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?

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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.

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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.

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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).

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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)

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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.

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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.