Evolution of Caribbean echinoderms during the Cenozoic: moving towards a complete picture using all of the fossils

Abstract

In the Antillean region, collecting bias towards complete tests has engendered a lop-sided view of the diversity of the echinoderms through time. Crinoids, asteroids and ophiuroids, as well as certain echinoid groups, are widespread as fossils, but are only preserved as disarticulated or broken fragments that have commonly been ignored in studies of faunal diversity. Utilization of these disarticulated remains and collection of specimens from units that have hitherto been considered to lack common fossil echinoderms (such as the Plio-Pleistocene of many islands) is resulting in a more consistent synthesis of their regional diversity during the Cenozoic. Both echinoids and stalked crinoids show distinctive faunal changes following the Eocene–Oligocene extinction events, with the roots of their modern components appearing in the region at that time, at least at the generic level. In contrast, distinctive asteroid species that are recognised from the Oligocene do not appear to have persisted into the later Cenozoic of the region. Incorporation of data from disarticulated elements commonly indicates that regular echinoids were at least as diverse as irregular echinoids. Some taxa that are common in the Caribbean at the present day, such as diadematoids, that apparently lacked a recognisable fossil record due to taphonomic factors, are recognised from throughout the Cenozoic on the basis of distinctive disarticulated ossicles.

Introduction

Our knowledge of the fossil record is as good as our ability to identify its components, that is, fossils (Paul, 1998). However, palaeontological methodology, taxonomic practice and the limitations imposed by incomplete preservation of individuals impede an understanding of the true diversity through time of certain fossil groups that have a skeleton that easily disarticulates following death. When presented with a complete, articulated fossil specimen, it is usually a relatively straightforward task for the expert to identify and classify it. However, this is by no means always the case with the fragments of skeleton produced by the biological, physical and chemical breakdown of what were originally complete organisms. This is particularly true of taxa which have a complex, multi-component skeleton, such as vertebrates, plants, arthropods and echinoderms. Even between these groups, fragmentary components have been studied to a different degree depending upon a variety of factors; for example, vertebrate bones and teeth, and plant leaves, seeds and pollen, are better known taxonomically than crab claws and brittlestar arm vertebrae. Thus, when Benton and Simms (1995) argued that the quality of the vertebrate and echinoderm fossil records was comparable, they did not take into account the much greater effort that has been expended historically in identifying disarticulated bones when compared to that spent on recognizing individual echinoderm ossicles to even a high taxonomic level (Donovan, 1996). Not surprisingly, complete echinoderms are usually much easier to identify to the level of genus or species than their myriad disarticulated components.

Kier (1977) (see also Greenstein, 1993, Table 1) used a global database to show that the irregular echinoids have a superior fossil record to the regular echinoids, implying a bias in the echinoid fossil record that has received various explanations. This skew is apparently true for the Cenozoic of the Antillean region, where tests of irregular echinoids are far more abundant and diverse than those of regular echinoids. Indeed, some widespread, extant taxa of regular echinoids, such as the diadematoids, do not have a fossil record of complete specimens within the study area and are only preserved as disarticulated or broken fragments (see below). However, this pattern also applies to other echinoderm groups; crinoids, asteroids and ophiuroids are almost unknown from the Antillean region, even though these all form common components of modern Caribbean communities in shallow to deeper waters. No complete specimens of any of these non-echinoid groups have been documented from the Cenozoic of the Antilles. Apart from other taphonomic considerations (see, for example, Donovan, 1991a, for review), the poor preservation of many (most?) echinoderm taxa in the Antillean fossil record has undoubtedly been influenced by high tropical temperatures which accelerated decay and disarticulation (Kidwell and Baumiller, 1990).

The disarticulated ossicles of non-echinoid echinoderm groups, hitherto at best very poorly recognized from the Antillean region, are now being collected from a broad range of facies and stratigraphic intervals. Of these, to date only the crinoids are being studied in detail. Asteroids and ophiuroids cannot, as yet, be confidently examined in this manner. Similarly, echinoid groups with an apparently poor fossil record are now being found to be more widespread as their disarticulated elements become recognised, a result that parallels the actuopalaeontological research of Nebelsick (1996, p. 185), who has shown that “… the study of fragments within bulk sediment samples greatly increased the possibilities for determining echinoid presence and distribution.”

The purpose of the present paper is to utilise all lines of evidence currently available to produce a new interpretation of the geological history of the echinoderms in the Antilles during the Cenozoic. Four of the five extant classes are now recognised from the Cenozoic fossil record of the region and the fifth, the holothurians, are probably present, but no micropalaeontological investigation to find their disarticulated ossicles has so far been undertaken.