Synapsids

The synapsids ('single-arched') are distinguished by presence of a single opening behind the eye socket and one temporal arch. The most primitive synapsids, the pelycosaurs, beginning from the end of the Carboniferous Period, became most diverse in the Early Permian of North America. In the Late Permian of East Europe very few representatives of the pelycosaurs are known, among them are the Ennatosaurus and the Mesenosaurus. Very diverse advanced synapsids, the therapsids ('beast-arched'), inhabited all continents during the Late Permian, but only few survived to the Triassic. These animals are the ancestors of mammals. At the end of the Paleozoic Era, the therapsids dominated the fauna of terrestrial vertebrates in both the southern continent, Gondwana, and the northern one, Laurasia. The richest records of therapsids were collected in the Permian deposits of East Europe. The earliest forms come from the early Late Permian and belong to the eotheriodonts ('early beast-toothed'). In the 'Russian Dinosaur Exposition' they are presented by a predatory Eotitanosuchus and a huge herbivorous Estemmenosuchus. Descendants of theriodonts formed several large groups. The Dinocephalia ('horrible-headed'), distinguished by an extraordinary thickened skull roof, replaced the eotheriodonts till the middle of the Late Permian. Like the eotheriodonts, the dinocephalians included large predatory forms (Titanophoneus) as well as large herbivorous animals (Ulemosaurus). The Anomodontia ('abnormal-toothed') lived at the same time as the dinocephalians. The advanced representatives of the anomodonts, the dicynodonts ('double-fanged') have only two very large fangs in the upper jaw. All known anomodonts are herbivorous (Dicynodon, Lystrosaurus). The principal therapsid group is the theriodonts ('beast-toothed'). In the exhibition, they are presented by a large predator Inostrancevia and two therocephalians ('beast-headed'): piscivorous Annatherapsidus and small insectivorous Silphedosuchus. The most important in evolution are advanced theriodonts, the cynodonts ('dog-toothed'), represented in the exhibition by Dvinia. Cynodonts are related most closely to the primitive mammals. These two groups share many common features, in particular, broadened teeth with a number of cusps, suitable for food chewing, and presence of bony palate, separating the respiratory tract from the mouth cavity. Possibly, the theriodonts were also covered by primitive fur that could prevent drying rather than provide insulation. Moreover, the cynodonts, like mammals, probably possessed the diaphragmatic breath. Specialization of the jaw apparatus in the theriodonts 'released' some bones of the lower jaw, which in strongly reduced and transformed state make up the structure of the middle ear in mammals. However, the most essential aspect in the mammalian evolution was a progressive development of the brain that begun already in some advanced theriodonts.