Paleoecological Analysis of Cheilostome Bryozoa from Venezuela-British Guiana Shelf Sediments

The Venezuela–British Guiana continental shelf is a broad, gently seaward-sloping plain with surface sediments characterized by a near-shore silty clay facies which grades seaward into a calcareous sand. In the southern portion of the study area, streaks and prominences of biohermal carbonates occur near the shelf edge. These are considered Pleistocene biohermal relicts of a lower stand of sea.

The cheilostome Bryozoa found in sediment samples collected from 40 stations along the shelf were described and their occurrence was numerically investigated. Q-mode clustering techniques were employed to establish biofacies on the basis of stations bearing similar faunules, and R-mode clustering methods were used to establish “associations” from grouping mutually occurring species. Biofacies and “associations” of Bryozoa and their distribution and composition are discussed and compared to several environmental parameters. Two meaningful “associations” and four biofacies were rendered by the clustering technique.

A strong interrelationship between bryozoan “associations,” biofacies, and sediment substrate was noted. The silty clay facies along the inner shelf was virtually devoid of Bryozoa. Between the barren silty clays and the bryozoan-rich calcareous sand of the outer shelf there is a transitional biofacies that is essentially devoid of Bryozoa other than Nellia tenella.

A bryozoan “association” characterized by species having lunulitiform zoaria, dominantly Cupuladria canariensis, Discoporella umbellata, and Mamillopora cupula, in association with large quantities of N. tenella, was found along the outer shelf. A “sub-association” on the somewhat finer substrate along the outer shelf east of the Orinoco Delta was identified, and is characterized by Reteporellina marsupiata and the conical variant of M. cupula.

At stations near the shelf edge in the vicinity of the biohermal prominences an “association” of Canda retiformis, Gemelliporina glabra, Cellaria hassleri and Setosellina goesi was recorded. Species diversity attains a maximum in the proximity of the bioherms. The bioherms may provide a source for a redistributed, reworked faunal contribution to the “association” characteristic of the outer shelf.

It was concluded that sedimental substrate is the principal factor influencing the distribution of Bryozoa along the shelf. The effect of the measured environmental hydrographic parameters of temperature and depth on the faunal distribution could not be discerned. If present at all, it is completely masked by the influence of the sediment substrate.