Jawless fish that swam the seas some 400 million years ago may have relied on the same mechanism to repair bone damage as modern mammals do to mend their teeth.

A team led by Zerina Johanson at the Natural History Museum in London analysed fossils of the large fish Psammolepis from rock formations in Estonia and Latvia. Psammolepis are covered by bony shields ornamented with a thin layer of dentine, the same tissue that makes up teeth. One specimen had a damaged armour plate, possibly from a predator attack. The researchers were surprised to find that the bony plate was plugged not with bone, but with dentine.

Specialized stem cells speed the regrowth of dentine in extant mammals such as humans and mice, and the researchers suggest that a similar mechanism existed in this toothless fish to repair its bony armour.

Biol. Lett. http://dx.doi.org/10.1098/rsbl.2013.0144 (2013)