Don't judge a dinosaur by its ankle.

Posted by Nima On Tuesday, April 25, 2017 0 comments

A change of pace from the sauropods for a bit: after a few questions from a follower of my work, I looked at Hutchinson (2011) again - the paper that estimated Sue the T. rex at 9 tons; I can't tell you how many times over the years I've had to point out the flaws in that extremely chubby model. (It's not that I don't like plump dinosaurs - when they're titanosaurs, they're downright gorgeous. It's plump predators that don't make sense - their survival depended on catching another dinosaur as fast as possible, and at any size, especially multi-ton megafauna, extra weight was exponentially costly).

It seems one of the reasons why Jack Horner and John Hutchinson don't accept a fast Tyrannosaurus is because the lower leg (i.e. ankle) supposedly aren't all that proportionately large, limiting its top speed (and that of other animals). While Horner isn't as hardcore in pushing the "T. rex was mainly a scavenger" theory as in the past, it's clear that this idea still informs much of his thinking and that of his proteges and colleagues.

Hutchinson basically relies on under-exaggeration. He makes arguments like "T. rex metatarsals are proportionally much shorter than those of ostriches, therefore T. rex can't run". Well nobody claimed T. rex was topping 70mph like a cheetah, but acting as if shorter metatarsals imply it can't run at all, is a red herring. And in large part it has to do with the Hornerites' love of straw men.

They take Bakker completely out of context, and pretend that paleontology today is full of people claiming that dinosaurs were supercharged cyborgs outracing hurricanes and crossing dimensions. That simply is not the case. Even the most radical paleontologists don't believe that, nor did Bakker ever make such bizarre claims. He simply stated the case for dinosaurs being warm-blooded, and at least as active as most mammals today (which shouldn't be such a big deal - lions are lazy, dogs are lazy, most mammals sleep a lot... there just don't get torpid like lizards and crocs can). There's nobody claiming that T. rex was running fast all the time - like most predators, it likely only hunted for a small portion of the day. All that we're saying is that a big 'rex was easily capable of 35mph when the time came to actually hunt and kill prey. Which is actually slower than an ostrich.

We're not saying this:

We're actually saying this:

Greg Paul 1988. Used for educational purposes only.

Yes they're fast, but not too fast to track mud or keep at least one foot on the ground for most of the stride. Again, not such a big deal when you consider how T. rex legs were actually built (long toes, huge muscle crests, metatarsals far longer than in any modern mammal over 2 tons, plus they had a built-in shock absorber with the interlock and the 5th metatarsal splint was basically a spring-loader for the outer ankle tendons to make running much more energy-efficient).

Why the "short ankle" problem isn't really a speed-killer:

What Hutchinson fails to pay attention to, is that tyrannosaur metatarsals are actually VERY long as juveniles, and shorten a bit as they grow into adults - they are not going from ostrich proportions to elephant proportions. In fact the "shortness" of T. rex adult metatarsals doesn't get anywhere near as short as in "elephant proportions". They're a lot longer in T. rex, and the actual toes are immensely longer than those of elephants. The toes of T. rex are about as long as the metatarsals themselves! Hutchinson seemingly ignores the impact of long toes and huge cartilage anchor surfaces on boosting speed and stride length.

This is what I call the "blind men and the elephant fallacy" - where you look at one part of an animal and make big sweeping assumptions about the animal based on just that one part, largely ignoring how it works together with the other parts. Ostriches have more elongated metatarsals but much shorter toes, whereas tyrannosaurs gradually shorten the metatarsals but lengthen the toes (as well as having much bigger muscle crests on the knee) - this reduces the stress on the metatarsus and helps distribute the higher mass more evenly, as well as boosting the stride length back up. It all evens out in the end, and the 'rex can still run - just more easily in the 35mph range than a 40 or 50mph range. There is a bit of reduction in speed versus the ostrich, but not THAT drastic like Hutchinson claims. Smaller tyrannosaurs with longer metatarsals like Albertosaurus and Alectrosaurus may well have been able to rival the ostrich in speed, at least for short bursts. And of course tyrannosaur phalanges are far longer and more flexible than those of either elephants or rhinos (and rhinos are flexible runners despite their short toes).

The metatarsals of T. rex are still much longer than what you get in an elephant. On top of that they are interlocked, which only happens in cursorial animals, and it's a biped, which means no weight-bearing forearms to limit the hindlimb stride length. And finally the ankle joint is a lot more flexible than in an elephant, so clearly there was some running going on. And yet Horner and Hutchinson keep arguing that their speed and movement was basically one and the same.

Tendons and muscles:Most of the lower leg segments are covered in tendons rather than muscles. However, we need to realize the whole thing is interconnected, so that we avoid the blind men and elephant fallacy. The main thing to remember is that these long tendons on the lower leg are like big cables hooked to the muscles further up. Generally longer lower leg tendons imply more speed and flexibility, but they do need big muscles on the femur and shin to power them. Now compared to a spindly ostrich, T. rex had attachment surfaces for thigh and shin muscles in spades. And an huge caudofemoralis. And a huge tail to anchor all that and make running even more energy-efficient. What does an ostrich have to anchor its rear thigh muscles, that tiny pygostyle? With all of these advantages for T. rex, we soon realize the shorter metatarsus is not as big of a disadvantage as Hutchinson makes it out to be.

The main enemy for any big theropod is mass. You need to get bigger to tackle bigger or more well-armored prey, but as mass increases, limb segments must become more robust, the ankle gets shorter and more compact because most of the shear stresses of running are directed there - if no other changes were made to the leg as tyrannosaurs evolved to get bigger, this on its own would reduce stride length, and require a major sacrifice of speed. But of course other changes were made. Considering all the ways that T. rex compensated for the shorter metatarsus (longer toes for distributing the stress and increasing stride length, tightly interlocked metatarsals, expanded muscles on the hips and knees, the MT5 spring-loader, the huge caudofemoralis) it's pretty clear that the net total sacrifice in speed wasn't all that great, and fast running was still priority #1. Again, we have to look at how the entire leg evolved and functioned, over-focusing on one segment is very misleading.

The toes of T. rex were not flat-footed or stiff - they were flexible, active parts of the leg stride, and in fact proportionally oversized relative to other giant theropods of similar mass. Note the huge caudofemoralis muscles. Even at half the size depicted here, they would yield considerably more torque than an ostrich scaled up to the same hip height. Image by Scott Hartman, used for educational purposes only.

I don't know if Hutchinson ever argued that not having big muscles on the ankle made T. rex slow, but if he did, that would be an incredibly bogus argument. The ankles of fast-running animals NEVER carry big muscles. They are almost entirely covered in compact, elastic tendons which are powered by muscles much higher up on the leg. Ostriches, horses, big cats, it makes no difference, the lower 50% of the leg is all bone and tendons. Big muscles on the ankle would serve no purpose, as the ankle itself doesn't drive the leg stride, the femur does! The only things that ankle muscles could affect are the toes, and hence having a really big muscle there would be pointless, unless the toes needed to be super-prehensile for climbing trees and the like. I don't think Hutchinson ever argued that. Hutchinson's main argument about ankles seems to be that T. rex's ankle bones were too short to allow the strides needed for high speeds - that's still a weak argument because (a) it ignores toe length, which is very substantial in tyrannosaurs, and (b) it ignores the fact that T. rex metatarsals are far longer and more flexible than what you get in elephants, and animals don't evolve such specialized metatarsals just to waste them or keep them immobile.

T. rex's metatarsals were ALSO much longer than those of duckbills like Edmontosaurus, which were the fastest large herbivores of its day.

The real paradox of the Hutchinson/Horner T. rex: What's really odd is how these people spend so much time and effort trying to throw T. rex under the bus merely because of its size, and omit all mention of at least 75% of its high-speed adaptations, when there were other large theropods (some of them smaller than T. rex) that actually were designed to be slow. Majungatholus and Rajasaurus were clearly slow animals, they have much less metatarsal and toe length than T. rex, and in fact they look more like a zeppelin with legs than a hunter, yet they clearly filled in a top predator niche. Then we have giant allosauroids like Acrocanthosaurus and Giganotosaurus, which actually did have relatively small toes for the leg length, and relatively stiffer legs than tyrannosaurs with less cartilage attachment area (makes sense, they were hunting sauropods for crying out loud!) and yet were still probably topping 20mph easy if their trackways are any clue.

A real slow-running giant theropod - Acrocanthosaurus, hunting Pleurocoelus. Note the compact feet and short metatarsals. Even with this leg design, it could easily outrun a human. Painting by Greg Paul, used for educational purposes only.

And Spinosaurus... a born slowpoke, eating giant lungfish and amphibians, no way to imagine how a spinosaurid could outrun a tyrannosaurid with that low-hunched body... but I get the idea that Hornerism is less about hard facts and more about getting famous (if Horner's input in JP3 was any clue) and the quickest way to do that is to start discrediting the anatomy and fearsome reputation of the world's most famous and "over-fanboyed" dinosaur species.

Spinosaurus by Miyess - one of the more reliable reconstructions out there. It is unlikely that this relatively long-bodied, short-legged animal could run fast, though contrary to some recent data-masking papers, it was almost certainly still a biped.

Well perhaps there's actually a good reason T. rex is so famous. It's not the biggest predator, it's not the fastest, and sure as hell isn't the prettiest, but simply the way it's built, the toughness of the skull and teeth, the binocular vision, its unusual speed for its size, and the sheer amount of abuse its body could take above and beyond other theropods before giving out, is truly remarkable for so many reasons. Of course other big theropods weren't forced to evolve to deal with 90% of their prey being a double-sized charging ceratopsid with a solid bone shield on its neck, but had the same pressures existed in other mesozoic faunas, T. rex would have had many imitators.

The funny thing is, none of this ever made me a "fanboy" of T. rex. I was never crazy about theropods, and even among tyrannosaurs I don't like T. rex all that much (ironically the same sentiment Horner expresses). I never had any delusions about it being "invincible" (indeed you could argue the only reason it developed in such an "overkill" direction was due to its prey animals nearly doubling in size, and sometimes armor, over the past few million years, and there were at least two contemporary species in the southern part of its range - not to mention many more in foreign lands and epochs - that could annihilate even the largest T. rex in one blow). Yet I can still admit that it's a very exceptional species, heavily specialized for crushing rather than slashing, and yet sacrificed less stride length and speed than just about any theropod of similar mass. The conditions that produced T. rex were very unusual. Some theropod would inevitably fill this sort of role. It turned out to be T. rex. I can admit this. What's stopping the Hornerites?


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