The Earliest Osteostracan Kalanaspis delectabilis Gen. Et Sp. Nov. from the Mid-Aeronian (Mid-Llandovery, Lower Silurian) of Estonia

The Osteostraci are an early clade of jawless vertebrates widely regarded as the sister group of all jawed vertebrates. Despite their prominent role in the Silurian to Devonian marginal marine ecosystems, the early evolutionary history of this subclass remains poorly understood. Here we report the discovery of a head shield (part with broken right margin and counterpart) of Kalanaspis delectabilis gen. et sp. nov. from the Kalana Quarry in Central Estonia. Reliably dated to the Pranognathus tenuis Conodont Zone, Raikküla Regional Stage, mid-Aeronian, mid-Llandovery, the specimen is about 10 million years older than the previously known oldest record of the Osteostraci. Kalanaspis delectabilis gen. et sp. nov. is identified as belonging to the new Family Kalanaspididae fam. nov., Order Ateleaspidiformes. It has a relatively large dorsal head shield, which is somewhat wider than long, with very small and very closely placed orbits, short prepineal length, one pair of notably long lateral fields, and gently undulating posterior margin. It lacks a posteromedian dorsal ridge as well as cornual processes and pectoral sinuses. No impressions of nasohypophyseal and pineal features were detected. The preservation of the specimen is unusual; the energy-dispersive spectroscopy (EDS) elemental analysis revealed that calcium phosphate, the original major constituent of the dermal skeleton of vertebrates, has disappeared and has been substituted by carbonaceous matter.