Extinction and origination patterns change after mass extinctions

Date:
October 6, 2021
Source:
Stanford University
Summary:
A sweeping analysis of marine fossils from most of the past
half-billion years shows the usual rules of body size evolution
change during mass extinctions and their recoveries. The discovery
is an early step toward predicting how evolution will play out on
the other side of the current extinction crisis.



FULL STORY ========================================================================== Scientists at Stanford University have discovered a surprising pattern
in how life reemerges from cataclysm. Research published Oct. 6 in
Proceedings of the Royal Society B shows the usual rules of body size
evolution change not only during mass extinction, but also during
subsequent recovery.


========================================================================== Since the 1980s, evolutionary biologists have debated whether mass
extinctions and the recoveries that follow them intensify the selection criteria of normal times -- or fundamentally shift the set of traits that
mark groups of species for destruction. The new study finds evidence
for the latter in a sweeping analysis of marine fossils from most of
the past half-billion years.

Whether and how evolutionary dynamics shift in the wake of global
annihilation has "profound implications not only for understanding the
origins of the modern biosphere but also for predicting the consequences
of the current biodiversity crisis," the authors write.

"Ultimately, we want to be able to look at the fossil record and use it
to predict what will go extinct, and more importantly, what comes back,"
said lead author Pedro Monarrez, a postdoctoral scholar in Stanford's
School of Earth, Energy & Environmental Sciences (Stanford Earth). "When
we look closely at 485 million years of extinctions and recoveries in the world's oceans, there does appear to be a pattern in what comes back based
on body size in some groups." Build back smaller? The study builds on
recent Stanford research that looked at body size and extinction risk
among marine animals in groupings known as genera, one taxonomic level
above species. That study found smaller-bodied genera on average are
equally or more likely to than their larger relatives to go extinct.



==========================================================================
The new study found this pattern holds true across 10 classes of marine
animals for the long stretches of time between mass extinctions. But mass extinctions shake up the rules in unpredictable ways, with extinction
risks becoming even greater for smaller genera in some classes, and
larger genera losing out in others.

The results show smaller genera in a class known as crinoids -- sometimes called sea lilies or fairy money -- were substantially more likely to be
wiped out during mass extinction events. In contrast, no detectable size differences between victims and survivors turned up during "background" intervals. Among trilobites, a diverse group distantly related to modern horseshoe crabs, the chances of extinction decreased very slightly with
body size during background intervals -- but increased about eightfold
with each doubling of body length during mass extinction.

When they looked beyond the marine genera that died out to consider
those that were the first of their kind, the authors found an even more dramatic shift in body size patterns before and after extinctions. During background times, newly evolved genera tend to be slightly larger than
those that came before. During recovery from mass extinction, the pattern flips, and it becomes more common for originators in most classes to be
tiny compared to holdover species who survived the cataclysm.

Gastropod genera including sea snails are among a few exceptions
to the build- back-smaller pattern. Gastropod genera that originated
during recovery intervals tended to be larger than the survivors of the preceding catastrophe.

Nearly across the board, the authors write, "selectivity on body size is
more pronounced, regardless of direction, during mass extinction events
and their recovery intervals than during background times." Think of
this as the biosphere's version of choosing starters and benchwarmers
based on height and weight more than skill after losing a big match. There
may well be a logic to this game plan in the arc of evolution. "Our next challenge is to identify the reasons why so many originators after mass extinction are small," said senior author Jonathan Payne, the Dorrell
William Kirby Professor at Stanford Earth.



========================================================================== Scientists don't yet know whether those reasons might relate to
global environmental conditions, such as low oxygen levels or
rising temperatures, or to factors related to interactions between
organisms and their local surroundings, like food scarcity or a dearth
of predators. According to Payne, "Identifying the causes of these
patterns may help us not only to understand how our current world came
to be but also to project the long-term evolutionary response to the
current extinction crisis." Fossil data This is the latest in a series
of papers from Payne's research group that harness statistical analyses
and computer simulations to uncover evolutionary dynamics in body size
data from marine fossil records. In 2015, the team recruited high school interns and undergraduates to help calculate the body size and volume
of thousands of marine genera from photographs and illustrations. The
resulting dataset included most fossil invertebrate animal genera known
to science and was at least 10 times larger than any previous compilation
of fossil animal body sizes.

The group has since expanded the dataset and plumbed it for
patterns. Among other results, they've found that larger body size has
become one of the biggest determinants of extinction risk for ocean
animals for the first time in the history of life on Earth.

For the new study, Monarrez, Payne and co-author Noel Heim of Tufts
University used body size data from marine fossil records to estimate
the probability of extinction and origination as a function of body
size across most of the past 485 million years. By pairing their body
size data with occurrence records from the public Paleobiology Database,
they were able to analyze 284,308 fossil occurrences for ocean animals belonging to 10,203 genera. "This dataset allowed us to document, in
different groups of animals, how evolutionary patterns change when a
mass extinction comes along," said Payne.

Future recovery Other paleontologists have observed that smaller-bodied
animals become more common in the fossil record following mass extinctions
-- often calling it the "Lilliput Effect," after the kingdom of tiny
people in Jonathan Swift's 18th- century novel Gulliver's Travels.

Findings in the new study suggest animal physiology offers a plausible explanation for this pattern. The authors found the classic shrinking
pattern in most classes of marine animals with low activity levels and
slower metabolism. Species in these groups that first evolved right
after a mass extinction tended to have smaller bodies than those that originated during background intervals. In contrast, when new species
evolved in groups of more active marine animals with faster metabolism,
they tended to have larger bodies in the wake of extinction and smaller
bodies during normal times.

The results highlight mass extinction as a drama in two acts. "The
extinction part changes the world by removing not just a lot of
organisms or a lot of species, but by removing them in various selective patterns. Then, recovery isn't just equal for everyone who survives. A
new set of biases go into the recovery pattern," Payne said. "It's only
by combining those two that you can really understand the world that we
get five or 10 million years after an extinction event." Payne is also
a professor of geological sciences and, by courtesy, of biology.

Support for this research was provided by the U.S. National Science
Foundation and Stanford's School of Earth, Energy & Environmental
Sciences.

========================================================================== Story Source: Materials provided by Stanford_University. Original written
by Josie Garthwaite. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Pedro M. Monarrez, Noel A. Heim, Jonathan L. Payne. Mass extinctions
alter extinction and origination dynamics with respect to body size.

Proceedings of the Royal Society B: Biological Sciences, 2021;
288 (1960) DOI: 10.1098/rspb.2021.1681 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211006143434.htm

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