Zombie worms caught in the fossil record boring into Pliocene shark teeth

On a Pliocene seafloor that is now the clayey badlands of Tuscany, central Italy, zombie worms did something no palaeontologist had ever caught them doing: they bored into shark teeth. A new study in the journal Fossil Record reports the first fossil evidence that Osedax, the bone-eating annelid worms known as zombie worms, exploited the dentine of shark tooth roots in the wild. Until now, that behaviour had been seen only once, under experimental conditions, when two Osedax species were found colonising the tooth roots of thresher shark jaws deployed near a whale fall off Monterey, California. The Italian specimens show that the behaviour is not a laboratory curiosity but part of the natural repertoire of these worms, with consequences that may ripple through the entire fossil record of sharks.

The discovery rests on four fossil requiem shark teeth of the genus Carcharhinus, surface-collected from two sites near Pienza in the Siena-Radicofani Basin and housed in the public collections of Gruppo AVIS Mineralogia e Paleontologia Scandicci. The teeth come from offshore mudstones of the Argille Azzurre Formation, deposited during the Zanclean age of the Early Pliocene in a semi-enclosed embayment on the outer shelf and upper slope, where low oxygen and abundant organic matter favoured the preservation of marine vertebrates. Three of the teeth are lowers and one is an upper, identified by their serrated cutting edges, narrow cusps and roots with widely separated lobes bisected by a nutrient groove, characters typical of a genus that counted at least eight Mediterranean species in Pliocene times.

What turned these ordinary-looking teeth into evidence was micro-computed tomography. Scanned at the CNR Institute of Applied Physics in Sesto Fiorentino with voxel sizes down to 18 micrometres, the teeth revealed tree-like cavities hidden inside the root dentine, invisible or barely visible from the outside as tiny subcircular holes a fraction of a millimetre across. Each cavity opens at the surface through a single aperture with no rim or platform, extends inward through a short neck and swells into a globular or irregular chamber studded with exploratory branches, some bulbous, some tapering to a point. That architecture is the unmistakable fingerprint of Osedax, whose branching root tissues, housing symbiotic bacteria, are what the worm sinks into the substrate it digests.

In formal terms, the traces belong to the ichnogenus Osspecus, established in 2012 for fossil borings of this exact morphology. Five well-preserved borings were reconstructed in three dimensions, one on a lower tooth from the site of Lucciola Bella and four on an upper tooth from the same locality, with apertures between 0.16 and 0.47 millimetres and chamber volumes all below half a cubic millimetre. A sixth trace, opening within the nutrient groove of a lower tooth from the site of Selvoli, could not be fully segmented because it merges with the groove and the natural vascular canals of the dentine. Based on their short tapering necks and far-from-spherical chambers, the reconstructed borings are referred to Osspecus tusciae, an ichnospecies whose holotype also comes from the Tuscan Pliocene, from the historic whale-bone locality of Orciano Pisano.

The worms were selective eaters. Every boring excavates the root dentine, while the enameloid-coated crowns remain essentially untouched, mirroring exactly how living Osedax fed on deployed shark teeth in the Monterey experiment. The preference makes biological sense: shark tooth roots are built of osteodentine, a porous, organic-rich, bone-like tissue loaded with collagen, the protein the worms’ bacterial symbionts are equipped to digest, whereas the enameloid cap is far more mineralised. Several of the borings have since collapsed into crater-like pits called pockmarks, where the thin roof of dentine over the chamber gave way, and on some teeth these craters coalesce into large damaged areas along the root lobes, a decay pattern familiar from Osspecus traces on fossil whale bones.

The finding widens the known menu of an animal already famous for eating beyond its niche. Osedax was described only in 2004 from whale bones on the deep seafloor of California and lacks a gut entirely, relying on Oceanospirillales bacteria in its root tissues to digest collagen and lipids. Deployment experiments have since shown it feeding on the bones of terrestrial mammals, seals, bony fishes, turtles, birds and alligators, and trace fossils have placed its distinctive borings on fossil birds, bony fishes, whale tooth roots and Cretaceous plesiosaurs and marine turtles. Shark teeth, however, had never yielded the trace, despite three earlier suggestions: contentious circular borings on a Cretaceous lamniform vertebra from Italy, tiny microborings on Lower Cretaceous shark teeth from Austria, and a rootless Megalodon tooth from the deep Pacific whose loss was speculatively pinned on zombie worms. The Tuscan teeth settle the question with three-dimensional morphology, the standard of proof the earlier candidates lacked.

The taphonomic stakes are considerable. Dense Osedax colonisation can pit, degrade and ultimately destroy entire whale skeletons, an influence on the cetacean fossil record so significant it has its own name, the Osedax effect. The authors argue that a parallel, selachian version of this effect likely exists, one that progressively destroys the dentine of shark teeth, including the root and the inner core of the crown, while leaving the enameloid cap pristine. Supporting evidence may already sit in museum drawers and seafloor trawls: roughly half the shark teeth dredged from the seamounts of the southern Pacific are rootless, many deep-sea tooth assemblages are similarly depleted, and some time-averaged coastal accumulations inland contain teeth missing their roots and much of the dentine core yet bearing perfectly preserved enameloid cusps, a preservation style so localised it points to a specific agent. Fungal and bacterial bioerosion are usually blamed, but zombie worms may share the blame.

There is a paradox in the traces themselves: they are self-destroying evidence. Osedax consumes its own substrate, so a boring only survives if the tooth is buried halfway through its journey to the seafloor sediment. The Italian borings are strikingly small, volumes far below one cubic millimetre, compared with the tens of cubic millimetres living species achieve on bone, and the largest boring recorded in the literature exceeds the entire root volume of one of the studied teeth. The authors conclude the traces are probably incomplete, snapshots of worms that would have grown much larger had burial not interrupted them, meaning complete destruction of small, inconspicuous tooth roots is likely common and often total. That invisibility, compounded by the fact that nobody thought to look for Osspecus on shark teeth, explains how a behaviour coexisting with sharks for at least 100 million years went unnoticed until now.

The discovery also feeds a deeper debate about where zombie worms came from. The Monterey experiment prompted the suggestion that shark tooth dentine might be the primordial food source of the genus, and the fossil record of shark-like teeth stretches back some 425 million years, offering a deep-time stage for such an origin. The authors are sceptical: molecular clock estimates and the first appearance of Osspecus in the Albian, roughly 100 million years ago, converge on a Cretaceous origin for Osedax, and both the living worms and their fossil traces concentrate overwhelmingly on the bones of marine tetrapods, whales today, marine reptiles in the Mesozoic. Shark teeth, they conclude, were unlikely to have been the ancestral substrate. Instead, the teeth probably served as benthic islands and ecological stepping stones, small peripheral colonies seeded near whale falls or reptile carcasses that connected distant, isolated food patches across the ocean floor.

One final question hangs over the end-Cretaceous mass extinction. A stratigraphic gap separates the youngest Mesozoic Osspecus, of latest Cretaceous age, from the oldest Cenozoic record, of late middle Eocene date, and how the lineage bridged it is unknown. Sea turtles, the only Mesozoic marine vertebrate group with known Osedax traces whose descendants persist, are one candidate refuge, but shark teeth are an intriguing alternative: the K-Pg event appears to have left some elasmobranch communities largely unscathed, and many ocean floors are carpeted with shed teeth. Whether a carpet of shark dentine helped zombie worms walk through the extinction that killed the dinosaurs remains open, but the Pliocene teeth of Tuscany now prove that wherever dead whales lay, the worms were also nibbling at the sharks.

Subject of Research: First fossil evidence of Osedax zombie worms boring into shark tooth dentine, from Pliocene Carcharhinus teeth in Tuscany

Article Title: Trace fossil evidence for Osedax Rouse et al., 2004 exploiting shark tooth dentine on a Pliocene seafloor: broadening our understanding of a major taphonomic agent

Article References: Collareta, A., Mezzasalma, O., Agresti, J., Barucci, A., Bosio, G., Mulè, F., Casati, S., Di Cencio, A., Bianucci, G., Pieri, A., & Nobile, F. (2025). Trace fossil evidence for Osedax Rouse et al., 2004 exploiting shark tooth dentine on a Pliocene seafloor: broadening our understanding of a major taphonomic agent. Fossil Record, 28(2), 359-375. https://doi.org/10.3897/fr.28.167615

Image Credits: AI Generated

DOI: 10.3897/fr.28.167615

Keywords: Osedax, zombie worms, Osspecus, shark teeth, trace fossils, Carcharhinus, Pliocene, Tuscany, micro-CT, taphonomy, whale fall, Osedax effect