The Cretaceous–Paleogene (K-Pg) boundary -- formerly known as the Cretaceous–Tertiary (K-T) boundary -- is often remembered as a violent time that took out the dinosaurs around 65 million years ago. Now new work shows how the K-Pg extinction event may have actually helped establish our modern reef fish communities.
Today, about 4,500 fish species live in reefs, which house one of the largest, most diverse collections of vertebrates on Earth. About 7 percent of all vertebrate species have made their homes in coral reefs, which only make up less than 0.01 percent of Earth’s surface. Yet, their origin and early evolution isn’t very well understood. The earliest known fossil assemblage of modern coral reef fish dates back 50 million years. The rest of the patchy fossil record suggests that the major colonization of reef habitats must have occurred in the Late Cretaceous and early Palaeogene.
A team led by Samantha Price from the University of California at Davis used a phylogenetic approach (with evolutionary trees) to analyze the early dynamics of modern reef fishes. About 92 percent of reef fish belong to a single group called acanthomorphs, the spiny-rayed fishes. By looking at niches they occupy as well as DNA data, the team hoped to pinpoint when different acanthomorphs colonized the reefs.
Their results show that the mass extinction at the end of the Cretaceous had a major, and positive, impact on the evolution of reef fishes: Reef lineages successively colonized reef habitats throughout the time periods that straddle the K-Pg boundary.
They found two waves of reef colonization on either side of the mass extinction event: one in the Late Cretaceous from 90 to 72 million years ago and the other immediately following the end-Cretaceous mass extinction.
Early colonization of reefs by acanthomorphs were made rapidly by anatomically distinct lineages. Those transitions were followed by a decrease in colonization rate and the eventual filling of ecological niches, the “saturation” of space. Makes sense: The earliest fish to arrive occupied different niches, and later arrivals filled in the gaps. These patterns match a classic niche-filling scenario. The reef invasion surge continued for another 10 million years, after which the pace of transitions to reef habitats slowed.
"Before the mid-Cretaceous, the early antecedents of the fish species we associate with reefs today -- wrasse, parrotfish, clownfish -- were unlikely to be living on reefs," Price tells New Scientist. Instead, prior to the two peaks of acanthomorph colonization (or invasion), reefs were inhabited by unrelated, now-extinct groups of fish. Although, that doesn't necessarily mean they were outcompeted by Nemo characters. "All we have is a pattern, we have to be careful suggesting a process,” Price adds.
The first peak of acanthomorph reef colonization coincided with a decline in the number of coral reefs and the rise of a different kind of reef built from mollusk shells. Maybe acanthomorphs adapted better to these newer, mollusk reefs than the original fish – or maybe they were just generally more adaptable. The impact that killed off the mollusk-based reefs, triggered the return of coral reefs -- possibly enabling the second wave of acanthomorph colonization.
The work was published in Proceedings of the Royal Society B last week.
[Via New Scientist]
Image: S.A. Price et. al., Royal Society 2014