Receptaculitids are extinct high-level fossils that provide a window into the history of life. After the discovery and analysis of a deposit of phosphatized receptaculitids on the Baltic Sea island of Öland, the authors conclude that receptaculitids possess an attribute not found in any other group of organisms, living or fossil.

Problematic fossils--those groups of organisms that do not fit conveniently into any existing phylum--play a pivotal role in the reconstruction of the history of life, being in effect "early experiments." This lavishly illustrated volume provides careful analyses and descriptions--in anatomical, functional, and developmental terms--of most of the major problematic fossil taxa. Sixteen original papers, written by internationally recognized scholars, discuss the features that make these taxa problematic and that provide clues to their phylogenetic relationships. Since Precambrian groups have been well covered in the existing literature, Hoffman and Nitecki focus on Paleozoic, and especially Early Paleozoic organisms, although Precambrian biota are also discussed.