PREDATION ‘DOWN-UNDER’: DRILLING IN THE IRREGULAR ECHINOID, FIBULARIA SP., FROM OLIGOCENE, NEW ZEALAND

Biotic interactions help shape the structure of modern ecological communities, and have been tied to evolutionary and ecological concepts, such as escalation and coevolution. The fossil record holds evidence of biotic interactions in the geologic past providing important details about the organisms involved, the nature of the interaction, and, in some instances, quantitative estimates of their intensity. Holes left in the shells of prey by drilling predators have proven especially useful because they can be easily differentiated from taphonomic processes by their shape, are readily preserved, and are relatively common. The fossil record of drill holes in mollusks and brachiopods has been studied extensively so that patterns of drilling intensity through time for these taxa are relatively robust. However, for many other taxa such as echinoids that are also known to have been subject to drilling predation, the record is less well known.

We examined 813 tests of the epifaunal, irregular echinoid Fibularia sp. collected at a single Oligocene locality on South Island, New Zealand. All specimens were collected by bulk and surface sampling of the Otekaike Limestone at Haughs’ Quarry in the Hakataramea Valley, South Canterbury. They were measured for size and examined for drill holes; if a drillhole was present, its size, shape, and location were also recorded.

Specimens of Fibularia were drilled at a frequency of 7.0% (57 drilled specimens/813 total) and 38% of these had two or more drill holes (22/57; 2.7% of all specimens). All holes were complete, circular to sub-circular in outline, and in some instances showed a degree of beveling, although the thin test of Fibularia generally made beveling difficult to see. The distribution of holes on the test was non-random, with the central aboral area showing a significantly higher (p value = 0.005), frequency of holes suggesting site-selectivity by the predator. No evidence of size selectivity was detected: the relationship between drill hole size and test size of Fibularia was not significant. These findings are consistent with other studies of drill holes in both Recent and fossil Fibularia. The driller was most likely a gastropod, probably the cassid Galeodea or the parasitic eulimid Niso, given that these taxa are known to be drillers and were extant in New Zealand at this time.