Mammaliaformes

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Mammaliaformes
Temporal range: Carnian – Present 225–0Ma
Life restoration of Castorocauda lutrasimilis
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Order: Therapsida
Clade: Cynodontia
Clade: Prozostrodontia
Clade: Mammaliaformes
Rowe, 1988
Subgroups

Mammaliaformes ("mammal-shaped") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts.[1] 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.[2] Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium as well as the Triassic Tikitherium, the earliest known member of the group.[3][4]

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 Sinoconodon 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.[2] This wider group includes some families that trait-based taxonomy does not include in Mammalia, in particular Tritylodontidae and Brasilodontidae.

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

Mammaliaformes in life[edit]

Early mammaliaforms were generally shrew-like in appearance and size, and most of their distinguishing characteristics were internal. In particular, the structure of the mammaliaform (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.[5] 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 mammaliaforms. To compensate, mammals developed prismatic enamel, characterized by crystallite discontinuities that helped spread out the force of the bite.[6]

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 mammaliaforms 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.[7] Prior to hatching, the same glands would provide moisture to the leathery eggs, a situation still found in monotremes.[8]

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;[9] 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.[10]

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.[11]

Hadrocodium lacks the multiple bones in its lower jaw seen in reptiles. These are still retained, however, in earlier mammaliaforms.[12]

Phylogeny[edit]

Cladogram based on Rougier et al. (1996)[13] with Tikitherium included following Luo and Martin (2007).[3]

  Mammaliamorpha

Tritylodontidae




Adelobasileus




Sinoconodon


Mammaliaformes

Morganucodontidae  

Morganucodon






Tikitherium


Docodonta


Haldanodon



Castorocauda





Crown-group Mammalia
Monotremata

Ornithorhychus (Platypus)



Tachyglossidae (Echidna)



Theriiformes

Triconodonts, Multituberculates,
Marsupials, and Placentals









See also[edit]

References[edit]

  1. ^ 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. 
  2. ^ a b Rowe, T. S. (1988). "Definition, diagnosis, and origin of Mammalia". Journal of Vertebrate Paleontology 8 (3): 241–264. doi:10.1080/02724634.1988.10011708. 
  3. ^ a b Luo, Zhe-Xi; Martin, Thomas (2007). "Analysis of Molar Structure and Phylogeny of Docodont Genera". Bulletin of Carnegie Museum of Natural History (39): 27–47. doi:10.2992/0145-9058(2007)39[27:AOMSAP]2.0.CO;2. 
  4. ^ Datta, P. M. (2005). "Earliest mammal with transversely expanded upper molar from the Late Triassic (Carnian) Tiki Formation, South Rewa Gondwana Basin, India". Journal of Vertebrate Paleontology 25 (1): 200–207. doi:10.1671/0272-4634(2005)025[0200:EMWTEU]2.0.CO;2. 
  5. ^ 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.). New York: Wiley. ISBN 978-0-471-38461-8. 
  6. ^ Line, S. R. P.; Novaes, P. D. (2005). "The development and evolution of mammalian enamel: Structural and functional aspects". Brazilian Journal of Morphological Sciences 22 (2): 67–72. ISSN 0102-9010. 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ 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. 
  10. ^ 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. PMID 16497926. 
  11. ^ Hurum, J.H.; Luo, Z-X & Kielan-Jaworowska, Z. (2006). "Were mammals originally venomous?". Acta Palaeontologica Polonica 51 (1): 1–11. 
  12. ^ Kemp, T. S. (2005). The Origin and Evolution of Mammals. Oxford University Press. p. 149. ISBN 0-19-850760-7. 
  13. ^ 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. 

External links[edit]