A paleontologist answers The product of the earliest divergence of amniotes and index fossils of the Cenozoic

Mammals are the only extant descendants of the synap-sids—the first well-established group of amniotes, named after a rounded temporal opening behind the orbit bordered by the jugale and squamosum bones. Since the beginning of amniotes, evolution of synapsids proceeded separately from the other amniotes, which later diversified in particular reptile lineages including dinosaurs and birds. The first amniotes recorded from the middle Carboniferous (320 million years ago) were just synapsids and just this clade predominated in the fossil record of the terrestrial vertebrates until the early Triassic. A large number of taxa appearing among early synapsids represented at least two different clades: Eupelycosauria and Caseasauria. The former included large carnivorous forms and the latter were generalized small- or medium-sized omnivores. Since the middle Permian (260 mya), another group of synapsids called Therapsida dominated the terrestrial record. In comparison with pelycosaurs, therapsids had much larger temporal openings, a single pair of large canines, and clear functional and shape differences between the anterior and the posterior teeth. Two lineages of that group, Di-cynodontia and Cynodontia, survived the mass extinction at the Permian/Triassic boundary (248 mya).

Immediate ancestors of mammals are found among the cynodonts. Mammals are closely related to cynodont groups called tritylodontids and trithelodontids, which first appeared during the late Triassic. All three groups, including mammals, had additional cusps on posterior teeth, a well-developed ramus mandibulae, and a complete secondary palate. In some of them (Diarthrognathus), the jaw joint was formed both by the original articulation (articulare-quadra-tum) and by the mammal-like process (dentary-squamosal). In the oldest true mammals, the former jaw articulation is abandoned and removed in the middle ear. These characters are the index diagnostic features of a mammal in the fossil record (no. 23, 26, 27 of the above list).

The oldest mammals, Sinoconodon, Adelobasileus, Kuehneo-therium, or Morganucodon (about 200-225 million years old), were all very small, with long heterodont dentition and a triangular arrangement of molar cusps designed for shearing. They were most probably quite agile night creatures resembling today's insectivores. The relative brain volume in the earliest mammals was close to that found in extant insectivores and about three times higher than in cynodonts. Of course, they still differed from the modern mammals in many respects. The derived characters of modern mammals (as reviewed in the preceding text) did not evolve together but were subsequently accumulated during the long history of synap-sid evolution.

In contrast to the medium- to large-sized diurnal dinosaurs, birds, and other reptiles that had dominated the terrestrial habitats, the early mammals were quite small, nocturnal creatures. Nevertheless, since the Jurassic period they grew in greatly diversified groups and at least four lineages of that radiation survived the mass extinction at the Cretaceous/ Tertiary boundary (65 mya). Three of these groups, mono-tremes, marsupials, and placentals, are extant; the fourth group, multituberculates, survived until the end of Oligocene. Multituberculates resembled rodents in design of dentition (two pairs of prominent incisors separated from a series of cheek teeth by a toothless diastema), but their cheek teeth and skull morphology were quite different from those in any other groups of mammals.

The major radiation of mammals appeared at the beginning of Tertiary, in the Paleocene. That radiation produced many groups that are now extinct (including nine extinct orders) as well as almost all the orders of modern mammals. During the Paleocene and Eocene, other groups occupied the niches of current mammalian groups. In Eurasia and North America it was Dinocerata, Taeniodonta, and Tillodontia as herbivores and Pantodonta and Creodonta as their predators. All these are extinct lineages not related to any of the recent orders. The most isolated situation was in Australia, which had been cut-off from the other continents since the Cretaceous and was not influenced by the intervention of the eutherian mammals. The mammalian evolution in South America after its separation from Africa at the early Paleocene was equally isolated. Besides the marsupials (clade of Ameridelphia) and edentates with giant glyptodonts, mylodonts, and megalony-chids, whose relatives survived until recently, a great variety of strange eutherians appeared here during the Paleocene and Eocene. This includes the large herbivores of the orders No-toungulata, Astrapotheria, Litopterna, and Xenungulata, as well as the Pyrotheria (resembling proboscideans) and their giant marsupial predators, such as Thylacosmilus, resembling the large saber-toothed cats. The mammalian fauna of South America was further supplemented by special clades of hys-tricognathe rodents, haplorhine primates, and several clades of bats, particularly the leaf-nosed bats. These groups probably entered South America during the Paleocene or Eocene by rafting from Africa. The evolution in splendid isolation of South America terminated with the appearance of a land bridge with North America some 3 mya, which heavily impacted the fauna of both continents. The impact of African and Asian fauna on the European mammalian evolution by the end of Eocene was of a similar significance.

It is important to remember that the fossil record of mammals, including detailed pathways of evolutionary divergences and/or the stories of particular clades, is much more complete and rich in information than in any other group of vertebrates. This is due to the fact that the massive bones of mammals, and in particular their teeth, which provide most information on both the relationship and feeding adaptation of a taxon, are particularly well suited to be preserved in fossil deposits. Due to this factor, the fossil record of mammals is perhaps the most complete among the vertebrates. Also, during the late Ceno-zoic, Neogene, and Quaternary, the fossil record of some mammalian groups (such as rodents, insectivores, and ungulates) is so rich that the phylogeny of many clades can be traced in surprisingly great detail by the respective fossil record. For the same reason, some of these fossils (e.g., voles in the Quaternary period) are the most important terrestrial index fossils and are of key significance not only for local biostratigraphies and precise dating of the late Cenozoic deposits, but also for large-scale paleobiogeography and even for intercontinental correlations. The late Cenozoic period is characterized by gradually increasing effects of climatic oscillations, including repeated periods of cold and dry climate—glacials—followed by the evolution of grass and the treeless grassland country. Many clades of mammals responded to these changes and produced the extreme specialists in food resources of the glacial habitats, such as mammoths, woolly rhinos, lemmings, cave bears, and cave lions.

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