Glyptodonts, extinct relatives of armadillos that were very similar in size and shape to a Beetle, emerged thanks to an evolutionary explosion that is quite rare among mammals, says a new study signed by a trio of Brazilian researchers.
The accelerated evolution of critters, which may have reached about a ton in weight and disappeared around 10,000 years ago, appears to have been a phenomenon relatively as rapid as the emergence of the human lineage from their primate ancestors. The most likely hypothesis is that glyptodonts made this leap by adapting to a new kind of way of life, that of grazing — they became, for all intents and purposes, immense lawnmowers.
Such conclusions are in a recent article in the British scientific journal Proceedings B. The study is signed by Alex Hubbe, from the Federal University of Bahia, Gabriel Marroig, from USP, and Fabio Machado, from the Polytechnic State University of Virginia (USA). The researchers compared the skull structure of Ice Age “super armadillos” (members of the genus glyptodon) with that of 14 species of their much more modest cousins ​​today.
One of the team’s goals was to examine a relatively well-established idea for anyone studying evolution. It turns out that, in most of the millions of years over which species evolve, what seems to prevail is the so-called stabilizing selection, which prevents the characteristics of living beings from changing excessively.
This is probably due to the fact that, after the organism has adapted reasonably accurately to a certain way of life, the chances that a very large change will be more disruptive than helping it to survive and reproduce. It’s as if evolution followed the maxim “In a winning team, don’t move”.
However, fossils clearly show that sometimes more radical changes work. This is how land mammals turned into whales or bats, for example. “The problem is that, in these examples, the time scale in which this happened is quite large, and we don’t have very good analogues of the ancestors of these animals still living today”, explains Machado. “Glyptodonts get around that because the DNA data shows that they’re right there in the middle of the armadillos, so to speak. It’s a lot easier to do that kind of analysis.”
Our row of bottom teeth ends just before the ear. In their case, it is as if the jaw starts at the height of the Adam’s apple and ends behind the ear. They violate all supposedly general rules of mammalian skull biomechanics.
Another factor is that the skull of the animals, compared to the armadillos we know, is very peculiar. It’s as if the central part of the skull has turned into modeling clay and been pulled down. Also, the position of the teeth at the back of the mouth also seems to be completely wacky.
“Our row of bottom teeth ends a little before the ear. In their case, it’s as if the jaw started at the height of the Adam’s apple and ended behind the ear”, compares the Brazilian researcher. “They violate all the supposedly general rules of mammalian skull biomechanics.”
All this was evidence of a somewhat different evolutionary process, but the researchers were able to quantify and compare this oddity with what happened throughout the evolution of “classic” armadillos. The process involves mathematically analyzing how the distances between different points on the skull of all these species have changed. This helps to compute the joint evolution of the cranial bones, which, after all, need to transform in relative harmony as the characteristics of an animal lineage change over time.
With this, it is possible to estimate the rate of evolutionary transformations over millions of years, that is, to have an idea of ​​which animals underwent more intense changes in the same time interval. Result: glyptodonts are, by far, the champions of the so-called directional selection. That is, an evolutionary process that clearly pushed them to a certain side, instead of stabilizing selection, relatively averse to change.
The specialization to devour grass indicated by the skull wasn’t the group’s only evolutionary oddity. The enormous size, the solid hull and the tail that looked more like a medieval knight’s mace indicate that the animals became occupants of a very specialized ecological niche. And that’s not always a good idea, judging by the group’s demise.
“The factors that led to their disappearance were probably several, but extreme food specialization may have been one of them because at the end of the Ice Age South America experienced a large loss of undergrowth areas and gained many forests”, which would not be good for glyptodonts, says Machado.
