It’s often been remarked that the group of fish known as sturgeons are living fossils — having remained more or less unchanged for the past 100 million years. But now, new research has shown this to be far from true — sturgeon are actually one of the fastest-evolving group of fish on the planet with regards to changes in body size
The new research — from the University of Michigan — has helped to clarify the origins of modern sturgeon species, and has also resulted in the creation of one of the largest evolutionary trees ever created for any group of animals.
“Sturgeon are thought of as a living fossil group that has undergone relatively slow rates of anatomical change over time. But that’s simply not true,” said Daniel Rabosky, assistant professor in the U-M Department of Ecology and Evolutionary Biology and a curator of herpetology at the Museum of Zoology.
“Our study shows that sturgeon are evolving very quickly in some ways. They have evolved a huge range of body sizes. There are dwarf sturgeon the size of a bass and several other species that are nearly as big as a Volkswagen.”
The new findings on sturgeon evolution are the result of a larger, wide-ranging study on the rates of species formation and anatomical change in different types of fish. The work involved putting together an evolutionary tree mapping out the different evolutionary relationships “between nearly 8,000 species of fish are delineated in the branches of the tree, allowing the researchers to make inferences about all 30,000 or so species of ray-finned fish.”
The primary purpose of the project was the testing of “a longstanding idea in evolutionary biology that has anecdotal support but which had never been rigorously evaluated. It was Charles Darwin who coined the term ‘living fossil’ to describe extant creatures, such as the gar (another Great Lakes resident) and the lungfish, which have been present for many millions of years in the fossil record yet appear to have undergone very little anatomical change.”
The press release continues:
Paleontologists have long suspected that these observations reflect a fundamental coupling between the rates of species formation and anatomical change: groups of organisms that contain lots of species also seem to have greater amounts of anatomical variation, while groups with only a few species, such as the gar, lack much morphological variety.
Rabosky and his colleagues assembled a time-calibrated evolutionary tree for 7,864 living fish species using DNA sequence data and body-size information from publicly available databases. Their data set was so large that they had to develop new computer programs from scratch to analyze it.
The new computer models and the vast amount of data enabled the team to study the correlation between how quickly new species form and how rapidly they evolve new body sizes on a scale that had not previously been possible.
What the research found is that there is a large correlation between rates of species diversification, and body size evolution, “across the more than 30,000 living species of ray-finned fish, which comprise the majority of vertebrate biological diversity.”
“We’re basically validating a lot of ideas that have been out there since Darwin, but which had never been tested at this scale due to lack of data and the limits of existing technologies,” Rabosky said.
Almost all of the fish groups studied can be easily categorized as being one of only two different types — fish that form new species very slowly and have relatively little range in body size, or fish that form new species rapidly and exhibit a large range of different body sizes. “Fish like the gar form species very slowly and show little range in body size. Others, like the salmon family — which includes salmon, trout, whitefish and arctic char — do both: they form species quickly and have a wide range of body sizes.”
“Sturgeon have been around more than 100 million years and today consist of 29 species worldwide, including the lake sturgeon found in the Great Lakes. They don’t fit the general pattern found by Rabosky’s team; there are few sturgeon species but a great variety in body size.”
“In that sense, they’re kind of an outlier,” Rabosky said.
But perhaps in the relatively recent past there were a great many more sturgeon species? And they only recently experienced a large loss in total species numbers? It is known for certain that many sturgeon species have already been lost in just the last 10,000 or so years — as a part of the 6th Great Mass Extinction Event. And many sturgeon species are currently endangered, many critically.
The new research was recently published online in Nature Communications.