Tim
David Krentz On Oct 17, 2007, at 12:25 AM, Tim Williams wrote:
David Krentz wrote:
Marisol Montellano-Ballesteros (2003) mentions remains of a titanosaur from Chihuahua in Mexico. The age of the rocks in the formation are said to have ranged from Campanian to early Maastrichian. So...if it were a titanosaur from the Campanian would that make the reintroduction of the group from South America earlier than once thought?
The idea has come up before. Sullivan & Lucas (2000; JVP 20: 400– 403) described indeterminate titanosaurid material ("Alamosaurus") from the late Campanian of New Mexico (De-na-zin Member, Kirtland Formation). Before that, McCord (1997; JVP 17:620–622) described a titanosaurid caudal from Arizona, which Sullivan & Lucas regard as of late Campanian age (Fort Crittenden Formation). So, if titanosaurids entered North America some time in the Late Cretaceous, this would have been no later than the late Campanian.
As you say, at this stage it seems unlikely that _Alamosaurus_ was a "home-grown" titanosaurid. Although there were plenty of titanosauriforms in the Early Cretaceous of North America (e.g., _Venenosaurus_, _Cedarosaurus_, _Sauroposeidon_, _Paluxysaurus_, _Astrodon_), there are no known basal titanosaurs from North America. AFAIK, anyway.
Were they re-introduced via Asia?
Not sure. Some studies, such as Wilson (2002; Zool. J. Linn. Soc. 136: 217-276), have allied _Alamosaurus_ most closely to the Asian genus _Opisthocoelicaudia_ within the family Saltasauridae (a family that includes many of the most derived "titanosaurids"). Other analyses also tend to unite _Alamosaurus_ with the saltasaurids, either inside this clade or close to it. Saltasaurids are most common in South America, but the derived titanosaur _Opisthocoelicaudia_ is a close relative of these forms. A South American origin for _Alamosaurus_ seems the most parsimonious.
The problem is that the genus _Alamosaurus_ has tended to be used as a wastebasket for late Campanian-late Maastrichtian titanosaurid material from the southern/SW United States. More than one derived titanosaurian taxon is probably represented by the sum total of the "Alamosaurus" material. AFAIK, the above analyses used the _Alamosaurus sanjuanensis_ type material, as well as material that can be confidently assigned to this genus (like the "Big Bend" juvie _Alamosaurus_ from Texas.)
In other words...can I place classic Campanian dinosaurs ( gorgos, centrosaurs, etc..) together with Alamosaurs in a drawing and not have to hang my head in shame?
Says Sullivan & Lucas (2000): "Both units [see above] contain a sparse dinosaur fauna, mostly of indeterminate hadrosaurs and theropods (cf. _Albertosaurus_)." But this is just the units in the United States of America, not its neighbor, the United States of Mexico.
Cheers
Tim
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