Paper No. 7
Presentation Time: 3:20 PM
STRATOFACIES DISTRIBUTION OF HADROSAURIAN DINOSAURS IN THE TWO MEDICINE/JUDITH RIVER CLASTIC WEDGE SUGGESTS BOTH ANAGENIC AND CLADOGENIC EVOLUTIONARY PATTERNS
Over the past 30 years the Upper Cretaceous (Campanian) Two Medicine/Judith River clastic wedge complex has yielded an impressive inventory of hadrosaurid (Ornithopoda) dinosaur specimens, each recorded with chronostratigraphic and sedimentological data. Specific taxa have been found confined to particular stratigraphic horizons and facies related to the regressive-transgressive cycle of the Claggett-Bearpaw Sea. Hadrosaurids from the basal, initial regressive sediments include the two primitive taxa Acristavus and Gryposaurus latidens. Acristavus is hypothesized to be the cladogenic sister taxon of the derived, contemporary genera Maiasaura and Brachlophosaurus. Maiasaura is found in red, calichified mudstone sediments representing the proximal (“upland”) wedge facies of the regressive maximum, whereas Brachylophosaurus canadensis is found in the distal, near marine sediments of the regressive maximum. A second species of Brachylophosaurus (B. sp.) is also found in regressive sediments located more proximal within the “lowland” portion of the wedge. B. sp is hypothesized to be an anagenic precursor of B. canadensis. The genus Gryposaurus presents a more cosmopolitan pattern where G. latidens persists in upland sediments from the stratigraphically lower, initial regressive sediments, up into the stratigraphic middle red-bed sediments of the regressive maximum. An undescribed, derived Gryposaurus (G. sp. A) co-existed in the regressive sequence alongside B. sp., whereas a second undescribed taxon (Gryposaurus. sp. B) is found in the upper transgressive sequence alongside the hadrosaurid meta taxa, Prosaurolophus “blackfeetensis” and Hypacrosaurus stebingeri. Apparent time-overlapping of some sister taxa suggest cladogenic evolutionary events whereas the closely related, stratigraphically separated taxa suggest that they were derived from potential anagenic evolutionary events.