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

[David Marjanovic <david.marjanovic@gmx.at>]

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
>  water.
>
>  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.

>  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 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.

>  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.)