More than two hundred and fifty years ago, in 1758, Carlos Linnaeus coined the term “mammal” to refer to one of the large groups of vertebrates. Mammals are characterized because they are covered in hair, they nurse their young and, above all, because they have three little bones in the middle ear. It’s easy to distinguish living mammals from other vertebrates, such as birds and reptiles, but as soon as we add fossils, things get complicated. Because very few fossils preserve hair or breasts, and not so many include ear bones; of many small ancient mammals, all we have are teeth.
In 1988, the American paleontologist Timothy Rowe defined mammals as a terminal group, which in phylogenetic terms refers to the group formed by the common ancestor of all living mammals and all their descendants, extinct or not. There are other definitions, but in any case, the problem remains. There are some groups of fossils whose position in the evolutionary tree is not clear, we do not know if they are true mammals, or if they branched out before the appearance of these. One of these groups is the Gondwanatherians, who take their name from the supercontinent of Gondwana, made up of South America, Africa, Arabia, Madagascar, India, Australia and Antarctica, which began to break up in the Jurassic. Gondwanaterians lived on these continents from the Cretaceous, about 70 million years ago, to the Miocene, 17.5 million years ago. Until very recently, we only knew of them from teeth and jaw fragments, and from a single skull. Although at first they were classified with sloths, armadillos and anteaters, today it is clear that they are a more primitive group, which is either situated between monotremes and marsupials and placentals, and in that case they would be mammals. , or else it separated previously from the common trunk that later gave rise to these three groups, and then they would not be.
Gondwanatherians are herbivorous animals that are characterized by their thick and robust snout. Among his teeth there is no clear distinction between molars and premolars; their chewing teeth are collectively called molariform. Their way of chewing was different from that of other mammals: they moved their jaws back and forth. Most of the Gondwanatherians described belong to the Sudamericidae family, which, thanks to the high crowns of their molariform teeth, were the first specialized grazers. There are two other families: the feruglioteridae and the groeberidae, which were more generalist herbivores.
Among the South Americans, the best known is Vintana, which lived in Madagascar at the end of the Cretaceous. Only one skull without a jaw has been found, but it is supposed to have been an animal with the appearance and size of a marmot, with highly developed sense of smell; weighing about nine kilos, it was one of the largest mammals of its time. It was also a South American Patagonian, the last gondwanatherium, which lived in Argentina at the beginning of the Miocene, about twenty million years ago; It was a burrowing animal that, like modern rodents, had rootless incisors that never stopped growing. Other genera in the family are Gondwanatherium, from the Late Cretaceous of Patagonia; South America, which lived in Argentine Patagonia and Antarctica from the Middle Paleocene to the Lower Eocene; Lavanify, Malagasy for “long tooth”, which is only known from two teeth found in Madagascar and dated to the late Cretaceous; Bharattherium, “animal of India”, from the Sanskrit term for India, “Bharat”, which also lived in India at the end of the Cretaceous and seems to have survived the extinction of the dinosaurs; Galulatherium, “Galula animal”, after the name of the site where it was found, from the Middle Cretaceous of Tanzania; and Greniodon, from the Eocene of Argentina, of which only two teeth have been found.
Feruglioterids are only known from a few teeth and a jaw fragment. They lived in swampy, fluvial, or coastal environments of southern South America during the Late Cretaceous, and were herbivorous, omnivorous, or insectivorous. They are small animals, less specialized than the South Americans; the crowns of their molariforms are not as tall as theirs, although some have a sharp, blade-like crest, which may have given rise to the tall crests of South Americans. Ferugliotherium, named after the 20th century Italian naturalist and geologist Egidio Feruglio, explorer of Patagonia, was a small animal weighing about seventy grams, with long incisors similar to those of rodents. Other genera are Trapalcotherium, “animal from the Trapalca basin”, due to the place where it was discovered; Argentodites, “Argentine traveller”, both known for only one tooth each; and Magallanodon, “tooth from the Magallanes region”. All of them have been found in Argentina, except for Magallanodon, discovered in southern Chilean Patagonia.
The third family, that of the groeberids, lived in southern South America during the Eocene and Oligocene periods, and were initially taken to be marsupials related to the South American runcho mice. They were also herbivores. The genera Groeberia, from the Eocene, and Klohnia, Epiklohnia and Praedens, from the Oligocene, form part of this family.
Sometimes the discovery of a single fossil skeleton revolutionizes our knowledge of an extinct group. This is the case of the Adalatherium gondwanatherium, from the Late Cretaceous of northwestern Madagascar, whose discovery was announced in April 2020. The almost complete skeleton belonged to an immature youngster the size of a cat and badger-like, about two feet tall. long, and with an estimated weight of about three kilos. It is the only gondwanatherium skeleton that we know of.
The extraordinary number of foramina, ducts for nerves and blood vessels, indicates that the snout of Adalatherium is very sensitive, with a large number of vibrissae, those stiff hairs that some mammals have on their whiskers, and that have a tactile function. In the bone at the top of the snout there is a hole whose function is unknown; no other mammal has it. The vertebral column is made up of at least thirty vertebrae, more than any other Mesozoic mammal. The tibia is curved. Adalatherium’s posture is more erect than that of other mammals of its time, although not as erect as modern mammals. It is supposed to have lived in burrows, although it could also be a fast runner. It lived with dinosaurs, primitive birds, and the giant toad Beelzebufo, which we have already talked about in Fossil Zoo.
Adalatherium means “crazy animal”, from the Malagasy “adala”, which means “crazy, crazy”, and the ancient Greek “terio”, “animal”. Because their anatomy is a strange mixture of primitive characters with others that did not appear in more advanced mammals until much later, and some of their features are unique, and their function is still not well understood. So his discovery raises more questions than it answers. We still need more fossils to better understand this group of primitive mammals or proto-mammals.
CONSTRUCTION OF GERMAN FERNÁNDEZ:
reticular infiltrate
Reticular Infiltrator is the first novel in the trilogy The Saga of the Borelians. Do you want to see how it starts? Here you can read the first two chapters.
The Karnak file. Ed.Rubeo
The hanged man and other fantastic tales. Ed.Rubeo