Experimental disintegration of regular echinoids: roles of temperature, oxygen, and decay thresholds

Abstract

Laboratory experiments on regular echinoids indicate that low water temperatures retard organic decomposition far more effectively than anoxia, and that the primary role of anoxia in the preservation of articulated multi-element calcareous skeletons may be in excluding scavenging organisms. When tumbled at 20 rpm, specimens that were first allowed to decay for two days in warm seawater (30°C) disintegrated more than six times faster than specimens treated at room temperature (23°C) and more than an order of magnitude faster than specimens treated in cool water (11°C). In contrast, the effects of aerobic versus anerobic decay on disintegration rates were insignificant. The longer the period that specimens were allowed to decay before tumbling, the greater the rate at which specimens disintegrated, until a threshold time that appears to mark the decomposition of collagenous ligaments. This required a few days at 30°C, about two weeks at 23°C, and more than 4 weeks at 11°C for Strongylocentrotus. Up until this threshold, coronas disintegrate by a combination of cross-plate fractures and separation along plate sutures; cross-plate fractures thus can be taphonomic in origin and are not necessarily related to predation. Specimens decayed for longer-than-threshold periods of time disintegrate virtually instantaneously upon tumbling by sutural separation only. Undisturbed coronas can remain intact for months, sufficient time for epibiont occupation. Rates of disintegration were documented semi-quantitatively by recognizing seven stages of test disarticulation, and quantitatively by tensometer measures of test strength and toughness. The effects of temperature and oxygen on decay and the existence of a decay threshold in disintegration should apply at least in a qualitative sense to many other animals whose skeletons consist of multiple, collagen-bound elements.Regular echinoids should still be perceived as taphonomically fragile organisms, but our results suggest the potential for latitudinal as well as bathymetric gradients in the preservation of fossil echinoid faunas. Echinoid preservation under any given set of conditions should also be a function of taxonomic differences in test construction (particularly stereom interlocking along plate sutures) as suggested by previous workers, although our experiments indicate that these effects should only be significant among post-threshold specimens. A survey of regular echinoids from Upper Cretaceous white chalk facies of Britain substantiates the basic experimental patterns, yielding examples of all disarticulation stages and significant taxonomic differences in quality of preservation. A diverse array of borers and encrusters on fossil coronas also corroborates the post-mortem persistence of some tests on mid-latitude seafloors.