LASTING IMPRESSIONS—SOFT-BODIED MOLDS OF ARTICULATED VERTEBRATE FOSSILS

Soft tissue preservation is extremely rare in the fossil record, especially for vertebrates, and is almost always overlooked when preserved as a soft-body mold. This mode of preservation is novel and unexpected occurring as body or soft part impressions surrounding articulated fossils. The impressions are interpreted as carcass imprints in the soft sediment and are found as cavities after lithification. The body molds were formed in soft sediments such as carbonate oozes or volcanic ash. Microbial mats and volcanic ash provide death masks or shrouds preserving bodies of extinct organisms and provide new insights concerning their soft tissue anatomy.

Certain preparation techniques allow body molds to be recognized in Mesozoic birds from China and Germany as well the classic examples from the Pompeii human body molds found in Italy. The process of formation of the soft-body impressions is different for each particular sediment type or locality.

In the case of the Chinese bird, transfer preparation of the skeleton revealed it was draped by a thin layer of white volcanic ash several millimeters thick and was overlain by dark lacustrine sediments. It was only by the contrasting color differences of the matrix that enabled the identification the body mold.

The Solnhofen Formation (Bavaria, Germany) preserves body molds found as impressions around the skeletons in otherwise featureless lithographic limestones. This phenomena occurs in nearly all the Archaeopteryx specimens as preserved outlines of soft parts such as torso impressions, wing and tail patagia. Silicone peels provide positive casts that are more easily studied and allows restoration of certain parts of the anatomy.

Pompeii body molds were formed by a volcanic ash fall that covered the bodies. The ash conformed to outer body contours leaving an interior cavity containing the skeleton. Preserved details have been detected on the inside of this natural mold by pouring a casting media into the hollow cavity.