| Abstract: | Changes in the environment have modified the
ecological role of reef builders through geologic time,
and major extinctions have reset evolutionary patterns.
In Cretaceous reefs, the relative ecologic role of
scleractinian corals and rudist bivalves shifted during
the middle Cretaceous, and rudist bivalves took over as
the dominant skeletal organisms for the remainder of
the period.
In previous works related to Cretaceous reefs, species
diversity of rudist bivalves was quantified, reef lines
based on the distribution of the bivalves were
established on plate reconstructions, and fluctuations
in the reef lines were interpreted in terms of thermal
changes attributed to ocean heat transport. A dynamic
rather than stable environmental history was proposed
for the tropics, and the role of large-scale disturbance
in the evolution of tropical ecosystems was proposed. However, the role of bivalves as proxies for reefal
conditions remains questionable, and the time periods
of the Cretaceous for which hypotheses of biotic
change were proposed were temporally and
geographically variable across the globe.
In this work, the geographic locations of scleractinian
coral species were plotted for the Caribbean and
circum-Caribbean region for all stages of the
Cretaceous. Species occurrences were noted for each
stage and plotted on present-day mercator maps.
Occurrences were grouped and transferred per five
degrees paleolatitude to the 130, 95 and 80 my plate
tectonic reconstructions. Number of occurrences,
number of species, and number of paleolatitude
increments comprised the observed data base used
for statistical analyses of colonial corals.
Results of the statistically analyzed coral database, and
comparison to rudist bivalve data and interpretations,
include the following. 1. Coral data fill in the reef line for
the earliest Cretaceous stages. 2. The geographic
extent of the Caribbean reef line defined initially on
rudist bivalves was conservative. Colonial corals
expand the paleolatitudinal extent of the tropics and
display temporal patterns of geographic expansion and
contraction similar to those expressed previously by
rudists. 3. Dispersion patterns for corals and rudists
infer that the geographic extent of the Cretaceous
tropics was greater than that of our present-day
interglacial period. 4. Knowledge of the paleolatitudinal
extent of the ancient tropics is essential for analyzing
the role of the tropics in ocean and atmospheric heat
transport.
The larger-scale focus of this investigation is to
question the driving mechanism for evolution in the
tropics. A first-order inquiry concentrates on dynamic
environmental conditions as opposed to tropical
stability, and subsequent investigations analyze the
processes driving tropical evolution under greenhouse
versus icehouse conditions. Paleobiological questions
center on macroevolutionary factors involving both the
environment and the ecological characteristics of the
co-existence of the two groups, specifically,
morphologic and taxonomic changes expressed during
the critical mid-Cretaceous faunal transition. |