Mammal

The mammals are the class of vertebrate animals characterized by the presence of mammary glands, which in females produce milk for the nourishment of young; the presence of hair or fur; and which have endothermic or "warm-blooded" bodies. The brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass some 5500 species, distributed in about 1200 genera, 152 families and up to 46 orders, though this varies depending on the classification scheme adopted.

Origins

Mammals belong among the amniotes, and in particular to a group called the synapsids, distinguished by the shape of their skulls, in particular the presence of a single hole where jaw muscles attach, called temporal fenestra. In comparison, dinosaurs, birds, and most reptiles are diapsids, with two temporal fenestrae; and turtles, with no temporal fenestra, are anapsids.

Related Topics:
Amniote - Synapsid - Temporal fenestra - Dinosaur - Diapsid - Anapsid

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From synapsids came the first mammal precursors, therapsids, and more specifically the eucynodonts, 220 million years ago (mya) during the Triassic period.

Related Topics:
Therapsid - Eucynodont - Million years ago - Triassic

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Pre-mammalian ears began evolving in the late Permian to early Triassic to their current state, as three tiny bones (incus, malleus, and stapes) inside the skull; accompanied by the transformation of the lower jaw into a single bone. Other animals, including reptiles and pre-mammalian synapsids and therapsids, have several bones in the lower jaw, some of which are used for hearing; and a single ear-bone in the skull, the stapes. This transition is evidence of mammalian evolution from reptilian beginnings: from a single ear bone, and several lower jaw bones (for example the sailback pelycosaur, Dimetrodon) to progressively smaller "hearing jaw bones" (for example the cynodont, Probainognathus), and finally (possibly with Morganucodon, but definitely with Hadrocodium), true mammals with three ear bones in the skull and a single lower jaw bone. Hence pelycosaurs and cynodonts are sometimes called "mammal-like reptiles", though this is strictly incorrect since in modern parlance these two are not reptiles, but rather synapsids.

Related Topics:
Incus - Malleus - Stapes - Jaw - Pelycosaur - Dimetrodon - Cynodont - Probainognathus - Morganucodon - Hadrocodium - Mammal-like reptile - Synapsid

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During the Mesozoic Period mammals diversified into four main groups: multituberculates, monotremes, marsupials, and placentals. Multituberculates went extinct during the Oligocene, about 30 million years ago, but the three other mammal groups are all represented today. Most early mammals remained small and shrew-like throughout the Mesozoic, but rapidly developed into larger more diverse forms following the Cretaceous-Tertiary extinction event 65 mya.

Related Topics:
Mesozoic - Multituberculate - Monotreme - Marsupial - Placental - Oligocene - Shrew - Cretaceous-Tertiary extinction event

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The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory that Placentalia were descendants of Marsupialia, which were in turn descendants of Monotremata, but this theory has been refuted. However, Eutheria and Metatheria are often used in paleontology, especially with regards to mammals of the Mesozoic.

Related Topics:
Prototheria - Metatheria - Eutheria - Placentalia - Marsupialia - Monotremata - Paleontology

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Mammal evolutionary progression is below:

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• Jawless fish: Cambrian period to mid Ordovician periods
• Bony fish: mid-Ordovician period to late Devonian period
• Amphibians: late Devonian period to early Carboniferous period
• Reptiles: late Carboniferous period
• Pelycosaurs (synapsids, or "mammal-like reptiles"): late Carboniferous period to very early Triassic period
• Cynodonts: Permian-Triassic
• Mammals: mid-Triassic period to today

In the Mesozoic

Most early mammals were small shrew-like animals that fed on insects. However, in January 2005, the discovery was reported of two fossils of Repenomamus around 130 million years old, one more than a meter in length, the other having remains of a baby dinosaur in its stomach (Nature, Jan. 15, 2005

Related Topics:
Shrew - Repenomamus - Nature

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http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v433/n7022/full/433116b_fs.html.) The earliest mammals include:

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• Eozostrodon: Triassic and Jurassic
• Deltatheridium: Cretaceous
• Jeholodens: mid-Cretaceous
• Megazostrodon: late Triassic and early Jurassic
• Triconodont: Triassic to Cretaceous
• Zalambdalestes: late Cretaceous

Although mammals existed alongside the dinosaurs, mammals only began to dominate after the mass extinction of the dinosaurs 65 mya, in the Cenozoic.

Related Topics:
Dinosaur - Mass extinction - Cenozoic

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In the Paleocene

During the next 8 million years, the Paleocene period (64–58 mya), mammals exploded into the ecological niches left by the extinction of the dinosaurs. Small rodent-like mammals still dominated, but medium and larger-sized mammals evolved.

Related Topics:
Paleocene - Ecological niche - Rodent

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• Ptilodus: multituberculate
• Pucadelphys andinus: an opposum-like marsupial
• Purgatorius: a primate-like mammal, placental
• Ectoconus: an early hoofed mammal, placental