Fossil discovery reveals that trilobites had clasper-like limbs used for
Boulder, Colo., USA: Thanks to their easily fossilized exoskeleton,
trilobites largely dominate the fossil record of early complex animal life.
However, trilobite appendages and the anatomy of the underside of their
body are typically not well preserved, which makes it difficult to infer
their mating and reproductive behaviors.
Until now, modern arthropods have been heavily used as an analog to infer
the mating behavior of trilobites, but a new study published Friday in Geology described the discovery of a specialized limb in a mature
male trilobite species that sheds light on trilobite’s mating behaviors for
the first time.
Detailed study of a fossil specimen of the trilobite species Olenoides serratus revealed two sets of peculiarly reduced
appendages in the middle of its body. Each of these appendages is
interpreted to be a clasper-like limb, which mature males would use to
grasp females during mating to ensure that the male is in the best position
for external fertilization of the eggs.
“Occasionally you will get fossil specimens that actually died and got
preserved in the act of copulation, and there’s a couple of insects that
are preserved during mating, but short of that, it’s hard to infer mating
behaviors,” said Sarah Losso, lead author of the study. “There’s about
20,000 described trilobite species, but less than 40 species have preserved
appendages. This is the first time that really significant appendage
specialization is seen in trilobites. The discovery teaches us more about
the behavior of trilobites and shows that this type of complex mating
behavior already existed by the mid-Cambrian.”
The O. serratus fossil specimen is approximately 500 million years
old from the Cambrian Period and was originally collected from the Burgess
Shale in British Columbia, Canada, which is a world-renowned fossil deposit
for its soft-bodied preservation of Cambrian organisms. The fossil specimen
is currently housed at the Royal Ontario Museum in Toronto, Canada.
Losso systematically studied and photographed over 60 trilobite specimens
from the Burgess Shale that had preserved appendages. The one specimen of O. serratus was the only one to display these uniquely specialized
limbs. According to Losso, the fossil specimen is broken and is missing
most of the exoskeleton that covers the head and half of the body, but the
fact that it was broken helped them to see the clasper-like limbs, which
otherwise would not have been visible and would have been completely
concealed. Until now, this special clasper-like appendage was otherwise
unknown for O. serratus or other trilobite species to date.
Horseshoe crabs are commonly used as modern analogs for trilobites given
their similar appearance and lifestyles, and this special trilobite limb
appears functionally similar to the claspers that male horseshoe crabs have
and use to hold onto the spines of a female during external fertilization.
While it was previously presumed that trilobites had mating behaviors
similar to horseshoe crabs, this fossil discovery provides evidence for the
similarities in their reproductive strategies based on their anatomical
“Trilobites and horseshoe crabs are not particularly closely related to
each other, but they share a similar overall organization, and they live in
similar marine environments. It’s a little bit like how a bat can fly and a
bumblebee can fly. They both use wings, but the wings themselves are quite
different in how they are made and how they work. This discovery suggests
to some extent that if you are a helmet-looking marine animal that lives on
the sediment, there are only so many ways you can effectively mate
externally,” said Javier Ortega-Hernández, co-author of the study.
The discovery of this clasper-like limb in trilobites reveals that the
complex mating behaviors that are observed in modern arthropods originated
during the Cambrian Explosion over 500 million years ago. It is the
earliest record of an appendage of this type used for reproduction and
represents a degree of limb specialization for a non-feeding function.
“Traditionally, trilobites are looked at as examples of primitive animals.
This discovery shows that they could actually display complex behaviors for
reproduction, similar to what some of the animals that we have today are
doing,” said Ortega-Hernández. “It is really contributing towards a better
understanding of the Cambrian environment as actually being a thriving,
ecologically complex system, rather than a lesser version of the biosphere
we have today.”
Claspers in the mid-Cambrian Olenoides serratus indicate
horseshoe crab–like mating in trilobites
Sarah R. Losso and Javier Ortega-Hernández
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