TESTING FOR A LATITUDINAL GRADIENT IN ESCALATION, MIDDLE JURASSIC SUNDANCE SEAWAY, U.S.A

Recent tests of the escalation hypothesis demonstrate that geographic scale and environmental conditions are important determinants of escalation. Building on previous tests that have found differences in escalation among continents, we test here for geographic differences in escalation within a single epicontinental seaway. The Jurassic Sundance Seaway of western Northern America exhibits evidence of a temperature and salinity gradient, with more open marine conditions to the north. As such, it is expected that species diversity should therefore increase towards the north, which in turn would promote escalation. This study tests if there is an escalation gradient in the Sundance Seaway by testing if 1) species diversity was greater to the north along the Sundance Seaway during the Bajocian and Bathonian and 2) comparing changes in proportions of 6 escalation-sensitive life habit groups from the Bajocian to Bathonian using data from Utah (southern dataset) and Wyoming, Montana, and South Dakota (northern dataset).

As expected, results reveal that species diversity increases to the north. However, temporal evidence of escalation does not parallel the environmental gradient, as 5 of the 6 proportions of the 6 escalation-sensitive life habit groups change opposite to what was predicted to the north. In particular, we find that in the south, there is an increase in infaunal and mobile taxa and a decrease in cementing epifaunal taxa, whereas in the north, infaunal and mobile taxa decrease but cementing epifaunal taxa increase. The lack of evidence for escalation despite the presence of an environment gradient could be explained by changes in the availability of shelly substrates from the Bajocian to Bathonian, as shellier substrates result in fewer infaunal taxa and an increase in cementing epifaunal taxa. Whereas there is evidence of escalation during this time interval in the western Tethyan realm during the Jurassic, these results demonstrate the variability that can occur within a single seaway that might affect the promotion of escalation, and they underscore the importance of regional tests of escalation to account for this variability.