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Crown Mammals with Multituberculates?
Another paper featured in _Paleontological Journal_, is one by Agadjanian
in the same issue as that of the hadrosaur tooth. This paper offers a
theory that crown mammals in the strictest sense are paraphyletic in
relation to theriodonts.
Agadjanian, A.K. 2003. Questions of early adaptive radiation [sic] of
mammals. _Paleontological Journal_ 37 (1): 76-88.
It was originally published in _Palyeontologichyeskii Zhurnal_ 2003 (1):
78-91: "Voprosaya rannaya izluchyenaya mlyekopitayushcheye
"Morphological and physiological differences and paleontological data
suggest the independent origin of the Prototheria and the Theria. The
Prototheria gave rise to the most specialized groups of Mesozoic mammals.
The reasons for this are discussed. An important role in the evolutionary
fate of these subclasses was played by the structural difference in the
region of the middle ear, since it influenced the structure of the
basicranium. The development of a perfect placental pattern in the therian
stem at the end of the Cretaceous was a key character determining the fate
of mammals. This gave rise to changes in the central nervous system and
provided for the domination of the Placentalia in the Cenozoic."
The paper proposes the following phylogeny on the basis of some shared
and rather plesiomoprhic conditions outlined in the abstract:
Numbers refer to apomorphies, as in the text and figure 7 of the paper:
1) tricuspid cheek teeth;
2) multituberculate cheek teeth;
3) docodont tooth pattern;
4) teeth no longer symmetrodont in form, with expansion of upper crowns
and axial elongation of lower crowns;
5) tribosphenid tooth crowns;
6) expanded hypocone makes for a quadricuspid crown in molars;
7) reduction of postdentary bones into middle ear, Meckel's cartilage,
resorption into jaw complex and vanished;
8) angular process of lower jaw;
9) incisors developed with diastema;
10) "perfect placental pattern";
11) abrupt increase in brain size.
Agadjanian's thesis primarily concerns the shape of the incus and its
relationship, evolving with the petrosal and malleus, and brainsize in
monotremes and other therians, with the tympanicum.
A rather outre, if competantly done paper.
Jaime A. Headden
Little steps are often the hardest to take. We are too used to making leaps
in the face of adversity, that a simple skip is so hard to do. We should all
learn to walk soft, walk small, see the world around us rather than zoom by it.
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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