THE INFLUENCE OF SIZE ON THE NUMBER OF REPAIR SCARS IN GASTROPODS FROM THE MIOCENE ST. MARY’S FORMATION

Studying past predator-prey interactions using fossils can reveal implications for modern human-induced ecological devastation. Repair scars in gastropod shells indicate unsuccessful attacks, likely from crabs. Using the gastropods Ilyanassa, Mangelia, Turritella, and naticids from the Miocene St. Mary’s Formation of Maryland, we examined the relationship between shell size and the number of repair scars as well as shell size and repair frequency. Additionally, we searched for trends in repair frequency through time at a high stratigraphic resolution.

Ilyanassa (n = 3736) had a repair frequency (RF) of 56%, Mangelia (n = 935) 45%, Turritella (n = 2858) 53%, and naticids (n = 377) 40%. Ilyanassa, Mangelia, and Turritella all showed a significant positive relationship between the number of repair scars and shell size (p < 0.001; R² = 0.03, 0.02 and 0.60 respectively) as well an increase in RF with size. We also found that Turritella, Ilyanassa, and Mangelia all followed similar trends in RF through time. Turritella demonstrated a marked jump in repair frequency with size. Turritella 5-10mm in length had a RF of 20%, but Turritella 10-15mm had a RF of 50%. This jumped to 78% for Turritella 15-20mm in length. This suggests either that smaller specimens were less likely to be attacked by crushing predators, or that they were attacked more but did not survive. Among naticids, there was no significant relationship between number of repairs and shell size (p = 0.81) or between repair frequency and shell size. Naticids’ RF through time followed an inverse trend compared to the other three taxa. These contrasting results may be due to one of two reasons; first as naticids are drilling predators, it’s possible that they experienced different predator-prey interactions than the other three taxa. According to previous studies, the most likely explanation for naticids being the only taxa that had inverse trends through time is that naticids avoid their predators by hiding in the lower substrate. This behavior is unique to naticids as Turritella, Mangelia, and Ilyanassa typically stay closer to the substrate surface. Secondly, the manner in which naticids grow may cause past repair scars to be covered up by new whorls, thus limiting our ability to see multiple repair scars easily.