What is the reason for the absence of trilobites in the sea at the present time?

 

For more than 270 million years, trilobites were one of the most successful groups of animals on Earth. They inhabited ancient oceans, survived multiple ecological crises, and left behind an extraordinary fossil record. Yet today, not a single trilobite lives in the oceans. What happened?

---

What were trilobites?

Trilobites were marine arthropods that lived from the Cambrian period (about 521 million years ago) until the end of the Permian period (252 million years ago) (Fortey, 2000; Whittington, 1992).

                                 

 Ordovician (about 445 Myo) trilobite Flexicalymene ouzregui Destombes, 1966 (Photo by J.Sanchez)

 Their name comes from the three longitudinal divisions of their bodies: a central lobe (the axial lobe) and two lateral lobes (pleural lobes). They had a segmented, calcified exoskeleton and, in many species, highly developed compound eyes that have been important for understanding the early evolution of vision in arthropods (Clarkson, 1998).

During their long history:

• They occupied nearly all marine environments.

• They diversified into more than 20,000 described species.

• They were key components of Paleozoic marine food webs.

---

The event that changed rverything: The End-Permian extinction

The final disappearance of trilobites occurred during the mass extinction at the boundary between the Permian and the Triassic, about 252 million years ago.

This event is considered the largest biological crisis in Earth's history, eliminating roughly 90% of marine species (Erwin, 1994; Benton & Twitchett, 2003; Burgess, Bowring & Shen, 2014).

 Asaphiscus wheeleri Meek 1873 with some tiny Elrathia kingii Meek 1870 (Photo by J.Sanchez)

What caused this extinction?

The most widely supported hypothesis points to a combination of extreme environmental changes linked to massive volcanic eruptions in what is now Siberia (Wignall, 2001; Burgess & Bowring, 2015):

• Large-scale, prolonged volcanism

• Massive release of CO₂ and other gases

• Rapid global warming

• Ocean acidification

• Ocean anoxia (loss of oxygen)

These changes dramatically altered marine ecosystems and were particularly devastating for benthic organisms such as trilobites.

---

A decline before the catastrophe

Importantly, trilobites had already been declining in diversity since the Late Devonian. After the mass extinction at the end of the Devonian, many trilobite lineages disappeared and only a few groups survived into the Permian (Fortey & Owens, 1999).

During the late Paleozoic they faced several pressures:

• Competition with newly evolving marine arthropods and vertebrates

• Increasing predation pressure

• Environmental and habitat changes in marine ecosystems

  
   

   Ordovician (~466.0 to 443.7 Myo) trilobite Panderia beaumonti Rouault, 1847 (Photo by J.Sanchez).

By the time the crisis at the end of the Permian occurred, trilobite diversity was already low, which may have reduced their evolutionary resilience to extreme environmental disruption (Erwin, 2006).

---

Could trilobites ever return?

No. Trilobites are completely extinct and left no direct living descendants. Although they belonged to the phylum Arthropoda—like modern crustaceans, insects, and arachnids—the trilobite lineage itself ended entirely (Whittington, 1992).

 

Devonian (~400 Myo) trilobite Gerastos granulosus Goldfuss, 1843 (Photo by J.Sanchez)

---

A Fossil Legacy

Even though they no longer inhabit modern oceans, trilobites remain extremely important for science. Their fossils are widely used in biostratigraphy, particularly for dating rocks from the Cambrian and other Paleozoic intervals, because they evolved rapidly and had a broad geographic distribution (Fortey, 2000).

In addition, exceptional fossil deposits such as the Burgess Shale have preserved remarkable details of their anatomy and early ecology.

---

Scientific References

Benton, M. J., & Twitchett, R. J. (2003). How to kill (almost) all life: the end-Permian extinction event. Trends in Ecology & Evolution, 18(7), 358–365.

Burgess, S. D., & Bowring, S. A. (2015). High-precision geochronology confirms voluminous magmatism before, during, and after Earth’s most severe extinction. Science Advances, 1(7), e1500470.

Burgess, S. D., Bowring, S., & Shen, S. Z. (2014). High-precision timeline for Earth’s most severe extinction. Proceedings of the National Academy of Sciences, 111(9), 3316–3321.

Clarkson, E. N. K. (1998). Invertebrate Palaeontology and Evolution. Blackwell Science.

Erwin, D. H. (1994). The Permo-Triassic extinction. Nature, 367, 231–236.

Erwin, D. H. (2006). Extinction: How Life on Earth Nearly Ended 250 Million Years Ago. Princeton University Press.

Fortey, R. A. (2000). Trilobite! Eyewitness to Evolution. Alfred A. Knopf.

Fortey, R. A., & Owens, R. M. (1999). Feeding habits in trilobites. Palaeontology, 42(3), 429–465.

Whittington, H. B. (1992). Trilobites. Boydell Press.

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).

GRAPTOLITES: ANCIENT MARINE WONDERS AND THEIR ROLE IN BIOSTRATIGRAPHY

 

Silurian graptolite (about 430 million years old) Oktavites spiralis (Geinitz, 1842) from Żdanów (Poland) (Photo by Jordi Sanchez).

Graptolites are extinct marine organisms that inhabited the oceans from the Lower Cambrian to the Lower Carboniferous (approximately 500 to 400 million years ago). They were colonial animals that played a significant role in biostratigraphy, helping geologists date and correlate sedimentary strata from different regions.

---

🧬 Morphology

Graptolites formed colonies called rhabdosomes, made up of individual units called zooids. These colonies could take various forms, ranging from branched (dendroid) shapes to simpler ones. Each zooid was housed in a tubular or cup-shaped cavity, and the colonies were connected by living tissue. In life, the colonies floated in the water, but after death, they sank and were preserved in marine sediments.

 

Silurian graptolite Linograptus posthumus (Richter, 1875) from the silurian of Żdanów (Poland) (Photo by Jordi Sanchez).

---

🌍 Distribution and Biostratigraphy

Fossils of graptolites are common in sedimentary rocks worldwide, especially in slates and shales, where their preservation is optimal. Their rapid evolution and abundance made them essential guide fossils for dating strata from the Upper Cambrian to the Lower Carboniferous. The extinction of many species at the end of the Ordovician and their subsequent diversification in the early Silurian are key events in their biostratigraphy.

---

🧭 Paleontological Importance

Graptolites are critical in biostratigraphy due to their rapid evolution, wide distribution, and abundance in the fossil record. Their study allows paleontologists and geologists to:

• Precisely date sedimentary strata.
• Correlate rock layers from different geographical regions.
• Reconstruct ancient marine environments and paleoenvironmental changes.

---

🧪 Conservation and Preservation

Fossils of graptolites are mainly found in slates and shales, which formed in deep marine environments with low circulation, favoring their preservation. In some cases, they have been found in three dimensions when minerals like pyrite infiltrated them. Their preservation can range from flat impressions to detailed three-dimensional structures.

Silurian graptolite Oktavites spiralis (Geinitz, 1842) from the silurian of Żdanów (Poland) (Photo by Jordi Sanchez).