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.

History, if we define it as the mere transcription of the written records of former generations, can go no farther back than the time such records were first made, no farther than the art of writing. But now that we have come to recognize the great earth itself as a story-book, as a keeper of records buried one beneath the other, confused and half obliterated, yet not wholly beyond our comprehension, now the historian may fairly be allowed to speak of a far earlier day. For unmeasured and immeasurable centuries man lived on earth a creature so little removed from "the beasts that die," so little superior to them, that he has left no clearer record than they of his presence here. From the dry bones of an extinct mammoth or a plesiosaur, Cuvier reconstructed the entire animal and described its habits and its home. So, too, looking on an ancient, strange, scarce human skull, dug from the deeper strata beneath our feet, anatomists tell us that the owner was a man indeed, but one little better than an ape. A few æons later this creature leaves among his bones chipped flints that narrow to a point; and the archæologist, taking up the tale, explains that man has become tool-using, he has become intelligent beyond all the other animals of earth. Physically he is but a mite amid the beast monsters that surround him, but by value of his brain he conquers them. He has begun his career of mastery.

History, if we define it as the mere transcription of the written records of former generations, can go no farther back than the time such records were first made, no farther than the art of writing. But now that we have come to recognize the great earth itself as a story-book, as a keeper of records buried one beneath the other, confused and half obliterated, yet not wholly beyond our comprehension, now the historian may fairly be allowed to speak of a far earlier day. For unmeasured and immeasurable centuries man lived on earth a creature so little removed from "the beasts that die," so little superior to them, that he has left no clearer record than they of his presence here. From the dry bones of an extinct mammoth or a plesiosaur, Cuvier reconstructed the entire animal and described its habits and its home. So, too, looking on an ancient, strange, scarce human skull, dug from the deeper strata beneath our feet, anatomists tell us that the owner was a man indeed, but one little better than an ape. A few æons later this creature leaves among his bones chipped flints that narrow to a point; and the archæologist, taking up the tale, explains that man has become tool-using, he has become intelligent beyond all the other animals of earth. Physically he is but a mite amid the beast monsters that surround him, but by value of his brain he conquers them. He has begun his career of mastery.