for National Geographic News
Australian scientists say they have found morsels of fossilized muscle—the oldest vertebrate tissue ever known—in the remains of two fish that lived 380 to 384 million years ago.
Unearthed in western Australia 20 years ago, the specimens belong to two species of an extinct group of primitive, armored fish known as placoderms (map of Australia).
The fish's remarkably well-preserved soft tissues include bundles of muscle cells, blood vessels, and nerve cells. They were found during recent electron microscope scans, the research team reported last week in the British journal Biology Letters.
Fossilized muscle is quite rare, and the new finds are even more exceptional, because they weren't flattened but rather preserved with their three-dimensional shape intact, the researchers say. (Related: "Fossils Yield 10-Million-Year-Old Bone Marrow -- A First" [July 25, 2006].)
The remains shed light on the evolution of placoderms, which ruled the world's oceans, rivers, and lakes for 70 million years until they died out about 360 million years ago.
"On the evolutionary tree, they're the first jawed animal, and we're the last. So they're our first jawed ancestors," said lead study author Kate Trinajstic, a paleontologist at the University of Western Australia.
The fossil fish's muscle tissue grew in W-shaped blocks—a trait also seen in lampreys, a modern-day remnant of other primitive fish—the scans revealed. (Related: "Bloodsucking Lamprey Found to Be 'Living Fossil'" [October 25, 2006].)
"There has been some discussion as to whether or not placoderms were the most primitive fish or whether sharks were more primitive," Trinajstic said.
"These muscles show us that placoderms were the most primitive fishes and the most primitive jawed fishes."
Ranging in length from 6 inches to 6 feet (15 to 180 centimeters), placoderms lacked bony, internal skeletons and were put together a bit like lobsters.
The fish had thick plates that interlocked like suits of armor over their heads and bodies, while sharklike tails sprouted from their backs.
SOURCES AND RELATED WEB SITES