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

Re: Kickboxing Cassowary

2008/10/3, David Marjanovic wrote

> These features suggest that the jaws & teeth were not used to kill large
> prey (*Varanus*, being venomous, cheats). They do, however, suggest that
> prey was _eaten_ that was too large to be swallowed whole or ripped to
> pieces.

No, small teeth do not suggest killing prey that cannot be swallowed
entirely, and large size for teeth indicate possibility of cutting off
larger chunks. There is just an example of a carnivore that can cut
flesh from larger prey with small teeth (Komodo dragon) against many
examples of carnivores that for small relative size of the teeth eat
smaller prey. In addition, the kinematics in the Komodo and the
dromaeos cutting likely differ given the functionally important
flattened snout of the dragon vs. the transversely compressed one of
dromaeos, so that the analogy is not complete.

> Cats, especially sabertooths, have extremely strong forelimbs for holding
> prey in place when the bite is being done (saber teeth were likely too
> fragile to be used on struggling prey, and as we're talking about mammals
> here, they weren't replaced).

At least for sabertooths, it is not known the true function of the
forelimb. I think it has to do with something else than grabbing prey
forelimbs robust as those of the lion suffice to hold the flesh of the
victim to deliver a bite. So did I suppose climbing, ot at least,
greater resistance for hanging on. If large teeth are to be used
against large prey, I suppose yo have to climb upon it. And the
sabertooth do not seem to have the jumping abilities of Felinae.

>Dromaeosaurid forelimbs are adapted for
> maximum speed of movement -- perhaps stabbing movements using the large
> finger claws, or holding the prey just long enough for a kick.

But for prey larger than themselves, they should require, unless for
ankylosaurs, to jump and hang from the flanks of the taller herbivore.
I doubt of these claws stabbing.

> The limb bones were not pneumatic, and eagles can do quite some wrestling
> despite having almost completely pneumatic skeletons.

Eagles do not wrestle with prey larger than themselves, except for
trained eagles that can grasp wolves. However, I doubt this being
natural behavior. And theropod limb bones, while not pneumatic
according to Britt (1991), have thin walls according to Sereno (1999).

> Are you sure about this lack? *Achillobator* is named for having a tendon
> attachment site on its sickle claws that is as large as the one for the
> Achilles tendon in other tetrapods.

All dromaeos have and enlarged ventral process. But these are
insertion sites, not origins, where the larger bulk and transverse
width of the musculature is commonly located. And the enlarged flexor
process might have to do with maintaining leverage for flexors given
that when the claws are curved, their basal part is more dorsodistally
directed, forcing the flexor tendon to round around the ventral
articulation in order to reach the ungual - which can put the tendon
to compression, which can result in the formation of sesamoids in
other case, which are not seen.

> I do think it did that. Why else is the cutting edge serrated?

Edges help penetration. It has been considered by many that saberteeth
are fragile, so supposing they cutted when sunk on prey would
seemingly lead to breakage - teeth is more fragile than bone when
submitted to shearing forces.

>> f)-general inferiority of keratin and bone materials when compared with
>> steel.
> That doesn't mean they aren't still good enough.

just that the propierties of the ghurkha dagger and human sickles are
superior and not extrapolable to dromaeos.

> The lack of sickle claws anywhere outside Deinonychosauria makes all of this
> pointless.

No, it doesn't, as the appearence of a novel structure does not
indicate the appearence of a new behavior if behaviors more compatible
to those indicated by the EPB are consistent with the novel structure.

> Not that much, especially in ornithischians. Also, what about cutting the
> leg muscles to immobilize, rather than kill, the prey?

In the ornithischians not the ribs, but I said that in ornithischians
the anteroventrally facing femur overlaps that part of the abdomen in
non-thyreophorans, that have more flexed knees. To admit they cutted
through the limb, you have to counter the argument by Manning et al.
(2006) of the poor performance of the claw against thick portions of
pork meat.

> *Dromaeosaurus* seems to have used the tyrannosaur method: attack by strong
> bite. This method is probably more limited by the size of the predator than
> generally slashing methods (including the hacksaw method of *Allosaurus*)
> are.

If there is a correlation between relative bite force (RBF in future)
and sheer size, it may be only within small groups, such as among
hyenids, but not always (e.g., varanids, Moreno et al., 2008).
However, there are many examples of smaller animals with stronger bite
than larger taxa. For example, Alligator has a greater RBF than
Tyrannosaurus, Crocuta than any Panthera, Panthera than any Ursus,
Canis than any Felis, and Lycaon than the wolf, the Tasmanian devil
than the thylacine, etc. Bakker is kind of an idol to me, but I doubt
of his analogy beteen sabertooths, allosaurids, and Pacific ocean
natives' maces.

> Jumping height is proportional to leg length (proportional to body length),
> proportional to muscle cross-sectional area (proportional to square of body
> length), and inversely proportional to body weight (proportional to cube of
> body length). All else being equal, it is therefore independent of body
> size. Elephants can't jump for the same reason turtles can't jump -- they're
> graviportal --, not because they're too large.

Generally in agreement, but with size, the distal limb elements tend
to become shorter in digitigrades, including dinos. Note that the
animals that jump much have relatively long pes, as frogs, tarsiers,
galagos, some lemurs, small rodents, Ratites, kangaroos, etc.
A digitigrade of the wight of rhinoceros, however capable of running
better than the elephant, cannot jump, for example. Nor the
long-limbed giraffe, as far as I know. In addition, as you note, the
capacity of exerting force increases with the square of linear
increase in size, while the weight increases with the cube of the
linear increase (much more). Thus, for larger animals, you have
proportionally less force to move your body, and this increases with
size. That is the reason by which when taxa increase in size, they
lost some abilities: e.g. lion cannot climb while the similarly built
leopard can, or chimps can brachiate fastly while gorillas apparently
not, or by which gazellas jump relatively higher than the eland.

The correlation is not perfect, as you note with your turtle example,
but this does not mean that there is a relation between body size and
possibility of engaging certain physical prowesses.

In addition, bone resistance increases also with the square of the
linear increase (as it is given by the cross section of the bone).
Thus, for larger animals to maintain a similar morphology, it
indicates bone can tolerate less the shearing forces imposed by the
weight of the individual and its locomotion. That's why at some point,
jumping and running is difficult for large animals. And that's why
large animals tend relatively thicker long bones than smaller animals
of similar body form (e.g., compare Allosaurus with small theropods).


Augusto Haro