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

[Jaime Headden <qi_leong@hotmail.com>]

David Marjanovic completely misunderstands what I wrote and ran with what is 
essentially a false analogy. On herons, he wrote:

<I would rather say it has a pointy beak _because_ it lacks teeth.>

  No, it has a pointly beak because the jaws terminate into triangular apices. 
*Rhynchops* (skimmers) have a beak, but they are not "pointy," and only the 
lower bill in *Rhamphastos* sp is "pointy." A large number of other birds, 
including one that I mentioned earlier (*Diomedeidae*, and in fact most other 
members of *Procelleriiformes*, but also *Accipitridae*, *Psittaciformes*, and 
I could go on), possess similarly rounded jaw tips (at least for the lower jaw) 
while the uper jaw terminated into a blunt rounded face with a down-turned 
extension (the nail) which forms a "hook." Yes, this makes the jaw "pointy," 
but hardly in the same aspect that heron jaws are "pointy" (i.e., they are 
triangular).

<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.>

  And here David confused "beak" with "rhampohotheca." One may have a beak and 
the keratinous covering of its margins as well, or possess the pointed jaw tips 
and lack those coverings (I'm looking at you, *Cyamodus*! Here's a cyamodontid 
skull: http://s51.radikal.ru/i133/0810/62/392dd4e551a0.jpg -- I am not 
rejecting an hypothesis they may have rhamphotheca). I supposed this would work 
as well to contradict, say, azhdarchid stork-like feeding models, in which a 
strong triangular "beak" is useful for "stabbing at prey" and is almost 
certainly sheathed in some extent of rhamphotheca. However, my analogy does not 
require "beak" to mean "rhamphotheca," as I was referring to the relative 
shapes of the jaws in comparison, which David ignores up until...

<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.>

  Herons use a stabbing motion, a quick "snatch" movement, but like angingas, 
jacanas, and marabous, this motion is only in the approach; prey is acquired 
generally just posterior to the jaw tips, and while they may impale the 
occassional item or two, they typically grab prey between the jaw tips. This 
morphology does not require loss of teeth, and in fact, only a lack of teeth at 
the tips would be useful to this mode of feeding. This may be the method by 
qith triangular pointy-beaked birds like *Ichthyornis* and *Hesperornis* 
acquired prey, and all of these also had pointy-tipped beaks, triangular-shaped 
jaws, and a rhamphotheca-covered jaw tip ... and they had teeth. The same 
morphology is likely true for *Germanodactylus cristatus* (edentulous and 
pointy jaw tips, teeth posterior to this region).

<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.>

  I wanted to pick a fairly recognizable analogy (the image grab was of an 
albatross [*Diomedea* sp.] skull). I could have gone with cormorants, gulls, 
etc., the former which like the anhingas and jacanas mentioned above, have a 
very similar feeding style or catch prey which swimming, the latter being 
opportunistic feeders that will wade, scavenge, predate on the ground, and dive 
after food (like many albatrosses, in fact). It is actually something of a myth 
that albatrosses (*Diomedeidae*) only catch food while on the wing, and while 
this may generally hold for most terns, it is certainly not true for 
albatrosses.

<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.>

David wrote in the first post to which I responded:

<Like the spinosaurs, it may have been a heron analogue.>

  While this was all about *Masiakasaurus*, an analogy I won't get to now 
because it requires a different level of refutation (and is equally erroneus), 
David did in fact compare spinosaurines favorably to herons, so we are going 
with this.

<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 wish I could just set aside my own comparisons at will as though they never 
happened. Now David is expanding his "heron" to "extant birds," without 
regarding the constraints that jaw morphology and jaw shape have on feeding and 
diet, and well as what these things allow us to infer about bird diet (and test 
accordingly), and the same is true for crocs (precision biting is found in 
hook-billed birds just as much as it is in gharials simply because of the same 
expanded tips of the jaws -- "jaws" here disregarding the absence of teeth in 
birds). Taking the animal out of the medium in which it might be placed, while 
attempting to use the same biomechanics of biting and jaw function in another 
medium and then inferring diet is, to my understanding, a practice that has 
NEVER been done on any dinosaur. It certainly isn't used to help infer diet in 
heterodont taxa, or ornithischians with their edentulous jaw tips (simply being 
herbivores with broad or narrow jaws is ALL previous analyses have done as far 
as assessing styles of feeding). That gharials have a jaw shape that permits 
precision biting as well as quick lateral sweeps of the head (including 
low-profile rostrum), isodont dentition posterior to the rosette that 
contradict any sort of regionalization of the dentition as seen in *Crocodilus* 
sp. that would infer handling of diverse prey in diverse media, we are able to 
use the entire jaw as factors in assessing diet. But with herons, we seem stuck 
on it lacking teeth.

<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.>

  This is a false comparison in that "beaks versus teeth" is a _non sequitur_. 
Ignoring that I was talking about the array of the jaws in total (including any 
rhamphothecal or dental emargination), we can actually infer dietary effects of 
dental arrays and have have little to no understanding of jaw function without 
seeing them in situ. Similarly, an edentulous jaw in any given bird is only as 
good as its habitat (the same is true for any potential predator). This 
reasoning is borne out in a spinosaurine jaw, for as may have been glossed over 
at some point, spinosaurines (using specifically the dal Sasso et al. specimen) 
lack isodonty and may even be termed heterodont: they possess rostrally 
straight teeth, while further posteriorly the teeth become incredibly tiny, 
then increase in size to the rostral dentition but are curved, then become 
smaller but retain more curvature than in the rostral dentition.

  In *Spinosaurus aegyptiacus*, for which we only have a mandibular example 
(I'll leave the reference of upper cranial material to the side, as all such 
specimens are either regionally or temporal displaced from the Baharija 
deposits and may only belong to a related animal), the teeth are entirely 
straight front to back, but differ in size more extremely than even in 
gharials, which is indicative of partitioning of jaw function. Despite this, 
the inference of a heron-like habitus (due to an estuarine or deltaic or 
palustrine-like habitat) seemed to be more important than actually assessing 
the jaw anatomy (regardless of teeth). But even were you to remove all teeth 
from the jaws, the festooned jaw shape implies directly greater variability in 
jaw strength at different points of the jaw margin (and tested by Rayfield et 
al.) -- unlikeherons, because of their jaw shape, they do not grade evenly from 
one end to the other, but increases (amazingly enough) at sites that also have 
the largest teeth. This infers both a rostral precision bite and a more 
posteriorly "gripping" bite -- only in the upper jaw ... no one seems to test 
lower jaws). 

<spinosaurine teeth lack carinae[]>

  *Spinosaurus aegyptiacus*, based on the holotype and backed up by recovered 
teeth across the north of Africa (Tunisian, Algerian and Moroccan samples) have 
carinae. What they lack are serrations, and at least *Irritator challengeri* (? 
incl. *Angaturama limai*) has carinae with beaded but apparently not serrated 
sculpturing. And I am not confusing carinae with fluting here. Just clearing 
this up. Not that it means much.

<Teeth make ripping and tearing (if not cutting[)] 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.>

  A carina should have little effect on either a precision bite or gripping 
hold, save that it may also increase the wound size during entry and removal of 
the tooth given jaw and head action relative to the impaled substrate. This is, 
however, just another way in which spinosaurines are not heron analogues: 
herons consume their prey hole, and are essentially required to hunt for prey 
they can swallow ... spinsoaurines need not be so picky. But even a serrated or 
undulating or festooned jaw margin would not help the analogy, and it does not 
help the analogy for birds which have teeth or serrated rhamphotheca, as these 
birds are generally pelagic feeders (either hesperornithiforms as divers or 
pseudodontornithids as divers/surface snatchers). Likewise, this has nothing to 
do with heron size: secretary birds manage prey about as big or often bigger 
than herons do (and indeed, so may marabous) and manage to use their feet quite 
a bit (although they are much larger feet), but they have a different type of 
jaw than herons.

Cheers,

Jaime A. Headden
The Bite Stuff (site v2)
http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
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