The Rapanuian Faunal District (Easter and Sala y Gómez): In Search of Ancient Archipelagos

The shore fauna of Easter and Sala y Gómez Islands displays a remarkably high degree of endemism, primarily at the specific level and recently estimated at 42% for marine mollusks. It is difficult to explain how such high endemicity could have evolved in situ because of the relatively high rate of extinction expected on small, isolated islands. The islands are also very young (2.5 and 2 million years [My], respectively), so high rates of speciation for marine forms would have been required. By contrast, the Hawaiian Archipelago, where marine mollusk endemism is estimated at 20%, is at least 20 My old. Furthermore, some of the endemics of Easter-Sala y Gómez are relicts stemming from older Indo-West Pacific groups, which are likely older than these islands. An alternative explanation seems to be required to account for the origin of the endemics.

A parsimonious hypothesis for the origin of the endemics of Easter-Sala y Gómez is that most if not all were acquired from elsewhere in the region. An analysis of the numerous seamounts of the Sala y Gómez and Nazca Ridges, which extend from Easter to near South America, shows that most were once likely islands. It also indicates that there could have been a chronological continuity of shores among these islands for at least the past 29 My, shores upon which the endemics of the region could have evolved and been perpetuated up to present times. This hypothesis can be tested by dredging the guyots of the region.