IS HALF ENOUGH? THE VALIDITY OF USING SINGLE VALVES IN STUDIES OF PREDATION ON BRACHIOPODS

Predation in the fossil record is often studied using repair scars, which are formed when the prey is attacked by a predator but survives to repair its shell (valve). Repair frequency (the proportion of scarred individuals out of total individuals in the assemblage) has been shown to be a proxy for predation pressure in living systems. When calculating repair frequencies for fossil brachiopods, both valves are normally examined. There are many processes (e.g., disarticulation, encrustation, spalling, sediment filling, slab preservation) in which only one valve can be observed. Consequently, those brachiopod species and paleoenvironments that encourage these types of preservation are underrepresented in predation studies. Examination of single valves might enable study of many new predation systems. However, because brachiopod valves are not symmetrical, predators may prefer specific valves because of the relative strength of the valve or position of the internal organs relative to the valve. If only one valve were examined, this scenario would result in higher repair frequencies on the preferred valve and different apparent rates for the same assemblage.

This study tests whether it is possible to use single valve brachiopod samples by simulating using only one valve. Repair rates of individual valves (pedicle vs brachial) of articulated specimens to repair rates of the whole specimen. The study examines 18 genus-unit combinations spanning multiple morphologies from the Ordovician to the Permian. If there is little variation in repair rates between valves, single valves alone may be used to generate reliable repair rates.

Repair frequencies on brachiopod taxa from the Middle Devonian onwards varied little between valves, regardless of time or taxon, supporting that single valves can be used in predation studies. Furthermore, most repair scars occur on both valves and in the majority of cases for which there is a valve preference, it is for the more convex valve, regardless of overall morphology. The rank order of the samples’ repair frequencies did not change between using convex single valves or both valves (Spearman’s rho = 0.98, p << 0.001). These results suggest that single valves can be used in predation studies and that single valve studies can be made more accurate by using convex valves.