Re: Lack of Running Giant Theropod Tracks

[Augusto Haro <augustoharo@gmail.com>]

2010/12/7 Sim Koning <simkoning@msn.com>:
>
>> Maybe, but in this case, as a problem for the Triceratops snout, you
>> still have the close to the midline hindlimbs, which are largely made
>> in bone and very massive. Being that the torso may have been nearly
>> horizontal in Tyrannosaurus, perhaps in a frontal attack horns may
>> first found the chest, and perhaps here the snout is less of a
>> problem.
>
> Exactly what I was thinking. In a face off, a T. rex would be taking a 
> massive risk if it tried to turn to escape, as it would expose its flank and 
> the above scenario would be nearly perfect provided the Triceratops was 
> careful to miss the legs in the charge.
>
If it took the chest, good. But the torso is short and the legs would
move fast in the worried predator, so it would not be easy to miss the
legs. In addition, the target is relatively small if catched
frontally. Anyway, this would be less of a problem in specimens with
the horns at 90º from the long axis of the skull. I do not trust much
in the capabilities of the small nasal horn, overall because its base
is so close to the point of the snout weakened by the enlargement of
the external nares.

On the other hand, I was yesterday reading that the antelope which
better defends itself from dhole packs is one with short horns. This
suggests to me that relatively short horns are better when dealing
with predators because they permit more maneuvrability at close range.
Long horns, as those of impala, eland and gazelle may be quite an
impedement, requiring a greater space of motion, and may explain why
they do not rely too much in them, even when they can also charge and
do at times. And in this comparison, Triceratops most closely reflects
the long-horn ungulates.

I do not know if the direction of the horns varies with ontogeny, with
sex or the individual. In the latter case, it would suggest that they
were not so useful for defense, as the horns forming a lesser angle
with the snout would put it in danger in the event of thrusting a
moving predator. The frontally directed horns mostly resembles the
configuration in the Jackson's chameleon, which is used in streght
contests between males. In fact, in the section on Ceratopsia in the
second edition of the Dinosauria, it is posited that both the frill
and horns are more likely of use in intraspecific contest than for
defense. The frill is also full of superficial vessels (with a
possible thermorregulatory function), and some bite directed at it may
imply dangerous blood loss.

>> Well, how erect you are is not necessarily related to velocity.
>> Sprawling lizards can be faster than mice or birds in their same size
>> league. Perhaps even large lizards can be as fast as or faster than
>> semi-erect crocodilians of their same size.
>
> Yes, but how many lizards can actually 'gallop'? As far as I know Crocs are 
> the only (so called) reptiles that can and they have a more erect posture. 
> Now I don't want to take that too far, since my statement about galloping 
> crocs was more a comment on credulity than actual animal morphology, which is 
> hardly scientific (or even good philosophy) for that I apologize.
>
Lizards do not have a symmetrical gait like gallop as far as I know
but can assymetrically run, which may outperform galloping at least
regarding velocity. The point I referred to with this is that we
cannot infer an at least similar (if not greater) velocity (if viewed
as needed for a charge) just because some taxon (like ceratopsians) is
more erect than other (like crocodilians). For example, elephants are
more erect than both lizards crocodilians but are less able to run
(with a phase suspended in the air). Anyway, inferring galloping in
dinosaurs because of its presence in crocodilians is not unambiguously
supported according to extant phylogenetic bracketing because the only
relatives which seem to gallop are crocodilians, and perhaps not all
of them (granted, birds cannot gallop because of being bipedal, but
they do not support the hypothesis else). And crocodylians do gallop
by laterally swinging the forelimbs, which ceratopsians do not seem to
be able to do, so there would not be a functional similarity.

>> However, I think a problem in comparing crocodilians with Triceratops
>> is that the former have saddle-shaped shoulder joints which permit to
>> protract the humerus by rotating it laterally, so as to direct
>> cranioventrally the forelimb to lessen the impact of the fall after a
>> jump. In ceratopsians, the glenoid is not saddle shaped and directs
>> caudally, precluding the lateral excursion of crocodylians to direct
>> the humerus cranioventrally. The most protraction the humerus can
>> perform is the vertical, as far as I can check, along its limits in
>> the glenoid, assuming the scapulocoracoid to direct obliquely
>> (caudodorsally). For the humerus to be directed cranioventrally, it
>> seems to me that the scapulocoracoid should be set close to the
>> horizontal, which is unlikely anatomically. If Triceratops did not
>> have its humerus cranioventrally directed, it would be more difficult
>> for it to receive the impact with the ground after a jump of those
>> that trotting implies.
>
>  I was reading through Greg Paul's paper on ceratopsians and how they are 
> most analogous to rhinos in morphology, and I noticed that in rhinos the 
> humerus doesn't seem to pass the vertical in a gallop.
>
> http://www.youtube.com/watch?v=0J0Xq0X_7lU

Yep, that's right. I just supposed that in large animals the humerus
would better withstand the impact with a humerus more aligned with the
oblique reaction force from the ground. Although the humerus is stout,
also is the mass of the rhino and thus its inertia. Anyway, the charge
may not require a gallop, just a forward jump may suffice, overall for
better recovering if missing the first attack.

> However, the palms of the ceratopsid manus seem to be rotated somewhat 
> medially. How much do you think that may have impacted their capacity to 
> gallop at high speeds?
>
Well, crocodylians may gallop with greatly medially facing palms,
because of lacking complete pronation as mammals, so that does not
seems like an impedement. This basic archosaurian condition perhaps
explains why archosaurs hands only bear claws on the medial three
digits, because these are the most cranialwards oriented, which can
better grasp the soil against the forces retracting the limb.

>> That may well be, but I do not know much modern cases of archosaurs
>> hunting grupally, although crocs are gregarious and there seem to be a
>> few birds with social hunting strategies.
>
> I really don't like that argument at all. Imagine if there was another mass 
> extinction and bats were the only mammals that survived; now imagine some 
> future paleontologist decided that it was silly to think of Lions hunting in 
> prides all because they do not observe pack hunting among extant "neobats". I 
> think it's more helpful to ask *why* animals hunt in packs.
>
I said it may be, but phylogenetic bracketing on living forms provides
you the more parsimonious hypotheses on the basis of the current
evidence. More parsimonious does not imply more certain. Many birds
hunt animals, not only raptors, and in most of them no collective
organization is seen. Carnivorous dinosaurs may have hunted in groups,
or not - the last being more parsimonious to infer. That they can be
found in close association may indicate gregariousness (or
preservational bias), as in crocs and many birds, not necessarily
cooperative hunting. Although at times, it may be difficult to
differentiate hunting in a gregarious mielieu from cooperative hunting
(for example, if a gnu is targeted by more than one crocodile, many
end up biting it and thus collaborate in killing it in practice, but
it does not imply that the intention was to rely in other
crocodilians, and may just be a case of intended unique predation
coinciding with the attempts of other crocs - perhaps the origin of
cooperative hunting and cognitive association of these occasions with
success in killing by the individual predators?).

Your example of the bats indicates the dangers of inferring from the
Recent only. But, in lack of more conclusive evidence either way,
parsimony is all you have. In any case, it seems that theropods mostly
ate young, as many Recent predators, and thus would not have required
grupal hunting. Whether the adult ceratopsians did or did not put much
effort in the defense of their youngs is unknown.

And, regarding the question "why animals hunt in packs?" as being more
helpful than invoking phylogenetic bracketing, I do not see why it is
of more relevance when dealing with the behaviour of the T. rex. You
may answer that because the prey is difficult to take individually,
but there is the alternative way to take: do not attack the dangerous
prey. As far as I know, that is what leopards and cheetah attempt to
do with warthogs if not confident enough to hunt them, and this also
applies in the ex-Soviet Union even with grupally hunting wolves and
boar, for the adult boar is so agressive and fights in so closed
spaces that the wolves prefer to attempt killing the piglets.