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RE: How herbivory developed among dinos



From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of philidor11 
 
Some time ago there were mentions on the list that various people were working on this topic.  Has anything been published?  Is the current hypothesis that there was a single radiation, apparently starting in South America, or did herbivory more likely develop among different groups in different places?
Thanks! 
 
 
It looks as if herbivory twice early in Dinosauria (and many more times, independantly, later on within advanced maniraptoriforms, esp. birds).  Ornithischia and Sauropodomorpha seem to have evolved the condition convergently, as all the numerical phylogenetic analyses to date place sauropodomorphs closer to theropods than to ornithischians.  (In an alternative, appealing, but as yet unsupported scenario, Ornithischia and Sauropodomorpha form a clade, Phytodinosauria, which would support a single origin of herbivory).
 
However, the place of origin is something altogether different.  Remember that South America, North America, etc. are not independant entities at the dawn of the Age of Dinosaur, but simply parts of a single giant landmass, Pangaea.  We don't know where in Pangaea the first populations of ornithischians or sauropodomorphs arose.  Argentina has an excellent fossil record for the early Carnian (early Late Triassic) in its Ischigualasto Formation (which contains _Pisanosaurus_, the oldest known ornithischian), and somewhat more spotty evidence is from the same age in Brazil (including _Saturnalia_, a very primitive sauropodomorph).  However, there are lots of herbivorous dinosaur teeth and fragmentary skeletal elements from around the Late Triassic all over the world, not of all of which is confidently dated WITHIN the Late Triassic.  Some might be older than the Ischigualasto, for example.
 
Recently some sauropodomorph material was described from Madagascan rocks interpreted as being Middle Triassic, making these the oldest dated dinosaurs.  However, the age dates were not established independant of the vertebrate fauna: instead, the age date is assigned primarily on the presence of forms (traversodont cynodonts, dicynodonts, rhychosaurines) which are typical of the Middle Triassic or the Carnian.  However, as they admit, it could be that these are late surviving members of these forms rather than some particularly early prosauropods.  Some data independant of the vertebrates (pollen, radiometric, etc.) would be extremely useful in resolving this issue.
 
Hope this helps.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742      
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