RE: Nature: Pseudo-tribosphenic vs tribosphenic mammals

[Tim Williams <twilliams_alpha@hotmail.com>]

Although mammal-oriented, the thread is potentially relevant to dinosaurs.  
Fossorial adaptations may be primitive for crown-group Mammalia, and burrowing 
underground was one way a little mammal could evade (or avoid) sharp-eyed 
predators.  It would be interesting if certain theropods became specialized for 
digging up burrows.  This has been suggested for _Deinocheirus_, but not 
published AFAIK.

Another possible way to escape theropod predators was to climb up trees or to 
head to the water - at least until theropods learned the knack.  As well as 
_Pseudotribos_ (fossorial), the Jiulongshan Formation of Daohugou has also 
yielded the beaver-like _Castorocauda_ (semi-aquatic) and the 
flying-squirrel-like _Volaticotherium_ (arboreal/glissant).  It's possible that 
small, possibly arboreal Daohugou theropods like 
_Epidendrosaurus_/_Scansoriopteryx_ and _Pedopenna_ may have crossed paths with 
_Volaticotherium_.  (Though I'm not absolutely certain that they came from the 
same beds - I'll have to check.)  The _Pseudotribos_ paper maintains a Middle 
Jurassic date for this formation, before the first birds evolved - unless 
scansoriopterygids or _Pedopenna_ were birds (or at least avialans, sensu 
Senter).

Cheers

Tim







> Date: Thu, 1 Nov 2007 00:32:10 +0000
> From: john.bass@ntlworld.com
> Subject: RE: Nature: Pseudo-tribosphenic vs tribosphenic mammals
> To: twilliams_alpha@hotmail.com; dinosaur@usc.edu
>
> 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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