When Trilobites Ruled the World
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WASHINGTON — Trilobites
may be the archetypal fossils, symbols of an archaic world long swept
beneath the ruthless road grader of time. But we should all look so
jaunty after half a billion years.
At the Smithsonian’s National Museum of Natural History, Brian T. Huber, chairman of paleobiology, points to a flawless specimen of Walliserops, a five-inch trilobite that swam the Devonian seas
around what is now Morocco some 150 million years before the first
dinosaurs hatched. With its elongated, triple-tined head horn and a
bristle brush of spines encircling its lower body, the trilobite could
be a kitchen utensil for Salvador Dalí. Nearby is the even older Boedaspis ensifer, its festive nimbus of spiny streamers pointing every which way like the ribbons of a Chinese dancer.
In
a back room of the museum, Dr. Huber opens a drawer to reveal a dark,
mouse-size and meticulously armored trilobite that has yet to be
identified and that strains up from its sedimentary bed as though
determined to break free.
“A lot of
people, when they see these fossils, don’t believe they’re real,” said
Dr. Huber, who is 54, fit from years of fieldwork, and proud that the state fossil of his native Ohio is a trilobite. “They think they must be artists’ models.”
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The
fossils are real, and so, too, is scientists’ unshakable passion for
trilobites (TRY-luh-bites), a diverse and illuminating group of marine
animals, distantly related to the horseshoe crab, that once dominated
their environment as much as dinosaurs and humans would later dominate
theirs — and that still have a few surprises up their jointed sleeves.
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In
a series of recent reports, scientists describe fresh insights into the
trilobite’s crystal-eyed visual system, unique in the animal kingdom,
and its distinctive body plan, a hashtag of horizontal segments arrayed
along three vertical lobes that allowed the trilobite to roll up into a
defensive ball against predators and sea squalls.
Other
researchers have found evidence that some trilobites were highly
social, migrating long distances in a head-to-tail procession as they
searched for food, or gathering together during molting season at a kind
of Trilo’s Retreat, where the trilobites could simultaneously shuck off
their carapaces and seek out mates.
“It looks like a lot of trilobite mating behavior happened when they were in a soft-shelled form,” said Carlton E. Brett, a professor of geology at the University of Cincinnati, who has presented research on trilobite assemblages to the Geological Society of America and elsewhere. “They did it in the nude.”
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To
investigate trilobite social life, Dr. Brett and his colleagues
analyzed numerous examples of mass burial sites, where congregations of
trilobites had been trapped in place by the sedimentary upheavals from
violent sea storms, just as the residents of Pompeii were smothered in midscream by Vesuvian ash.
“You
feel a little bad for the trilobites, but it’s incredible seeing these
things preserved in the act of life processes,” Dr. Brett said. “It’s
frozen behavior.”
Trilobites: Variations on a Theme
Over 300 million years, trilobites evolved a diverse and successful array of forms while maintaining a simple, common body plan.
On
a similarly erotic note, some researchers have proposed that many of
the more gothic features identified in the trilobite fossil record — the
oversized head horns, the curlicue shoulder spines, and maybe the
eyestalks that look like a couple of periscopes plunked on either side
of a trilobite’s face — are the trilobitic equivalent of a peacock’s
tail, results of sexual selection rather than adaptations to the
environment.
By this argument, the
showstoppers in a given trilobite collection are probably males, their
appurtenances having evolved to impress females or intimidate rival
males. “If you look at the diversity of life now, most of the weird,
exaggerated things we find are sexually selected,” said Robert J. Knell
of the Queen Mary University of London. “There’s no reason to think
evolution was working differently in the past.”
Dr. Knell and the renowned trilobite expert Richard A. Fortey of the Natural History Museum in London reported their ideas about sexual selection among trilobites in the journal Biology Letters.
The
sticking point: Researchers have no clue how to determine a trilobite
fossil’s sex. If a big trident marks a Walliserops as male, for example,
scientists have yet to identify who his drab peahen may have been. As
Dr. Brett said, the “X-rated parts” don’t readily fossilize.
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contrast, the fossil record brims with trilobite PG-rated parts, the
mineralized remains of the hard outer sheath, or exoskeleton, that
covered much of the trilobite’s body. Not only did trilobites persist
for close to 300 million years — almost twice as long as the dinosaurs —
and thus had ample opportunity to leave heaps of their carcasses
behind, but all the molted shells they discarded while alive were
likewise fair game for stratigraphic immortality.
“Trilobites,” Dr. Fortey has written, “were veritable fossil factories.”
Researchers
particularly appreciate the trilobite’s developmental consistency, the
way it avoided any complicating, butterfly-style metamorphosis and
instead grew larger by simply generating new segments toward the rear
and then “jacking itself apart,” said Nigel Hughes,
a trilobite specialist at the University of California, Riverside.
“They changed relatively little as they went through their molts,” he
said, “so we’ve been able to assemble growth series, stage after stage,
for a number of trilobite species.”
That
accordion approach to maturation played out in a symphonic diversity of
forms. Researchers have identified some 20,000 trilobite species, which
range in adult size from a quarter-inch to the dimensions of a kitchen
tabletop. Some scurried on the sea floor or buried into the sediment and
ate detritus. Others swam or floated near the surface and may have
hunted small invertebrates.
“They
can have scoops or shovels, be fantastically spiny or beautifully
streamlined,” Dr. Hughes said. “They diverged to really explore their
evolutionary space, but they maintain that common body plan” — the three
vertical lobes, or trilobes, that give the class its name.
Researchers
were long at a loss to explain the origins of all the architectural
partitioning. They believed that early trilobites lived flatly, like
flounders, and only later would take advantage of their longitudinal
seams to begin enrolling — curling up into a ball, armadillo-style, to
protect their soft underparts.
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But last fall a team from the University of Cambridge and the Chinese Academy of Sciences reported the discovery
of fully enrolled fossils dating to the trilobite’s advent, in the
Cambrian period roughly 510 million years ago, suggesting that the lobes
were about enrollment from the start. Later still, with the rise of
jawed fish and other fierce predators, trilobites evolved increasingly
elaborate enrollment techniques, including tips and sockets that locked
together and made the rounded trilobite almost impossible to pry apart.
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Scientists
can also spy the escalating threats that trilobites confronted by
studying the evolution of their eyes. Trilobite eyes were unlike those
of virtually any other known animal, the lenses built not of protein but
of calcite crystals, lending the animals a “stony stare,” as Dr. Fortey
put it.
In most trilobites, each
compound orb held hundreds of tiny calcite lenses, arranged in a
tightknit honeycomb pattern, like the eye of a fly. But fairly late in
trilobite evolution one group developed a different sort of eye,
composed of a smaller number of larger, separated calcite lenses.
As they described last spring in the journal Scientific Reports, Brigitte Schoenemann of the Universities of Cologne and Bonn in Germany and Euan N. K. Clarkson of the University of Edinburgh, used advanced scanning techniques, including synchrotron radiation,
to examine specimens of these later, larger-lensed trilobite eyes. On
the back of the lenses, the scientists were astonished to see traces of
the sensory receptor cells that once linked the eyes to the brain.
“It
was extraordinary,” Dr. Schoenemann said. “As far as we know, these are
the oldest receptor cells that have ever been seen in any fossil
animal.”
Analyzing the
microstructure of the receptor tracings, the researchers concluded that
the eyes were designed to work optimally in lowlight, murky conditions, a
sign that some trilobites were turning reclusive, descending to deeper
waters or burrowing farther into the mud to escape the proliferation of
toothy marine predators and new crustacean competitors.
Toward the end of the Paleozoic Era, the once-thriving trilobite tribe had been reduced to a scattering of species. And they, too, vanished in the great Permian extinction 252 million years ago.
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Yet
the trilobite’s appeal is undimmed. “People always like trilobites,”
Dr. Schoenemann said. “They find them sweet.” Those big eyes. That
rounded head. Trilobites “show a scheme of childlike characteristics,”
Dr. Schoenemann added. “You want to protect them.”
Childlike, you say? Just wait till they molt.
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