DRILL HOLES IN SERPULID POLYCHAETES FROM THE PLIOCENE OF SOUTHEASTERN SPAIN

Drilling predation/parasitism, one of the key sources of data on biotic interactions in the fossil record, has been documented through the Phanerozoic for several major prey/host groups, including mollusks, brachiopods, and echinoderms. In contrast, reports on drilling in other groups (e.g., bryozoans, annelids, etc.) have been limited. Here, we report drillings in calcitic tubes of the serpulid polychaete Ditrupa spp. from the middle Pliocene of southeastern Spain. Eleven samples were collected from 6 levels of fine-grained sands at the Cope 2 outcrop, near Águilas.

In 9 samples, which yielded notable numbers of Ditrupa specimens (n>50), specimens with drill holes were observed (271 drilled specimens). The drill holes displayed distinct features, including regular circular outline, penetration from outside, penetration of one tube wall only, and the presence of single hole per tube. Most likely, these holes were bored by a predator or parasite in live Ditrupa (or in Ditrupa tubes inhabited secondarily by sipunculids).

Because specimens are mostly fragmented, it is difficult to estimate drilling frequency by count. Instead, we estimated specimen numbers by dividing the total mass of all tubes and tube fragments in a sample by the mass of a single complete specimen. Even most conservative estimates, based on the smallest complete tubes found, suggest that drilling exceeded 1% in all samples. More realistic estimates, based on average-sized specimens, suggest that drilling frequency usually exceeded 10%, and reached 40% in some samples.

The frequent presence of drilled specimens across all sampled levels suggests that drilling predation/parasitism was common in the studied sequence. Drill holes in Ditrupa have been reported previously from the Eocene of France and the Pliocene of Italy. There is, thus, growing evidence that drill holes in fossil annelids may be an important, but underutilized source of data on the fossil record of biotic interactions.