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Nature: Pseudo-tribosphenic vs tribosphenic mammals
Not only is this a very important paper in its own right, but it also relates
to a recent discussion on the DML - concerning the digging adaptations of
monotremes (living and fossil)...
Zhe-Xi Luo, Qiang Ji & Chong-Xi Yuan (2007). Convergent dental adaptations in
pseudo-tribosphenic and tribosphenic mammals. Nature 450: 93-97.
Abstract: "Tribosphenic molars of basal marsupials and placentals are a major
adaptation, with the protocone (pestle) of the upper molar crushing and
grinding in the talonid basin (mortar) on the lower molar. The extinct
pseudo-tribosphenic mammals have a reversed tribosphenic molar in which a
pseudo-talonid is anterior to the trigonid, to receive the pseudo-protocone of
the upper molar. The pseudo-protocone is analogous to the protocone, but the
anteriorly placed pseudo-talonid is opposite to the posterior talonid basin of
true tribosphenic mammals. Here we describe a mammal of the Middle Jurassic
period with highly derived pseudo-tribosphenic molars but predominantly
primitive mandibular and skeletal features, and place it in a basal position in
mammal phylogeny. Its shoulder girdle and limbs show fossorial features
similar to those of mammaliaforms and monotremes, but different compared with
those of the earliest-known Laurasian tribosphenic (boreosphenid) mammals. The
reveals a much greater range of dental evolution in Mesozoic mammals than in
their extant descendants, and strengthens the hypothesis of homoplasy of
'tribosphenic-like' molars among mammals."
Clade Yinotheria Chow and Rich, 1982
Family Shuotheriidae Chow and Rich, 1982
_Pseudotribos robustus_ gen. et sp. nov.
Locality and age: Daohugou locality, Ningcheng County, Inner Mongolia Region,
China, in the Middle Jurassic Jiulongshan Formation. Mammaliaforms from this
formation include the docodont _Castorocauda_ and the basal mammal
Holotype: CAGS040811A and CAGS040811B are the part and counterpart,
respectively, of a partial skeleton with impression and carbonized residues of
The paper also states: "The robust shoulder girdle of _Pseudotribos_ shows many
fossorial features that are also plesiomorphies for crown Mammalia... The
expanded muscular attachments suggest a powerful forelimb with a sprawling
posture, as in monotremes with burrowing adaptation, consistent with such
fossorial features as the expanded deltopectoral crest, the teres major
tubercle, and an expanded distal end of the humerus. The olecranon process is
long, and its length is 62% that of the ulnar length anterior to the semilunar
notch, similar to _Fruitafossor_ and extant fossorial mammals."
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