Ordovician Life on Land and Early Paleozoic Global Change

Abstract

Many Paleontologists share the opinion of McGhee (1996), who wrote “Prior to the Devonian, there was no terrestrial ecosystem to speak of. Some primitive plants precariously establishing a beachhead in protected coastal areas was about it. The interiors of the continents of the planet Earth were as barren as the rocky landscapes of Mars.” Thus, it was with trepidation that I reported paleosols containing trace fossils of early land animals in the late Ordovician, Juniata Formation, of Pennsylvania (Retallack and Feakes, 1987; Retallack, 1992a, 1992b, 1993). My late colleague, Jane Gray, engendered considerable debate by reporting Ordovician and Early Silurian spores like those of liverworts (Gray and Boucot, 1977; Gray, 1985). This spore, trace fossil and paleosol evidence for life on land in the Ordovician has remained controversial (Buatois et al., 1998; Shear, 1998), but evidence for Ordovician life on land has continued to accumulate. Especially important was discovery of myriapod trackways from mid-Ordovician (Llandeilian-Caradocian) Borrowdale Volcanics of the Lake District, England (Johnson et al., 1994). Abundant arthropod burrows and tracks, and a single body fossil of an euthycarcinoid in the fluvial-eolian Tumblagooda Sandstone of Western Australia (White 1990; McNamara and Trewin, 1993; Trewin and McNamara, 1995) are now thought to be late Ordovician in age (Iaksy et al., 1998). An enigmatic assemblage of arthropods and plants from a mid-Ordovician paleokarst in Tennessee (Caster and Brooks, 1956) is now thought to have been lacustrine (Gray, 1988a). The fossil record of Ordovician land plants also has improved with the discovery of possible megafossil mosses (Snigirevskaya et al. 1992), and possible late Ordovician trilete spores (Nøhr-Hansen and Koppelhus, 1998; Richardson 1988; Strother, 1991; Strother et al., 1996). But the most abundant evidence for Ordovician life on land remains fossil soils, now exploited by increasingly thorough and sophisticated studies (Retallack, 1985, 1992a, 1992b, 1993; Feakes et al., 1989; Driese and Foreman 1991, 1992a, 1992b; Driese et al., 1992, 1997; Mora et al., 1991, 1996; Mora and Driese, 1993; Yapp and Poths, 1992, 1994, 1996; Yapp, 1993, 1996). Mounting evidence from fossils and paleosols now presents an increasingly detailed view of Ordovician ecosystems on land.