13THE PALAEONTOLOGY OF THE MID PLIOCENE BALANUS BED OF TOBAGO
S.K. Donovan (Department of Geology, UWI, Mona
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THE PALAEONTOLOGY OF THE MID PLIOCENE BALANUS BED OF TOBAGO |
The principal palaeontological reference for the Balanus Bed of the Rockly Bay Formation
in Tobago (Figure 4) is still, over 50
years later, Trechmann's (1934) description of the major elements of the molluscan fauna, including some comments
on the balanid barnacles by T.H. Withers. The only other part of the Balanus Bed fauna that has received
detailed attention are the foraminifers, 59 species of which were identified by Saunders and Muller-Merz (1985).
It is the evidence of the foraminifers that indicates a mid Pliocene age for the bed.
However, although balanids, benthic molluscs and foraminifers are the most abundant elements of the fauna,
the diversity of organisms is somewhat greater. Indeed, the original description of the benthic molluscs by Trechmann
(1934) concentrated on those species which were large and well preserved, yet examination of the type section
in Rockly Bay shows the fauna to be much more diverse, although the numerous smaller species are generally only
preserved as moulds. The precise diversity of the balanid fauna is also in need of thorough re-examination.
Fieldwork in the summers of 1987 and 1988 has further increased our knowledge of the fauna of the Balanus Bed.
The principal reason for investigating the bed was palaeoecologic (Donovan 1988. in press). The Balanus Bed
appears to represent a diachronous deposit which rests unconformably on Mesozoic basalts, having been deposited
during the mid Pliocene rise in sea level.
The principal macrofaunal elements are encrusters, such as balanid barnacles and oysters. In the Rockly Bay area
there is a rapid lateral change from coarse-grained near-shore sediments, derived from the weathering of the basalts,
to a muddier, less proximal facies. Vertical burrows indicate that the latter was still a shallow water, high-energy
environment.
It is the muddier facies, exposed in the type section, which has produced some notable additions to the fauna.
Pride of place amongst these must go to a shark's tooth which was collected last summer (Ward and Donovan in preparation).
Shark's teeth are locally common in many deposits of Cenozoic age in Europe and North America, but are rare in
the Caribbean region. The Tobago specimen is thus particularly exciting. David Ward has identified the Tobago specimen
as belonging to a species known from the Miocene of Florida.
Two species of echinoid have been identified from the type locality (Lewis and Donovan in preparation). Commonest
is the cidaroid Eucidaris tribuloides (lamarek), a species still extant in the Caribbean region and elsewhere in
the western Atlantic. The large and distinctive spines of this species are common, although no fragments of test
have yet been found. Much rarer is the beautiful arbacioid Arbacia improcera (Conrad). Two reasonably complete
tests of this species have been recovered, plus a few test fragments. It is interesting to note that both species
are regular and therefore epifaunal. Presumably the plentiful large balanid plates which occur in the sediment
at this locality would have made the substrate inhospitable to burrowing irregular echinoids.
Barnacles at the type locality are found encrusting a variety of hard
substrates, including pebbles, shell fragments and other balanids, but they, in turn, acted as substrates for a
diverse fauna of cheilostome bryozoans. Research on these is currently in progress at the British Museum
(Natural History) (BMNH), but a preliminary faunal list was recently sent to the author by Dr. Paul D. Taylor and
includes the following genera:
Biflustra d 'Orbigny (very common):Conopeum Gray; Parellisina Osbum Ste gin op orella Smith;? Exechonella
Canu and Bassler, Microporella Hineks (very common) ?Schizomavella Canu and Bassler; Tremogastrina Canu: Celleporella
Gray, and Trypostega levinsen.
The macrofauna of the Balanus Bed is thus seen to be particularly diverse and it is certain that further discoveries
remain to be made. As an indication of this, rare fragments of crab carapace have been recognised as such by Dr.
S.F. Morris (BMNH), but further collecting is necessary if the species is to be identified.
REFERENCES
Donovan, S.K. 1988. The Pliocene Balanus Bed of Tobago. Recent Advances in Caribbean Geology, Kingston, Jamaica,
18-20 November. Abstracts: 4-5.
Donovan, S.K. in press. Palaeoecology and significance of barnacles in the mid Pliocene Balanus Bed of Tobago,
West Indies. Geological Journal.
Lewis, D.N. and Donovan, S.K. in preparation. Neogene Echinoidea of Tobago, West Indies. To be submitted to Bulletin
of the British Museum (Natural History).
Saunders, J.B. and Muller-Merz, E. 1985. The age of the Rockly Bay Formation, Tobago. Transactions of the 4th Latin
American Geological Congress, Trinidad and Tobago, 1979, 1 339-344.
Techmann, C.T. 1934. Tertiary and Quaternary beds of Tobago, West Indies. Geological Magazine, 71: 481-493.
Ward, D.J. and Donovan , S.K. in preparation. A shark from the mid Pliocene Balanus Bed of Tobago. To be submitted
to The Journal of the Geological Society of Jamaica.
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