Twenty-five new species of 65-million-year-old tiny fossils called ostracodes from Jamaica recently described in the journal Micropaleontology show affinities to species from Chiapas, southern Mexico, and indicate that the two landmasses were once connected. The fauna is remarkably distinct from those of both North and South America and appears to have evolved in isolation. These observations contribute to understanding the very complex plate tectonic history of the Caribbean region.
While conducting field investigation in Jamaica, my guide, Professor Simon Mitchell at the University of the West Indies, suggested that I visit Chiapas, as he was aware of faunal connections between the two localities. I was able to get two Mexican geologists, Gerardo Carbot Chanona and Marco Antonio Coutiño from the Museo de Paleontología "Eliseo Palacios Aguilera," to help me in the field. Although ostracodes were abundant in several samples, the preservation was generally poor. Nonetheless, their similarity to the new species from Jamaica is unmistakable.
Observations indicating a Jamaican-Mexican faunal connection have been accruing over several decades. In the late 1950's, L.J. Chubb, a geologist with the University College London and later with the Geological Survey Department of Jamaica, noted that an extinct group of bivalves known as rudists from Jamaica were very similar to ones in the Ocozocoautla Formation (Ok, say that one fast three times!) of Chiapas. In contrast to common bivalves today, rudists had highly asymmetrical shells, with one being cone- or horn-shaped and the other essentially a lid that closed over the open end of the cone. The similarity of the Jamaican-Mexican fauna was so great that Chubb was able to use information from Jamaica to date the Ocozocoautla deposits. In the late 1960's, Edward Robinson, a geologist with the Geological Survey Department and later at the University of the West Indies, observed a very strong Jamaican-Mexican connection with a group of microfossils known as foraminifera. Robinson honored Chubb's previous contribution by naming a new genus after him, Chubbina. Later observations of Jamaican-Mexican faunal connections also include corals and algae.
Ostracodes, which are very small crustaceans (usually up to one mm in length) with two shells that often look like peanuts, are well suited for studies of plate tectonics. Whereas many types are very evolutionarily conservative and widespread geographically -- some are living fossils and have changed very little in hundreds of millions of years -- others are highly ornamented and unique to specific areas. The conservative types are almost always smooth, blind and cosmopolitan, whereas the geographically restricted types are often highly ornamented and have eyes. The eyes indicate that they lived in shallow water (at least within the photic zone). Many studies have shown that shallow water ostracode faunas may differ significantly between areas that are geographically close if the areas are separated by deep water, indicating that deep water is a barrier for migration of these types. This is the key to their usefulness in plate tectonics. If, for example, there are geologic terranes in a single landmass that contain different ostracode taxa of the same age, it suggests the possibility that the ostracodes evolved on different terranes that have since accreted. Alternately, if the same shallow-water ostracode fauna occurs in areas now separated by deep water, it indicates the possibility that they evolved in the same shallow water area that has since separated. This appears to be the case with the faunas in Jamaica and Mexico.
Prior to the 1970's, the mechanisms of plate tectonics and the specific information required to reconstruct the tectonic movements of the Caribbean region were largely unknown. The causes of the faunal similarities in the Caribbean were, therefore, poorly understood, and attributed vaguely to their occurrence along the same latitudes or dispersal of larvaevia oceanic currents. By the late 1980's, a key piece of the puzzle was revealed by studying the Cayman Trough. This complex structure, which extends nearly east-west from just north of Honduras to just south of Cuba, is not only a left-lateral transform fault but is also a spreading zone. This latter feature is really handy in this case. In spreading zones, magma rises in the center of the structure and then spreads laterally away, cooling and forming new oceanic crust. As the new crust is forming, the magnetic minerals in the rocks record the direction of Earth's magnetic field, which flips at irregular intervals. The ages of these magnetic reversals being known, the timing of the movements of the structure can be calculated precisely. If these magnetic reversals can be thought of as images in a film, running the film backwards can reveal the configuration of the structures at former times. Running this film all the way back reveals that Jamaica was attached to southern Mexico--Chiapas, in fact -- prior to roughly 50 million years ago, when it began its long trek (nearly 1500 km) to the east.
The geographically restricted distribution of many of the ostracodes also presents the opportunity to study their evolution. If these ostracodes evolved in isolation, then the ancestor-descendant relationships must occur there. This is an area of future research. In any case, this study underscores the close association between plate tectonic movements and biological evolution.
Mark Puckett, Ph.D., is an Associate Professor of Geology at the University of North Alabama. His areas of research include paleontology, stratigraphy and tectonics.
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