BUFFALO, N.Y. -- Tiny sea creatures called rhabdopleurids reside
on the ocean floor, building homes of collagen on the shells of
dead clams. Rhabdopleurid colonies are small, and the critters are
by no means the dominant animals in their ecosystem.
But they have lived this way -- and survived -- for more than
500 million years. And in doing so, they have outlasted more
elaborate species that also descended from a common ancestor,
according to a
new study in the journal Lethaia.
Though rhabdopleurids' age and modern existence are
well-documented, the paper breaks new ground by identifying them as
a predecessor to ancient zooplankton -- known as pelagic
graptolites -- that went extinct about 350 million years ago.
The lesson, according to lead author Charles Mitchell: Newer
isn't always better.
"We think that change is always going to lead us to a better
place, that evolution is always going to lead to something better,"
said Mitchell, a University at Buffalo geology professor. "But all
this progress in making all these wonderful pelagic graptolites
didn't lead them to take over the world. They didn't survive, but
these simple dudes, these bottom-dwelling creatures, did."
Mitchell's partners on the research included Michael J. Melchin
from St. Francis Xavier University in Nova Scotia, Canada; Chris B.
Cameron of the Universite de Montreal; and Jorg Maletz from the
Frei Universitat Berlin.
The paper, which appeared online on Aug. 2, used rhabdopleurids'
structure and form to determine that they were some of the most
primitive graptolites that ever existed.
While their zooplankton relatives evolved rapidly, splitting
into many new species and evolving many new traits, rhabdopleurids
pretty much stayed the same over the course of history.
As the zooplankton developed ways to live closer to the ocean's
surface, the rhabdopleurids continued dwelling on the ocean floor.
The zooplankton became important players in their new ecosystems.
The rhabdopleurids remained inconspicuous.
Ultimately, the conservative approach won out: The
rhabdopleurids survived and are still around today, living in areas
from Bermuda to the Bering Sea. The zooplankton graptolites went
"High speciation rates generally go hand in hand with high
extinction rates, and likewise low with low," Mitchell said.
"Conservative lineages may weather the storms of climate change and
other events, but do not become big parts of the ecosystem, whereas
the major players are impressive but often brought low by mass
extinction and other 'slings and arrows of outrageous
The idea that conservative approaches can bear rewards over time
is one that holds true not only in biology, but in other fields of
study as well, Mitchell said. He pointed to financial markets as
"You can pick 'safe' investments like bonds and blue chip
stocks, and so expose your money to low risk of decline in values,
but the yield is low, as well: Values do not grow much," Mitchell
said. "On the other hand one can pick high-yield tech stocks like
Facebook and Apple, but the risk of declines in value, especially
in bad economic times, is also high."
Though humble, rhabdopleurids and the colonies they build are
beautiful to behold under a microscope.
The creatures themselves are about a millimeter long and
Y-shaped, with a pair of tentacled arms extending from a narrow
body to filter food from the water. The colonies they fashion are
whimsical-looking structures, consisting of a network of
copper-colored tubes that resemble tiny elephant trunks, each one
bearing numerous ridges.
The knowledge that rhabdopleurids are ancient graptolites will
enable researchers to gain insight into poorly understood aspects
of graptolite biology. Studying rhabdopleurids could reveal new
clues about how early graptolites looked and reproduced, and even
what they ate.
Support for the research that appeared in Lethaia came from the
Natural Sciences and Engineering Research Council of Canada
Discovery Grant program, the U.S. National Science Foundation and
the St. Francis Xavier University James Chair Visiting
Professorship. The paper is titled "Phylogenetic analysis reveals
that Rhabdopleura is an extant graptolite."