What it takes to be a mammal, according to the LRT

Earlier we nested
the monotremes, Akidolestes and Ornithorhynchus as basalmost mammals (contra traditional nestings) in the large reptile tree (LRT). It is notable that both of these taxa display atavistic (reversed) traits, including splayed hind limbs and the loss of teeth. Akidolestes also appears to converge with therians in molar occlusion patterns, while Ornithorhynchus converges with expansion of the braincase and fusion of the cranial elements.

It is possible
that known monotremes developed advanced molar occlusal patterns by convergence with derived mammals. This may be one reason for the difference in the nesting of monotremes in other studies and the LRT.

Basic and traditional traits
associated with being a mammal include:

  1. Mammary glands on the female
  2. Single tooth replacement
  3. Dentary/squamosal jaw joint

Rowe (1988) defined Mammalia phylogenetically 
as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therians and all descendants of that ancestor. According to Wikipedia, that excludes all pre-Middle Jurassic forms.

Kemp (2005) defined mammals by their key traits:
Synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement. This becomes the last common ancestor of Sinoconodon and living mammals. The earliest known synapsid satisfying Kemp’s definitions is Tikitherium, (Late Triassic, Datta 2005).

Here
in the large reptile tree, not all known taxa are listed. Here monotremes, like Ornithorhynchus and Akidolestes, appear after ChiniquodonSinocodon nests within the Mammalia. The following are traits separate monotremes from Chiniquodon in the large reptile tree, which employs traits not specific to synapsids and mammals. Other mammals may not have these traits. Traits specific only to monotremes are not listed.

  1. Parietals fused, frontals fused
  2. Quadrate becomes an ear ossicle
  3. Supraoccipital fused to tabulars and opithotics (data missing for Chiniquodon)
  4. Premaxillary teeth, more than four (lost in Ornithorhynchus, of course).
  5. Last maxillary tooth below posterior orbit
  6. Sacral vertebrae reduced to two (not Akidolestes)
  7. Interclavicle evolves from long ‘I’ shape to short ‘T’ shape
  8. Sternum present
  9. Ulna > 3x radius + ulna width
  10. Longest metacarpals: 3 and 4 (except Ornithorhynchus where 2-4 are subequal)
  11. Longest manual digit is not 4
  12. Ilium posterior process is not longer
  13. Pubis orientation is not strictly medial
  14. Prepubis bone present
  15. Tarsus < 0.6x pedal digit 4 length
  16. Longest pedal digits 2-4 (rather than 4, not known from Chiniquodon)
  17. Pedal 4.1 proportions: length/width not < 3:1

Most of these, 
it goes without saying, are provisional subject to additional taxa and more precise data.

References
Datta PM 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.
Kemp TS 2005. The Origin and Evolution of Mammals. United Kingdom: Oxford University Press. p. 3. ISBN 0-19-850760-7. OCLC 232311794.
Rowe T 1988. “Definition, diagnosis, and origin of Mammalia”(PDF). Journal of Vertebrate Paleontology 8 (3): 241–264. doi:10.1080/02724634.1988.10011708.

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