| Mammaliaformes Temporal range: Late Triassic–Recent, 220–0 Ma |
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| Adelobasileus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Synapsida |
| Order: | Therapsida |
| Suborder: | Cynodontia |
| Superfamily: | Chiniquodontoidea |
| (unranked): | Mammaliaformes Rowe, 1988 |
| Clades | |
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See text |
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Mammaliaformes ("mammal-shaped") is a clade that contains the crown group mammals and their closest extinct relatives. Phylogenetically, it is defined as the clade originating from the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia.[1] The precise phylogeny is disputed due to the scantness of evidence in the fossil record. However, it is thought that Mammaliaformes comprises four major groups: Allotheria, Docodonta, Morganucodonta, and Trechnotheria. Mammaliaformes radiated from earlier probainognathian cynodonts.
In phylogenetic nomenclature, not all mammaliaforms are considered mammals, as the latter term is restricted to the crown group, i.e. the last common ancestor of all extant mammals and its descendants.[2] In traditional classifications based on traits rather than phylogeny, the term "mammals" includes all mammaliaforms.
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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] Being warm-blooded requires more calories than cold-blooded animals, 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, small mammals generally being 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 mammaliaformes were probably nocturnal.[3]
Mammaliaforms have several common structures. Most importantly, mammaliaforms have highly specialized molars, with cusps and flat regions for grinding food. This system is also unique to mammals, although it seems to have evolved convergently in stem-mammals multiple times.
Lactation and fur, along with other characteristically mammalian features, are also thought to characterize the Mammaliaformes, but these traits are difficult to study in the fossil record. The fossilized remains of Castorocauda lutrasimilis are a unique exception. While the mammaliformes likely had some form of lactation, they would not have their mammary glands forming distinct mammae with nipples, but rather as have the glands distributed in patches on the belly side, 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] Fur, being necessary to keep a small homeotherm animal warm, would have been present in all Mammaliformes, as all of them were relatively small animals.[6] Even the largest of them, Repenomamus would have evolved from a mouse-sized furry ancestor. Considering their phylogenetic position, the fur would look like that of primitive therians and the platypus, with a single layer of hair.
Like primitive extant mammals, the legs of the various mammaliaforms was somewhat sprawling, giving a somewhat "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.[7]
Hadrocodium lacks the multiple bones in its lower jaw seen in reptiles. These are still retained, however, in earlier mammaliaforms.[8]
Cladogram from an online synthesis by Mikko Haaramo.[9]
| Mammaliaformes |
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