Re: Where have all the ornithischians gone?

["Jaime A. Headden" <qilongia@yahoo.com>]

Dan Varner (Danvarner@aol.com) wrote:

<I'm posting this from vrtpaleo. I seem to  remember Mesozoic crocs with 
ornithichian-like teeth being illustrated years  ago. Can anyone refresh my
ageing memory?>

  *Simosuchus* teeth resemble those of many ankylosaurs in having a
"pectinodont" or comb-like arrangement of cusps/denticles, and some
non-mammalian eucynodonts have multiple rows of cusps on their molariform
molars and premolars that are reflected in some other Mesozoic notosuchians
such as *Uruguaysuchus* and *Chimaerisuchus*. Some Jurassic ornithischians also
have multiple rows of cusps, a featured best left at convergence due to there
being only two or three ornithischians with teeth of this morphology (*Drinker*
included) where a secondary row occurs along the cingulum. Sometimes, it seems
likely, accessory cusps aid in mastication, so may be selected for in
herbivores. Note that carnivores such as felids and *Thylacoleo* select against
multiple rows of cusps to favor a single row, though some of the nascent rows
may be worn by carnassial action.

  Anyways, crocs with mammal or ornithischian-like teeth:

  Buckley, G. A., C. A. Brochu, D. W. Krause and D. Pol. 2000. A pug-nosed
    crocodyliform from the Late Cretaceous of Madagascar. _Nature_ 405:941-944.

  Clark, J. M., L. L. Jacobs & W. R. Downs. 1989. Mammal-like dentition in a
    Mesozoic crocodylian. _Science 244:1064-1066.

  Gomani, E. M. A crocodyliform from the Early Cretaceous dinosaur beds,
    northern Malawi. _Journal of Vertebrate Paleontology_ 17:280-294.

  Rusconi, C. 1932. _Sobre reptiles cretáceos del Uruguay (*Uruguaysuchus
    Aznarezi*, n. g. n. sp.) y sus relaciones con los notossúquidos de
    Patagonia_. _Bol. Inst. Geolog. Perf. Uruguay_ 19:3-64.

  Wu X.-c., Sues, H.-D. & Sun A.-l. 1995. A plant-eating crocodyliform reptile
    from the Cretaceous of China. _Nature_ 376:678-680.

  These all form a small clade within the Notosuchia, with the following
general topology:

  (*Notosuchus*, *Libycosuchus* (*Candidodon* (*Malawisuchus*,
*Chimaerasuchus*) (*Simosuchus*, *Uruguaysuchus*)))

  Basal ornithischians later revised to other groups include portions of the
jaws and teeth of *Technosaurus* (mostly a plateosaur?), *Revueltosaurus* (a
pseudosuchian that may or may not be an aetosaur-like animal), *Galtonia* (a
plateosaur?), and so forth. Few basal ornithischians are securely known from
their teeth alone, and even *Fabrosaurus* has jaw material that permits the
order and dentary/maxilla distinction of teeth to be tested against. I will be
discussing *Azhendohsaurus* at SVP this year (if I'm lucky), as well as other
tooth-based taxa, and these are the lucky ones. *Pisanosaurus* has teeth that
are extremely worn and thus do not allow most of the unworn or slightly worn
"diagnostic" ornithischian teeth described to date from Triassic/Jurassic beds
to be compared to the "basal ornithischian condition" unless they are
comparable to *Lesothosaurus* and the "thyreophore-grade" ornithischians such
as *Scutellosaurus*.

  Dzik in his *Silesaurus* paper mentioned that it was a likely herbivore, and
Parker et al. further note:

  "*R. callenderi* and *Silesaurus* (Dzik 2003) are examples that show the
   danger of assigning isolated herbivore-like teeth to specific groups. Both
   animals hint at a poorly understood diversity of Triassic herbivorous
   archosaurs with similar tooth morphology. All herbivore-like archosaur teeth
   must have evolved from a laterally compressed, serrated and recurved
   plesiomorphic tooth form, so there is limited variability in the _Bauplan_
of
   herbivore-like archosaur teeth. This can be seen in the similarity among
   teeth in a variety of archosaurs such as *Revueltosaurus*, aetosaurs,
   *Silesaurus*, ornithischians, prosauropods, therizinosaurs and some
   crocodylomorphs (Buckley et al. 2000; Harris et al. 2000)."

  Specifically, Parker et al. state: "We question the assignments of taxa such
as *Galtonia*, *Tecovasaurus*, *Lucianosaurus*, *Pekinosaurus* and
*Technosaurus* to the Ornithischia, because at present no synapomorphies
support these referrals." This implies that a great deal of basal ornithischian
evolution is unknown, and recovery in EJ and MJ fossil beds (restricted mostly
to the southwest of the US and to China) have not permitted enough recovery to
make broader phylogenetic states.

  So, indeed, where have they gone? A lot of potentially ornithischian skeletal
material may be what lies in drawers as scrap and junk bone, so worn and
weathered it is virtually unidentifiable. Triassic theropod bone is apparently
less rare, and this may suggest that carnivorous dinosaurs are phylogenetically
confused, and may pertain to non-theropod clades. Like this is new: *Eoraptor*
has been considered a non-saurischian dinosaur, and *Herrerasaurus* and
*Staurikosaurus* have been considered non-theropod saurischians, based on
skeletal plesiomorphies. What we may call "ornithischians" may be MUCH more
broader a group, and a grade of herbivorous dinosauromorph, rather than just
ornithischians.

  Cheers,

Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

"Human beings, who are almost unique in having the ability to learn from the 
experience of others, are also remarkable for their apparent disinclination to 
do so." --- Douglas Adams


                
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