Plesiosaurs breathed air. At intervals, among every body of water they inhabited, the marine reptiles rose to exhale mucus-flecked and oxygen-depleted air as they took another enriching breath. The need left them vulnerable. A tooth driven deep into a plesiosaur neck bone is the proof.

FMNH PR 187 is a stunning skeleton in its own right. The plesiosaur, collected 77 years ago near West Green, Alabama, was one of the gawky, long-snouted forms paleontologists know as Polycotylus latipinnis. After death, the marine reptile drifted down into oxygen-depleted bottom waters in the Late Cretaceous and was blanketed so thoroughly that experts have most of the skeleton and a suite of skull pieces to study. But it’s one particular bone, a neck vertebra from the middle of the series, that drew the attention of paleontologist Stephanie Drumheller and colleagues.

The fact that the hapless Polycotylus was bitten by something has been clear since the time of the plesiosaur’s discovery. But bites can happen for all sorts of reasons, at various times. A bite mark, or even embedded tooth, might represent scavenging rather than predation, or perhaps even a fight. The experts had to get a better look at the embedded portion of the tooth to start picking apart what must have happened. Thankfully, we live at a time when CT scanning fossils is relatively accessible and commonplace. Lo and behold, the essential details were held safe in the bone that was so viciously chomped.