Nature: Pseudo-tribosphenic vs tribosphenic mammals

[Tim Williams <twilliams_alpha@hotmail.com>]

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
_________________________________________________________________
Climb to the top of the charts!  Play Star Shuffle:  the word scramble 
challenge with star power.
http://club.live.com/star_shuffle.aspx?icid=starshuffle_wlmailtextlink_oct