YSM Issue 86.1

BY JASON YOUNG
PALEONTOLOGY
A Hard Case: Shucking the Mollusk Mystery
Specimen of Kulindroplax perissokomos,
the shelled aplacophoran
from the Silurian of Herefordshire,
as seen in the split
concretion before reconstruction
by grinding. Courtesy of Derek
Siveter.
New findings suggest that one version of the proverbial chickenor-egg
dilemma has been resolved. The answer, however, applies not
to poultry but to mollusks.
A team of researchers led by Mark Sutton of Imperial College
London and Derek E. G. Briggs, Director of the Yale Peabody
Museum of Natural History,
recently discovered a new fossil
which has helped elucidate the
evolutionary history of mollusks,
a category of invertebrate
species that includes octopuses,
chitons, snails, and oysters. The
find represents the only known
sample of the ancestral species
Kulindroplax perissokomos. The
living examples of the Aplacophora
class of mollusks, characterized
as worm-like creatures,
are believed to have evolved
from an animal like Kulindroplax.
Before the discovery, scientists
have long debated the origins of
the Aplacophora class of mollusks,
whose modern-day members
are a collection of shell-less
species. One hypothesis posited
that the aplacophorans deviated
early on in the evolution
of the mollusks, representing a
“primitive” line of organisms.
The other theory, known as the
Aculifera hypothesis, argued
that Aplacophora evolved from shelled ancestors and are closely related
to other shelled species in the Polyplacophora group, such as chitons.
Ultimately, these two hypotheses touch upon much deeper questions
regarding the origin of shells in mollusks and the evolutionary relationship
between shelled and unshelled species.
Though recent molecular studies corroborated the Aculifera model,
researchers lacked concrete fossil evidence of an ancestral species
common to both shelled and shell-less mollusks that would help verify
this hypothesized evolutionary relationship.
“The prediction that [this] model makes is that we should find some
kind of intermediate morphology between chitons (Polyplacophora)
and worm-like mollusks (Aplacophora),” Briggs said.
The fossil discovery of the Kulindroplax specimen at a deposit on
the English-Welsh border known as the Herefordshire Lagerstätte
was exactly the kind of physical evidence needed to corroborate the
Aculifera hypothesis. Briggs calls this “the missing link.” Using a
grinding apparatus that removed layers of rock only a few microns
thick, the researchers were slowly able to reveal the fossil and produce
a three-dimensional digital reconstruction. According to Briggs, the
fossil was “so remarkably preserved” in the concretion that the form
of Kulindroplax could be accurately described, almost as if it were a
living animal.
The formal report was published in Nature by Sutton, Briggs, and
fellow researchers David Siveter, Derek Siveter, and Julia Sigwart. The
study presents a morphological analysis demonstrating that Kulindroplax
perissokomos is an ancestral species that possesses both aplacophoran
and polyplacophoran characteristics. The most striking physical trait
of the four centimeter-long specimen is a series of seven valves or
shells coating the exterior of the organism, which suggests that Aplacophora
and Polyplacophora evolved from a common, shelled ancestor
and that modern day shell-less aplacophorans arose after losing
their shells in the course of evolution. In fact, the round body shape
of Kulindroplax resembled that of existing aplacophorans, while the
valve morphology resembled that of modern day polyplacophoran
chitons. The specimen was also covered in spicules or short spines; the
researchers hypothesized that these were used for movement through
thick sea-floor sediment.
As a consequence of the find, researchers now believe that aplacophorans
emerged more than 50 million years after the Cambrian
Explosion — a period of time approximately 540 million years ago
that witnessed a sudden increase in animal diversity.
According to Briggs, the finding not only allows researchers to better
understand the evolutionary history of mollusks, but also highlights
how the fossil record continues to provide new insights into the evolution
of living groups.
Virtual reconstruction of Kulindroplax perissokomos (upper and
side views). The specimen is about 4 centimeters long and there
has been some loss of detail due to decay at the front and rear.
Courtesy of Mark Sutton.
www.yalescientific.org January 2013 | Yale Scientific Magazine 11