Ordovician vertebrate habitats: a Gondwanan perspective

The Middle-Upper Ordovician represents a significant period in the early evolution of fishes. During this time, many of the major lineages, including jawless and putative jawed taxa, made their first appearance in the fossil record, marking a series of diversification events. As a number of studies have focused on the habitat of Laurentian fish during this interval, work has been undertaken at a number of known Gondwanan vertebrate localities in order to provide new perspectives on the ecological preferences of early fish from the Southern Hemisphere. Ichnological and sedimentological data collated from these localities enable reconstructions of the habitats of a number of Ordovician fish, most notably those of the arandaspid-bearing successions of the Anzaldo Formation of Bolivia, the Stairway Sandstone of central Australia, and the Amdeh Formation of Oman, from which articulated or macroscopic fragmentary fossil remains are recorded. These data indicate that the arandaspids were constrained to very shallow marine habitats and prone to seasonal influxes of freshwater and terrigenous sediment. It is proposed that this narrow paleoecological range may be used as a prospecting tool to search for other Ordovician vertebrate-bearing horizons.