UNDERSTANDING RUDISTS: THEIR LIFE, EXTINCTION AND ACTUAL USES.

 

Rudist bivalve, Maurens Formation, Upper Cretaceous, southwestern France (Photo by Wilson44691 - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=59877823).

Rudists were fascinating bivalves that played a significant role in marine ecosystems during the Cretaceous period. Here’s a detailed look at their distribution, mode of life, and eventual extinction:

• Filter-Feeding Organisms: Rudists were primarily filter feeders, meaning they obtained their nutrients by filtering small particles from the water. This feeding strategy required them to be anchored firmly to a substrate to effectively filter the water around them.
• Habitat Preferences: They thrived in environments with hard substrates, where one of their valves could be cemented to the surface. This anchoring provided stability, allowing them to continue their filtering process efficiently. However, they struggled in soft sediment environments, as their coiled valves could not elevate the opening enough to avoid contamination from the substrate.

 

 Rudist disparity and external morphology. Representatives of the major families: a. Caprinula  (Caprinidae), b. Diceras (Diceratidae), c. Toucasia (Requieniidae), d. Monopleura (Monopleuridae), e. Ichthyosarcolites (Ichthyosarcolitidae), f. Durania (Radiolitidae), g. Vaccinites (Hippuritidae) Right valves are shaded in grey (Hernández, J. O. (2011). Rudists. Geology Today, 27(2), 74-77).

 

• Adaptations for Survival: Over time, some rudist genera developed adaptations that allowed them to adopt a semirecumbent or elevated stance. This change helped keep their feeding structures above the sediment-water interface, enhancing their ability to consume food and grow.
• Extinction Events: Rudists faced a dramatic extinction at the end of the Maastrichtian period, around 65 million years ago, coinciding with the mass extinction event that wiped out the nonavian dinosaurs. This extinction was likely triggered by a combination of factors, including meteorite impacts, volcanic activity, and significant global sea level regression.

 

Evolutionary history of rudists (Hernández, J. O. (2011). Rudists. Geology Today, 27(2), 74-77).

• Vulnerability to Climate Change: The strong provincialism of rudists, which meant they were primarily found in warm, shallow waters, made them particularly susceptible to drastic climatic changes. A significant regression of sea levels due to tectonic activity left many shallow reefs exposed, leading to asphyxiation and dehydration of rudists and other marine life.

• From Fossil Reefs to Building Blocks: These ancient rudist reefs eventually lithified into rudist limestone, a sedimentary rock rich in fossil fragments and calcium carbonate. Over millions of years, tectonic activity and erosion exposed these formations, making them accessible for human use.

In modern times, rudist limestones are not prized for their fossils, but for their composition. Their uses include:

·        Cement Production: Limestone is a primary ingredient in Portland cement. The calcium carbonate from rudist limestones is ideal for this process.

·        Aggregates: Crushed limestone is used as a base material in road construction, concrete mixes, and railway ballast.

·        Decorative Stone: In some regions, especially where rudist limestones are visually interesting due to fossil patterns, the stone is used in architecture and interior design as tiles, facades, or ornamental features.

 

                                          Detail of a rudist limestone staircase (Photo by Jordi Sanchez).

• Global Occurrence and Economic Importance: Rudist limestone deposits are found in various parts of the world, particularly around the Mediterranean (Spain, Italy, the Balkans), parts of the Middle East, and some areas in the Americas. These rocks are often quarried not only for local construction but also for export, depending on their quality and abundance.

 Rudist limestone (Photo by Jordi Sanchez).