Life in a nutshell: New species found in the carapace of late cretaceous marine turtle

Date:
August 25, 2020
Source:
Waseda University
Summary:
Scientists have identified a new ichnospecies from the shell of an
extinct marine turtle fossil, the first known species coexisting
on living marine vertebrates.



FULL STORY ========================================================================== While paleontologists have a wealth of vertebrate fossils at their
disposal, their knowledge of the ecology of ancient extinct species, particularly regarding their relationship with invertebrate species,
is relatively poor. As bones and hard shells "fossilize" much better
than soft tissues and cartilage, scientists are limited in their ability
to infer the presence of parasitic or symbiotic organisms living in or
on these ancient vertebrates. As a result, relatively little is known
about the evolutionary relationships between these ancient "clades"
and their modern descendants.


==========================================================================
All hope is not lost, though, as researchers can infer the presence
of these small organisms from the footprints they left behind. These
records are called trace fossils, or ichnofossils. One clear example of
such ichnofossils is the boreholes that many mollusks make in the turtle
shell remains and whale and fish bones on the ocean floor. However, to
this date, there have been no indications that such species also lived
in the shell while the turtle was alive and well.

In their recent study published in the journal Palaios, Assistant
Professor Kei Sato from Waseda University and Associate Professor Robert
G Jenkins from Kanazawa University focused on the trace evidence left
on the carapace (shell) of an extinct basal leatherback marine turtle (Mesodermochelys sp.). The fossil was recovered from an Upper Cretaceous formation in Nio River, Japan, and the evidence in question were 43 tiny, flask-shaped boreholes all over the turtle shell fossil.

Eager to learn more about the organisms responsible for this, the
scientists formulated a hypothesis, based on previous borehole evidence
found on ancient marine turtle shells. After observing the fossil up
close and measuring the morphological characteristics of the boreholes,
they produced a 3-dimensional reconstruction of the carapace and the cross-section of one of the boreholes, which allowed them to observe
the intricate details left by the species.

Sato, who is the lead author of this study, elaborates on the surprising evidence they found, "We saw that there were signs of healing around
the mouth of boreholes, suggesting that the turtle was alive when
the organisms settled on the carapace." Based on the morphology and
positioning of the boreholes, they determined that the likely culprits
for these boreholes were "bivalves" from the superfamily Pholadoidea,
creatures similar to the modern clams. These "sessile" (or immobile)
organisms normally require a stable substrate to bore into, and the
turtle carapace was a suitable host. The fact that the host animal was
swimming around freely probably helped, as this allowed exposure to
new environments.

Sato and Jenkins identified the boreholes called Karethraichnus; however,
they were unable to match the characteristics of the boreholes they found
with those made by any currently described species. This only meant one
thing: that they had stumbled onto a completely new species! They have accordingly named this new species as Karethraichnus zaratan.

Sato is excited about the implications of their findings, stating, "This
is the first study to report this unique behavior of boring bivalves
as a symbiont of living marine vertebrate, which is a significant
finding for the paleoecology and evolution of ancient boring bivalve
clades." Previously, no such species had been shown to live on the
carapace of living vertebrates. Instead, they were often reported to
occur on the remains of marine turtles and other vertebrates, laying on
the ocean floor alongside various decomposing organisms.

By attaching themselves on a live, free-swimming substrate, such as the carapace of a marine turtle, these pholadoid bivalves may have paved the
way for a novel, yet-unknown evolutionary path of accessing previously unexplored niches and diversifying into new species. As the tracemaker
bivalves of Karethraichnus zaratan are considered to belong to one of the
basal groups for Pholadoidea, this knowledge is crucial for understanding
the evolutionary history of extant organisms in this group.


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


========================================================================== Journal Reference:
1. Robert G. Jenkins, Kei Sato. Mobile Home for Pholadoid Boring
Bivalves:
First Example from a Late Cretaceous Sea Turtle in Hokkaido Japan.

PALAIOS, 2020; 35 (5): 228 DOI: 10.2110/palo.2019.077 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200825110740.htm

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