Theria
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Giant anteater ( Myrmecophaga tridactyla ) |
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| Theria | ||||||||||||
| Parker & Haswell , 1897 | ||||||||||||
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The Theria ( ancient Greek θηρίον Therion "wild animal") are a taxon within the Mammals (Mammalia). It summarizes the marsupials (Metatheria) and the higher mammals (Eutheria) together and is the Schwestertaxon the monotremes .
features
The theria are defined, among other things, by the following synapomorphies , which are common derived features, but which can be reduced in individual cases:
- They are viviparous , so in contrast to egg-laying monotons , they give birth to living offspring.
- The outlet of the urinary and genital organs and the anal opening are separated by a dam (perineum). Cloaks, on the other hand, have a common outlet, the cloaca .
- The shoulder girdle only has the shoulder blade (scapula) and collarbone (clavicula) - but the collarbone is reduced in many species. The monotons have additional bones in the shoulder area like the coracoid .
- The mammary glands lead to teats , whereas the monotremes are teatless.
- The (secondary) side wall of the skull consists mainly of the sphenoid bones (alisphenoid), in monotones it is mainly formed from the petrous bone.
- Theria have whiskers (vibrissae).
- The molars have a common (“tribosphenic”) basic pattern.
Development history
In terms of evolutionary history, the Theria are classified within the Tribosphenida , a group that is well developed compared to the primitive, early mammals. In addition to the Theria, these include some extinct, little-known collateral lines such as the Aegialodontidae , which mainly lived in the early Cretaceous period . Within the Theria, at the latest 125 million years ago, the division into sac and higher mammals occurred, since the earliest known representative of both taxa ( Sinodelphys szalayi and Eomaia scansoria ) lived at this time.
Some researchers, however, reject the Theria, they consider the monotremes and marsupial mammals to be more closely related and place them in a taxon Marsupionta . For this taxon morphological features such as the pouch bones - which are probably an original mammalian feature - and certain genetic data are given. The overwhelming morphological and genetic data speak against this thesis, so that the Marsupionta are only represented by a minority.
literature
- TS Kemp: The Origin & Evolution of Mammals. Oxford University Press, Oxford 2005. ISBN 0-19-850761-5
- W. Westheide and R. Rieger: Special Zoology. Part 2: vertebrates or skulls. Spektrum Akademischer Verlag, Munich 2004. ISBN 3-8274-0307-3
