Speculations on the origin of the invertebrate faunas of the lower continental slope

Recent studies of the benthic faunas of the continental slope of western North America, highlighted by the collection of additional specimens of Neopilina galatheae Lemche, 1957, off Baja California have led to speculations regarding the origin of invertebrate faunas on the continental slope of the Eastern Pacific. It is suggested that many of the shelf and epicontinental bottom faunas migrated down the slope during the Paleozoic and early Mesozoic times in response to competition and population pressure from the newly evolved forms. In areas of relatively old, stable, deep-sea topography, some of these early forms still exist, with minor changes in external morphology. Since middle Cretaceous times, many new topographic features of the Eastern Pacific, particularly trenches, ridges, and borderlands, presumably have been formed along the continental margins. In as much as these features were formed after the Paleozoic and early Mesozoic invasion of invertebrates into the deep sea had occurred, a new migration of Tertiary and Holocene faunas may have taken place. Recent collections of benthic invertebrates in depths of 1000 to 4000 metres support this thesis, as ‘ancient’ or relict forms of invertebrates, exemplified by Neopilina, were found on outer slopes that descend directly to the sea floor, or on the undisturbed abyssal bottom. Modern, or more recently evolved faunas, which appear to have originated from fairly recent (Tertiary) shallow-water environments, as well as from older abyssal regions were found at equivalent depths in trenches and in borderland basins, inshore of the Paleozoic relicts. Animals from Arctic and Antarctic waters may have invaded abyssal and hadal depths during the cooling of the Pleistocene.