Tiny fossil of crustacean is oldest record of male animal Well-endowed sea creature is nearly half a billion years old David Perlman, Chronicle Science Editor Friday, December 5, 2003 �2003 San Francisco Chronicle | Feedback | FAQ
URL: <<sfgate.com/article.cgi?file=/chronicle/archive/2003/12/05/MNGT93GQS21.DTL>>
[Photo of specimen]
Scientists studying the evolution of common sea creatures whose ancestors date back nearly half a billion years have discovered what must be the oldest animal fossil whose gender is unequivocally male.
An animal's soft body parts hardly ever remain apparent as the organs fossilize and are gradually replaced by stone. But in the case of the 425 million-year-old crustacean, only two-tenths of an inch long, the scientists were able to clearly discern a penis.
"The copulatory organ is large and stout," said the team of scientists, led by David J. Siveter of the University of Leicester in England, in their report today in the journal Science. "The prominent copulatory appendage indicates that the specimen is a sexually mature adult male."
The scientists have named their fossil species Colymbosathon ecplecticos, which means "swimmer with an astoundingly large penis."
The specimen, a new member of a widespread class of tiny shelled animals called ostracodes, was discovered in a deposit of Silurian period rocks in Britain's Hertfordshire.
To determine the shapes of the fossilized soft tissues, Siveter and his colleagues developed a technique they call "shave and photo." Under a microscope they shaved thin layers of the indecipherable rock, photographed each layer and then assembled the images into what amounted to a virtual reconstruction of each separate organ, they said.
What is striking about the tissues the scientists recovered, including its compound eyes and the soft appendages it must have used to swim and scavange for prey, is how closely they resemble the same organs among many of the modern ostracode species.
"This is a demonstration of unbelievable (evolutionary) stability," said Thomas M. Cronin, an ostracode specialist with the U.S. Geological Survey in a commentary also published in Science. Although ostracodes have evolved and diversified into some 30,000 living or extinct species, "these guys have just been plodding along totally unfazed."
Ostracrode shells are common in oceans all over the world. Their fossils are often used by scientists to determine the age of ancient seabeds where they are found, as well as to study ancient climate changes and as timetables for the pace of evolution in other animals.
At UC Berkeley's Museum of Paleontology, Kenneth Finger, who specializes in the evolution of ostracodes, noted that given the right kind of stable environment, many other organisms might undergo little evolutionary change for millions of years.
The horseshoe crab, for example, has remained stable for at least 250 million years, and so have several other crustaceans, Finger said.
On the other hand, he said, "while many of the soft tissues in the ostracode from the Silurian period show a striking similarity to modern ones, I bet if you could get a DNA sample from the old one, you'd find plenty of differences."
Like their ancestors, modern ostracodes have penises that, relative to their body size, are larger than almost any other animal, he said. In the modern species at least, the penis is normally retracted in a coil that extends swiftly during copulation. The reconstruction of the fossil ostracode's organ does not show that configuration.
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