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.

Two of the greatest evolutionary events in the history of life on Earth occurred during Early Paleozoic time. The first was the Cambrian explosion of skeletonized marine animals about 540 million years ago. The second was the "Great Ordovician Biodiversification Event," which is the focus of this book. During the 46-million-year Ordovician Period (489–443 m.y.), a bewildering array of adaptive radiations of "Paleozoic- and Modern-type" biotas appeared in marine habitats, the first animals (arthropods) walked on land, and the first non-vascular bryophyte-like plants (based on their cryptospore record) colonized terrestrial areas with damp environments. This book represents a compilation by a large team of Ordovician specialists from around the world, who have enthusiastically cooperated to produce this first globally orientated, internationally sponsored IGCP (International Geological Correlation Program) project on Ordovician biotas. The major part is an assembly of genus- and species-level diversity data for the many Ordovician fossil groups. The book also presents an evaluation of how each group diversified through Ordovician time, with assessments of patterns of change and rates of origination and extinction. As such, it will become the standard work and data source for biotic studies on the Ordovician Period.