Jamaica’s Fossil Rudist Bivalve Assemblages from the Late Cretaceous

The Late Cretaceous rudist bivalve or clam assemblages of Jamaica’s Guinea Corn Formation (18° 09' 18'' N077° 23' 12'' W) from the Late Maastrichtian are dated between 72.1 - 66.0 million years ago were named in 2022 among the “First 100 Geological Heritage Sites” designated by the International Union of Geological Sciences (IUGS).

A Fossilised Reef of Rudist Bivalves

The Maastrichtian Age was a division of the Cretaceous Period and the last of the six ages within the Late Cretaceous Epoch preceded by the Campanian Age and succeeded by the Danian Age of the Palaeogene Period.

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First named by the French naturalist Jean-Baptiste Lamark in 1819 the Rudistae or Rudists derives its name from the Latin word rudus meaning rubble or broken stone.

Around the world, rural communities have been known to use the outcrops of fossilised rudists to build walls and fences especially in Europe.

From the Late Jurassic Period until the Late Maastrichtian this bizarre group of molluscs were abundant across the shallow tropical marine carbonate seabeds of the world.

This entire group would later be wiped out leading up to and during the Cretaceous-Paleogene (K-Pg) mass extinction event.

At the time of the extinction event, rudists inhabited warm shallow water carbonate platforms that were influenced by sea level change. These carbonate platforms were generally flat-topped platforms bordered by a rim formed by a reef or carbonate sand shoal on passive continental margins.

Throughout the Caribbean, the limestone rocks containing rudists are succeeded by either volcanic deposits or unstable deep-water turbidite sequences. Current research suggests that rudists started to become extinct several million years before the end of the Cretaceous Period through a combination of cooling of tropical temperatures, and a drop in sea level exposing the carbonate platforms to excessive heating from carbonate platforms and bleaching.

The impact of the asteroid only amplified the change in global climate. This period of transition triggered corals to re-establish themselves and become the dominant tropical reef builder.

Rudist bivalves were a box, tube, and ring-shaped and significant reef-builders in the Tethys Ocean.

Their outer shells were either made from Aragonite (CaCO3) or from the more stable calcite that makes up the bulk of the fossil record.

Rudists resemble corals and it has been suggested that the rudists displaced corals from reefs during the Cretaceous Period.

Illustration of rudist bivalves

The Rudist Bivalve or clam assemblages of Jamaica formed a large equatorial reef. Each rudist was attached to the sea floor sediment. They were filter-feeders that evolved a massive asymmetric fixed or sessile valved shell.

A thicker valve was known as the umbo or attached valve is a rounded protrusion found just above the hinge. The hinge is the pivoting point where it meets the thinner and smaller free valve.

Rudist morpohology - Image by Schumann & Steuber

A feature of the valves is their teeth and sockets that are found along the hinge to prevent shell misplacement. Rudists have been classified according to their hinge structure, bottom attachment, and several internal features giving them generally three morphotypes known as elevators, clingers, and recumbents.

Each morphotype reflects a specific environmental condition whether that was related to the sediment, sedimentation rate, or sea currents.

Cross section of an elevator rudist bivalve

On the island of Jamaica, there is a central inlier of Cretaceous Period rocks defined geologically as an area where a succession of older rocks is found to be surrounded by younger rocks.

First referred to as ‘St. John’s trap’ by Lyme Regis geologist Henry De la Beche in 1824 who inherited the slave-worked sugar plantation of Halse Hall in Clarendon in Jamaica.

This area contains a succession of igneous lava flows and minor intrusives, extensive volcaniclastic sediments, and subordinate carbonates.

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British geologist and naturalist Henry De la Beche (1837-1862) who was also the director of the first geological survey of Jamaica and the youngest Fellow of the Geological Society of London at the age of 18 years was among the early scientists to study rudists.

At the time the large species of Rudists found in the northern Blue Mountains were classified as Hippurite rudist and importantly they were used to age the rocks from the Cretaceous Period rather than previously thought as Palaeozoic Age.

The River Rio Minho In Jamaica

The fossil hunting exploits of Lucus Barrett were later followed up by Charles Taylor Trechmann (1884–1964) and in 1949 as the Geological Survey Department in Jamaica was established the stratigrapher, educator, and historian of Jamaican geology Lawrence Chubb (1887–1971) published 'Rudists of Jamaica' in 1971 shortly before his death.

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Known as the Guinea Corn Formation the succession is exposed along the bed and banks of the Rio Minho which is Jamaica's longest river.

This formation is made up of eight stratigraphic sections designated A to G exposed along the Rio Minho between Grantham and Guinea Corn.

The Guinea Corn Formation has three well-defined sedimentary rhythms of shale, a limestone that contains abundant corals or algal oncolites, and limestone in ascending order. Along with rudists other 'reef-type' fauna including barnacles, brachiopods, calcareous nannofossils, corals, foraminifera, gastropods, and ostracods are also present.

The highest concentration of rudist-bearing limestones appear at Logie Green in northern Clarendon within the Logie Green Beds. This limestone has a highly diverse assemblage of rudist bivalves including numerous species and genera of small radiolitids, antillocaprinids, large radiolitids, giant radiolitids, hippruitids, and various antillocaprind associations.