VASE-SHAPED MICROFOSSILS OF THE TOGARI GROUP, TASMANIA AND IMPLICATIONS FOR BIOSTRATIGRAPHIC SUBDIVISION OF THE TONIAN PERIOD

Vase-shaped microfossils (VSMs) are a distinctive group of morphologically diverse microfossils with resistant shells (tests). They are considered to be early representatives of the polyphyletic modern group known as the testate amoebae (comprising rhizarian and amoebozoan taxa). VSMs are widely distributed in Neoproterozoic rocks of varying lithologies including carbonates, shales, cherts and phosphatic nodules, having been described from shallow marine lagoonal facies as well as from deeper, more distal marine environments. Their fossil record begins ca. 750 Ma when diverse VSM species appear in tremendous abundance in geographically widespread locations. For a brief and glorious few million years they dominated the fossil record. And then they were gone—absent from the fossil record for ≥400 million years before testate amoebae fossils reappeared in the freshwater and terrestrial environments that they most commonly inhabit today.

The occurrence of VSMs in the fossil record is noteworthy, not least as evidence for diversification within major eukaryote clade(s) and as evidence for predation in the Neoproterozoic Era, but also as index fossils marking the interval of time between the two major negative carbon isotopic excursions of the latest Tonian Period, the Bitter Springs and Islay anomalies.

During my talk I will present new results of a systematic paleontological analysis of a chert-hosted VSM assemblage from the shallow marine Neoproterozoic (~750 Ma) Togari Group of northwestern Tasmania. This assemblage is well-preserved in varying modes including siliceous casts and molds and as pyritic coats and framboidal infillings, and the species composition is strikingly similar to the VSM assemblage of the Chuar Group, Grand Canyon and most other VSM localities reported worldwide. I will discuss the global VSM fossil record in terms of species distribution, preservational modes and what they may tell us about habitat, depositional environment and test composition and in terms of the significance of VSMs to biostratigraphic subdivision of the Tonian Period.