DESCRIPTION AND SYSTEMATIC RELATIONSHIPS OF †TOMOGNATHUS, AN ENIGMATIC FISH FROM THE ENGLISH CHALK

Abstract

The Upper Cretaceous fish †Tomognathus mordax Dixon, 1850, is redescribed, based on newly acid-prepared material that reveals many new structures and reinterpretations of some parts of anatomy previously described. The cranium shows the absence of a supraoccipital, a double jaw joint involving a symplectic and quadrate, a lower jaw incorporating a coronoid, prearticular and a surangular, concave shape of the posterior margin of the maxilla, a single supramaxiila, a 'V'-shaped median rostral with lateral horns and an interopercle that lies in series with the subopercle. These features suggest affinities with the Halecomorphi. Within Halecomorphi, character conflict means that †Tomognathus may only be placed as Superfamily Amioidea incertae sedis. It is highly apomorphic in showing parietals, frontals, premaxillae and vomers all fused into median elements, a parasphenoid with scythe-like processes developed along the ascending wings and an infraorbital series reduced to tube-like elements. †Tomognathus is one of only three halecomorphs recorded from the English Chalk.