New Jurassic mammal

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

This paper passed me by, until it was pointed out to me this morning by a
certain Portsmouth-based British palaeontologist.
 
OLIVER W. M. RAUHUT, THOMAS MARTIN, EDGARDO ORTIZ-JAUREGUIZAR & PABLO PUERTA
(2002) A Jurassic mammal from South America.  Nature 416: 165-168.

Describes the new Jurassic australosphenid _Asfaltomylos patagonicus_, known
from a single dentary of Mid Jurassic age (from the Canadon Asfalto
Formation - isn't this one where _Patagosaurus_ was found?).  As proposed
previously, the Australosphenida evolved the tribosphenic molar independent
of the Boreosphenida (tribosphenic mammals from the Northern Hemisphere).
The Ausralosphenida group includes an array of Mesozoic critters
(_Shuotherium_, _Asfaltomylos_, _Ambondro_, _Ausktriboshenos_, _Bishops_),
which collectively represent a paraphyletic assemblage leading up to the
Monotremata (_Steropodon_, _Monotrematum_, _Obdurodon_, and the modern
platypus and echidnas).   

The tribosphenic molar allowed a much more effective way of processing food
over competing tooth morphologies.  Hence, the tribosphenic molar has often
been regarded as one of those "key innovations" - an  evolutionary novelty
that gives its bearers an edge against the competition ("...red in tooth and
claw..."), and propelling the group into rapid diversification.  However, as
noted by Rauhut et al., the australosphenids had only limited success, and
dwindled after the Late Jurassic - in stark contrast to the fame and fortune
enjoyed by the boreosphenids (which includes the placentals).  Modern
monotremes have actually reduced their dentitions.

Anyway, it ain't dinosaurs, but I thought it was interesting.  


Tim


------------------------------------------------------------ 

Timothy J. Williams 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 3163 
  



"The Jurassic period is an important stage in early mammalian evolution, as
it saw the first diversification of this group, leading to the stem lineages
of monotremes and modern therian mammals1. However, the fossil record of
Jurassic mammals is extremely poor, particularly in the southern continents.
Jurassic mammals from Gondwanaland are so far only known from Tanzania2, 3
and Madagascar4, and from trackway evidence from Argentina5. Here we report
a Jurassic mammal represented by a dentary, which is the first, to our
knowledge, from South America. The tiny fossil from the Middle to Late
Jurassic of Patagonia is a representative of the recently termed
Australosphenida, a group of mammals from Gondwanaland that evolved
tribosphenic molars convergently to the Northern Hemisphere Tribosphenida,
and probably gave rise to the monotremes1. Together with other mammalian
evidence from the Southern Hemisphere2-4, 6-8, the discovery of this new
mammal indicates that the Australosphenida had diversified and were
widespread in Gondwanaland well before the end of the Jurassic, and that
mammalian faunas from the Southern Hemisphere already showed a marked
distinction from their northern counterparts by the Middle to Late
Jurassic."