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
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:
301-405-4084 Email: tholtz@geol.umd.edu
Fax
(Geol): 301-314-9661 Fax (CPS-ELT):
301-314-7843