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Re: What did Spinosaurus eat? New species of Lepidotes found

Don Ohmes <d_ohmes@yahoo.com> wrote:

> I think there is a size constraint here -- taking as working assumptions
> that the ability to nest competently on dry land was critical to individual
> dinosaurs, and also that the occasional forced overland trek is a part of
> most semi-aquatic lifestyles -- the larger dinosaurs were logically limited
> to a foot design that functioned well on land, whatever their daily habits.

I don't disagree - at least where large theropods are concerned.
However, theropods don't need to be large.  _Masiakasaurus_ was only
2m long, and has craniodental features (like procumbent front teeth)
that suggest that it might have been a fish-eater.  If correct, this
in turn suggests that _Masiakasaurus_ habitually entered the water.
Is this the kind of animal that could have given rise to a fully
aquatic lineage of non-avian theropods?  (Not _Masiakasaurus_ itself,
given that it lived in the Maastrichtian.)

Birds have been very successful at exploiting aquatic habitats.  Is
there something about birds that allowed them to exploit aquatic
niches in the Mesozoic?  It can't simply be flight, because the
hesperornithinean birds were flightless.  Is it something to do with
the development of three separate (but integrated) locomotor modules -
pectoral, pelvic, caudal?  If so, there's no reason why small
non-avialan maniraptorans could not also have turned to an aquatic
existence.  Or are the aquatic adaptations of birds a spin-off of
powered flight?  Hesperornitheans were secondarily flightless, and an
aquatic lifestyle is likely to have preceded the loss of flight.  I
don't have any answers to these questions - I'm just putting forward
the sorts of questions that might be worth asking.

> Heh -- as to spinosaurs -- given the potential of a spinosaurid body to clog
> a shallow water column w/ silt, and their general tactical capacity to adopt
> heron-like or bear-in-a-salmon stream tactics, I doubt they had to swim well
> to catch lepidotes or nile perch anyway.

AFAIK, spinosaurids also lack pachyostatic adaptations - which is
unusual for large tetrapods that apparently spent much of their time
immersed in water.