Fleshy tube feet preserved in a rare fossil suggest an ecological shift through time, and may settle a long-standing debate about the preservation of soft parts, say paleontologists at the University of Illinois.
Discovered in the Hunsrueck Slate of Germany by an amateur collector, the specimen is a brittle star, Bundenbachia beneckei, of the phylum Echinodermata, which includes starfishes and sea urchins.
In life, the tube feet were fleshy extensions of an internal plumbing system called the water vascular system, and projected from the animal like so many small water balloons.
"The preservation of delicate soft parts is extremely uncommon in the fossil record," said Daniel Blake, a UI professor of geology. "This is the first time unequivocal evidence of soft-tissue tube feet has been found in these animals."
The fossil was most likely created when the brittle star -- which inhabited a shallow, muddy sea bottom -- was smothered by a debris flow. Buried in thick sediment, the animal's corpse was protected from large scavengers. Over time, the fine-grained mud was transformed into slate, which was later quarried for roofing shingles.
"As millions of bacteria fed upon the decaying flesh, their waste products combined with sulfur and iron dissolved in the seawater to form pyrite," said Alexander Glass, a doctoral student at the UI. "The degradation process coated the animal with a thin veneer of pyrite, also known as 'fool's gold,' creating a mineral cast that accurately preserved the original shape of the soft tissues."
The specimen was prepared using an airbrasive technique developed in the last 10 years by German amateur fossil collectors especially for the Hunsrueck Slate fossils. Formerly a secret of amateurs, the technique -- which is similar to sandblasting, but uses smaller, softer particles of iron as the scouring agent -- allows preparation of extremely delicate pyritized structures without damage. Glass and Blake brought the technique to the United States.
The fully prepared specimen exposes long, thick tube feet extending from two ventral arm surfaces.
"The size and shape of the tube feet suggest that the animal was able to seize and manipulate larger items of food, possibly live prey," Glass said. "The tube feet may also have served as a primary means of locomotion, resembling those found on living starfishes. There are modern equivalents of brittle stars, but they have tiny tube feet. So it appears the ancient brittle stars were not living in the same manner as their descendants."
The length of the tube feet therefore suggests a functional shift through time, Blake explained. "Somehow, through the evolutionary process, the starfishes were able to grab this mode of life, and the brittle stars were unable to get it back."
The fact that the tube feet are preserved so well "also presents an interesting quandary where preservation of soft parts in the Hunsrueck is concerned in general," Blake said. Because traditional mechanical preparation methods often destroyed the delicate details preserved in fossils in the Hunsrueck Slate, X-ray techniques that peer into the rock have long been employed.
"But X-rays, especially older ones, can yield controversial images," Glass said. "Some paleontologists have argued that what appear to be soft tissues preserved in the slate are misidentifications of skeletal structures."
In the case of B. beneckei, however, the presence of tube feet "is an observable fact," Blake said. "We don't need X-rays to see them, so there is no question about identification."
The scientists' findings have important implications for the interpretations of similar fossils. The presence of soft parts in B. Beneckei serves as an interesting reference point when arguing for the existence of soft parts in the Hunsrueck in general. It provides additional evidence that the special chemical conditions necessary for such extraordinary preservation did occur in the muddy Hunsrueck sea.
The scientists will discuss the fossil and its implications for paleontology at a joint meeting of the North-Central and Southeastern sections of the Geological Society of America, being held April 3-5 in Lexington, Ky. - By James E. Kloeppel
[Contact: James E. Kloeppel]