Approximately 66 million years ago, a mass extinction event at the close of the Cretaceous period reshaped Earth’s biodiversity. However, its impact on marine fish has remained a topic of debate due to gaps in the fossil record. A new study by paleontologists describes a rich assemblage of fossil marine fish from the 62.2-million-year-old Ksar E.
Sana’a El-Sayed of the University of Michigan, the study’s lead author, stated, “We have a ten-million-year gap, and the fossil record is extremely sparse. We know an asteroid impacted the marine environment, but it was unclear how these modern fishes came to be in the oceans.”
Remarkably, this site helps us address questions about when, where, and what existed in the modern ocean, only a few million years after the dinosaur extinction.
At Ksar Es-Sana’a 3, El-Sayed and her colleagues discovered fossils representing 21 fish species from nine orders.
“Most of the fish belong to percomorphs, a large group in modern oceans that was relatively rare during the age of dinosaurs,” said co-author Prof. Matt Friedman, director and curator of the University of Michigan Museum of Paleontology.
The findings also support the idea that the asteroid impact-related biological crisis, the Cretaceous-Paleogene mass extinction, caused the disappearance of some fish lineages, followed by the rapid emergence of other fish groups that look distinctly modern.
The poorly fossilized stretch of the paleontology record from the Cretaceous-Paleogene mass extinction is known as the Patterson Gap, named after the paleontologist who first highlighted it.
Because of this temporal disconnect, it has been challenging to understand how the extinction affected fish populations.
“This gap in early Cenozoic deposits leads to two interconnected questions,” Prof. Friedman explained. “First, did the fishes that we typically think went extinct at the end of the Cretaceous actually either not survive into the subsequent Paleogene period, and we’ve simply missed them because the record is so poor? This gap represents a long period of time during which we have a poor understanding of what was happening, and it unfortunately coincides with one of the most interesting periods in Earth’s recent history. Here, we’ve found this remarkable deposit that sheds new light on this critical interval.”
Numerous skeletons were preserved, but none of the fish species that were thought to have gone extinct were found there.
The conclusions suggest that these fish likely disappeared during or around the time of that major Cretaceous-Paleogene event, rather than simply being absent due to the poor record.
At the same time, the site provides direct evidence that many of these modern-looking fish groups had established themselves quite early.
The researchers also considered what their findings mean in the broader context of the early fossil record following the Cretaceous-Paleogene mass extinction.
By comparing their findings with information from other fossil sites, they discovered that most percomorphs found in the immediate aftermath of the mass extinction were predominantly found in the tropics.
Fewer percomorph individuals were observed at higher latitudes.
Only much later after the extinction did percomorphs appear to become widespread.
“There’s a rough but intriguing geographic pattern to how these modern-looking animals originated,” Prof. Friedman remarked. “Perhaps they originated in the tropics, for example, and then they dispersed to higher latitudes as climates changed or these groups diversified. This is a question that will require more careful scrutiny as results improve.”
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