Concavo-convex brachiopods are generally assumed to have lived with their convex valves against the sediment. This orientation is based primarily on the a priori assumption that the upturned commissure would prevent fouling of the brachiopods' mantle cavity. Here, I present epibiontic and taphonomic evidence from orthids and nonproductid strophomenids that concavo-convex brachiopods lived in the reverse life orientation, with the convex valve on top.Ten testable epibiontic and taphonomic criteria are proposed to establish the uppermost valve during encrustation and whether epibionts encrusted primarily live or dead hosts. The criteria are evaluated for 11 collections of Ordovician and Devonian nonproductid concavo-convex brachiopods that contain a total of over 500 brachiopods and 4000 epibionts. In all cases, the results support a convex-up orientation and the encrustation of live hosts. Five criteria concern epibiont growth patterns and show that (1) epibionts predominate on convex valves, (2) epibionts are not restricted to shell margins, (3) distinct exposed and cryptic faunas, as predicted theoretically and described in previous studies, exist on convex and concave valves 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 morphologies, (2) internal surfaces of disarticulated specimens are rarely encrusted, (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 were probably firmer than usually assumed and that a more complex functional interpretation of concavo-convexity is required. Hydrodynamic stability was important to many concavo-convex, plano-convex, and unequally bi-convex brachiopods. Brachiopods with these morphologies probably lived in the more stable orientation on their concave or flat valve. Productid brachiopods, although also concavo-convex, lived convex valve down and are ecologically distinct from earlier concavo-convex taxa. Productids were anchored, semi-infaunally, by spines and were more similar to Mesozoic oysters in orientation and shell function.
Shell-encrusting assemblages have uniquely high ecological fidelity among paleocommunities because adjacent epibionts coexisted in ecological time, and the abundances and spatial relationships of the various members are accurately preserved. This study presents a quantitative reconstruction of communities on brachiopod shells (Composita and alate spirifers) from the Carboniferous of North America. Thirty-one epibiont communities, comprised of a total of 1180 encrusted brachiopods, are analyzed in terms of epibiont abundance (e.g., percent areal cover), ecological diversity (e.g., Shannon-Wiener Index), and the role of larval recruitment and spatial competition in structuring the communities.Mississippian epibiont communities contain from seven to 14 encrusting taxa that occupy up to 25 percent of host shell surfaces. These communities are spatially dominated by trepostome bryozoans, but also include common lower Paleozoic epibionts, such as encrusting ctenostome bryozoans, hederellids, cornulitids, and edrioasteroids. Pennsylvanian epibiont communities contain between eight and 16 epizoan taxa that occupy up to 34 percent of shell surfaces. They are dominated by fistuliporoid or trepostome bryozoans and increasing proportions of erect bryozoans (e.g., fenestellids), encrusting foraminiferans, and articulate brachiopods.Carboniferous epibionts, like their modern counterparts, exist in loosely organized ecological communities that vary widely in abundance and composition in space and geological time. There is little evidence for tight controls on community structure. Much of the variation is attributed to vagaries in the magnitude and selective nature of larval recruitment. Evidence of competition between community members is rare, except where a high percentage of the shell surface is covered. In these cases, sheetlike bryozoans are usually most abundant. Solitary epibionts are surprisingly successful competitors in one-on-one encounters, but apparently lose areal cover to longer-lived colonial encrusters during ecological succession.
The frequency and intensity of disturbance on living coral reefs have been accelerating for the past few decades, resulting in a changed seascape. What is unclear but vital for management is whether this acceleration is natural or coincident only with recent human impact. We surveyed nine uplifted early to mid-Holocene (11,000-3700 calendar [cal] yr B.P.) fringing and barrier reefs along ϳ27 km at the Huon Peninsula, Papua New Guinea. We found evidence for several episodes of coral mass mortality, but frequency was Ͻ1 in 1500 yr. The most striking mortality event extends Ͼ16 km along the ancient coastline, occurred ca. 9100-9400 cal yr B.P., and is associated with a volcanic ash horizon. Recolonization of the reef surface and resumption of vertical reef accretion was rapid (Ͻ100 yr), but the post-disturbance reef communities contrasted with their pre-disturbance counterparts. Assessing the frequency, nature, and long-term ecological consequences of massmortality events in fossil coral reefs may provide important insights to guide management of modern reefs in this time of environmental degradation and change.
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