Ichnology, palaeoecology and taphonomy of a Gondwanan early vertebrate habitat: Insights from the Ordovician Anzaldo Formation, Bolivia

Abstract

Sacabambaspis janvieri is the most completely known pteraspidomorph fish from the Ordovician in terms of its morphology, histology and palaeogeographical range. Restricted to Gondwana, Sacabambaspis has assumed a role as the text-book Ordovician fish as it is known from a large number of articulated specimens recovered from within the mid-late Ordovician Anzaldo Formation of central Bolivia. The research presented here provides insights into the nature of the habitats of this early vertebrate, utilising new palaeoecological, ichnological and taphonomic field data. The Anzaldo Formation is interpreted as having been deposited in a number of shoreface and offshore transition environments that can be divided into four ichnofacies containing thirteen ichnotaxa: ?Arenicolites isp., Cruziana furcifera, C. goldfussi, C. rugosa, Curvolithus isp., Monocraterion isp., Palaeophycus striatus, Phycodes curvipalmatum, Planolites montanus, Rusophycus isp., R. latus, Skolithos linearis, and Teichichnus isp. Two of the ichnofacies are bathymetrically-controlled (the Cruziana and Skolithos ichnofacies) and two are controlled by localised depositional and salinity conditions (a mixed Skolithos– Cruziana and restricted Skolithos ichnofacies). Articulated specimens of Sacabambaspis are confined to the restricted Skolithos ichnofacies where they have been found in association with concentrations of lingulid brachiopods. Other brachiopod concentrations are found within the archetypal Skolithos ichnofacies. Data is presented that suggests that both the fish and brachiopods were killed by a combination of freshwater inundations and influxes of terrigenous sediment that buried and preserved the organisms within nearshore obrution deposits. Subsequent colonization of these obrution deposits was then initially limited to opportunistic Skolithos tracemakers.