In the latest installment of the Jurassic saga, Jurassic World Dominion , many animals appear that we have not seen before. The list of new dinosaurs is long, but we have also had the privilege of seeing other prehistoric animals on the big screen, which many fans have been waiting for since the premiere of Jurassic Park in 1993 . One of them is Dimetrodon .
This peculiar animal already appeared in the official toys of the first installment of the saga. However, film after film, we had never seen it in the cinema, until now.
Who is Dimetrodon ?
Dimetrodon is a carnivorous animal that lived in the Early Permian , between 295 and 272 million years ago. It was discovered in 1878 by Edward Drinker Cope , one of the protagonists of ‘the war of the bones’, a sad episode in the history of paleontology. His name means “teeth of two measures”.
Dimetrodon has the appearance of a large lizard, with a laterally flattened and rounded head, although its distinctive feature is a huge sail on its back, supported by the expansions of the dorsal vertebrae.
This curious-looking animal is often called a “dinosaur,” but it isn’t. This is a fairly common confusion, and it can be resolved with a simple rule. Among many other features, all dinosaurs share a common characteristic: the legs are not placed on the sides of the body, as in a lizard or a crocodile, but in a vertical position, in the lower part of the body, as in mammals or in birds. Remember that birds are dinosaurs .
In Dimetrodon , the legs are placed laterally, like those of a crocodile , therefore, this feature would already be enough to exclude it from the group of dinosaurs. In addition, the diversification of the dinosaurs begins at the end of the Triassic period, about 210 million years ago. The time that elapsed since Dimetrodon lived until the diversification of the first dinosaurs is approximately the same as that elapsed since the extinction of Tyrannosaurus rex until the appearance of the human being.
The ‘mammalian reptiles’
The group of animals to which Dimetrodon belongs has traditionally been called ‘ mammalian reptiles ‘, because their anatomical features appear to be an intermediate version between reptiles and mammals, more similar to reptiles the older they are. But since phylogeny began to be used as a way of classifying living beings, this term fell into disuse and is now considered outdated.
Phylogeny is the study of the origin, formation, and evolutionary development of biological species and groups . The classical system –prior to phylogeny– classifies living beings based on their similarities and differences; however, the phylogenetic system classifies them based on their evolutionary origin. And when this new methodology is applied, sometimes the classic classification does not make sense, and it is necessary to correct or restructure it.
One of the rules of phylogenetic classification is that living things always belong to the same phylogenetic group as their ancestors. That is, all the descendants of the same ancestor belong, without exception, to the same group as their ancestor. This property is called monophyletism . If you group several living beings in a group to which their common ancestor does not belong — polyphyletic — or if you exclude some descendants — paraphyletic —, that classification is incorrect in terms of phylogeny.
Synapsids: Reptiles or non-reptiles?
If we look back and look at the ancestors of mammals, we find these animals that were traditionally called ‘mammalian reptiles’, and among which Dimetrodon is included. Following the rules of phylogeny, if these animals are reptiles, then mammals must also be reptiles , otherwise we would be excluding some descendants of the group, and remember that paraphyleticism, in phylogeny, is incorrect.
The logical alternative is to reject the category of “reptile” for mammals, but then no animal belonging to the evolutionary lineage that gave rise to mammals could be a reptile. So Dimetrodon could not be called a ‘reptile’ , or a ‘mammal’, or anything else.
Science, as usual, proposes a solution to this mess. A new group of animals was created: Synapsida , separated from the group called Sauropsida, which would be the reptiles. Both groups descend from a common ancestor, which designates the Amniota group, and designates all animals whose egg has an amniotic membrane —currently, those we call reptiles, birds and mammals—.
The sauropsids, let us remember, true reptiles, diversified into the scaled —lizards and snakes—, the testudines —turtles—, the crocodiles and the dinosaurs , in addition to many other groups that are now extinct, such as ichthyosaurs, pterosaurs or plesiosaurs.
For their part, the synapsids had their own diversification , very wide and varied, especially during the Permian —here would be our beloved Dimetrodon— , and one of its branches finally gave rise to the lineage from which the mammals descend.
That is to say, that we, the mammals, belong, after all, to that Synapsida group , because, let’s remember, monophyletism is maintained. And so Dimetrodon is, in evolutionary terms, much closer to us mammals than it is to any reptile, modern or extinct, dinosaurs included.
References:
Laurin, M. et al. 2012. Amniota: Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives. Tree of Life web project.
Souza, RBB de et al. 2021. Allometric relations of respiratory variables in Amniota: Effects of phylogeny, form, and function. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 252, 110845. DOI: 10.1016/j.cbpa.2020.110845
Werneburg, I. et al. 2009. Timing of organogenesis support basal position of turtles in the amniote tree of life. BMC Evolutionary Biology, 9(1), 82. DOI: 10.1186/1471-2148-9-82
