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Re: Dilophosaurus Forelimb Bone Maladies



> On Mar 3, 2016, at 5:13 AM, Darius Nau <dariusnau@gmx.at> wrote:
> 
> There is rather good direct evidence of _Allosaurus_ predation on 
> _Stegosaurus_, namely two documented cases of _Allosaurus_ pathology caused 
> by stegosaur thagomizers.
> The first one is the rather well-known (by means of being featured in a 
> certain BBC documentary) punctured _Allosaurus_ caudal described by Carpenter 
> et al. 2005

A significant source of mortality and morbidity among the subadults of living 
large predators is attempting to take prey out of their ability range. I'm not 
surprised that Allosaurus got mashed up sometimes by a Stegosaurus, but I 
seriously doubt adult stegosaurs were a regular target.

Also, keep in mind that juvenile stegosaurs probably had some tail weaponry. An 
injury interpreted as the tip of an adult weapon could easily be a deeper hit 
from a juvenile.


> Also a bite mark on a cervical plate is more consistent with predation than 
> with feeding (Carpenter et al. 2005); this is a part with very low 
> nutritional value, but placed in a way that would make it come in the way of 
> any attempt to bite the stegosaur’s neck.

Possibly, though as Hone (2014 I think) noted, low nutrition bits also get 
eaten during scavenging on late stage carcasses.


> Besides that, with the challenges of an effective kleptoparasitic lifestyle 
> in large carnivores, and the behaviour of extant predators, I think 
> defaulting to writing off cases of theropod bite marks on large prey as mere 
> scavenging traces is very risky.

That's fair, but assuming theropods were somehow not playing by the well 
studied rules of ecology is also risky, I think.


> All the modern predators that are most similar to large non-avialan theropods 
> in terms of their functional anatomy and inferred ecology (komodo dragons, 
> birds of prey, sharks, large mammalian apex predators) at least occasionally 
> hunt and kill prey their own size or larger.

Only very occasionally. I also don't think those animals are actually such 
great models for theropods, in reality, which was part of my point. Komodo 
dragon teeth have similarities, but they are otherwise not similar to theropods 
(so perhaps a good model for food processing, but maybe less solid analogy for 
hunting - also, Komodos are weird among varanids. Other varanids are small game 
specialists that can't carve meat well, and non-Komodo varanids may be better 
tooth models in some cases). Birds of prey can disengage if need be - those 
that hunt other flying vertebrates take prey smaller than themselves (typically 
much smaller).

The only mammalian carnivores that really specialize in big prey are large 
cats, and they are basically everything a theropod wasn't: flexible bodied, low 
profile, able to take falls easily, precision bite killers, and limb-first 
attackers.


> I don’t see what would stand in the way of a large theropod doing the same.

See above: they can't get out easily if the fight goes poorly, they can't take 
a hit/fall, they don't have a quick-kill mechanism against big animals (except 
maybe tyrannosaurs), and they were face-first predators. Most importantly, 
their world was full of juveniles (many likely with no parental care). Why 
would something run past the piles of 50 kg animals to attack the two ton one?

Living stiff-bodied, quick moving, high profile, face-first predator do exist 
today. They are highly successful as small prey specialists (storks, secretary 
birds, most canids, etc).

> Of course macropredation becomes more challenging with size, but in part 
> that’s because large prey is simply rare. Of course a majority of known 
> dinosaur specimens aren’t fully grown and could be classified as "juveniles" 
> if one likes that term, but there was still a considerable biomass of large 
> prey.

Sure, but that doesn't mean things were eating that biomass very often. And we 
also get into what constitutes "large". I'm not trying to suggest that a 4 ton 
theropod wouldn't make a meal of a 500kg animal, for instance, and 500 kg could 
be seen as large depending on your point of view.


> In terms of mere evidence of (at least) feeding, in think the best examples 
> are the surprisingly common theropod bitemarks on sauropod bones from the 
> Morrison Formation (Chure et al. 1998, Hunt et al. 1994, Jensen 1988, Matthew 
> 1908).
> The prevalent stance is to consider them traces of scavenging, although this 
> hasn’t been universal (Jensen 1988).

A two ton predator would have to be suicidal to take on a 20 ton animal. I'd go 
with the prevalent stance on this one, but that's just my take.

Thanks for the refs!

Cheers,

--Mike H