Mammalization of theiodonts

One of the first isolated groups of ancient amniotes in the second half of the Paleozoic is synapsids . They formed the basis for the formation of a subclass (other researchers distinguish them into a separate class) of beast-like ( Latin Theromorpha ), also called therapsids ( Latin Therapsida ). In the Permian period, a group of beetles ( Lat. Theriodontia ) was formed among them. They turned out to be close in level of organization to mammals.

However, the living conditions prevailing in the Mesozoic era favored evolutionary development and an increase in the diversity of sauropsids , and in the Mesozoic, archosaurs dominated. The body sizes of the animals were reduced, their numbers and distribution were reduced, they were forced to be content with secondary niches. Their potential was realized later, which, obviously, is associated with the mass extinction of archosaurs and climate change at the end of the Mesozoic.

Curiously, the appearance of the first dinosaurs was marked by the massive, but not complete extinction of therapies, whose higher forms were very similar in their organization to single-pass mammals and, according to some assumptions, had mammary glands and wool ^[2] .

The closest to the mammal among the animals was the cynodonts ( lat. Cynodontia ). The most striking features of skeletal changes were found among them in trinaxodon ( lat. Thrinaxodon ) from the early Triassic .

The mammary glands originated in ancient mammals, probably from mutated sweat glands . Perhaps the initial function of the mammary glands was not so much feeding as “feeding” the cubs, that is, supplying them with the necessary moisture and salts.

In the subsequent formation of mammals, paleontologists emphasize changes in the dental system. This led to the isolation of two groups - morganukodontid ( lat. Morganucodontidae ) and cuneotherides ( lat. Kuehneotheriidae ).

The descendants of early mammals found in sediments of the Upper Triassic include peculiar many-tuberous , which got their name in connection with the presence of numerous tubercles on molars. It was a specialized group of animals with very strongly developed incisors and without fangs. Many-tuberous ones represented specific herbivorous animals, and they cannot be considered the ancestors of subsequent groups of mammals. True, hypotheses have been advanced that their earlier forms gave rise to single-pass , but these hypotheses are outdated.

The second group was more successful in subsequent adaptive radiation . Their main line was eupantotherium ( lat. Eupantotheria ; this term, however, is almost out of use, since the group of mammals designated by it is paraphyletic and includes quite a variety of Mesozoic mammals). They were probably small animals that fed on insects, maybe other small animals and reptile eggs. Their brain was small, but larger than non-mammalian synapsids. At the end of the Mesozoic, in this group there was a separation into two independent trunks - the lower, marsupial ( lat. Metatheria , lat. Marsupialia ), and higher, placental ( lat. Eutheria ).

Placental mammals, like marsupials, arose in the Cretaceous . Studies have shown that in the Cretaceous period they have already evolved in very different directions.

The origin of mammals can be traced from the following chain ( cladogram ), which reflects the phylogeny of synapsids:

  Synapsida (Synapsids) ( Pelicosaurus )
  | - Caseasauria ( Eotirides and Caseids )
  `- Eupelycosauria
     | - Varanopseidae
     `- + - Ophiacodontidae
       `- + - Edaphosauridae
         `- Sphenacodontia
            | - Sphenacodontidae
            `- Therapsida ( Therapsids )
               | - Biarmosuchia
               |  `- Eotitanosuchus
               `-Eutherapsida
                  | - Dinocephalia
                  `-Neotherapsida
                    | - Anomodontia
                    `- Theriodontia ( Theriodonts )
                        | - Gorgonopsia
                        `-Eutheriodontia
                          | - Therocephalia
                          `- Cynodontia ( Tsinodonts )
                             | - + - Dvinia
                             |  `- Procynosuchus
                             `- Epicynodontia
                                | - Thrinaxodon
                                `- Eucynodontia 
                                   | - + - Cynognathus
                                   |  `- + - Tritylodontidae
                                   |  `- Traversodontidae 
                                   `- Probainognathia
                                      | - + - Trithelodontidae
                                      |  `- Chiniquodontidae 
                                      `- + - Prozostrodon
                                         `- Mammaliaformes
                                             `- Mammalia ( Mammals )

• Adelobazileus

• Carroll R. Paleontology and the evolution of vertebrates: In 3 vols. T. 2. - M .: Mir, 1993 .-- 283 p. - ISBN 5-03-001819-0 .
• Carroll R. Paleontology and the evolution of vertebrates: In 3 vols. T. 3. - M .: Mir, 1993 .-- 312 p. - ISBN 5-03-001819-0 .

• Tatarinov L. P. From the book “The Evolution of Theiodonts”