The Trilobite Paradoxides

 

Paradoxides davidis davidis (Specimen FMNH PE 28974, 200mm or ~8 inches long)

If you think of large arthropods along the rocky coast of New England you probably think of lobsters, but there is another large arthropod literally in these rocky coasts, Paradoxides, a trilobite.  Paradoxides is a large trilobite close to half a meter (one and a half feet) long. There are many species and subspecies of this genus and they have been collected from the Czech Republic, Scandinavia, Wales, Spain, Morocco, Newfoundland, New Brunswick, Maine and Massachusetts. Geologists use these species to correlate rock formations from these different areas.


This strange geographic distribution and the remarkable similarities of some trilobites from North America and Europe puzzled paleontologists. However, using these trilobites and other fossils, plus paleomagnetic evidence and other clues in the rock record, combined with the Theory of Plate Tectonics geologists have been able to work out the complex story of how these fossil trilobites were distributed to so many different areas.

Paradoxides davidis intermedius (Specimen FMNH PE 28971-B, 240mm or 9.25 inches long)

Paradoxides davidis trapezopyge (Specimen FMNH  PE 25557-A,160mm or 6.25 inches long)

 

During the Cambrian Period, 510 million years ago, before there were any lobsters these trilobites crawled along the seafloor, but not off the coast of North America or Europe. They lived in the shallow waters off the coast of a small continent that no longer exists called Avalonia by geologists.


Avalonia has a complicated geologic history. It started out as small continent in between Africa and the northern part of Europe. Over the next 200 million years it collides with Europe then they collide with North America and finally Africa collides with them sandwiching the small continent in the center of the supercontinent Pangaea.  When Europe and Africa break away from North America parts of Avalonia (and the fossil trilobites) are left on the Eastern coast of North America, the western half of Europe, Scandinavia, and Morocco.

Paleogeography map showing North America, Northern Europe, and Avalonia. Modified from Ron Blakey, Northern Arizona University Geology.

The specimens pictured are three subspecies of Paradoxides davidis from Middle Cambrian rocks of the Avalon Peninsula of Newfoundland (Avalonia gets its name from this peninsula), Paradoxides davidis davidis, Paradoxides davidis intermedius, and Paradoxides davidis trapezopyge. You can tell the difference between the three subspecies by examining their pygidium or tail. P. davidis davidis has a narrow pygidium while P. davidis trapezopyge has a wide trapezoid-shaped pygidium and P. davidis intermedius is in between the two.

The specimens were collected and donated to the Field Museum by Riccardo Levi-Setti a professor of physics at the University of Chicago and the inventor of the high-resolution scanning ion microprobe. Levi-Setti enjoys collecting trilobites and has published a book and several scientific articles on trilobites.

Reference

Riccardo Levi-Setti, 1993, Trilobites, University of Chicago Press, Chicago, 342 pages.