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Re: What did Spinosaurus eat? New species of Lepidotes found
I like Jaime Headden's critique, because it has happened far too often
that shallow analogies were casually thrown around in the literature and
turned into textbook wisdom without being tested.
Still, I disagree with it:
David Marjanovic wrote:
"Well, no. It suggests that _its snout_ habitually entered the
Like the spinosaurs, it may have been a heron analogue."
A heron lacks teeth and is encumbered with a large pointy beak.
I would rather say it has a pointy beak _because_ it lacks teeth.
Herons approach prey during acquisition by stabbing swiftly with
partially opened jaws, to capture the prey between the halves of the
beak. Rather unlike spinosaurids, any of them, in which the jaws are
BLUNT at the tips, broad, rounded, and with large premaxillary teeth
in front accompanying a mandibular "rosette."
Spinosaurid jaws were not covered by a beak (and indeed a beak wasn't
necessary with such teeth). This means there is no reason why the jaw
tips should have become pointed; pointed jaw tips would only have become
injured more easily and would not have conferred any benefits.
Seen from the other metaphorical direction: because herons lack teeth,
stabbing prey with their beak is the only option they have, so that is
what they are adapted for.
Descriptions of the terminal rosette, as in some crocs, implies that
all of the rostralmost premaxillary teeth form a neat rounded
colonnade, but this is misleading: spinosaurid teeth, especially the
two premaxillae-pairs referred to it (Taqcuet and Russell, 1995 and
dal Sasso et al., 2005) are longest in the first two pairs, greatly
diminished distally; this pairs with a large terminal array of the
mandible in which the largest teeth are NOT the most terminal, but
immediately following them, producing a mandible that "fits" behind
the premaxillae. Spinosaurids, in this sense, had something of an
overbite. This does not compare well with herons ... but it does
compare well with gulls. Gulls, especially albatrosses
(*Diomedeiidae*) have a large rostral premaxillary nail that
overhangs the dentary which opens rostrally into a trough; the
mandible is thus expanded upward lateral and posterior to the
temrinus right behind the nail, from where it declines into the
remainder of the jaw margin.
Fair enough; herons stab, gulls and albatrosses don't. However, the
latter two catch seafood on the wing, an option not open to a spinosaur.
(...And not that it matters, but albatrosses aren't gulls. Not even
close. And Diomedeidae has a single i.)
If we were to infer dietary habits from the mere shape of the
rostrum, as David does in comparing spinosaurids to herons,
Oh no. I use the general shape of the snout (more similar to crocodiles
than to herons or any toothless birds) only along with the helical jaw
joints documented by Christophe Hendrickx and the lack of known
adaptations to swimming.
I think extant birds are severely constrained in these respects by their
lack of teeth. Lacking this constraint, the spinosaurs were free to use
precision biting and other more crocodile behavior, even though, unlike
crocodiles, they did not (apparently) swim for a living.
then we would find that a snatching, probing, tearing process may be
projected. I think this may better describe spinosaurid diet,
especially as Rayfield et al.'s work implies a vertical bite with
high torsion constraints, permitting precision biting far better than
"snatch feeding" as in herons, anhingas, etc.
I am not sure about water-wading, but am fairly certain the dietary
qualities were far broader than a giant animal stalking for a quick
snatch: the jaws appear suited for processing larger animals, such as
lungfish larger than its own head.
This is another option not open to a heron. A toothless beak can either
be long, low and weak like a heron's or short, tall, hooked and powerful
enough for ripping and tearing like an eagle's; teeth allow both
stabbing/holding and cutting functions at any snout length and many
snout shapes. Teeth make ripping and tearing (if not cutting --
spinosaurine teeth lack carinae) easier at any snout length, because
they allow a predator to hold prey between the jaws strongly enough that
it rips apart instead of being pulled out when the predator, say, steps
on it or holds it with its large thumb claws. With a heron's smooth jaw
margins, this would have a low success rate.
(Of course, heron legs strike me as too weak for this anyway; but an
animal the size of a spinosaurid couldn't afford such fragile-looking
legs, so it would automatically have stronger ones no matter how it fed.)