Palaeogeography, Palaeoclimatology, Palaeoecology
Evolution of Caribbean echinoderms during the Cenozoic: moving towards a complete picture using all of the fossils
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.
Section snippets
Echinoids
“… there has been virtually no study of preservational bias in important tropical American deposits, but understanding taphonomic processes is critical to ecological and evolutionary interpretations of any paleontological pattern” (Jackson and Budd, 1996, p. 11).
I do not disagree with the above comment. Nevertheless, we do know quite a bit about the taphonomy of Recent (Greenstein, 1989, Greenstein, 1990, Greenstein, 1991, Greenstein, 1993 and fossil echinoids from the rock record of the
Crinoids
Crinoids are one of the ‘unloved’ groups of Caribbean fossils, perhaps with good reason. They are rare, occurring almost exclusively in deposits laid down in deeper water, and even when they do happen to be present they are almost invariably limited in diversity and numbers. Antillean fossil crinoids are never preserved as complete specimens and are known almost exclusively from single columnals or cirral ossicles or, less commonly, pluricolumnals; only very rarely are brachials or cups known.
Asteroids
Asteroids are a diverse and locally common component of the modern Caribbean marine fauna; for example, Downey (1973) noted 95 species belonging to 56 genera from the Caribbean and Gulf of Mexico. Yet asteroids are not commonly recognized in faunal lists of fossiliferous deposits from the Antillean region. This absence is only apparent and is driven by two factors. Firstly, asteroids, like other echinoderms, have a multi-element skeleton that disarticulates rapidly after death into myriad
Ophiuroids
Ophiuroids are the most diverse, shallow-water, extant echinoderms in the Antilles (Hendler et al., 1995), yet no complete fossil ophiuroids are known from the region. However, a few published accounts have recorded the presence of disarticulated ophiuroid vertebral ossicles in the Cenozoic records of Trinidad (Berry, 1935) and Jamaica (Donovan et al., 1993, Dixon et al., 1994, Donovan and Paul, 1998). Unlike asteroid marginal ossicles, ophiuroid vertebral ossicles are small, perhaps explaining
Discussion
As is apparent from the above, new finds of tests from horizons that were hitherto ignored and the record of fragmentary ossicles provides important supplementary evidence on the diversity, stratigraphic distribution and macroevolutionary patterns of Antillean fossil echinoderms which, when analysed in conjunction with existing data, has enabled the determination of a more complete picture of their distribution in the tropical western Atlantic. Perhaps counter-intuitively in the light of the
Acknowledgements
Part of this paper was researched and written during the period of National Geographic Society Grant #5722-96, which is gratefully acknowledged. I thank Walter R. Brown and Susann G. Braden (SEM Laboratory, National Museum of Natural History, Smithsonian Institution) for their assistance in the preparation of SE micrographs for Fig. 3. These were taken while the author was in receipt of a Smithsonian Institution Senior Research Fellowship in 1995, which I gratefully acknowledge. René Panchaud
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