Theropod dinosaurs, which appeared more than 230 million years ago, walked on two legs and reached a great diversity of sizes. Its dimensions ranged from gigantic —with sizes that no other terrestrial predator reached— to miniature species, such as Anchiornis huxleyi, which lived at the end of the Jurassic period and weighed just over 100 grams and measured about 30 centimeters in length. Although birds are direct descendants of coelurosaurs, a clade of these archaic animals, there were others that did not have ‘heirs’. These are non-avian theropods.
Now, an international team of researchers, led by Michael D. D’Emic of Stony Brook University (USA), in collaboration with Argentine institutions such as CONICET and the Universidad Nacional de Río Negro (UNRN), among others, has just presented the first comparative phylogenetic analysis that examines the developmental strategies that would explain the evolution of body size in these non-avian theropods.
The work, published this week in the journal Science, makes it possible to understand the dynamics of evolution through heterochronic mechanisms, that is, the speed and pace at which a species develops. In this case, they studied differences in growth rates and their duration in around fifty extinct species.
Theropods walked on two legs and reached sizes that no other land predator reached.
The scientists were able to reconstruct ancestral states of growth rate and body mass in a taxonomically rich dataset. Thanks to these measurements, they found a new key to enter that disappeared natural world. In this way, they know that within the same lineage of carnivorous dinosaurs, although the growth strategies were different, there were primitive forms (slow growth, but sustained over time) and more advanced modes (rapid growth that stopped at a certain time). moment in life), both growth styles appear equally distributed in evolution.
Both strategies are present in non-avian theropods and there is no imbalance between the appearance of one and the other throughout evolution, which gave rise to the great disparity in body sizes of these animals. This finding can be transferred to the evolution of amniotes (a branch of vertebrate tetrapods) in general, according to the researchers.
To put these ‘ways of growing’ in context, Rodolfo Coria, professor at the Department of Vertebrate Paleontology at UNRN, explains to SINC that, among vertebrates, “primates grow very quickly, with a high initial rate, which is truncated, as they also happens in birds (and in the theropods closest to birds).” That’s why “we don’t have giant birds,” he says.
On the other hand, “other gigantic forms of theropod dinosaurs, more basal, show a very slow growth rate that lasts over time; continuously, continue to grow until very advanced stages of life”, highlights coria. This type of slow, sustained growth over most of their existence led some non-avian theropods to “develop immense sizes, unprecedented in nature”, in the words of the paleontologist. Today there are no land animals that exhibit this style of evolution.
Downsize to eat better
It took about 50 million years of shrinking for a dinosaur lineage to find its way into birds, according to previous research released over the past decade. Thus, scientists pointed out that progress in body size would have been a liberating step for dinosaurs, allowing them to explore new lifestyles and habitats. Suddenly, more agile species with insulating wings and feathers could climb trees (to escape predators), chase insects, jump and glide to obtain new food resources, which would have given rise to this branch of evolution. . With smaller bodies, larger brains and eyes that enhanced three-dimensional vision, they could recognize masses of trees and engage in nocturnal activities.
Primates have very rapid and truncated growth; the most basal dinosaurs grew slowly but steadily over time
Although this study is limited to the other branch of theropod dinosaurs, it predicts that “various growth strategies will be recognized in other clades (branches) of animals”, so these discoveries open a new door to understanding the evolutionary history of “taxa that evolved very large and very small body sizes” and even “closely related species that may exhibit widely disparate sizes,” the authors write.
Asked about the magnitude of these discoveries, Coria says that “they provide a theoretical and verifiable context for identifying the evolutionary strategies of some dinosaur lineages, in particular, with regard to the development of size”.
Grow like a tree, almost forever
The predominant mechanisms that would explain the evolution of gigantism and miniaturization have been little studied within a comparative phylogenetic framework. The reason? There were few abundantly sampled long-lived clades containing a diversity of body sizes to answer these questions, according to the authors.
As the Argentine researcher puts it: “Many paleontological explorations were necessary to gather more evidence and allow the elaboration of cladograms or phylogenetic tables full of species, worked in detail, because this is an essential condition to carry out an analysis of evolutionary rhythms”.
Until half a century ago, without going any further, “we knew about a dozen non-avian carnivorous dinosaurs, and now there are hundreds,” he says. “This accumulation of evidence has allowed us to risk hypotheses that would be impossible without it,” says Coria, for which “the contribution is supported in a phylogenetic context that has only now been reached, through the analysis of many specimens discovered in the last 30 years”.
This study opens the door to discovering the growth strategies of closely related species that can exhibit very different sizes.
This time, Michael D’Emic and his colleagues performed a large-scale comparative phylogenetic analysis that allowed them to examine underlying developmental strategies in non-avian theropod dinosaurs, which can range from tiny (less than 0.5 meters long) to gigantic. (more than 12 meters).
To do this, the team took fossil measurements of 42 non-avian species, including cortical annual growth rings, and compiled comprehensive histological data on body size and growth rate, demonstrating that changes in growth rate and duration played a role. nearly equal roles in evolution. of size diversity.
On the possibilities of applying these results to the protection of current species, Coria replies: “Although I don’t see a direct implication, I imagine that what the phylogeny of current species tells us could be investigated at a molecular scale, and check if the results are comparable. However, our conclusions lead us to think that the size development strategies in theropod dinosaurs would be repeated in all amniotes”, he concludes.
D’Emic, M. et al. “Developmental strategies underlying gigantism and miniaturization in non-avian theropod dinosaurs”. Science (2023).