[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



In regards to some of these comments, I have a few things to say. Augusto Haro 
wrote:

<-Lenght of the skull and neck may have also permitted a reach advantage, 
relative to body size, over short-necked crocodiles or short-snouted theropods, 
good to increase the feeding area with less locomotor energy expanditure.>

  It should be noted that the length of the skull in spinosaurids is not that 
dramatically longer relative to body length than in most other theropods. This 
is true whether you are dealing with *Majungasaurus crenatissimus* (see: 
http://qilong.wordpress.com/2010/05/20/the-domed-and-the-toothless/) or with 
*Allosaurus fragilis,* both of which are relatively "normal" and which bracket 
the spinoasaurs. The skull is much lower and slightly longer than a typical 
theropod's, true, but not by much; the aspect of the low skull adds to the 
"longer" aspect.

<-The mentioned spacing of the large tooth may be useful to put more pressure 
into each large tooth. Regarding the size variation on the teeth, it seems the 
large tooth are more useful to grab a relatively large fish, and the small 
tooth located opposite them are better to grasp smaller fishes, perhaps to 
exploit more resources (a larger range of sizes). This may mean that the 
relatively larger and thicker the conical teeth of an ichthyophagous 
vertebrate, the proportionally larger the prey it hunted?>

  This is also where the gharial-comparison breaks down: regular spacing and 
size of teeth, with exception of the most rostral, is a better indicator of a 
piscivorous diet due to the purpose of the teeth. For gharials and other 
long-snouted aquatic predators, the profile and size of the teeth must remain 
in close morphospace, so as to impair a struggling prey fish no matter where on 
the jaw it is caught, or during the transit from the front of the snout 
rearward. Large differentiation of shape, orientation, and size in the tooth 
row indicates provincialization of the tooth row, and thus segmentation of 
tooth functionality or effectiveness. In birds, this is never an issue: even in 
toothed birds like *Jeholornis,* *Ichthyornis* or such, the teeth are all 
essentially the same size and shape -- otherwise, birds simply lose teeth and 
use the shape of the beak to allow dietary freedom (I might theorize that the 
longer and straighter the jaw, the more likely the bird is to be a generalist, 
and this model may hold for other jawed vertebrates). Gharials have a long, 
straight jaw, but also bear an expansion that distorts the jaw's shape and 
profile when swimming or moving sideways, and this actually influences their 
diet: gharials are otherwise opportunistic piscivores, although they will eat 
non-fish prey when it is available.

<-Alternatively, or additionally, although I hypothesized Spinosaurus to be 
worst than baryonychines in tearing, the irregular oral border in lateral view 
may help with the overbite, tearing effect Jaime hypothesized, but in a greater 
degree than in gulls and procellariiforms (and perhaps even baryonychines), 
because not only the premaxillary “hook” would form an overbite relative to the 
dentary, but also the dentary rostral expansion may form an “overbite” with 
respect to the maxillary expansion, so perhaps “two hooks” would be present, 
not only helping at a greater degree in prey retention, but also alleviating 
the stresses on each hook both at prey retention and tearing.>

  This is quite possible. Note that the rostral dentary and the maxillary tooth 
rows are very similar to one another -- the upper and lower snouts are only 
divergent in aspect when adding the premaxillae and their teeth onto the snout. 
This is very similar to the model in coelophysoids with their snout "kink" and 
merely enforces provincialization along the snout. Moreso when you consider 
that some of the upper snouts referred to *Spinosaurus* (Taquet and Russell for 
*Spinosaurus maroccanus* and dal Sasso et al. for *Spinosaurus aegyptiacus*) 
have very small posterior premaxillary teeth (in the region where the rostral 
end of the mandible and its expanded dentition would "fit" when the jaw closed) 
while the rostral premaxillary teeth are among the largest in the entire jaw. 
The two expansions typically oppose one another, and would act in accordance 
when the jaw closed, rather than as in many fish where the mandibular portions 
acts against the substrate (prey) and forced against the upper jaw (anvil to 
the jaw's hammer), emphasizing the mandibular dentition in most bony and many 
cartilaginous fish.

  But as with gharials, the jaws should have relatively consistent dentition 
when dealing with prey-retention -- high provincialization of the dentition, 
and a very irregular profile, impairs retention (or holding) simply because 
there would be portions of the jaw that were incredibly weaker at 
prey-retention than others. Not so for gharials. Spinosaurids likely developed 
a variable diet, utilizing portions of their jaws for different activities, 
much as any heterodont[*n1] carnivore does, and only a small portion of the jaw 
(say, the posterior 3/5s) appears close to the piscivore-retention model. The 
remainder, including the rostral hook of the upper, and the radial arrays of 
the maxillary and lower, and the "rosette" of the premaxillary and lower, 
indicate a variety of additional behaviors, and some may be piscivorous, but 
others may not be. I described the rostral hook as part of a precision bite, 
and I think I'm pretty secure on this model, but it doesn't explain most of the 
rest of the jaw. The same morphology is present to a lesser degree in 
baryonychine spinosaurids and in *Dilophosaurus* and *Coelophysis,* although 
the latter two show smaller dentition in the premaxillae than in the maxillae, 
which is not the case in spinosaurids. Thus, I would argue for a precision bite 
focused at the premaxilla, a concordant bite between the posterior maxillary 
and dentary dental series, and a potentially unique "rasping" effect for the 
rostral maxillary and dentary when the dentary row is radially arranged, and a 
concordant bite bewteen the premaxillary and rostral dentary when the teeth are 
not radial (as in spinosaurines).

[n1] Note that I recognize differential definitions for "heterodonty": 
http://qilong.wordpress.com/2010/07/07/a-definition-for-heterodonty/

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
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion 
Backs)





----------------------------------------
> Date: Wed, 9 Mar 2011 19:17:32 -0300
> From: augustoharo@gmail.com
> To: d_ohmes@yahoo.com
> CC: dinosaur@usc.edu
> Subject: Re: What did Spinosaurus eat? New species of Lepidotes found
>
> 2011/3/3 Don Ohmes :
> > Yes. Adopting temporarily the assumption that sails served to dump heat, I
> > would expect a lifestyle wherein the main body was usually immersed in warm
> > water and size was an advantage. The thermal conductance of water being much
> > higher than air, a vascularized flat plate sticking up in the breeze to take
> > advantage of evaporative cooling would allow for a larger body w/out
> > overheating.
> > I am a little skeptical of the "sail as heat absorber" btw, just because in
> > my experience flat plates w/out insulation make crappy solar collectors
> > (even with liquid tubes), especially when wet, only working well on hot days
> >
> Great idea... Although the Spinosaurus more likely did not transpired
> the way us mammals do, being an aquatic animal it may have submerged
> wetting the sail for the evaporation to relieve warmth. Is there
> something written about sails as bad solar collectors?
>
> Some further toughts on Spinosaurus
>
> -Lenght of the skull and neck may have also permitted a reach
> advantage, relative to body size, over short-necked crocodiles or
> short-snouted theropods, good to increase the feeding area with less
> locomotor energy expanditure.
>
> -From the figures I was able to see, it seems that the expansions of
> the snout and irregularities of the mouth borders may be simply
> results of the enlargement of a small numer of relatively spaced
> teeth. The transverse, ghara-like expansions of the snout of
> spinosaurids may be mostly a consequence of the simple expansion of
> the alveoli that harbor the teeth with a greater diameter. The
> irregular shape of the oral border in lateral view has to do with the
> dorsoventral expansion of the alveoli allowing the longer teeth, for
> the longer the teeth, if the root is not deepened, the more easily the
> tooth would be dislodged by the action of external forces. The longest
> teeth are apparently those with greater transversal diameter, surely
> because the increase in width may enhance resistance to breakage,
> surely to compensate the greater risk of breakage implied by their
> increase in length. These tooth dimensional factors may thus explain
> the coincidence in the snout of the points of greater transversal
> width with those of greater dorsoventral height.
>
> -The mentioned spacing of the large tooth may be useful to put more
> pressure into each large tooth. Regarding the size variation on the
> teeth, it seems the large tooth are more useful to grab a relatively
> large fish, and the small tooth located opposite them are better to
> grasp smaller fishes, perhaps to exploit more resources (a larger
> range of sizes). This may mean that the relatively larger and thicker
> the conical teeth of an ichthyophagous vertebrate, the proportionally
> larger the prey it hunted?
>
> -Alternatively, or additionally, although I hypothesized Spinosaurus
> to be worst than baryonychines in tearing, the irregular oral border
> in lateral view may help with the overbite, tearing effect Jaime
> hypothesized, but in a greater degree than in gulls and
> procellariiforms (and perhaps even baryonychines), because not only
> the premaxillary “hook” would form an overbite relative to the
> dentary, but also the dentary rostral expansion may form an “overbite”
> with respect to the maxillary expansion, so perhaps “two hooks” would
> be present, not only helping at a greater degree in prey retention,
> but also alleviating the stresses on each hook both at prey retention
> and tearing.
>
> -What about the possibility of Spinosaurus being able to project a
> mostly cranially directed narrow shadow, by getting the sun from
> behind and slightly from a side, close to the range of motion of the
> skull and neck?