The Prelude to Continental Invasion

Abstract

The colonization of land by animals was a major evolutionary transition. Ichnologic evidence suggests that this process may have begun at the end of the Ediacaran interval with incursions into very shallow, marginal-marine settings, shortly after the estimated time of the emergence of bilaterian taxa. Animals made their first unequivocal amphibious terrestrial forays during the Cambrian and may have managed to establish themselves in truly alluvial environments by the Late Ordovician. Following this early stage, the Silurian to Permian is characterized by an unequivocal explosion of diversity and expansion into new environments and niches. Subaqueous and transitional subaqueous to subaerial marginal-marine environments were colonized late in the Ediacaran but ecospace occupation was limited, extending to shallow and semi-infaunal tiers respectively. The Cambrian shows no ichnologic evidence that true continental environments were colonized but rather indicates brackish-water colonization as well as excursions into subaerial coastal dune environments by animals able to survive temporary periods of desiccation. In those environments that had begun to be colonized during the Ediacaran, the Cambrian shows a marked increase in ichnodiversity and the number of architectural designs, plus an expansion in ecospace exploitation to the deep infaunal tier. Of the phyla to colonize the land, the Arthropoda, Mollusca, and one or more of the Annelida, Nematoda, and Nemertea had already begun to adapt to marginal-marine habitats by the end of the Cambrian. What followed in the Ordovician, rather than an increase in ecospace occupation, was an increase in ichnodiversity and architectural designs within already exploited ecospace. The Ordovician yields arguably the first evidence for animals appearing in truly alluvial environments. A pattern emerges in which initial colonization of a new environment is followed by rapid filling of ecospace, after which animals establish new behavioral programs represented first by the appearance of original architectural designs, and then by a proliferation of ichnogenera representing variation upon these established themes. This pattern is consistent with the early burst model of diversification.