A review of the fossil seabirds from the Tertiary of the North Pacific: plate tectonics, paleoceanography, and faunal change

Abstract

Ecologists attempt to explain species diversity within Recent seabird communities in terms of Recent oceanographic and ecological phenomena. However, many of the principal oceanographic processes that are thought to structure Recent seabird systems are functions of geological processes operating at many temporal and spatial scales. For example, major oceanic currents, such as the North Pacific Gyre, are functions of the relative positions of continents and Antarctic glaciation, whereas regional air masses, submarine topography, and coastline shape affect local processes such as upwelling. I hypothesize that the long-term development of these abiotic processes has influenced the relative diversity and community composition of North Pacific seabirds. To explore this hypothesis, I divided the history of North Pacific seabirds into seven intervals of time. Using published descriptions, I summarized the tectonic and oceanographic events that occurred during each of these time intervals, and related changes in species diversity to changes in the physical environment. Over the past 95 years, at least 94 species of fossil seabirds have been described from marine deposits of the North Pacific. Most of these species are from Middle Miocene through Pliocene (16.0-1.6 Ma) sediments of southern California, although species from Eocene to Early Miocene (52.0-22.0 Ma) deposits are from Japan, British Columbia, Washington, and Oregon. During the history of the North Pacific seabirds, thee were many widespread (global) and local (California Current) oceanographic events, but the underlying physical processes that affected the diversity of seabirds throughout the North Pacific were tectonic changes that led to sequential stages of thermal isolation and refrigeration of Antarctica. Besides these broad-scale phenomena, the uplift of the Isthmus of Panama and the initiation and intensification of coastal upwelling in the California Current directly affected the diversity of seabirds in southern California. For the most part, Middle to Late Miocene and Pliocene seabird faunas from California resembled Recent communities, but with several exceptions: (1) the extreme diversity and abundance, but subsequent extinction of gannets (Morus) and flightless alcids (mancallids); (2) the comparatively low abundance and diversity of cormorants and shags (Phalacrocoracidae) until the Late Pliocene; and (3) the absence of marine Laridae until the Late Pliocene. By affecting both the physical and biological environments, geological factors such as Antarctic glaciation, eustatic changes in sea levels, and local tectonic activities influenced and will continue to influence the structure of seabird systems in the North Pacific.