Ear-piercing in a Miocene whale: the effect of marine borers in vertebrate skeletons

(this is a slightly modified translation of a post by the same author in the blog ‘Paleobiologia del Neogen Mediterrani’)

Last year, we published a paper in Geobios where we reported the finding of a fossil cetacean in the Middle Miocene (Serravalian, around 13 million years b.p.) rocks of the coast of Tarragona, the ancient capital of the eastern Roman province in the Iberian peninsula. In that paper, we intended to understand under which conditions the whale died and was buried. To do that we analyzed not only the taphonomic features of the whale skeleton, but also other sedimentologic and ichnologic data, and the body fossils of invertebrates that were found with the bones. Our conclusion was that the whale was a juvenile that died for unknown reasons (not ruled out a shark predation attack) and whose corpse was deposited on the sea floor at a depth of few tenths of meters where it was disarticulated and dispersed by the action of scavengers (shark and other fish) and/or by weak bottom currents. Nevertheless, the Tarragonian whale still had some interesting things to further explore, and that led us to publish a second article, in this occasion in Palaeo3:

Belaústegui, Z., Gibert, J.M. de, Domènech, R., Muñiz, F. and Martinell, J. 2012. Clavate borings in a Miocene cetacean skeleton from Tarragona (NE Spain) and the fossil record of marine bone bioerosion. Palaeogeography, Palaeoclimatology, Palaeoecology 323–325, 68–74.

In this publication, we studied in detail the presence of bioerosion observed in the tympanic bulla (one of the ear bones) of the cetacean. The three observed borings (below) were assigned to the ichnogenus Gastrochaenolites based on their club- or pear-shaped morphology with a narrow apertural neck opening into a chamber with circular outline. The most likely producers and inhabitants of these cavities bored into the cetacean bone are pholadid bivalves which took advantage of the presence of the bones on the otherwise soft, silty seafloor to use them as a substrate where to live.

The study of these trace fossils provided us the opportunity to review the state-of-the-art concerning research in bone marine bioerosion This has been a traditionally understudied topic that is, since recently, receiving some greater attention by scientists. Based on our own experience and what has been published, we were able to distinguish six main types of  marine bioerosion in vertebrate skeletal substrates:

1. Microbioerosion produced by microorganisms (fungi, bacteria, algae) occupying the shallowest layer of bone.
2. Superficial scratches, mostly bite marks, left by vertebrate predators or scavengers feeding upon the soft tissues attached to the bone.
3. Traces left by the action of osteophagous brachyurans such as those produced today by Tanner crabs.
4. Borings made by worms (sibloginind polychaetes, maybe sipunculans, …), either to feed upon bone nutritious stuff or simply to make a dwelling.
5. Bivalve dwellings such as those described in our paper.
6. Superficial scratches produced by herbivore invertebrates (sea urchins, gastropods or polyplacophorans) grazing upon the algal cover of some bones.

Each one of these types of bioerosion is produced with different purposes under different conditions of their skeletal substrate and thus, their observation is useful to better understand the taphonomy of vertebrate fossils in marine settings and the paleoecology of benthic communities associated with them.