Size matters – Analysis of shell repair scars in endocerid cephalopods

Nearly one third of all conchs of Anthoceras buchi (Lesnikowa, 1949) from Baltoscandia display healed apertural breakages. Often multiple of these repair scars can be found in single conchs. Most of the scars are less than 2 mm deep, but deeper slit-like repaired breakages occasionally occur. Small injuries are usually around the entire apertural circumference, but the larger scars are concentrated at the ventral side of the conch, which is interpreted as result of a protection from a hood. The adult diameter of Anthoceras buchi is > 40 mm. Conch regions that exceed 30 mm in diameter rarely contain evidence of multiple repair scars. The relative frequency of deep healed breakages is highest at regions with diameters of 25–30 mm. This pattern is interpreted as evidence for a size limitation of the predator: large specimens with conch diameters above 30 mm had a considerably lower risk to get injured and the preserved injuries were less severe. This potentially explains the strong evolutionary trend of size increase in endocerids during the Lower to Middle Ordovician as escalation between them and shell breaking predators. Additionally, a specimen of Anthoceras buchi is described and figured, which displays colour marks with a mottled pattern