Mammaliaformes
| Mammaliaformes Temporal range: Late Triassic–Recent, 220–0 Ma |
|
|---|---|
 |
|
| Life restoration of Castorocauda lutrasimilis | |
Scientific classification  |
|
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Synapsida |
| Order: | Therapsida |
| Suborder: | Cynodontia |
| Superfamily: | Chiniquodontoidea |
| (unranked): | Mammaliaformes Rowe, 1988 |
| Subgroups | |
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. Its extension is roughly the same as the group traditionally called Mammalia though, unlike the latter, it is defined without reference to traits. The term is used primarily by authors who restrict the term "Mammalia" to the crown group mammals. In traditional classifications that are based on traits rather than phylogeny, the term "mammals" includes all mammaliaforms.
Contents |
[edit] 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] 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,[citation needed] 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. 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 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. There is evidence of hair, however, in the cynodont Cynodontipus,[7], though the interpretation is disputed.[8] The docodont Castorocauda, further removed from crown group mammals than Hadrocodium, had two layers of fur, guard hairs and underfur, as do mammals today.[9]
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.[10]
Hadrocodium lacks the multiple bones in its lower jaw seen in reptiles. These are still retained, however, in earlier mammaliaforms.[11]
[edit] Phylogeny
Cladogram based on [12].
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||
[edit] References
- ^ Rowe, T. S. (1988). "Definition, diagnosis, and origin of Mammalia". Journal of Vertebrate Paleontology 8 (3): 241-264. http://www.geo.utexas.edu/faculty/rowe/Publications/pdf/010%20Rowe%201988.pdf.
- ^ Abdala, F. (2007). "Redescription of Platycraniellus Elegans (Therapsida, Cynodontia) from the Lower Triassic of South Africa, and the cladistic relationships of eutheriodonts". Palaeontology 53 (3): 591-618. doi:10.1111/j.1475-4983.2007.00646.x.
- ^ a b Minkoff, Edwin H. Colbert, Michael Morales, Eli C. (2001). Colbert's evolution of the vertebrates : a history of the backboned animals through time (5th ed. ed.). New York: Wiley. ISBN 978-0-471-38461-8.
- ^ Oftedal, O.T. (2002). "The origin of lactation as a water source for parchment-shelled eggs". Journal of Mammary Gland Biology and Neoplasia 7 (3): 253–266. doi:10.1023/A:1022848632125. PMID 12751890. http://si-pddr.si.edu/jspui/bitstream/10088/542/1/Oftedal2002a.pdf.
- ^ Oftedal, O.T. (2002). "The mammary gland and its origin during synapsid evolution". Journal of Mammary Gland Biology and Neoplasia 7 (3): 225–252. doi:10.1023/A:1022896515287. PMID 12751889.
- ^ Ruben, J.A., and Jones, T.D. (2000). "Selective Factors Associated with the Origin of Fur and Feathers". American Zoologist 40 (4): 585–596. doi:10.1093/icb/40.4.585. http://icb.oxfordjournals.org/cgi/content/full/40/4/585.
- ^ Ellenberger, Paul (1976). "Une piste avec traces de soies épaisses dans le Trias inférieur a moyen de Lodève (Hérault, France): Cynodontipus polythrix nov. gen., nov. sp. les Cynodontes en France". Geobios 9 (6): 769–787.
- ^ Olsen, Paul E. (2012): Cynodontipus: A procolophonid burrow - not a hairy cynodont track (Middle-Late Triassic: Europe, Morocco, Eastern North America)). Program for Northeastern Section - 47th Annual Meeting (18–20 March 2012) Hartford, Connecticut, the Geological society of America
- ^ Qiang Ji et al. (2006). "A Swimming Mammaliaform from the Middle Jurassic and Ecomorphological Diversification of Early Mammals". Science 311: 1123-27. doi:10.1126/science.1123026.
- ^ Hurum, J.H.; Luo, Z-X & Kielan-Jaworowska, Z. (2006). "Were mammals originally venomous?". Acta Palaeontologica Polonica 51 (1): 1–11. http://www.app.pan.pl/archive/published/app51/app51-001.pdf.
- ^ Kemp, T. S. (2005). The Origin and Evolution of Mammals. Oxford University Press. p. 149. ISBN 0-19-850760-7.
- ^ Rougier, G. W.; Wible, J. R.; Hopson, J. A. (1996). "Basicranial Anatomy of Priacodon fruitaensis (Triconodontidae, Mammalia) from the Late Jurassic of Colorado, and a Reappraisal of Mammaliaform Interrelationships". American Museum Novitates (American Museum of Natural History) (3183). ISSN 0003-0082.
[edit] External links
- Mammaliforms and Mammaliaformes from Palaeos
- Mesozoic Mammals; Basal Mammaliaformes, an internet directory
