Decapod crustaceans (crabs) are among the most attractive of fossils. Yet, the beautifully preserved specimens seen in museum displays and dealers’ catalogues are in stark contrast with the usual haul of the collector, that is, scraps, commonly claws or (more rarely) bits of carapace, which we all find in (mainly) Cretaceous and Cenozoic sedimentary rocks. However, these bits and pieces represent most of the fossil record of crabs and, as such, are of importance to the systematist and anyone with an interest in aspects such as taphonomy and palaeoecology. Just as it is possible to identify a shark from a tooth or a cidaroid echinoid from a spine, so a crab claw can commonly provide data that permits its identification to the level of genus or species (Collins, 1999).
The present authors, in collaboration with Roger Portell of the Florida Museum of Natural History at the University of Florida in Gainesville, have been collecting and studying the fossil crabs of Jamaica (and the wider Antilles) for over 20 years. Until the 1990s, reports of fossil crabs from the island were limited to a few fragmented specimens and rare, well-preserved carapaces (some retaining claws) or the isolated claws of mud shrimps (=Callianassa sensu lato), which were collected mainly from the Upper Cretaceous and Eocene by visiting geologists as an aside to their own research. They were sent to the British Museum (Natural History) for description. These early records were reviewed and augmented by Morris (1993), who undertook the sorting and description of a new collection of (mainly) minute fragments of carapaces and claws from recently washed residues from the Late Pleistocene Falmouth Formation (see below).
Most notable among the early twentieth century collectors in the Antilles was the wealthy English amateur Charles Taylor Trechmann (1884-1964), who undertook fieldwork in the Antilles every winter between the two world wars (Donovan, 2003, 2010). Trechmann’s principal research interests in the Antilles were the systematics of fossil Cretaceous and Cenozoic molluscs (for example, Trechmann, 1923, 1924), stratigraphy (for example, Trechmann, 1934), and tectonics (for example, Trechmann, 1955), but he fulfilled the praiseworthy role of a collector for experts on other fossil groups in the UK. It was in this way that Thomas Henry Withers (1883-1953) of the British Museum (Natural History) in London (Stearn, 1998, pp. 239-240) came to be involved in describing fossil crabs from Jamaica (Withers, 1922, 1924a, 1927), and crabs and barnacles from the wider Antilles (for example, Withers, 1924b, 1926a, b). However, we are unaware that he ever visited the region.
This article is not intended to be a field guide to the crab localities, but, rather, a brief illustration of the diversity and beauty of Jamaican fossil decapods. By diversity, we refer to both form and mode of preservation. For the collector considering a Jamaican holiday, a field guide to the principal crab localities already exists (Donovan, 2011) and is available online at http://www.caribjsci.org. (All papers published in Caribbean Journal of Science are available free on this site, including a number of others by the present authors.) We supplement a collection of images of Jamaican fossil crabs, arranged in stratigraphic order, with discussions of preservation, palaeoecology, associations and the history of research.
Jamaican Cretaceous crabs
The oldest rocks in the island are Early Cretaceous (Valanginian or older; Robinson, 1994, fig. 6.4), but the known record of Jamaican fossil crabs is somewhat younger, that is, Late Cretaceous (Campanian and Maastrichtian). There are only two nominal species from this interval, both collected by geologists from northeast England and described by Withers. Carcineretes woolacotti Withers (Fig. 1), was first collected by Dr David Woolacott (1872-1924), reader in geology at Armstrong College, University of Durham. It is a portunid, that is, a member of the group that includes modern swimming crabs (note the rear leg enlarged as a paddle in Fig. 1). However, Morris (1993, p. 116) explained that this development in early members of the group, like C. woolacotti, was for digging. Cretacoranina trechmanni (Withers), similarly named after its collector, is a raninid, a group that includes modern frog crabs. This taxon has a more elongate, more distally tapered carapace and lanceolate fifth limbs than C. woolacotti, and is more obviously a burrower, well adapted for digging backwards into a sedimentary substrate.
However, this part of the record of Jamaican fossil crabs is almost certainly incompletely known. Nobody has actively pursued fossil crabs in these Late Cretaceous strata since the 1920s, in part because of the difficulty of the terrain and the intractability of the limestones. All records pertain to more or less complete specimens (Figs 1, 2) and no new nominal taxa have been added since 1927. The record of fragmentary material in these rocks is waiting to be exploited, although Morris (1993, p. 120) added two further raninids in open nomenclature, Parecrocarcinus? sp. and Necrocarcinus sp.
Jamaican Paleogene crabs
Similar comments, regarding how poorly known they remain, might be made about the fossil crabs of the Jamaican Paleogene. One species is known from the Paleocene, about ten from the Eocene (mainly from the highly fossiliferous Yellow Limestone Group) and none from the Oligocene (Figs. 3 and 4).
The Paleocene (Danian) species is based on a single, allochthonous carapace preserved on the sole of a turbiditic sandstone (Fig. 3D-F). The unique holotype of Trechmannius circularis Collins & Donovan is the first crab from the mainly siliciclastic succession of the Lower Paleocene – Lower Eocene Richmond Formation (Donovan, 2011, locality Pa1), which outcrops extensively in eastern Jamaica. Trechmannius circularis presumably lived in shallower water, but was collected from the sole of a turbidite bed deposited in deeper water and is presumed allochthonous. It is the second and oldest known fossil dynomenid crab to be described from the Caribbean. In having three, well-developed transverse grooves, Trechmannius is strongly reminiscent of, for example, some species in the largely Cretaceous genus Palaeodromites A Milne Edwards and, more particularly, to the Albian-Cenomanian genus Trachynotocarcinus Wright & Collins.
READ MORE...To view the rest of this article, you need A subscription. FROM JUST £2.95.
If you are already a subscriber, login here.