THE LIFE ORIENTATION OF CONCAVO-CONVEX BRACHIOPODS - OVERTURNING THE PARADIGM

Concavo-convex brachiopods are generally assumed to have lived with th eir convex valves against the sediment. This orientation is based prim arily on the a priori assumption that the upturned commissure would pr event fouling of the brachiopods' mantle cavity. Here, I present epibi ontic and taphonomic evidence from orthids and nonproductid strophomen ids that concavoconvex brachiopods lived in the reverse life orientati on, with the convex valve on top. Ten testable epibiontic and taphonom ic criteria are proposed to establish the uppermost valve during encru station and whether epibionts encrusted primarily live or dead hosts. The criteria are evaluated for 11 collections of Ordovician and Devoni an nonproductid concavo-convex brachiopods that contain a total of ove r 500 brachiopods and 4000 epibionts. In all cases, the results suppor t a convex-up orientation and the encrustation of live hosts. Five cri teria concern epibiont growth patterns and show that (1) epibionts pre dominate on convex valves, (2) epibionts are not restricted to shell m argins, (3) distinct exposed and cryptic faunas, as predicted theoreti cally and described in previous studies, exist on convex and concave v alves respectively, (4) epibiont colonies were often truncated by the commissures and hinges of live hosts, and (5) growth patterns of some epibionts indicate live hosts. Five taphonomic criteria show that (1) concavo-convex shells have greater epibiont cover than other morpholog ies, (2) internal surfaces of disarticulated specimens are rarely encr usted, (3) interiors of articulated specimens are rarely mud-filled, ( 4) standard taphofacies indicators suggest little postmortem exposure, and (5) sedimentological reconstructions suggest rapid burial. Convex -up brachiopods suggest that the Paleozoic mud bottoms they lived on w ere probably firmer than usually assumed and that a more complex funct ional interpretation of concavo-convexity is required. Hydrodynamic st ability was important to many concavo-convex, piano-convex, and unequa lly bi-convex brachiopods. Brachiopods with these morphologies probabl y lived in the more stable orientation on their concave or flat valve. Productid brachiopods, although also concavoconvex, lived convex valv e down and are ecologically distinct from earlier concavo-convex taxa. Productids were anchored, semi-infaunally, by spines and were more si milar to Mesozoic oysters in orientation and shell function.