Late Cretaceous Euselachians from the Northern Region of the Western Interior Seaway.

摘要

Canadian deposits yielding marine euselachian (shark and ray) remains from the Western Interior Seaway, an epicontinental sea that extended north–south through the middle of North America during the last half of the Cretaceous, have received less attention than those from the United States. Numerous isolated teeth were recovered from several localities situated in northwestern Alberta, Canada. These high-paleolatitude assemblages include 20 species belonging to at least three orders, at least 12 families, and 17 genera. Reported here are the first Canadian occurrences of cf. Polyacrodus illingsworthi, Scapanorhynchus, Carcharias aff. C. striatula, Johnlongia parvidens, Protolamna carteri, and Pseudohypolophus mcnultyi. It is also the first North American report of Dwardius woodwardi and the first report of Cardabiodon ricki from the Northern Hemisphere. The recovered material extends the northern geographical range of these taxa and demonstrates that there was strong taxonomic homogeneity of pelagic sharks within the seaway when compared with temporally equivalent southern assemblages. However, the lamniform species Archaeolamna ex. gr. kopingensis, Cardabiodon ricki, Johnlongia parvidens, and Dwardius woodwardi have not been found from well sampled deposits of the southernmost region of the seaway and may have been restricted to cooler waters. To test this purported antitropical distribution, the latitudinal and thermal ranges of these species are compared to those of the extant antitropical shark, Lamna nasus (porbeagle).;The recovery of a partial skeleton of Archaeolamna kopingensis provides the first ever detailed description of this lamniform species. The specimen preserves the first unequivocal occurrence of a fossilized dental bulla, an expanded region of the mesial jaw that houses the anterior teeth and is a synapomorphy of the Order Lamniformes. The articulated tooth set demonstrates that the tooth morphology and dental arrangement is distinct from that of all other extinct and extant lamniforms, validating its placement into the previously proposed Archaeolamnidae. An amended set of diagnostic criteria to define the family Archaeolamnidae is provided. In addition, an analysis of the jaw circumference of this specimen suggests that this species grew to a size much larger than had previously been thought, exceeding that of a 3.2 m Isurus oxyrinchus (shortfin mako) and a 3.8 m I. paucus (longfin mako).