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theropod1's avatar

Allosaurus fragilis/Epanterias amplexus

By theropod1
A classic as far as theropods go this is still a remarkable animal. The common misconception that Allosauridae are some sort of primitive "prototype theropods" does not do them justice. In fact, they were every bit as advanced as the tyrannosauroids that are more often given credit as such.

The general proportions paint the image of a generalist rather than a specialist, and this is consistent with the known bite marks that support feeding on a wide variety of diplodocid and macronarian sauropods, as well as thyreophorans. Clearly this served it well, since Allosaurus was by far the most common and successful theropod of its ecosystem.
Almost 70% of all dinosaur fossils in some Morrison localities pertain to Allosaurus. And not all of that can be ascribed to being predator traps, because it also far outnumbers all other contemporary predators in these lagerstätten. In Cleveland Lloyd quarry, a minimim of 46 Allosaurus were recorded, but only a single individual of Torvosaurus and Ceratosaurus respectively (Gates 2005).
For a large-bodied theropod, its skull and teeth are only moderately sized and the forelimbs are comparatively long and very powerful. The light skull, compact body and powerful legs suggest a high level of agility for an animal of such size (averaging between 8 and 9m, some specimens exceeded 11m, and possible relatives based on footprints could have significantly exceeded 13m).

Why was it so successful? The answer is probably hidden in its jaw and neck, which are uniquely adapted for both quick and powerful neck-driven strikes, with a lightweight skull able to withstand tremendous forces despite a weak bite force and wide gape, and a neck adapted for both rapid and forceful cranial depression and retraction, while remaining very flexible and capable of quick movements. In short, an extremely powerful yet versatile weapon, equally suited for pushing and tearing deeply into the flesh of a sauropod, shredding the neck of a stegosaur, or striking at a small ornithopod.
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Megalosaurid's avatar

Based on the humerus of DINO 2560, the Saurophaganax OMNH 1935 would have measured 11.3 m, for the largest specimen, 13 m is no longer a credible figure.

it is 8.8 m and it has a humerus measuring 42.5 cm according to Scott Hartman, scaled isometrically it gives you an estimate of 11.28 m or less.

Maybe Saurophaganax had relatively shorter arms, but that still gives you a range of 11-12 m, rivaling AMNH 5767.

theropod1's avatar
There´s no reason Dino 2560 should necessarily be the best model, and others, like USNM 4734 or MOR 693, should be dismissed (these yield approximately 13m).
There´s also a 26.3cm claw that must have belonged to a pretty gigantic animal in live.
The problem is a different one, 55cm may be an erraneous measurement since it only seems to base on scalebars.
Megalosaurid's avatar

There´s also no reason to call DINO 2560 a worse example than both MOR 693 and USNM 4734, just that both the 11.2 and the 13 m figures have to be taken into consideration, despite that, DINO 2560 is closer in size.

The 545 mm figure might be erroneous? ok, so how big do you think it was?

the 26.3 cm claw is not necessarily from a giant animal, It might have had a relatively huge claw, much like Neovenatorids and Megalosaurids had, keep in mind that pretty much all the restorations of Saurophaganax are speculative, we don´t know how it looked exactly, its not like it looked exactly like Allosaurus. 

theropod1's avatar
We actually don't know what it looked like, as you mention, so we don't have reason to presume it differed much from Allosaurus. That was just an observation, there's a claw more tha 26cm long while normal big Allosaurus claws are about 15cm long.

No problem with taking both into consideration, I was actually doing that. I merely replied to you stating 13m was no longer a credible figure. and yes, I'd say it's more probable that as a larger animal Saurophaganax had relatively shorter arms than that it had longer ones (as is typical for theropods), thus I'd rather give the edge to a relatively short-armed (remember that's still long-armed for theropod standards) specimen as a scaling model.

The humerus as pictured in Chure 1995 is 55cm long, and Mickey mortimer gives a suspiciously similar figure. The problem is we don't really know where that came from, it might have been from that figure and scalebars are often inaccurate. Smith 1998 conducted a morphometric analysis of Allosaurus, supposedly including Saurophaganax material as allosaurus maximus, with the largest humerus that was shown being 48.8cm long. Now both are a bit vague for my taste...

I think the largest Saurophaganax specimen was somewhere between 11.5 and 13m long and accordingly 5-7t.
Megalosaurid's avatar

Yeah, but that allometry rule does not always apply for theropods, just take a look and compare MOR 693 and USNM 4734 to DINO 2560, the later is a lot larger but does have proportionally longer arms, and perhaps just a relatively longer humerus, Its not just size but adaptations that play a role.

There is no reason to believe Allosaurus is the best model, unless Saurophaganax is the same genus just from a different and older species.

If anything, every figure is unreliable untill one single skeleton is ever found. 

theropod1's avatar
There's simply no model that's better than Allosaurus (I presume nobody wants to read the kind of size figure you'd get based on Acrocanthosaurus, and using it makes no sense...). There'll never be a prefect model for anything in palaeontology, but we have to go with the best we have. and to date, there's simply nothing suggesting that it looked much different (that is, apart from the kind of changes you'll expect to see in a larger animal, eg.  more bulky, perhaps with a larger skull, more muscle mass, less cursorial limb proportions).

I think DINO 2560 simply has pretty odd proportions considering both specimens of A. fragilis and of A. jimmadseni notably differ. And seriously, it's arms almost reach the ground. At least, I wouldn't PREFER to use it as a model.

I fully agree that every figure is unreliable, all I'm doing is trying to minimize the error, not establishing facts.
SpinoInWonderland's avatar
Cool! Allosaurus is my favorite theropod. It's a shame that people don't see allosaurs as the amazing theropods that they are.

I'm not so sure about synonymizing Allosaurus fragilis and Epanterias amplexus...
theropod1's avatar
I haven't found differences anywhere that would justify erecting a new species, let alone genus.
SpinoInWonderland's avatar
So Allosaurus fragilis can reach up to ~12 meters and ~5 tonnes?
FredtheDinosaurman's avatar
Agreed. It's quite possible Allosaurus grew to such enormity. I believe Saurophaganax may be the missing link between Allosaurs and Giganotosaurs. Just a speculation. Great drawing by the way Clap 
theropod1's avatar
Thanks :)

I don't think allosauridae (and therefore Saurophaganax) are direct ancestors to carcharodontosauria. At least, in most analyses they turn out a valid clade, not just a paraphyletic grade ancestral to more derived carnosaurs.
BioGhost50's avatar
Very cool. As a dinosaur nut I just love this
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