Mammaliaformes

Mammaliaformes Temporal range: Norian - Recent PreЄ Є O S D C P T J K Pg N
Life restoration of Castorocauda lutrasimilis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Order:	Therapsida
clade:	Cynodontia
Node:	Prozostrodontia
Node:	Mammaliaformes Rowe, 1988
Subgroups
†Morganucodonta †Docodonta †Triconodonta †Multituberculata Monotremata Trechnotheria

Mammaliaformes ("mammal-shaped") is a clade that contains the crown group mammals and their closest extinct relatives. It is defined as the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia.^[1] Besides the Morganucodonta and the crown group mammals, the Mammaliaformes includes the Docodonta as well as Hadrocodium. Mammaliaformes radiated from earlier probainognathian cynodonts.^[2]

Mammaliaformes is a term of phylogenetic nomenclature. In contrast, the assignment of organisms to Mammalia has traditionally been founded on traits and, on this basis, Mammalia is slightly more inclusive than Mammaliaformes. In particular, trait-based taxonomy generally includes Adelobasileus and Sinocodon in Mammalia, though they fall outside the Mammaliaformes definition. These genera are included in the broader clade Mammaliamorpha, defined phylogenetically as the clade originating with the last common ancestor of Tritylodontidae and the crown group mammals.^[1] This wider group includes some families that trait-based taxonomy does not include in Mammalia, in particular the Tritylodontidae and the Brasilodontidae. The terms Mammaliaformes and Mammaliamorpha are used primarily by authors who restrict the term "Mammalia" to the crown group mammals.

Animals in the Mammaliaformes clade are often called mammaliaforms, without the e. Sometimes, the spelling mammaliforms is used.

Mammaliaformes in life

Early mammaliaforms were generally shrew-like in appearance and size, and most of their distinguishing characteristics were internal. In particular, the structure of the mammaliform (and mammal) jaw and arrangement of teeth is nearly unique. Instead of having many teeth that are frequently replaced, mammals have one set of baby teeth and later one set of adult teeth which fit together precisely. This is thought to aid in the grinding of food to make it quicker to digest.^[3] Warm-blooded animals require more calories than those that are cold-blooded, so quickening the pace of digestion is a necessity. The drawback to the fixed dentition is that worn teeth cannot be replaced, as was possible for the reptilian ancestors of mammaliforms. However, as small mammals are generally very short-lived compared to reptiles of the same size, this was not much of a problem during the early phase of their evolution, in which the trait was set. Early mammaliaforms were probably nocturnal.^{[3][not in citation given]}

Lactation, along with other characteristically mammalian features, is also thought to characterize the Mammaliaformes, but these traits are difficult to study in the fossil record. While the early mammaliforms likely had some form of lactation, their mammary glands probably were not associated with distinct mammae with nipples but rather were distributed in patches on the belly side with the young licking milk from the fur.^[4] Prior to hatching, the same glands would provide moisture to the leathery eggs, a situation still found in monotremes.^[5]

Some early mammaliaforms did have fur. An insulative covering is necessary to keep a homeothermic animal warm if it is very small, less than 5 cm (1.97 in) long;^[6] The 3.2 cm (1.35 in) Hadrocodium must have had fur, therefore, but the 10 cm (3.94 in) Morganucodon may not have needed it. The docodont Castorocauda, further removed from crown group mammals than Hadrocodium, had two layers of fur, guard hairs and underfur, as do mammals today.^[7]

Like monotremes today, the legs of early mammaliaforms were somewhat sprawling, giving a rather "reptilian" type of gait. In some forms the hind feet likely bore a spur similar to those found in the platypus and echidnas. Such a spur would have been connected to a venom gland for protection or mating competition.^[8]

Hadrocodium lacks the multiple bones in its lower jaw seen in reptiles. These are still retained, however, in earlier mammaliaforms.^[9]

Phylogeny

Cladogram based on Rougier et al. (1996).^[10]

References

External links

• Mammaliforms and Mammaliaformes from Palaeos
• Mesozoic Mammals; Basal Mammaliaformes, an internet directory

See also

• Evolution of mammals
• Permian–Triassic extinction event
• Therapsid
• Vertebrate paleontology