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Growth in early species of the conodont Ancyrodella and implications for correlation of the Middle-Upper Devonian boundary

Miller C.G.

Geological Quarterly

2007

Vol. 51, No 4

443-452

The first occurrence of the conodont Ancyrodella rotundiloba has been used extensively for correlating the Middle-Up per Devonian (Givetian-Frasnian) boundary in sections worldwide despite many arguments as to its precise taxonomic definition. These arguments are summarised herein and three ontogenetic series illustrated from three samples across the Givetian-Frasnian boundary within the Vorota Formation of the Kozhym River section, Sub-Polar Urals, Russia. General trends within the three ontogenetic series suggest that the ratio of basal pit width to platform width in Ancyrodella pristina, and Ancyrodella recta in creases through ontogeny but the morphology of lateral secondary keel extensions to the basal pit remains constant and is a useful taxonomic feature. Folds and collars on the basal surface occur only in the later stages of development. The out line of the platform within species is variable and controlled by the formation of incipient nodes in juveniles; gerontic specimens can also have very variable platform out lines. Measure ments of platform width to basal pit width ratios and platform shape out lines may not therefore be a good way to compare between species. The three ontogenetic series provide typical late Givetian and early Frasnian examples and increase the potential for correlation of the Middle-Upper Devonian boundary particularly using juvenile Ancyrodella specimens.