The T-rex-type reared up roaring, its massive head silhouetted against the sky, the ground shaking as the giant carnivore stomped on its huge hind legs in pursuit of its terrified prey. Its dinky little arms might also have been silhouetted against the darkening sky, at some angles at least, causing nobody’s heart to stop.
The thing is, it wasn’t only the iconic Tyrannosaurus rex that boasted this seemingly odd body structure of gigantic toothy head, bipedal locomotion and “ridiculously” tiny little arms. Some other unrelated giant carnivorous dinosaurs have the same, such as carcharodontosaurids and abelisaurids. And now a new one has been found.
Last week, paleontologists Juan Canale, Peter Makovicky and colleagues reported the discovery of yet another previously unknown giant mega-predatory dinosaur with these proportions: Meraxes gigas, in Patagonia, in the journal Current Biology. Meraxes also had tiny little forelimbs. It looked a lot like T. rex, whose proportions would be akin to a six-foot-tall person having arms five inches long.
But the lookalike mega-predator dinosaurs, now including Meraxes, are not closely related. Their huge heads, wee arms and bipedalism are a case of convergent evolution among giant carnivorous theropods, the new paper posits.
Convergent evolution is when unrelated animals evolve similar solutions, causing them to superficially look alike. Wings, for instance, evolved independently in birds, cockroaches and bats (yes, cockroaches can and do fly). Shark (fish) and the orca (mammal) have similar body shapes. Ditto the snake and the legless lizard, both of which had ambulatory ancestors.
In the case of flying, we instantly get the logic because we have wing envy. We are less envious of the snake: research suggests that lack of limbs actually facilitates burrowing and the subterranean life. Legless lizards “swim” through the ground rather than claw it out, experts explain. The point is that leglessness has separately evolved over 60 times in the squamate set, it is estimated, so clearly it does good.
Among mega-predator bipedal monster dinosaurs, not all follow the big-head small-arms format. But a bunch did, stemming from ancestors that had longer front legs. “Ancestrally, small tyrannosauroids like Dilong and Eotyrannus have relatively long arms with proportions similar to other theropods of equivalent size,” Makovicky explains. “The same is true of the early relatives of Meraxes like Allosaurus and Concavenator. It’s actually only the latest and largest members of the three short-armed lineages that show the remarkable degree of arm reduction.”
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What might be the selective pressure leading a mega-predator to all but lose arm function, theoretically speaking? And who was Meraxes?
A giant rises
We cannot say if the T. rex roared per se, though Hollywood is in thrall to that image. Some suspect that it vocalized like emus do today – cooing, hooting and possibly issuing deep booming sounds. But to be fair, who’d pay to watch a lost-world movie where a T. rex with downy ankles ambles along clucking, scavenging fly-blown carcasses and scratching at fleas?
The movies also tend to show the rex-type’s wee arms curled into a sort of fetal position. In addition, the deathless “Jurassic World” sequence pitting the imaginary Indominus rex against an imagined T-Rex shows them curling their tiny fingers into tiny fists and, when the CG animals strike at one another (roaring), they are shown angrily but futilely flailing their dinky little arms while attempting to bite each other in the jugular.
Which leads us back to the science and to the postulation that these animals were using their mouths in the stead of their arms, and hence could afford to lose their arms.
Meraxes is named “after a dragon of the ‘Song of Ice and Fire’ fiction series by George R.R. Martin,” better known as Game of Thrones, the authors write. The addition “gigas” (Greek for giant) refers to the species’ enormous size. Wondrously, the fossil the team recovered from the Upper Cretaceous Huincul Formation in northern Patagonia was relatively complete, including an almost-complete arm and skull.
The skull was 1.27 meters in length, about four feet long, and its arms half that length. The animal itself measured over 12 meters in length, just shy of a semitrailer.
It also had an “ornamented” face, which means lumpy. The team suspects its facial ornamentation played a role in social signaling, just as ours does.
Canale, Makovicky and the team believe the key evolutionary tradeoff is the growth of the head against the diminishment of the arms. The bigger the head and jaws, the more powerful the bite. They suspect the arms were actually reduced as a consequence of the skulls becoming massive.
Asked how confident he is in the cause and effect, Makovicky said he’s reasonably confident. “For decades, people have debated the reasons and selective pressures that might be responsible for arm reduction in large tyrannosaurs and other big theropods,” he says.
The suggestions have been many and myriad, including a new one dating to April: that the T. rex either lived or dined in packs, and puny arms meant they wouldn’t bite off each other’s limbs during a feeding frenzy. Makovicky explains that this theory is based on untested assumptions, such as group meals in large theropods.
“Another hypothesis is that the arms were not directly being selected for or against, but rather that selection for optimizing some other trait (e.g. skull size) had the downstream effect of arm length being reduced,” he says. “Skull size as the trait being directly selected for is an obvious choice for a large predator, which is why we tested it, but there is of course a small possibility that it is some other aspect of the anatomy that hasn’t been thought of yet.”
Supporting the association between skull growth and arm diminishment, Makovicky points out that other big theropods like Therizinosaurus and Deinocheirus have very long arms and small heads.
“This clues us in that it’s not the overall body size, but the size of the head that is a determining factor in arm length, for which we found some statistical support,” he says. “Hence, we hypothesize that as arm function is being transferred to the skull during the evolution of these big predatory lineages, the arms are used less and can therefore shorten.”
Meaning what? That these creatures grabbed prey with their mouths. “As dexterous primates, we humans find it hard to imagine an existence without arms, but among big theropods this may have been a very successful model as it evolved at least three times,” Makovicky sums up.
“There is an odd lower bound to how short arms get in non-avian theropods (about 40 percent of femur length),” he adds. “We are trying to figure out why.”
Their working hypothesis is that even in the species with the shortest forelimbs, like Meraxes, the shoulder bones are not reduced, and since the shoulder bones anchor muscles that insert on the arms, that places a structural constraint on how short arms can get, he explains. This is supported by parallels in descendants of dinosaurs – the birds. Hesperornithoform, an extinct diving bird, had reduced shoulder bones and the giant moas had none, he says.
The bottom line is that this subset of unrelated giant carnivorous dinosaurs developed the huge-head short-armed solution three times, so we can assume it worked well for them.
Further attesting to this model’s success, the professor points out that as many as four lineages of carcharodontosaurid overlapped in time in prehistoric Patagonia. For big dinosaurs requiring big ranges, that constitutes high diversity.
“Only Late Cretaceous tyrannosaurids of North America have comparable species richness over a relatively short time span,” Makovicky says.
Sadly, the carchardontosaurids, including our Meraxes, were not among the dinosaurs that survived the great extinction event 66 million years ago. But before they died out, they and their facial ornamentation were at their peak, and we can take comfort in the thought that they went out at the top of their game.