LOOKING BACK OVER 40 YEARS: HISTORICAL PERSPECTIVES ON THE DEVELOPMENT OF CAMBRIAN TO EARLIEST ORDOVICIAN CONODONT BIOSTRATIGRAPHY

Klaus Müller described Cambrian conodonts from the USA and Europe in 1959. Pete Palmer found the American specimens while dissolving scraps of trilobite samples to find early Late Cambrian phosphatic brachiopods. Müller's taxa were mostly paraconodonts, a few protoconodonts (Chaetognatha), and a euconodont, Cordylodus proavus. Many more Cambrian conodonts were named by the early 1970s in Asia, North America, and Australia. Paraconodonts are used in Asia to delineate Middle and early Late Cambrian zones, although their global utility is not yet demonstrated. As Cambrian euconodonts diversified, paraconodonts gradually became less common but persisted well into the Early Ordovician.

Euconodonts diversified rapidly during the Late Cambrian, with four major lineages identified: 1) Proconodontus–Eoconodontus–Cambrooistodus–Cordylodus; 2) Teridontus–Semiacontiodus–Monocostodus–Utahconus–Rossodus–Polycostatus; 3) Dasytodus–Hispidodontus–Hirsutodontus–Rotundoconus–Clavohamulus; and 4) Fryxellodontus. Rapid evolution of abundant, cosmopolitan faunas resulted in detailed biozonal schemes that can be correlated globally. Lineage 1 is used to delineate zones; lineages 2–4 are used mostly to delineate subzones. Sea-level rises produced diversification within lineages, and abrupt sea-level drops caused extinctions.

Many morphological innovations appeared among euconodonts during the Late Cambrian and continued into the Ordovician. Euconodonts began as single-element apparatuses with smooth surfaces, extremely deep basal cavities, and little white matter (Proconodontus). Two-element apparatuses developed with shallower basal cavities and with cusps made of white matter (Eoconodontus) and with geniculate elements (Cambrooistodus). Denticulate Cordylodus may have been the first euconodont to develop complex multi-element apparatuses that compare with Ordovician taxa. Finely striate coniform apparatuses (Teridontus) produced various multi-element descendant genera with diverse costate ornament. The Dasytodus Lineage had spinose (Dasytodus, Hispidodontus, Hirsutodontus) or granulose ornament (Rotundoconus, Clavohamulus); only Clavohamulus survived very long into the Ordovician. Fryxellodontus had a complex symmetry transition but it was short-lived.