New study of fish fossil records near extinction events contradicts previous models
The history of evolution is periodically marked by explosions in
biodiversity, as groups of species try out a wide range of shapes and
sizes. With a new analysis of two such adaptive radiations in the fossil
record, researchers have discovered that these diversifications
proceeded head-first.
By analyzing the physical features of fossil fish that diversified
around the time of two separate extinction events, scientists from the
University of Chicago and the University of Oxford found that head
features diversified before body shapes and types. The discovery
disputes previous models of adaptive radiations and suggests that
feeding-related evolutionary pressures are the initial drivers of
diversification.
Two
radiations of ray-finned fishes (Actinopterygii), Carboniferous forms
(facing left) and acanthomorph teleosts (facing right) underwent
distinct cranial (feeding) and later postcranial (habitat) stages in
trait diversification. Credit: (Photographs by Lauren Sallan and Matt
Friedman).
“It seems like resources, feeding and diet are the most important
factors at the initial stage,” said lead author Lauren Sallan, graduate
student in the Department of Organismal Biology and Anatomy at the
University of Chicago. “Strange heads show up first – crushing jaws,
animals with big teeth, with long jaws – but they’re all pretty much
attached to the same body.”Adaptive radiations underlie the evolution of dominant and diverse
groups. After a major disruption, such as an extinction event, surviving
species diversify into a myriad variety of forms. Modern examples of
this diversity are the fish family of cichlids, with more than 1,000
documented species, or “Darwin’s finches” of the Galapagos Islands,
which exhibit many different beak types.
Evolutionary biologists have used these living species to propose at
least two models of how adaptive radiations work. One model proposes a
single “burst” of divergence followed by a long period of relative
stability. Another, sometimes known as the “general vertebrate model,”
introduced the idea of staged divergences, with habitat-driven changes
in body type preceding diversification of head types.
However, these models had not yet been tested with the rich data sets available in the fossil record.
“There hadn’t been any tests of these things using fossils,” said
Sallan, a graduate student in the laboratory of University of Chicago
Professor Michael Coates. “You have all these analyses of
diversification, yet not one of them goes back to the fossil record and
says what’s happening at this time period, and the next time period, and
the one after that.”
Sallan and co-author Matt Friedman, PhD, lecturer in paleobiology at
the University of Oxford and a former member of Coates’ laboratory,
looked at two different adaptive radiations in the fossil record. The
first was the explosion of ray-finned fishes after the Hangenberg
extinction, an event 360 million years ago that decimated ocean life on
Earth. The second group was the acanthomorphs, a group of fish that
exhibited a burst in diversity around the time of the end-Cretaceous
extinction that ended the age of dinosaurs.
In both datasets, the researchers used a method called geometric
morphometrics to quantify differences in features such as body depth,
fin position and jaw shape between species. Crucially, Sallan and
Friedman separated head features from body features in their analysis,
to better detect the timing of when each compartment showed a burst of
diversity in the record.
The results of the two analyses were in agreement: Diversification in
cranial features preceded diversification in body types. Unusual head
features such as jaws lined with sharp teeth or blunt teeth for crushing
appeared before diverse body shapes on a spectrum from slender and
eel-like to broad and disc-shaped.
“We have these two entirely separate radiations, and in both of them
the pattern is heads first. So feeding might be more important to
diversification than habitat use,” Sallan said. “It’s against both the
adaptive radiation model and the proposed stage model.”
The pattern detected with the new analyses suggests that the
appearance of new sources of food drives a burst of diversity before
species begin to change to adapt to new habitats.
“Ecological limits are taken away,” Sallan said. “There’s more
opportunity out there, more available resources, and they’re taking
advantage of that. Later, they’re taking advantage of specializing to
new habitats. So it’s not something within the animals themselves; it’s
more opportunity that matters.”
While the new study offers two distinct examples of head-first
diversification separated by hundreds of millions of years, the
universality of the model remains to be conclusively proven.
“Evolution is really complex, and it’s not really clear that there
should be only one model,” Sallan said. “It might be that this model
might apply to fishes in certain time periods, or might apply to
vertebrates, but a lot more investigation is needed to see whether that
is actually true.”
_____________
The paper, “Heads or Tails: Staged Diversification in Vertebrate
Evolutionary Radiations,” was published online Dec. 21 by the journal Proceedings of the Royal Society B.
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