Echinoderm taphonomy, taphofacies, and Lagerstätten

Abstract

Taphonomic study of echinoderms provides useful information on sedimentary conditions before, during, and after burial. Taphonomic studies of Recent echinoderms indicate that much skeletal disarticulation occurs within a few days after death. However, experiments also indicate that within a short period after death echinoderm carcasses remain rather resistant to disarticulation, and thus may be transported a considerable distance by currents; following periods of a few hours of decay, more delicate portions of echinoderm skeletons are readily disarticulated. Some skeletal modules (e.g., crinoid pluricolumnals) may resist disarticulation for periods of months in quiet- and or cool-water environments. Anoxia promotes intact preservation by excluding scavenging metazoans. Echinoderm ossicles may undergo minor abrasion and/or corrosion if left exposed, and less dense stereom corrodes much more rapidly than dense plates, such as echinoid spines. However, heavily abraded ossicles may indicate prefossilization and reworking.Various groups of echinoderms (e.g., pelmatozoans, asterozoans, echinoids) have differing propensities for degradation and, therefore, produce different arrays of preserved fossil material primarily depending upon the relative rates of burial, bottom-water oxygenation, and turbulence. Echinoderms may be divided into three groups based upon the relative ease of skeletal disarticulation. Type 1 echinoderms include weakly articulated forms (e.g., asteroids and ophiuroids) that rapidly disintegrate into individual ossicles. Type 2 includes those echinoderms whose bodies contain portions in which are more tightly sutured, as well as portions in which the ossicles are somewhat more delicately bound (e.g., crinoids, regular echinoids). Such echinoderms display more varied taphonomic grades from fully intact to mixtures of isolated ossicles and articulated modules. Type 3 comprises those echinoderms (e.g., irregular echinoids) in which major portions of the skeleton are so resistant to disarticulation that they may be broken across sutures rather than coming apart at plate boundaries.Comparative taphonomy of particular types of echinoderm skeletal remains leads to recognition of distinctive taphofacies that characterize particular depositional environments. Taphofacies include two types of characteristic modes of fossil preservation: event taphonomic signatures and background taphonomic signatures. Depending upon normal conditions of environmental energy and rates of sedimentation, the background condition of various types of echinoderms for a given facies may range from articulated, unabraded skeletal modules (in Types 2 and 3) to highly corroded and/or abraded ossicles. Conversely, the occurrence of fully intact fossil echinoderms provides unambiguous evidence of rapid and deep burial of benthic communities. Such well-preserved fossil assemblages can provide a wealth of information regarding the paleobiology of echinoderms, as well as the nature of the depositional events and burial histories.This paper presents a preliminary classification and characterization of background and event aspects of echinoderm taphofacies for carbonate- (9 taphofacies, including reefs and hardgrounds) and siliciclastic-dominated (5 taphofacies) environments. In each case, we recognize a spectrum of echinoderm taphofacies that coincides with a gradient of environments, ranging from nearshore, high energy shoreface through proximal and distal storm-influenced shelf, to deeper ramp and dysoxic basinal settings. Most taphofacies also feature particular styles of obrution (smothered bottom) Lagerstätten. These range from scattered lenses of articulated fossils in some high energy sandstone and grainstone facies to bedding planes of articulated, pyrite coated specimens in dark shales. We classify and discuss the genesis of these types of Lagerstätten and list typical examples. Finally, we present a simple model that integrates the occurrence of various echinoderm taphofacies with concepts of cyclic and sequence stratigraphy.