Convergence in Trilobites

Name any name and then remember everybody you ever knew who bore that name. Are they all alike? I think so. –Gertrude Stein

Broggerolithus broggerei, Harnage Shales, Ordovician Period, Welshpoole, Wales, United Kingdom
Broggerolithus broggerei (Family Trinucleidae), Harnage Shales, Ordovician Period, Welshpoole, Wales, United Kingdom. Trilobite is 2.5 cm across the genals.

When unrelated or only distantly related organisms evolve a similar form as an adaptation to a common way of life, you have convergence. And convergence is one of the great patterns in the history of life–and one of the clearest lines of evidence that evolution by means of natural selection is real.

Distinguishing features that are identical by descent (blood relationship) from those that are convergent is the central challenge in reconstructing the evolutionary histories of living things.

Aristoharpes sp., Devonian Period, Morocco
Aristoharpes sp. (Family Harpidae), Devonian Period, Morocco. Trilobite is 4.5 cm long.

Evidence of convergence is to be seen throughout the Trilobita. An easy place to recognize it is among the filter feeders. All the trilobites in this post were likely filter feeders, their large cephalons used as filtration chambers. Aristoharpes and Broggerolithus are not closely-related to each other, and Cordania is only distantly related to the others (they all belong to the Ptychopariida). Their superficial resemblance is likely due to a common way of life.

How many instances of convergence can you recognize in your collection?

Cordania wessmani, Bois d'Arc Formation, Devonian Period, Coal County, Oklahoma.
Cordania wessmani (Family Brachymetopidae), Bois d’Arc Formation, Devonian Period, Coal County, Oklahoma. This trilobite is more closely related to proetids than it is to any of the other trilobites in this post. Trilobite is 2.5 cm long.

©2017 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.

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