Early evolution of sea urchins
A genome-scale analysis of the evolutionary relationships and times of
origin of sea urchins and their relatives has prompted a re-evaluation of their fossil record

Date:
March 22, 2022
Source:
eLife
Summary:
New insight on the origins and early evolution of echinoids, a
group that includes the sea urchins, the sand dollars, and their
relatives, has been published.



FULL STORY ==========================================================================
New insight on the origins and early evolution of echinoids, a group
that includes the sea urchins, the sand dollars, and their relatives,
has been published today in the journal eLife.


==========================================================================
The study suggests that modern echinoids emerged approximately 300 million years ago, survived the Permo-Triassic mass extinction event -- the most
severe biodiversity crisis in Earth's history -- and rapidly diversified
in its aftermath. These findings help address a gap in knowledge caused
by the relative lack of fossil evidence for this early diversification.

There are more than 1,000 living species of echinoids, including sea
urchins, heart urchins, sand dollars and sea biscuits, which live
across different ocean environments ranging from shallow waters to
abysses. Throughout history, the hard spine-covered skeletons of these creatures have left an impressive number of fossils. However, despite
this remarkable fossil record, their emergence is documented by few
fossil specimens with unclear affinities to living groups, making their
early history uncertain.

"There are still debates among scientists about when the ancestors of
echinoids emerged and what role the mass extinction event that occurred
between the Permian and Triassic periods may have played in their
evolution," says first author Nicola's Mongiardino Koch, who completed
the work while he was at Yale University, New Haven, Connecticut, US,
and is now a postdoctoral fellow at Scripps Institution of Oceanography
at UC San Diego, US.

"We set out to help resolve these debates by combining genomic and paleontological data to disentangle their evolutionary relationships. The extraordinary fossil record of echinoids and the ease with which
these fossils can be incorporated in phylogenetic analyses make them
an ideal system to explore their early evolution using this approach." Mongiardino Koch and the team built upon available molecular resources
with 18 novel genomic datasets, creating the largest existing molecular
matrix for echinoids. Using this dataset, they were able to reconstruct
the phylogenetic relationships and divergence times of the major
lineages of living echinoids and place their diversification within
broader evolutionary history. They did so by applying a 'molecular
clock' technique to their dataset, whereby the rate at which mutations accumulated in the echinoid genomes is translated into geological time
with the use of fossil evidence, allowing researchers to determine when different lineages first diversified.

Their analyses suggest that the ancestors of modern echinoids likely
emerged during the Early Permian, and rapidly diversified during the
Triassic period in the aftermath of a mass extinction event, even though
this evolutionary radiation does not seem to have been captured by the
fossil record.

Additionally, the results suggest that sand dollars and sea biscuits
likely emerged much earlier than thought, during the Cretaceous period
about 40 to 50 million years before the first documented fossils of
these creatures. The authors say this result is remarkable, as the
tough skeleton of the sand dollars, their buried lifestyles, and their extremely distinct morphologies imply that their fossil record should faithfully reflect their true evolutionary history.

The team also developed a multivariate statistical approach called a 'chronospace' to help them visualise and assess the robustness of their evolutionary timeline to different choices in their analyses. They
found that different implementations of the molecular clock model had
the strongest impact on divergence times, while other decisions showed
minimal effects.

"Our work greatly expands the genomic data available for echinoids
and helps resolve some of the long-standing questions around their
evolutionary history," concludes senior author Greg Rouse, Professor of
Marine Biology at Scripps Oceanography. "Together, the results suggest
that we need to re-evaluate the echinoid fossil record, with future
studies of overlooked fossil remnants potentially providing further
support to our findings."

========================================================================== Story Source: Materials provided by eLife. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Nicola's Mongiardino Koch, Jeffrey R Thompson, Avery S Hiley,
Marina F
McCowin, A Frances Armstrong, Simon E Coppard, Felipe Aguilera,
Omri Bronstein, Andreas Kroh, Rich Mooi, Greg W Rouse. Phylogenomic
analyses of echinoid diversification prompt a re-evaluation of
their fossil record. eLife, 2022; 11 DOI: 10.7554/eLife.72460 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220322122555.htm

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