need to be "discovered" before any
other useful speculation could ensue. All we have is the osteological and
biomechanical data that looks like spinosaurids were specialized
piscivores (one need not say exclusive).

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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