Oct. 26, 2004 — New troves of "fool's gold" and phosphate fossils showing soft parts and even embryos of animals are giving scientists the clearest window yet into the first explosion of animal life more than a half billion years ago.

At Chengjiang in Yunnan Province, China, details as delicate as tiny leg hairs and nose spines on the earliest animals have been preserved by two forms of the iron-sulfur mineral pyrite, a.k.a. "fool's gold."

The "raspberry" form of pyrite, so-called for its nubby surface, quickly replaced the body tissues of the Early Cambrian animals before even bacteria could devour the soft tissues.

"Until the discovery in the Chengjiang, this type of preservation was thought to be very rare," said paleontologist Sarah Gabbott of the University of Leichester in England.

The only other known rocks with "pyritized" animal fossils are from later eras in New York State and Germany. "The fidelity of preservation in the Chengjiang is better than in those two deposits," she pointed out.

Gabbott co-authored a paper on the pyritized animal fossils of China in the October issue of Geology

Gabbott and her co-authors looked carefully at what the Chengjiang fossils did and did not manage to preserve, taking into account the chemical nature of soft tissues, to come up with a model for understanding how pyrite fossils are created.

"The key to this work was in detailed microscopic analytical investigation on a scanning electron microscope," said Gabbott, "and knowing a bit about chemistry and the composition of animal tissues."

"By pinning down these mechanisms of early replacement by pyrite (they) have helped greatly to understand how we can ever have a record of soft tissues preserved at all," said paleontologist Carlton Brett of the University of Cincinnati.

More enigmatic, and revealing even finer detail than pyrite fossils, are the 3-D phosphate fossils of arthropod embryos from the Early Cambrian, found on the Yangtze platform of China.

"The process of phosphatization is not well understood for these 3-D-preserved embryos," said paleontologist Michael Steiner of Technische Universität Berlin in Germany. Steiner co-authored a paper in the same issue of Geology on the phosphatized micro-fossils.

Somehow the extremely delicate embryos were gently buried — but not so much so that they were crushed — and quickly replaced with phosphates. The result is fossils of embryos so fine that individual cells are visible in early dividing eggs.

The discovery of thousands of such fossilized embryos could help in working out proportions of which types animals were present during the "Cambrian Explosion" of animal life, said Steiner.

That, in turn, could reveal details about how evolution chose the course that led to animal life today.