INVASIVE SPECIES IN THE LATE CAMBRIAN-EARLIEST ORDOVICIAN OF LAURENTIA: ANALYZING PATTERNS IN LINGULIFORM BRACHIOPODS

The Late Cambrian-earliest Ordovician was a time of lowered global diversity sandwiched between the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE). In Laurentia, this interval is characterized as a series of extinctions, migrations, and regional radiations (“biomeres”) first documented in trilobites. A similar pattern can be documented in linguliform brachiopods. This study examines linguliform brachiopod distribution across two of these biomere boundaries (Steptoean/Sunwaptan; Sunwaptan/Skullrockian) in two paleoenvironments (deep subtidal outer shelf: Nevada and Utah; shallow water cratonic: Texas, Oklahoma) in Laurentia. Invasive species, species whose with paleogeographic ranges were elsewhere but appear suddenly in the fossil record in Laurentia, appear after extinction in all study location after the later extinction, but only appear in the outer shelf environment after the earlier extinction.

Applying Stigall’s (2019) hierarchy of invasions, the earlier invasion could be considered an isolated invasion. Even though the invasion was accompanied by changes to the ecosystem, these changes were more likely the result of extinction and not the result of the appearance of a new species opportunistically increasing its geographic range, because both study areas experienced a reorganization, whether invasive species were involved or not. The immigration of invasive species after the later extinction event could be considered a coordinated invasion as the resulting geographic range was wider and included multiple paleoenvironments. As in the aftermath of the earlier extinction event, the ecosystem reorganized, but not necessarily due to the appearance of an invasive species. These case studies offer examples of the hierarchy of invasion as demonstrated in the fossil record, and also offer examples of the difficulty of distinguishing ecological changes that happen because of the appearance of invasive species from those that happen because of extinction when both processes are involved, as is seen in many modern marine and terrestrial ecosystems.