American Rudist Bivalves

American Rudists - an introduction

The rudist bivalves were a group of clams that appeared in the late Jurassic and by the early Cretaceous had become an important faunal component of carbonate platforms throughout the tropics. By the late Early Cretaceous through the Late Cretaceous they were probably the most important carbonate producers on tropical carbonate platforms. Although now known to be bivalves, some forms had earlier been classified with either the brachiopods or the corals.

The rudist bivalves of the Americas form a well defined province in the Cretaceous, which is known as the American or Caribbean bioprovince. They are found throughout northern South America (including parts of Brazil, Venezuela, Colombia and Peru), the Caribbean (Curacao, St. Croix, Puerto Rico, Hispaniola, Cuba and Jamaica), Central America (Costa Rica, Nicaragua, Honduras and Guatemala), and North America (Mexico, California, Texas, New Mexico and Florida - with sporadic occurrences further to the north).

Rudist bivalves had one valve cemented to the substrate, and interestingly, there is a distinctive bifurcation in their early development. One group (Requiuniidina) were attached to the substrate by their left valves (LV), whereas the other group (Hippuritidina) were attached to the substrate by their right valves (RV). The former group remained conservative during their evolution. In contrast, the latter group showed an explosion of morphological adaptations which created some of the strangest bivalves that have ever existed on the planet.

Rudist bivalves had an outer shell layer composed of (prismatic) calcite, and an inner shell layer composed of aragonite. The outer (calcitic) shell layer is often well preserved, whereas the inner (aragonitic) shell layer is often recystallied. Shell morphology ranges from forms in which the two valves (LV and RV) are equal to subequal, to forms in which the RV is elongate and the LV is smaller. In the latter forms, the LV ranges from tightly coiled, to loosely coiled, to cap-like.

The current classification of ruist bivalves was presented by Peter Skelton in the thematic volume for the rudist conference that was held in Jamaica in 2011. The classification (with an admendment) can be downloaded from the Caribbean Jounral of Earth Scinece at the following links:

Skelton, P. W. 2013. Rudist classification for the revised Bivalvia volumes of the 'Treatise on Invertebrate Paleontology.' Caribbean Journal of Earth Science, 45, 9-33. [PDF]

Skelton, P. W. 2013. Rudist classification: nomenclatural correction of 'Suborder Radiolitidina Skelton, 2013' to 'Suborder Hippuritidina Newell, 1965' Caribbean Journal of Earth Science, 45, 34. [PDF]

The different shell morphologies relate to different lifestyles. Skeleton and Gilli came up with a classifiaction of rudist bivalves based on morphotypes (i.e., lifestyles). Three morphotypes were recognized:

1. Elevators. Forms with an elongate RV and a shorter to tightly coiled to cap-like LV that lifted the commissure well above the sea floor. These forms often grew in bouquets. In some cases they seemed to have formed a blanket across the sea floor. These forms lifted their commissures away from the sea floor and, therefore, they were less at risk of catastophe from influxes of sediment during storm events.
2. Recumbents. These are forms which often have equal-sized (croissant-shaped) or sub-equal (hook-shaped) shells. These lay flat on the sea floor with the commissure adjacent to the sediment-water interface. Mitchell (et al., 2021) demonstarted how some of these forms developed tubes and novel feeding strategies. Sediment influx would have been a problem to these forms.
3. Clingers. These forms were attached to the substrate, and in many cases represent encrusters. As with recumbents, sediment influx would have been a problem to these forms.

It is important to realise that all rudists were attached to a substrate in their earlier life, but some became 'free-living' later in life. This is certainly the case with the large recumbents that are epitomysed by the giant Caribbean genus Titanosarcolites and its allies.

Both the inner shell layer and the outer shell layer, of one or both valves, may be modified. The modifications include: an increase or decrease in the thickness of the inner or outer shell layer; the development of a cellular structure in either shell layer; or the development of a series of pallial canals in either shell layer. Additionally, some forms may develop tubes (e.g., Titanosarcolites). The types of modification to the different shell layers are important in rudist classification.

The valves are connected by a ligament, teeth and muscles. The ligament in many bivalves holds the valves open, and they are closed by the contraction of the aductor muscles. Many different lineages of rudist show a reduction and eventual loss of the ligament, presumbably related to a change from active to passive filter feeding. The teeth hold the shells together and in most rudists there are three teeth: two in one valve and one in the other. Modifications are seen to this arrangement, and in some forms additional dental elements (toothlets) may be present. The muscles hold the two valves together, and complex muscle platforms (myophores) are developed in many rudists. The arrangement of the teeth and the form of the myophores are important in rudist classification.

The rudists' ancestors has two siphons, an inhalent and an exhalent siphons, which were related to feeding strategies. In some rudist clades, analogous structures may be present and these are now called the radial bands. These were originally labelled 'E' and 'S', but are now referred to as the Posterior Band (PB) and Ventral Band (...). In the hippuritids, these bands are folded into the inner shell layer and are called Pillar 1 (P1) and Pillar 2 (P2). A third Pillar (P0) may be present in some hippuritids and corresponds to the ligamental infold.

Rudist bivalves and Ocean Chemistry

Rudists with a thick, calcitic, outer-shell layer have proved vaulable in understanding the chemistry of the Cretaceous oceans. If well prsereved, these forms preserve the original oxygen and carbon isotope compositions of the worlds oceans and help us understand the climate of the Cretaceous period.

Radiations and Extinctions

Durning their existence, the rudists show repeated episodes of diversification and extinction. These events might be due to the spread of anoxic waters onto the carbonate platforms where the rudist bivalves lived, or to changes in sea level.

The three most important extinction events in the Americas (and elsewhere) were:

1. The early Aptian extinction event (c. 120 million years ago);
2. The Cenomanian-Turonian extinction event (c. 94 million years ago); and
3. The Cretaceous-Paleogene extinction event (c. 66 million years ago).

Rudist Formations

The term 'rudist formation' has often been used for partucular rock units that yield rudist bivalves. Frequently these are platform carbonates that contain in situ rudists buildups. Occasionally, these include other deposits where the rudists have been transported to deep-water envirnoments by mass flow deposits.

Publications

I have spent the last 25 years looking at rudists and rudist formations in the Americas. Below I provide a list and links to papers on the general Cretaceous of Jamaica, the taxonomy of various rudists, and the palaeoecology of rudists. Some of these papers are available free online, but others can be requested through Research Gate.

Cretaceous rocks of Jamaica

Mitchell, S. F. 2016. Geology of the western margin of the Benbow Inlier - implications for the relationship between the Yellow Limestone and White Limestone groups (with the description of the Litchfield Formation, new name). Caribbean Journal of Earth Science, 48, 19-25. [PDF] Erratum: [PDF]

Mitchell, S. F. and Edwards, T. C. P. 2016. Geology of the Maastrichtian (Upper Cretaceous) succession of the Jerusalem Mountain Inlier in western Jamaica. Caribbean Journal of Earth Science, 48, 29-36. [PDF]

Mitchell, S.F. 2013. The lithostratigraphy of the Central Inlier, Jamaica. Caribbean Journal of Earth Science, 46, 31-42. [PDF]

Fisher, J.D. and Mitchell, S.F. 2012. Lithostratigraphy of the Grange Inlier, Westmoreland, Jamaica. Caribbean Journal of Earth Science, 44, 19-24. [PDF]

Mitchell, S. F., Ramsook, R., Coutou, R. and Fisher, J. 2011. Lithostratigraphy and age of the St. Ann's Great River Inlier, northern Jamaica. Caribbean Journal of Earth Science, Volume 42, 1-16.[PDF]

Gunter, G.C. and Mitchell, S.F. 2005. The lithostratigraphy of the Maldon Inlier, parish of St. James, northwestern Jamaica. Caribbean Journal of Earth Science Volume 38, 1- 10.[PDF]

Mitchell, S.F., Miller, D.J. and Maharaj, R. 2003. Field guide to the geology and geomorphology of the Tertiary limestones of the Central Inlier and Cockpit Country. Caribbean Journal of Earth Science Volume 37, 39-48.[PDF]

Mitchell, S.F. 2002. Field Guide to the Geological Evolution of the Maastrichtian Rocks of the Central Inlier, Jamaica. Caribbean Journal of Earth Science Volume 36, 27-38.[PDF]

Mitchell, S. F. and Blissett, D. 2001. Lithostratigraphy of the Late Cretaceous to ?Paleocene succession in the western part of the Central Inlier of Jamaica. Caribbean Journal of Earth Science Volume 35, 19-31[PDF]

Mitchell, S. F. 1999. Stratigraphy of the Guinea Corn Formation (Upper Cretaceous) at its type locality in the Rio Minho between Grantham and Guinea Corn, Northern Clarendon Jamaica. Journal of the Geological Society of Jamaica Volume 33, 1-12.[PDF]

Mitchell, S. F. 1999. Report of a Field Meeting to the Guinea Corn Formation, northern Clarendon Jamaica 25th October 1997. Journal of the Geological Society of Jamaica Volume 33, 21-23.[PDF]

Taxonomy of rudists bivalves

Mitchell, S.F. 2009. A new rudist from the Santonian of Jamaica. Caribbean Journal of Earth Science, 40 (2009), 15-20.[PDF]

Mitchell, S. F. 2010. Revision of three large species of Barrettia from Jamaica. Caribbean Journal of Earth Science, Volume 41, 1-16.[PDF]

Mitchell, S. F. 2013. A revision of selected Lower Cretaceous American caprinoid rudists: implications for phylogeny and biostratigraphy. Caribbean Journal of Earth Science, 45, 47-75. [PDF]

Mitchell, S. F. 2013. A new rudist bivalve Curtocaprina clabaughikinsorum gen. et sp. nov. from the Middle Albian of Texas and its bearing on the origin of the Ichthyosarcolitidae Douvillé. Caribbean Journal of Earth Science, 45, 85-89. [PDF]

Masse, J.-P., Frenerci-Masse, M. and Mitchell, S.F. 2013. Revision of Praecaprina? pennyi (Harris and Hodson) (Caribbean caprinoid rudist): its placement in Pantojaloria Alencaster; taxonomic, stratigraphic and evolutionary implications. Caribbean Journal of Earth Science, 45, 99-105. [PDF]

Mitchell, S.F. and Skelton, P.W. 2013. A new rudist bivalve, Polytorreites gen. nov., from the Campanian of Puerto Rico demonstrating iterative evolution of American multiple-ray Hippuritidae. Caribbean Journal of Earth Science, 45, 119-123. [PDF]

Palaeoecology of rudist bivalves

Mitchell, S.F. Gunter, G. and Mulder, W. H. 2020. The palaeoecological significance of tubes, ribs, pallial canals and flanges in caprinoid rudist bivalves. Palaeogeography, Palaeoclimatology, Palaeoecology, 562 Article 110078, 12 pp. [LINK]

Mitchell, S.F. 2020. Exceptionally well-preserved silicified hippuritid rudist bivalves from the lower Maastrichtian of Puerto Rico. Carnets Geol. , 20 , 333-366.[PDF]

Mitchell, S.F. 2021. Rudist bivalves (Hippuritoidea) from the Clifton Limestone (Lower Campanian) of western Jamaica and a reassessment of the genus Vaccinites in the Americas. Carnets Geol. , 21 , 315-341.[PDF]

Mitchell, S.F. 2021. Revision of the hippuritid rudist bivalves from the Peters Hill Formation (late Santonian/early Campanian) of Jamaica. Journal of South American Earth Sciences , 112 , 12 pp.[LINK]

Mitchell, S.F. Authors/date Titles. Journal , vol , pp.[PDF/LINK]

Mitchell, S.F. Authors/date Titles. Journal , vol , pp.[PDF/LINK]

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