An embryonic mandibular tooth plate and associated remains of a Late Jurassic chimaeroid (Holocephali, Chimaeriformes) from the Iberian Peninsula

Abstract

ABSTRACT Remains of the chimaeroid fish Ischyodus are described from the early Kimmeridgian (Late Jurassic) of northeastern Spain. This is the first fossil holocephalan record from the Iberian Peninsula. The material comprises a mandibular tooth plate, two incomplete and isolated symphyseal tritoral rods, and almost 20 frontal tenacular denticles. On the basis of size and morphological features, the material represents different ontogenetic stages. The mandibular tooth plate is the smallest of a chimaeroid that has been described up to date. It is identified as an embryonic tooth plate, in which the pleromic tissue of the tritoral pads still is covered completely with dentine. The fragmentary symphyseal tritoral rods, which are almost as large as the tooth plate, and the frontal tenacular spines come from adults. The mandibular tooth plate from Spain described in this study most probably represents an embryonic stage of Ischyodus egertoni. The differences in tritor number, arrangement, and morphology are related to the early ontogenetic stage of the specimen. We thus tentatively assign the tooth plate, tritoral rods, and tenacular denticles to this species. Ischyodus egertoni was a common species inhabiting the epicontinental seas covering wide areas of England and France during the Kimmeridgian. The Spanish record extends the known geographic distribution of European Late Jurassic chimaeroids southwards.