BIOGEOGRAPHY OF THE JURASSIC SUNDANCE SEAWAY OF WESTERN NORTH AMERICA: A TEST OF THE SINGLE NORTHERN ENTRANCE

The Sundance Seaway was a Jurassic epicontinental seaway that covered portions of the United States and Canadian Western Interior. Reconstructions of the Seaway have varied in size, shape, and connections to the proto-Pacific Ocean, owing to an incomplete stratigraphic record of its limits. While most reconstructions depict a single, narrow, northern entrance from paleolatitudes 55-60° N, other reconstructions show a single wider entrance or multiple entranceways extending into subtropical latitudes. Because different entranceway configurations would drive different circulation patterns in the basin and lead to different faunal compositions within the Seaway, faunal data from the Paleobiology Database were used to evaluate the plausibility of a single northern entrance. A total of 13,709 global Jurassic occurrences of the 88 macrofossil genera previously identified from the Seaway were downloaded. Of these 88 genera known from the Seaway, 44% occur globally at paleolatitudes that would allow access via the single, northern entranceway. The remaining 56% are known globally only from latitudes south of the hypothesized entranceway. However, taxa that occur at or north of the single entranceway generally have more occurrences in the database and have a wider geographic span than taxa that occur only south of the entranceway, raising the possibility rarer taxa may have had unsampled ranges that would have permitted their access to the northern Seaway entrance. Resampling was conducted on taxa with at least 25 occurrences to test whether their ranges would have excluded the entranceway if they occurred less frequently in the database. For 46% of these abundant taxa, their 95% confidence interval on their northernmost occurrence lies south of the entranceway, suggesting that most of the taxa in the Sundance Seaway could have entered through a single northern entranceway. Support for a single northern entranceway, given the long length and shallow depth of the Sundance Seaway, suggests the likelihood of strong temperature and salinity gradients in the seaway, which would have greatly influenced the composition and taxonomic structure of its marine communities.