Origin and Evolution of Mammals. Permian Theromorphic Reptiles as Mammalian Ancestors. Formation of Chewing Teeth

Origin and evolution of mammals. Permian theromorphic reptiles - ancestors of mammals. Formation of chewing teeth This article will discuss the evolution of mammals and the formation of chewing teeth. The main ancestors of mammals are theromorphic reptiles, whose remains date back to the Upper Carboniferous, cotylosaurs. They lived in humid biotopes, as they retained many reptilian organizational features. This explains the large number of glands in the skin of mammals, as well as many other peculiarities. Mammalian ancestors had the following characteristics:

Amphicoelous vertebrae; Mobile cervical and lumbar ribs; Small brain size.

But along with these features, their teeth, which were located in alveoli, began to differentiate into such types of teeth as incisors, canines, and molars. At that time, the following features were also observed in mammal-like reptiles;

Secondary bony palate; Bicuspid or tricuspid occipital condyle; Double articulation of the jaw with the skull due to the articular and quadrate, as well as the dentary and maxillary bones;

Due to the articulation, there was an increase in the dentary bone, simultaneously with a decrease in the articular and quadrate bones. But at the same time, the articular bone did not fuse with the lower jaw. With the course of evolution, bones such as the articular and quadrate were modified, becoming part of the bony apparatus of the middle ear. In the Permian and the main part of the Triassic period, this type of reptile evolved and formed three groups:

Herbivorous; Carnivorous; Omnivorous.

All three species thrived in terrestrial biocoenoses. Their extinction dates back to the Jurassic period. The main reason for extinction is the inability to compete with archosaurs, which were more progressive at that time. Theromorphs of relatively small size were most likely driven into biotopes such as swamps, thickets, and the like, which were less favorable for life. In such conditions, there was a need to develop various sensory organs, complicate behavior, and strengthen communication between individuals of the same species. Due to displacement, theriodonts, or mammal-toothed reptiles, began a new line of development that was necessary for survival. Despite the division of groups of this type of reptile, structures and traits developed in them that eventually became characteristic of mammals. These traits included:

Tricuspid teeth formed; Upper nasal concha formed in the nasal cavity, which moistened and warmed the inhaled air; Soft lips formed, allowing mothers to suckle their young; An additional articulation of the lower jaw with the skull formed, running parallel to the reduction of the articular and quadrate bones; The cerebral hemispheres of the forebrain increased, etc.

Both extant subclasses of mammals could have originated from a single group of mammal-like reptiles with primitive tricuspid teeth – cynodonts, which were predators. This event is marked by the Triassic period. At that time, they already had a number of features characteristic of mammals:

Secondary palate, which strengthened the jaw apparatus; Differentiated dentition; Mammal-like physique.

{"translated_text": "Both subclasses of mammals that exist at the present time could have originated from a single group of mammal-like reptiles that had primitive three-cusped teeth – cynodonts, which were predators. This event is dated to the Triassic period.\n\nAt that time, they already had a number of features that are characteristic of mammals:\n\nSecondary palate, which strengthened the jaw apparatus;\nDifferentiated dental system;\nMammal-like body build.\n\nMost likely, they already had a diaphragm at that time, which separates the body cavities.\n\nHadrocodium, known at the present time, is the most ancient mammal, but it was smaller than a rat in size. Presumably, it lived 195 million years ago, alongside dinosaurs. The weight of this mammal was two grams. The skull length was 12 mm, which became the reason for the name of the mammal Hadrocodium, or \"Large Skull.\" Most likely, in terms of development level, it greatly surpassed all creatures that existed at that time. Based on the skull structure, one can hypothesize a developed brain, possession of extremely keen hearing and smell. However, despite this, its main diet consisted of worms and insects.\n\nThe future of this mammal is unknown. Hadrocodium could have been either an evolutionary dead end or our ancestor.\n\nHowever, the time and pathways of evolutionary development of the two subclasses of mammals remain unknown to this day.\n\nRegarding the subclasses of mammals, it can be said that they formed in the Upper Triassic. In both lineages, such features as double jaw articulation developed, as well as the establishment of occlusion of the lower and upper jaws, which contributed to the mechanical processing of food.\n\nThe subclass Prototheria, namely the remains of small animals with three-cusped molars Triconodontia, were discovered in Triassic deposits. They became ancestors of such groups as Multituberculata, or multituberculates, and Monotremata, which are known as monotremes. The first became extinct at the end of the Cretaceous period, while the second has such representatives as the echidna and the platypus.\n\nThe subclass Theria gave rise to the overwhelming majority of mammals known at the present time.\n\nConsiderable time was required to establish such a class as mammals. The development of the brain proceeded just as slowly. The ancestors of mammals are reptiles, and the most developed part of the brain was the cerebellum. Therefore, they can be called metencephalic animals. However, with evolutionary development, the brain developed and increased in size, and therefore mammals can be called telencephalic animals. This was a serious distinction of mammals from cynodonts, mammal-like lizards.\n\nAs was determined, mammals were small in size and resembled rats for two-thirds of their geological history, and could not play an important role in nature. The most serious progress was in the Cenozoic. But it was connected with sequential development, which led them to endothermy, increased energy levels, live birth, and nursing of their own young with milk. But the most important development was connected with the development of the sensory organs, the hormonal system, and the central nervous system. In the end, this led to the perfection of the organism as a holistic system, as well as provided for the complication of behavior. The final result of all these adaptations was the development of connections between individuals, along with the formation of dynamic groupings. The formation of social relations among mammals allowed them to find new ways to compete for position in biocenoses, and together with that, new methods of survival were created.\n\nAt the end of the Mesozoic – beginning of the Cenozoic, the Alpine cycle of mountain formation greatly changed the Earth: high ridges appeared, the climate became continental, and seasonal contrast increased considerably. The expression of seasons led to strong cooling throughout the planet. This led to the establishment of angiosperms, particularly dicots, while cycads and gymnosperms became impoverished. Similar changes in flora created big problems for all large reptiles that existed at that time, but warm-blooded mammals and birds, which were smaller in size, adapted much more easily to the new changes. For successful competition with reptiles, mammals and birds switched to feeding on small animals, high-calorie fruits, as well as vegetative parts of angiosperms, which allowed them to begin intensive reproduction. Each of these factors led to the extinction of reptiles, which marked the end of the Mesozoic era. Development, wide adaptation, and spread of mammals along with birds opened the Cenozoic era.\n\nTatarinov L.P. Origin of Mammals, \"Priroda\", 1975, No. 8\nhttps://www.youtube.com/watch?v=Iij0Fe1fAQs\nhttp://medbiol.ru/"}