300-million-year-old fish resembles a sturgeon but took a different evolutionary path

Date:
June 22, 2020
Source:
University of Pennsylvania
Summary:
A re-examination of a 300-million-year-old fish, Tanyrhinichthys
mcallisteri, revealed that its lifestyle more closely resembled
that of the bottom-dwelling sturgeon, rather than the stealthy pike,
as was previously believed.



FULL STORY ========================================================================== Sturgeon, a long-lived, bottom-dwelling fish, are often described as
"living fossils," owing to the fact that their form has remained
relatively constant, despite hundreds of millions of years of evolution.


==========================================================================
In a new study in the Zoological Journal of the Linnean Society,
researchers led by Jack Stack, a 2019 University of Pennsylvania graduate,
and paleobiologist Lauren Sallan of Penn's School of Arts & Sciences,
closely examine the ancient fish species Tanyrhinichthys mcallisteri,
which lived around 300 million years ago in an estuary environment
in what is today New Mexico. Although they find the fish to be highly
similar to sturgeons in its features, including its protruding snout,
they show that these characteristics evolved in a distinct evolutionary
path from those species that gave rise to modern sturgeons.

The find indicates that, although ancient, the features that enabled Tanyrhinichthys to thrive in its environment arose multiple times in
different fish lineages, a burst of innovation that was not previously
fully appreciated for fish in this time period.

"Sturgeon are considered a 'primitive' species, but what we're showing
is that the sturgeon lifestyle is something that's been selected for in
certain conditions and has evolved over and over again," says Sallan,
senior author on the work.

"Fish are very good at finding solutions to ecological problems,"
says Stack, first author on the study, who worked on the research as
a Penn undergraduate and is now a graduate student at Michigan State University. "This shows the degree of both innovation and convergence
that's possible in fishes. Once their numbers got up large enough, they
started producing brand new morphologies that we now see have evolved
numerous times through the history of fishes, under similar ecological conditions. " The first fossil of Tanyrhinichthys was found in 1984 in
a fossil-rich area called the Kinney Brick Quarry, about a half hour
east of Albuquerque. The first paleontologist to describe the species
was Michael Gottfried, a Michigan State faculty member who now serves
as Stack's advisor for his master's degree.



==========================================================================
"The specimen looks like someone found a fish and just pulled on the front
of its skull," Stack says. Many modern fish species, from the swordfish to
the sailfish, have protuberant snouts that extend out in front of them,
often aiding in their ability to lunge at prey. But this characteristic
is much rarer in ancient fishes. In the 1980s when Gottfried described
the initial specimen, he posited that the fish resembled a pike, an
ambush predator with a longer snout.

During the last decade, however, several more specimens of Tanyrhinichthys
have been found in the same quarry. "Those finds were an impetus for
this project, now that we had better information on this enigmatic and
strange fish," Stack says.

At the time that Tanyrhinichthys roamed the waters, Earth's continents
were joined in the massive supercontinent called Pangea, surrounded by
a single large ocean. But it was an ice age as well, with ice at both
poles. Just before this period, the fossil record showed that ray-finned fishes, which now dominate the oceans, were exploding in diversity. Yet
300 million years ago, "it was like someone hit the pause button," Sallan
says. "There's an expectation that there would be more diversity, but
not much has been found, likely owing to the fact that there just hasn't
been enough work on this time period, especially in the United States,
and particularly in the Western United States." Aiming to fill in some
of these gaps by further characterizing Tanyrhinichthys, Stack, Sallan,
and colleagues closely examined the specimens in detail and studied
other species that dated to this time period. "This sounds really simple,
but it's obviously difficult in execution," Stack notes, as fossils are compressed flat when they are preserved. The researchers inferred a three- dimensional anatomy using the forms of modern fishes to guide them.

What they noticed cast doubt on the conception of Tanyrhinichthys as
resembling a pike. While a pike has an elongated snout with its jaws
at the end of it, allowing it to rush its prey head-on, Tanyrhinichthys
has an elongated snout with its jaws at the bottom.



==========================================================================
"The whole form of this fish is similar to other bottom dwellers,"
Stack says.

Sallan also noticed canal-like structures on its snout concentrated in
the top of its head, suggestive of the locations where sensory organs
would attach.

"These would have detected vibrations to allow the fish to consume its
prey," says Sallan.

The researchers noted that many of the species that dwelled in similar environments possessed longer snouts, which Sallan called "like
an antenna for your face." "This also makes sense because it was an
estuary environment," Sallan says, "with large rivers feeding into it,
churning up the water, and making it murky.

Rather than using your eyesight, you have to use these other sensory
organs to detect prey." Despite this, other features of the different
ancient fishes' morphology were so different from Tanyrhinichthys that
they do not appear to have shared a lineage with one another, nor do
modern sturgeon descend from Tanyrhinichthys.

Instead the long snouts appear to be an example of convergent evolution,
or many different lineages all arriving at the same innovation to adapt
well to their environment.

"Our work, and paleontology in general, shows that the diversity of life
forms that are apparent today has roots that extend back into the past,"
says Stack.


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


========================================================================== Journal Reference:
1. Lauren Sallan, Spencer G Lucas, John-Paul Hodnett, Jack Stack.

Tanyrhinichthys mcallisteri, a long-rostrumed Pennsylvanian
ray-finned fish (Actinopterygii) and the simultaneous appearance of
novel ecomorphologies in Late Palaeozoic fishes. Zoological Journal
of the Linnean Society, 2020; DOI: 10.1093/zoolinnean/zlaa044 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200622133022.htm

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