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Trilobite Migration

Article written by: Jayson Kowinsky -

Understanding Trilobite Migration

Understanding Trilobite Migration by Looking at Ancient Relatives and Modern Crustaceans.

Ampyx priscus trilobite linear formation

This is Ampyx priscus in linear formation (Moroccan Lower Ordovician Fezouata Shale) Notice an asaphid trilobite in the migration line. Image Credit: Jean Vannier, Laboratoire de Geologie de Lyon: Terre, Planetes, Environnement (CNRS / ENS de Lyon / Universite Claude Bernard Lyon 1) Fossil used in Vannier et al., 2019

Multiple instances of preserved trilobites walking in single file lines have been observed over the years. Recently, examples of this behavior have been discussed in publications. Publications include Blazejowski et al., 2016 which show Trimerocephalus chopini trilobites from the Late Devonian of Poland, and more recently, Vannier et al., 2019 which show Ampyx priscus trilobites from the Lower Ordovician of Morocco. Overall, these examples show a variety of trilobite genera over a wide range of time periods, so it appears this behavior was relatively common. Why did trilobites march and when did they start this complex behavior?

Fossils show these animals were alive and well when marching in these long lines. All indications show the trilobites were rapidly buried by some kind of sediment collapse and killed quickly. Unfortunately, it is very difficult to determine the behavior of extinct animals based solely on their fossils.

To find out why, we can look at the behavior of similar animals today. Trilobites are arthropods that belong to the crustacean class. Some modern crustaceans today do this exact same thing. A good example, as brought up by Blazejowski et al., 2016, is the seasonal migration of the Caribbean Spiny Lobsters. During fall, when the Caribbean waters get rough, these lobsters will form a long train and march toward deeper waters, out of harm's way. Making a train greatly decreases the water drag and increases their migratory speed. Also, moving in a large group adds extra safety.

Figure 2 from Blazejowski showing a reconstruction of Trimerocephalus chopini in a queue,  a short migratory queue of spiny lobsters, and an example of T. chopini queue used in the research

This is figure 2 from Blazejowski et al., 2016 Showing A: Reconstruction of Trimerocephalus chopini in a queue. B: Short migratory queue of spiny lobsters. C: Example of T. chopini queue used in the research. Scalebar is 10 mm.

It looks like these trilobites were doing the exact same thing, possibly moving from rough waters to calmer ones. Unfortunately for the fossilized ones, they were too late. What's interesting is the oldest trilobite migration fossils date back to the Lower Ordovician (Vannier et al., 2019). This is the currently oldest occurrence of a crustacean migration.

linear clusters of the trilobite Ampyx priscus from the Lower Ordovician of Morocco

These images show linear clusters of the trilobite Ampyx priscus from the Lower Ordovician of Morocco. Notice section a has an asaphid trilobite in the migration line. Scale bars are 1 cm. 2a and 2e have been cropped from Figure 2 of Vannier et al, 2019. Image Credit: Jean Vannier. (CC-by-sa-4.0)

Although that's the oldest occurrence of a crustacean migration, arthropods, in general, have been doing this since the beginning. As it turns out, this complex behavior was present when arthropods first appeared in the Cambrian. 525 million year old Cambrian fossils from China reveal migration chains of early arthropods (Hou et al., 2008). The research indicates these chains were probably not for feeding or reproduction, but were "possibly associated with migration" (Hou et al., 2008). The main difference between these Cambrian arthropods and the trilobite migrations is the early arthropods were assembled in the water column and sank to the bottom to fossilize, whereas the trilobites were migrating on the sea floor. Despite this, it appears that arthropods, including trilobites have been performing these complex behaviors since they first appeared in the fossil record. Vannier et al. concludes this collective behavior probably evolved through natural selection during the Cambrian radiation and then developed more extensively during the Great Ordovician Biodiversification Event (Vannier et al., 2019).

This image shows early arthropods (euarthropods) in a migration chain from the Chengjiang Lgerstatte of China, which is Early Cambrian (525 million years old).  Image credit: Derek Siveter from Hou et al., 2008.

This image shows early arthropods (euarthropods) in a migration chain from the Chengjiang Lgerstatte of China, which is Early Cambrian (525 million years old). Image credit: Derek Siveter from Hou et al., 2008.

Image of a migratory line of Ampyx preiscus trilobites with a flexicalymene also in tow.  Image by Rama  (CC-by-SA-3.0).

Image of a migratory line of Ampyx preiscus trilobites with a flexicalymene also in tow. Image by Rama (CC-by-SA-3.0).

Works Cited:

Blazejowski, Blazej & Brett, Carlton & Kin, Adrian & Radwanski, Andrzej & Gruszczynski, Michal. (2016) Ancient animal migration: a case study of eyeless, dimorphic Devonian trilobites from Poland. Palaeontology. 59 (5), 743-751.

Xian-Guang Hou, Derek J. Siveter, Richard J. Aldridge, David J. Siveter. (2008) Collective Behavior in an Early Cambrian Arthropod. Science (New York, N.Y.). 322. 224. 10.1126/science.1162794.

Jean Vannier, Muriel Vidal, Robin Marchant, Khadija El Hariri, Khaoula Kouraiss, Bernard Pittet, Abderrazak El Albani, Arnaud Mazurier & Emmanuel Martin. (2019) Collective behaviour in 480-million-year-old trilobite arthropods from Morocco. Scientific Reports. 9, Article number: 14941. DOI: 10.1038/s41598-019-51012-3.

Recommended Books

The Trilobite Book: A Visual Journey
by Dr. Riccardo Levi-Setti, 2014

This is an updated (2014) hardcover (kindle available) of his famous 1994 book. It now has color images instead of black and white ones. The images are of prefectly prepared trilobites from all over the world. this book is geared toward the beginner and does not get overly technical. However, it's wonderful just to see the pictures, and a must for any trilobite enthusiast.

Trilobite: Eyewitness to Evolution
by Dr. Richard Fortey, 2001

Dr. Fortey is a famous natural history writer from the British Museum of Natural History. He brings trilbites to life in this well crafted and enjoyable narrative. He merges science and history together to show us the big picture about trilobites. It's a nice read for anyone interested in Trilobits.

Trilobites: Common Trilobites of North America (A NatureGuide Book)
by Jasper Burns, 2000

I love Jasper Burns. His Fossil Collecting in the Mid-Atlantic book is still one of my favorite fossil books. His drawings are spectacular and the books are well laid out. This book serves as a field guide and identification guide to North American trilobites. If you fossil hunt for trilobites in North America, you should have this book!

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