RE: Nature: Pseudo-tribosphenic vs tribosphenic mammals

[john hunt <john.bass@ntlworld.com>]

Perhaps someone should start a Mammal Mailing List!

-----Original Message-----
From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] On Behalf Of
Tim Williams
Sent: 31 October 2007 23:50
To: dinosaur@usc.edu; twilliams_alpha@hotmail.com
Subject: 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 find
 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."

Class Mammalia
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
_Volaticotherium_.

Holotype: CAGS040811A and CAGS040811B are the part and counterpart,
respectively, of a partial skeleton with impression and carbonized residues
of furs.

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."




Cheers

Tim
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