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Size Isn't Everything (WAS Re: cause of Gigantism in sauropods)



The big, well known Morrison theropods (Allosaurus, Ceratosaurus, and Torvosaurus) have strikingly different teeth. Allosaurus has relatively small, standard fare theropod teeth. Ceratosaurus has long and very blade-like tooth crowns. Torvosaurus has large teeth with crowns that remarkably robust. The Torvosaurus teeth from Dry Mesa Quarry described by Brooks Britt are way too large to fit in the mandible of the largest Allosaurus (the roots would stick out of the ventral surface of the dentary). Saurophaganax has one incomplete crown referred to it and that seems to resemble Allosaurus as best one can tell. Nothing is known of the teeth of Epanterias.

It has long been recognized that Morrison sauropods have spoon shaped teeth running the length of the jaws (Camarasaurids) and pencil like teeth restricted to the front part of the jaws (diplodocoids). This has generally been regarded as reflecting some partitioning of the food sources. Might not the same be true of Morrison theropods and that the widely differing tooth structure reflects either different prey or hunting strategies? The Morrison fauna is unusual (say compared to many Cretaceous faunas) in that there are multiple, co-existing, large theropods, with multiple genera co-occurring in the same quarries.

So guys, don't get hung up on size --- it isn't everything.

Dan



On 2/7/2011 6:11 PM, Habib, Michael wrote:
On Feb 7, 2011, at 7:29 PM,<vultur-10@neo.tamu.edu>  wrote:

Yes, elephant-lion is rare.

But Cape buffalo are a *major* part of lion diet, IIRC more than half of some lion 
populations' diet (Lake Manyara National Park is one, I think<60% Cape buffalo) 
and that's something like 3-4x the lion's mass; especially since lionesses do most 
of the hunting, and they're often like 120kg animals, not 200kg. *Allosaurus 
amplexus* or *Saurophaganax* were larger in comparison to *Diplodocus carnegii*.
Group attacks by lion on cape buffalo are common in some populations, true, but 
I am under the impression (admittedly from older literature) that solo attacks 
on such animals is extremely rare.  Taking into account that the average 
hunting group of lions is at least 3-4 individuals strong, that means that the 
mass ratio is actually close to 1:1, with the added advantages that a group of 
attackers naturally has on a single target.


And what were *Allosaurus amplexus* and *Saurophaganax* --doing-- if they 
weren't specialist sauropod, possibly big-sauropod, killers? Everything else 
*A. fragilis* was quite big enough to deal with.
I suggest that perhaps they were killing larger juveniles, primarily.  Under 
that model animals such as A. fragilis would be killing smaller juveniles and 
the adults of small ornithopods, etc.  There's no reason to expect that any 
animals must have regularly predated the adults of large sauropods.  And there 
is no reason that the prey sizes could not overlap significantly; prey sizes 
taken by large carnivorans in semi-arid African habitats overlap quite a bit 
among multiple predators.  Far more is made of niche-partitioning that probably 
aught to be.


But yes, the real titans were probably safe most of the time barring really 
hungry / desperate theropods. Still, the idea you sometimes see that 'adult 
sauropods were basically immune to predation' needs, at the least, 
qualification.
True enough.  However, the reverse notion, that theropods regularly mauled 
giant sauropods to death, is seen very regularly, and seems highly implausible. 
 Adult sauropods were probably not immune to predation, but some of them were 
likely close.

Cheers,

--Mike



William Miller

----- Original Message -----
From: "Michael Habib"<MHabib@Chatham.edu>
To: vultur-10@neo.tamu.edu
Cc: "dinosaur"<dinosaur@usc.edu>
Sent: Monday, February 7, 2011 12:11:46 PM GMT -06:00 US/Canada Central
Subject: Re: cause of Gigantism in sauropods


On Feb 7, 2011, at 12:45 AM,<vultur-10@neo.tamu.edu>  wrote:

So in the Morrison, the sauropod-theropod size gap seems smaller than the 
elephant-lion one. I see little reason to believe that Saurophaganax or A. 
maximus could not take down even Giraffatitan or Supersaurus.
Neat comparison with the body mass estimates (thanks for punching the numbers!) 
but I'm not sure I quite agree with your conclusion.  It seems reasonable that 
something like Saurophaganax could take down something like Giraffatitan under 
very rare, extreme circumstances, just as living terrestrial macro-predators 
(or groups of them) very rarely kill much larger animals than themselves.  
However, I see no reason to expect that such events were common, or even 
occurred with a high enough frequency for us to seriously consider them as 
major factors in our  reconstructions of Mesozoic ecology.  Living terrestrial 
vertebrate predators rarely take prey even three times their own mass, much 
less 6-8 times.

The elephant-lion size ratio probably does not represent the ratio at which 
predation is regular or ecologically important; at best it is a ratio at which 
a very rare predation event is still barely feasible - and that is for a 
specific guild of predators and herbivorous mammals.  The more important size 
ratio is the maximum predator:prey mass ratio among *regular* predation events. 
 Phrased as a question: Of those large terrestrial animals that are predated as 
adults with a high enough frequency for its impact on total population 
mortality to be measurable, how large are their smallest predators (or total 
mass of packs, if they are predated by groups)?

I don't know exactly what the answer to that question is, but qualitative 
observation suggests that the size gap is pretty small.  The vast majority of 
predators, even large ones, mostly take prey smaller than themselves.  Even 
animals like water buffalo, which are a fraction of the size of elephants, are 
large enough as adults to be predated upon rarely (albeit more often than 
elephants).

Cheers,

--Mike


Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
mhabib@chatham.edu
(443) 280-0181

Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
mhabib@chatham.edu
(443) 280-0181