The braincase of Pholidophorid and Leptolepid fishes, with a review of the Actinopterygian braincase

Abstract

The Mesozoic pholidophorid and leptolepid fishes stand at the base of the Teleostei. In this paper the neurocranium and the dermal bones intimately associated with it, the roofing bones and the vomer and parasphenoid, are described more or less completely in six Jurassic pholidophorids (Pholidop , Pholidophorus , P. germanicus, P. minor, P. macrocephalus and an unnamed Callovian Pholidophorussp.) and four Jurassic leptolepids (Leptolepis coryphaenoids, L. dubia, and two unnamed Leptolepisspp., one from the Sinemurian and one from the Callovian). These species provide a morphological series which illuminates many problems of actinopterygian evolution and anatomy, and a detailed comparison is made with teleosts and living and fossil holosteans and chondrosteans. In the course of these comparisons, new material ofPerleidus,parasemionotids,Heterolepidotus, Caturus, Lepidotes and Pachycormusis described, and previously described fossil holostean braincases are reinterpreted. The braincase of relatively advanced leptolepids proves to be teleost-like in every way, but the pholidophorids include forms with far more primitive braincases. There are few absolute differences in the braincase between pholidophorids and leptolepids, for the two groups intergrade in the Lower Jurassic and the SinemurianLeptolepisis in many ways intermediate betweenPholidophorus becheiand more advanced leptolepids. Among the braincases described here, 36 trends are recognized, from a relatively primitive pholidophorid condition to a more advanced leptolepid condition. Almost without exception, the leptolepid condition is the one found in generalized members of all teleostean cohorts. Evidence from the braincase strongly favours a monophyletic origin of extant teleost groups within the leptolepid grade, and there is no indication that any teleost is more closely related to pholidophorids than to leptolepids. Although far more primitive than leptolepids, pholidophorids are related to leptolepids and teleosts by several specializations in the braincase, of which the most important are extension of the myodome into the basioccipital, presence of a stm t across the subtemporal fossa formed by the prootic and intercalar, presence of a median mesethmoid and anterior myodome bone and a median vomer, and the form of the basipterygoid process. The most striking primitive features of the pholidophorid braincase are the obliteration of sutures between the cartilage-bones, terminating growth in full-grown individuals; the presence of a complete cranial fissure, perichondrally lined from the vestibular fontanelle upwards; the presence of two ossification centres absent in most leptolepids and in teleosts, an endochondral intercalar and an opisthotic; the primitive position of the ossification centre of the pterotic, in the postero-dorsal shoulder of the otic capsule; the extensive basisphenoid; the presence of an aortic canal; the presence of an endoskeletal partition separating the post-temporal fossa and fossa bridgei; the presence of a spiracular canal and a lateral cranial canal; the narrow, endochondral lateral commissure; the short posterior portion of the parasphenoid; and the presence in the snout of a pair of dermal bones, primitively toothed, which line the anterior part of the nasal pit, beneath the rostral, and separate the premaxillae. The new term lateral dermethmoids is introduced for these bones: they also occur inIchthyokentemaandPachycormus, while in most leptolepids and in teleosts they are fused with the rostral and more or less completely incorporated in the mesethmoid. These bones are held to be the homologues of the nasal process (previously called ascending process) and medial part of the premaxilla of living and fossil holosteans: separation of the premaxilla into a lateral dermethmoid and a mobile premaxilla of pholidophorid or teleost type is a specialization. Among the primitive features of the pholidophorid braincase, some are typical of holostean level actinopterygians, but others, especially the complete perichondrally lined fissura otico-occipitalis, are only found in palaeoniscoids andAustralosomus, and pholidophorids closely resemble these forms in the shape and proportions of the braincase. The principal anatomical conclusions concern the history of the cranial fissure, the ossification pattern of the actinopterygian braincase, and the history of the actinopterygian snout. Closure of the fissura otico-occipitalis in pholidophorids and leptolepids involves loss of the opisthotic and the endochondral portion of the intercalar (primitively the intercalar is entirely endochondral), a shift in the ossification centre of the pterotic, and extension of the occipital bones into the otic region (the new term epioccipital is introduced for the ‘epiotic’ of teleosts and some holosteans, which primitively had no connexion with the otic capsule, and is probably not homologous with the ‘epiotic’ ofLepisosteus). In some other actinopterygian groups closure of the fissure has had similar consequences, while in some groups the fissure persists after closure as a suture, without extension of neighbouring bones across it. The pholidophorids have more endochondral ossification centres in the braincase than leptolepids and teleosts, butPerleidus, parasemionotids and caturids are found to have an ossification pattern similar to that in pholidophorids. It is proposed that the number of ossification centres in the neurocranium has generally decreased, not increased, during actinopterygian evolution, and the ossification pattern of all groups is interpreted along these lines. The ‘ postrostral ’ of palaeoniscoids appears to be the homologue of the rostral of holosteans and pholidophorids, and the history of the snout bones of actinopterygians is reviewed, culminating in the compound ossifications in the complex rostrum of teleosts. Other anatomical conclusions include the precise homology between the spiracular groove on the parasphenoid of primitive actinopterygians and the ‘ prespiracular ’ groove of rhipidistians, and the inferred presence in many primitive actinopterygians of subcephalic muscles of the type found inPolypterus.