Paleoecology of an Estuarine, Incised-Valley Fill in the Dinosaur Park Formation (Judith River Group, Upper Cretaceous) of Southern Alberta, Canada

Abstract

Although portions of ancient estuarine paleochannels have high theoretical potential for burial and preservation of vertebrate skeletal remains, relatively few fossiliferous examples have been documented. A recently described, mud-filled incised-valley system (herein abbreviated OMFIV) in the Dinosaur Park Formation (late Campanian) of southeastern Alberta is a spectacular example of this potential. So far, this deposit has yielded thousands ofdisarticulated vertebrate fossils from basal and internal lag deposits, and tens of associated to articulated specimens from internal bedding surfaces. The OMFIV formed in the freshwater (upstream) portion of the central zone of low energy in a stretched-out, estuarine setting that extended >200 km up into a plain of very low slope. The high abundance and concentration of fossils is probably an artifact resulting from an absence of a diluting sandy bedload. Combined paleontologic and sedimentologic data indicate that (1) disarticulation and reworking of fossils took place within the OMFIV and (2) the assemblage is largely autochthonous to parautochthonous. The OMFIV vertebrate assemblage is more similar in overall composition to previously described coastal rather than inlandpaleocommunities of the Judith River Group. However, the OMFIV assemblage differs from coastal assemblages in that: (1) it is overwhelmingly aquatic compared to assemblages from other Dinosaur Park Formation paleochannel deposits; (2) the taxa Hybodus, Atractosteus, Belonostomus, and Paralbula dominate the vertebrate assemblage; (3) teleost fish are very abundant; (4) it includes taxa generally considered more typical of marine settings (ratfish, elasmosaurs, ?Lophochelys, and ?Baptornis); and (5) although terrestrial vertebrates are rare, ceratopsians and marsupials are present in unusual abundance, indicating that these taxa were common in paleoenvironments adjacent to the OMFIV On the basis of sedimentologic and paleontologic data, we conclude that: (1) the OMFIVfossil assemblage represents a distinct subset of the nonmarine Judithian coastalpaleocommunity comprising a number of taxa adapted to the freshwater portion of the central zone of low energy in an estuarine setting; (2) among Cretaceous vertebrates, teleost fish are sensitive paleoenvironmental indicators; and (3) because of high preservational potential, estuarine facies are important potential sources ofpaleoecological data.