Evolution of mammals

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placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes.[1] Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia
, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.

After the

birds being the only surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders
had appeared.

The synapsid lineage became distinct from the

terms, since they were not reptiles, nor part of reptile lineage. The modern term for these is stem mammals, and sometimes protomammals or paramammals.

Throughout the

endothermy and hair. Later in the Mesozoic, after theropod dinosaurs replaced rauisuchians as the dominant carnivores, mammals spread into other ecological niches. For example, some became aquatic, some were gliders, and some even fed on juvenile dinosaurs.[4]

Most of the evidence consists of

molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.[citation needed


mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation as these soft tissues are not often preserved in the fossil record. Most research concerning the evolution of mammals centers on the shapes of the teeth, the hardest parts of the tetrapod body. Other important research characteristics include the evolution of the middle ear bones, erect limb posture, a bony secondary palate, fur, hair, and warm-bloodedness.[citation needed

Definition of "mammal"


While living mammal species can be identified by the presence of milk-producing

, because mammary glands and other soft-tissue features are not visible in fossils.

One such feature available for

occipital condyle; they have two knobs at the base of the skull that fit into the topmost neck vertebra, while other tetrapods have a single occipital condyle.[5]

In a 1981 article, Kenneth A. Kermack and his co-authors argued for drawing the line between mammals and earlier synapsids at the point where the mammalian pattern of molar occlusion was being acquired and the dentary-squamosal joint had appeared. The criterion chosen, they noted, is merely a matter of convenience; their choice was based on the fact that "the lower jaw is the most likely skeletal element of a Mesozoic mammal to be preserved."[7] Today, most paleontologists consider that animals are mammals if they satisfy this criterion.[8]

The ancestry of mammals


Salamandra salamandra (white background).jpg


Sauropsids (including dinosaurs) Deinosuchus illustration Andrey Atuchin.jpg




Ed novomexicanus1DB.jpg


Dimetrodon grandis.jpg


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The first fully terrestrial

common Suriname toad, have evolved other ways of getting around this limitation). The first amniotes apparently arose in the middle Carboniferous from the ancestral reptiliomorphs.[9]

Within a few million years, two important amniote lineages became distinct:

sauropsids, from which lizards, snakes, turtles/tortoises, crocodilians, dinosaurs, and birds are descended.[2] The earliest known fossils of synapsids and sauropsids (such as Archaeothyris and Hylonomus, respectively) date from about 320 to 315 million years ago. The times of origin are difficult to know, because vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier than the first fossil.[10]


hole behind each eye
, in a fairly low position on the skull (lower right in this image).

holes behind each eye
, which served the following purposes:

  • made the skull lighter without sacrificing strength.
  • saved energy by using less bone.
  • probably provided attachment points for jaw muscles. Having attachment points further away from the jaw made it possible for the muscles to be longer and therefore to exert a strong pull over a wide range of jaw movement without being stretched or contracted beyond their optimum range.

A number of creatures often - and incorrectly - believed to be dinosaurs, hence part of the reptile lineage and sauropsids, were in fact synapsids. This includes the well-known dimetrodon.[11][12]

Terms used for discussing non-mammalian synapsids

When referring to the ancestors and close relatives of mammals, paleontologists also use the following terms of convenience:

  • Edaphosaurus cruciger
  • Stem mammals (sometimes called protomammals or paramammals, and previously incorrectly called mammal-like reptiles) — all synapsids, and all of their descendants, except for mammals themselves.[11] Stem mammals therefore include all pelycosaurs, and also all non-mammalian therapsids. Traditionally, these were known as "mammal-like reptiles", but this is incorrect and outdated;[12] terms such as "stem mammal" are preferred instead, because these synapsids were neither reptiles, nor even part of reptile lineage.[11][12]

Pelycosaur and "mammal-like reptile" are both

terms (and in the latter case also based on incorrect historical belief that mammals evolved from reptiles rather than in parallel to them), and for that reason are disfavored and outdated terms rarely used in modern literature.


Bonacynodon schultzi, a probainognathian cynodont related to the ancestors of mammals[13]