Evolution Icon Evolution
Paleontology Icon Paleontology

Fossil Friday: The Devonian Nekton Revolution

Photo credit: James St. John, CC BY 2.0 , via Wikimedia Commons.

This Fossil Friday features the acanthodian “shark” Diplacanthus striatus from the Lower Devonian of Scotland. It illustrates yet another “explosion” in the history of life.

Klug et al. (2010) described a previously overlooked radical change in the composition of the marine fauna of the Early Devonian, which they called the Devonian Nekton Revolution. Prior to this abrupt event, the marine ecosystems were dominated by organisms that lived either close to the seafloor (demersal) or passively drifting as plankton. Between 410-400 million years ago, a very sudden and enormous expansion of actively swimming (nektonic) animals occurred in the Devonian era, when groups such as ammonoid cephalopods and jawed fish made their first appearance. Within just 10 million years such active swimmers increased from only 5 percent to about 75 percent of the marine faunal biodiversity (see the chart below).

Image credit: Günter Bechly, redrawn and modified after Klug et al. 2010.

The authors commented in a later paper that “this macroecological event corresponds to an explosive trend from planktonic and demersal marine animals toward true nekton as represented by the great diversification of jawed fish and ammonoids, reflecting a selection for swimming capabilities. It coincided with macroevolutionary transformations among various mollusc groups” (Monnet et al. 2011) and “is strongly linked with the rise of predatory jawed vertebrates, which also became more active swimmers in the same interval” (Klug et al. 2017, also see Anderson et al. 2011).

Accepted and Included

The Devonian Nekton Revolution became widely accepted and included in modern textbooks on paleobiology (e.g., Benton & Harper 2020). Of course, evolutionary biology would not be evolutionary biology without a dissenting position, which denies the whole phenomenon. Whalen & Briggs (2018) published a study that disputed the concept of the Devonian Nekton Revolution and claimed that “new data indicate that the metazoan colonization of the water column was considerably more complex and gradual than previously understood.” The popular science media triumphantly reported “Ancient Swimming Revolution May Not Have Happened” (Gramling 2018) and “New evidence suggests the Devonian Nekton Revolution never occurred” (Yirka 2018).

Since Christian Klug is an old university friend of mine, I asked him about this new study. He told me (Klug pers. comm., May 29, 2021) that he is currently working on other questions, but that he is still getting a lot of positive feedback and confirmation by colleagues on the Devonian Nekton Revolution, including new views on ecological factors that give even more weight to this revolution such as the vertical transport of oxygen and nutrients. He was not impressed at all by these “Yalies’ attempt to shoot down [his] paper” and thinks that the Whalen & Briggs study lost itself so much in details that the larger pattern was made invisible. I would add that getting rid of inconvenient facts like explosive events in the history of life may have played a significant role as well. The truth is that such explosions and revolutions dominate the history of life, which was rather a series of abrupt saltations than the gradual change predicted by Darwinism.