[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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.

 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.)