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Re: Theropod bluff & hunting (was re Dilophosaurus Forelimb Bone Maladies



> On Mar 4, 2016, at 11:35 AM, Darius Nau <dariusnau@gmx.at> wrote:
> 
>> Most varanids, some of which have teeth that are also good matches for 
>> theropods, are obligate small prey hunters (they cannot cut effectively with 
>> their teeth). Komodos are the exception, and their ability to carve muscle 
>> is not so much a matter of their teeth as it is a special form of cranial 
>> kinesis that allows them to saw with the jaws (see work by Domenic D'Amore).
> D’Amore’s works are one of the main sources for the analogy between theropod 
> and komodo dragon dentitions. I think in his dissertation he made it quite 
> clear that he envisions them to have functioned in a similar manner.

Up to a point. I’ve spoken with Dr. D’Amore extensively about this (Domenic and 
I do fieldwork together on the Augustyn Expeditions in New Mexico). Komodo 
dentition is only a piece of the story. Other varanids have similar teeth but 
cannot cut muscle the way that Komodo dragons do. Crocodile monitors, for 
example, despite being very sizable animals (longer than a Komodo, in fact) are 
miserably poor at cutting muscle. If you give them a very large piece of meat, 
croc monitors really struggle with it. Now, there are some subtle differences 
between the teeth, and Domenic is working on those to see how different 
theropods compare to the the variation across varanids. The point, though, is 
that the specific kinesis in Komodo skulls is a major part of how they cut 
muscle. They basically saw through using each side of the jaws (i.e. right, 
then left) as semi-independent reciprocating units. 

Theropods definitely couldn’t cut this way, so that leaves other mechanisms 
such as the head-depression “hatchet” option suggested a couple of times in the 
literature (which you astutely noted earlier). I actually don’t have a problem 
with the suggestion that things like allosaurids used such a mechanism to help 
disable prey of decent size (say, 30% of the mass of the allosaur), but I have 
serious skepticism about using it to bleed out or carve up a really big prey 
item. That would be a good way to get killed.


>> Most sharks are also small game hunters. Even large adult great whites 
>> mostly hunt juvenile pinnipeds that can be consumed in a bite or two.
> Yet they still closely follow the trend observed in other animals; Great 
> whites can also take Elephant seals and right whale calves.

I’m not sure that’s a trend so much as an isolated feature of a very few number 
of particularly famous species. Great whites can, from time to time, take 
elephant seals (though usually, even then, the seal is smaller than the shark). 
But no other sharks come anywhere near great whites for large prey capacity. So 
a diverse, successful lineage has one outlier species that, as a large adult 
(subadults eat mostly fish) can very rarely kill things larger than itself. 
This is the same thing in varanids. In raptors, there are handful of species 
that take large prey. In cats, there are proportionally more, but they are also 
specialists in large prey capture with a slew of specific features that enable 
that specialization. 


> 
>> The adults are unusual prey
> Hayward & Kerley 2005 cite Scheel (1993) as confirming that only 25% of kills 
> in the Serengeti are juveniles, and go on to estimate the preferred prey 
> sizes of African Lions at over 200kg.

I just checked Scheel, and it looks like this might have been a misread by 
Hayward and Kerley. What Scheel (1993) actually says is this:

"For each species except Grant's gazelles, at least 25% of kills were calves or 
juveniles. The weights (kg of meat) and times spent stalking per kill for these 
species incorporate the small size and ease of capturing young individuals in 
the observed proportions.”

So the 25% was a minimum and the numbers in that Table indicate that most of 
the prey were actually juveniles for most species eaten. There were two buffalo 
of 240 kg killed (which is large prey for a lion, but also a relatively small 
buffalo - so probably young). After that, there were a number of young 
wildebeest and zebra at 82-85 kg. The most common food items were wart hogs, 
listed at an average of 37.2 kg


> I’m afraid that’s not the way I’m referring to it. I was specifically 
> referring to the findings of Rayfield et al. 2001. These seem to clearly 
> corroborate Bakker’s earlier hypotheses about _Allosaurus_, even if the 
> details have been subject to some disagreement.

Got it. Yes, Rayfield et al. 2001 found that Allosaurus had a strong skull in 
some respects, but not necessarily in the ways I’d predict for a large game 
hunter.  The authors note that Allosaurus had “weak muscle-driven bite force, a 
very 'light' and 'open' skull architecture” and "the skull of Allosaurus seems 
to be designed to resist large vertically directed forces applied along the 
tooth row”.

This led the authors to suggest the “hatchet” style feeding stroke. I think 
other equally plausible explanations exist, but even if we accept the 
hatchet-stroke model, that doesn’t mean that Allosaurus ate particularly large 
prey items. Prey on the order of two to three hundred kilograms would be plenty 
large enough that a hatchet stroke would work. In fact, a very large prey item 
would pose some problems for the hatchet model (penetration issues and the 
strong potential for unexpected torsional loads that might break the skull, 
since it was mostly strong to vertical loading, only).


>> But they don't have a great number of particularly specific reinforcements 
>> to twisting or even compressive loads along the dorsal margin of the skull.
> Because they didn’t need them, the most efficient way to avoid injury from 
> prey is to avoid prolonged physical contact. Cats don’t do it because their 
> teeth aren’t suitable for bite-and-let-go behaviour, but every predator that 
> hunts large prey with teeth that are remotely analogous to carnosaurs does.

But an even better way to avoid injury from prey is to attack small things that 
are naive (i.e. juveniles of much smaller mass than the predator). I’m also 
hesitant to consider large cats as a sort of “best of the worst” set of 
adaptations to big game predation, since they seem to be the best big game 
hunters among all living predators. We might expect that a hit and run is the 
best way to kill large prey, but what we actually measure in the wild is that a 
grapple and precision bite mode is the one used by the most notable group of 
specialized big game predators. 

The “every predator” here is basically just Komodo dragons, which are an 
outlier even within varanids. Shark teeth are only very loosely analogous to 
theropod teeth, and even then, only adult great whites are really big game 
hunters (and it’s still not the norm for them).


>> The jaws of small prey specialists can actually look a lot like those of 
>> related large prey capable taxa. Varanids are a good example, as are cats.
> You implied that komodo dragon skulls are specifically adapted for 
> flesh-tearing, and I certainly won’t argue with that, in fact I think certain 
> theropods display analogous adaptions, but that is where you disagree.
> Are the same features present in other varanids and just go completely 
> unused? Otherwise, looks are just deceiving.

Non-komodo varanids differ from Komodo dragons in the details of their cranial 
kinesis. In this sense, the skulls of theropods are more like non-Komodo 
varanids (though not terribly similar to any varanids, as theropod skulls were 
never as kinetic as even the less kinetic varanid skulls).


> 
>> Actually, this raises an interesting question for discussion: what do we 
>> predict that a small prey adapted theropod skull should look like, and what 
>> should a large prey specialist skull look like? I suspect we have different 
>> predictions.
> Primarily shallower, more gracile, and also proportionately smaller, with 
> additional differences in tooth morphology (often adaptions for handling and 
> restraining small prey, as is present in _Dilophosaurus_). And less optimized 
> for handling loads than _Allosaurus_ or the like, which may or may not be 
> visible, but will surely be detectable using Finite Element Analysis.
> E.g. Spinosaurs have shallow skulls, inferred to have had relatively less 
> resistance to bending. Their teeth have lost much (Baryonychines) or all 
> (Spinosaurines) of their cutting function. And direct evidence of predatory 
> interactions and diet supports a diet of fish, pterosaurs and juvenile 
> dinosaurs, so they fit the presumed niche of "small-prey specialist" nicely 
> on every account. I am also not aware of any spinosaur bite marks on large 
> dinosaur skeletons, or spinosaur skeletons with peculiar pathologies inferred 
> to have been directly or indirectly caused by a large animal (while what 
> there is are foreign bodies from sawfish stuck in their jaws). Your 
> previously used analogue of a stork is quite fitting here, although I must 
> say I highly disagree with comparing _Allosaurus_ to a stork.
> 
> What would be your prediction?

For a big-game hunting theropod, I would predict, in no particular order: 
Skull capable of handling large loads in unpredictable directions (including 
torsion), adaptations to high bite forces, reinforced teeth, robust overall 
build and ability to survive falls and/or grapples (either through flexibility 
or thoracoabdominal wall reinforcement), reduced moments of inertia in the tail 
and trunk (maneuverability for GTFO moments), expansion of neck musculature, 
and enlargement of the teeth (penetration). Some locomotor adaptations might 
also be expected, perhaps those related to high accelerations (since getting in 
and hitting hard, quickly, is a good idea against big animals).

For a small-game hunting theropod, I would predict:
Weak muscle-driven bite forces, light skull, fast-geared neck and head 
mobility, some gape expansion (swallow whole), short teeth, poor armor 
penetration, narrow trunk, generalized locomotor capacity (unless they are a 
fast-prey specialist).


I agree with your breakdown of spinosaurids and Dilophosaurus. I do note, 
though, that spinosaurids are likely piscivory-adapted, not merely small prey 
adapted (though they did, of course, take prey other than just fish). For 
Allosaurus, the gracile skull with vertical load resistance and relatively 
“average” proportions make me think of a generalist predator that might take 
the occasional relatively large animal (maybe 30-40% of the mass of the 
predator). From a comparative prey size range, that’s a bit like a marabou 
stork, though I didn’t mean to imply that an allosaur was particularly 
stork-like.



>> I'm not sure why that would be difficult.
> It’s probably more difficult for a lion to catch a gazelle than it is to 
> catch a buffalo.

That does seem intuitive, but the data from Scheel show that gazelle were 
caught much more frequently than buffalo. Go figure. I do understand your point 
that particularly fast, nimble prey might be difficult for some predators to 
catch, however.


>> That's a good point, but since juveniles are still preferred in the modern 
>> world, I'd consider that to be the dominant trend.
> I fully agree that it is a dominant trend, but using a variety of adaptions 
> and techniques (others apart from doing it like a cat) extant top predators 
> are by no means limited to juveniles and small prey as you seem to imply 
> theropods were.

It’s not an absolute rule, but most extant predators are limited to juveniles 
and prey smaller than themselves (usually much smaller). Even the “top” 
predators don’t take big prey often. Those are famous cases that make headlines 
and show up on National Geographic. I don’t think we should use them as a guide 
for the typical feeding behavior of theropods or other fossil vertebrates.

> Everything from cats, to canids, to bears, to varanids, to extant theropods, 
> to delphinids and lamniforms at least occasionally kills prey its own size or 
> larger. Theropds in general, as well as carnosaurs are among the most 
> successful major predatory taxa ever, and they absolutely dominated 
> terrestrial predatory niches of the mesozoic with very few exceptions. So 
> from a purely parsimonious standpoint, it’s nothing far-fetched that I’m 
> suggesting here (and sure enough it has been suggested quite frequently).

I think the success and ecological “dominance” of theropods in the Mesozoic has 
led to a common image of them as super predators. But that sort of ecological 
role is rare to (as often perceived) basically mythological. Yes, each of the 
lineages you noted has one or two species that very rarely kills prey its own 
size. The vast majority of predator diversity, however, is made up of taxa that 
attack animals much smaller than themselves, and juveniles are the preferred 
prey at almost all size classes. Lions, Komodo Dragons, Orcas, etc are the 
exception, not the rule, and they have specializations that allow them to take 
large prey opportunistically (including group hunting, which evens the total 
mass score in many cases). I suspect that mammals are unusual in the number of 
big prey specialists they’ve produced (and even then, it’s not a large number). 
The Mesozoic must have had an unusually large biomass of juvenile prey (given 
the life histories of large dinosaurs), as 
 well, which I consider a critical part of the Mesozoic ecology. Even if we 
posit that the trends in modern ecology held in the Mesozoic, the expectation 
would be that the vast majority of theropods ate prey much smaller than 
themselves, with only a handful of species capable of taking prey near their 
own size under the right circumstances.

I don’t think what you’re suggesting is far-fetched, and it certainly isn’t a 
rare suggestion. However, I do think that previous “super predator” models of 
theropods, as popular as they may be, need serious reexamination.


Cheers, and thanks for the lively debate!

—Mike


Michael Habib, MS, PhD
Assistant Professor, Cell and Neurobiology
Keck School of Medicine of USC
University of Southern California
Bishop Research Building; Room 403
1333 San Pablo Street, Los Angeles 90089-9112

Research Associate, Dinosaur Institute
Natural History Museum of Los Angeles County
900 Exposition Blvd, Los Angeles, CA 90007

biologyinmotion@gmail.com
(443) 280-0181