Stephanie E. Pierce, Museum of Comparative Zoology, Harvard University
Paleontologists have followed the evolution of the spine of mammals, according to Science . The first, even in reptiles, changed the cervical region, then in cynodonts, the ancestors of mammals, appeared thoracic department, and the very last – already in mammals – the lumbar division was differentiated.
It is well known that mammals are different from other animals: besides the fact that they feed the cubs with milk, they are warm-blooded, the vast majority of them are viviparous, they have a neocortex (areas of the cerebral cortex) and hair or wool. Among other things, mammals have acquired a complex spine, consisting of several departments. The spine is divided into cervical, thoracic, lumbar, sacral and caudal divisions. While in the spine of the ancient amniotes (higher vertebrates, which now include reptiles, birds and mammals), there were no cervical, thoracic and lumbar divisions, but there was no differentiated general “trunk”. Presumably, the appearance of these three divisions was associated with the appearance of different types of gait and respiratory functions ( 1 , 2).
American paleontologists, led by Stephanie Pierce of Harvard University, decided to answer the question of when the spine sections differentiated. In order to find out, they studied 16 perfectly preserved spinal synapsids or fur-like ones – a group of amniotes, which appeared about 318 million years ago. Synapsides included extinct terapsids (brutal reptiles) and cynodonts , the ancestors of mammals. In addition to fossils, the authors of the article analyzed the structure of thousands of vertebrates of modern animals, including mammals, reptiles and amphibians.
The results showed that the differentiation of the spine sections began about 270-280 million years ago. Reptiles of pelicosaurs (one of whose representatives was edaphosaurus) changed the length of the ribs and the position of the forelimbs. Next, in cynadont (in particular, in trinaxodon) about 250 million years ago, an additional “module” of the thoracic spine was added, which was accompanied by a restructuring of the thoracic girdle. They also decreased spinal ribs and increased mobility of the shoulder girdle. Finally, already in mammals, there was a differentiation in the lumbar spine, a part of the vertebrae lost its ribs.
Previously, scientists have shown that mammals began to lead a daily life only after the extinction of dinosaurs 66 million years ago. Before that, they were active at night and, thus, avoided interaction with predatory reptiles.