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DENTAL HISTOLOGY IN ICHNOMYLAX KARATAJAE SP. NOV., AN EARLY DEVONIAN DIPNOAN FROM THE TAYMIR PENINSULA, SIBERIA, WITH ADISCUSSION ON PETRODENTINE

Abstract The fragmentary remains of a new dipnoan, Ichnomylax karatajae, sp. nov., have been discovered in the Lower Devonian sediments of Taymir Peninsula, northern Russia. This form is characterized by a prearticular tooth plate consisting of a large, elongated, strongly convex, heel-like posterior region, which is clearly demarcated from the anterior tooth zone. There are three or four anterolaterally-directed rows of large, rounded teeth. Numerous rounded, elongate, enamel-covered denticles are present anteriorly between the tooth rows, as well as medially, laterally, and posteriorly, near the outer edges of the plate. This new species, like Ichnomylax kurnai, can be interpreted as being a basal dipnoan, close to Speonesydrion, another Lower Devonian form. The discovery of this new species of Ichnomylax on the Siberian plate, widely separated from the Southern Australian type locality of I. kurnai, provides additional evidence for rapid dispersal of Early Devonian dipnoans. Histological examination of one of the prearticular plates revealed the presence of petrodentine in both the body of the plate and in the teeth. We provide a definition, brief description, and discussion on the significance of this unique type of hypermineralized dentine, and conclude that the discovery of petrodentine in such an early stage of dipnoan history has important implications for studies of dipnoan evolution.