Braincase of the Upper Devonian Shark Cladodoides Wildungensis (Chondrichthyes, Elasmobranchii), with Observations on the Braincase in Early Chondrichthyans
Total Page：16
File Type：pdf, Size：1020Kb
BRAINCASE OF THE UPPER DEVONIAN SHARK CLADODOIDES WILDUNGENSIS (CHONDRICHTHYES, ELASMOBRANCHII), WITH OBSERVATIONS ON THE BRAINCASE IN EARLY CHONDRICHTHYANS JOHN G. MAISEY Division of Paleontology American Museum of Natural History ([email protected]) BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 288, 103 pp., 38 ®gures Issued March 2, 2005 Copyright q American Museum of Natural History 2005 ISSN 0003-0090 2 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 288 CONTENTS Abstract ....................................................................... 3 Introduction .................................................................... 3 Materials and Methods .......................................................... 4 Anatomical Abbreviations ....................................................... 10 Description .................................................................... 11 General Features, Taphonomy, and Forensic Observations ........................ 11 Ethmoid Region ........................................................... 15 Extent of Polar Cartilage-Derived Region .................................... 18 Ophthalmic and Efferent Pseudobranchial Arteries ............................. 19 Anterior Palatoquadrate Articulation ......................................... 22 Bucco-Hypophyseal Fenestra and Internal Carotid Arteries ..................... 22 Optic Pedicel, Optic Nerve, and Oculomotor Nerve ........................... 25 Trochlear Nerve ........................................................... 26 Abducent Nerve ........................................................... 27 Profundal Nerve ........................................................... 30 Trigeminal Nerve .......................................................... 30 Facial Nerve .............................................................. 31 Octavolateral Nerves ....................................................... 34 Posterior part of Orbit ..................................................... 35 Postorbital Process ......................................................... 36 Dorsal Aortae ............................................................. 39 Orbital Artery and Palatine Nerve ........................................... 40 Roof of Orbitotemporal Region ............................................. 41 Posterior Tectum and Dorsal Fontanelle ...................................... 41 Dorsal Otic Ridges ........................................................ 42 Lateral Otic Process and Hyomandibular Fossa ............................... 42 General Features of Skeletal Labyrinth ....................................... 42 Occipital Arch ............................................................ 43 Cranial Endocast .......................................................... 52 Discussion .................................................................... 57 Ontogenetic Aspects of the Braincase ........................................ 57 Precerebral Fontanelle ...................................................... 58 Efferent Pseudobranchial Artery ............................................. 58 Anterior Palatoquadrate Articulation ......................................... 59 Trabeculae, Polar Cartilage, and Suborbital Shelf .............................. 64 Optic Pedicel and Position of Oculomotor Foramen ........................... 67 Trigeminal, Facial, and Octavolateral Nerves .................................. 68 Trigemino-Facial Recess .................................................... 71 Postorbital Process ......................................................... 72 Postorbital Articulation ..................................................... 77 The ``Extra Foramen'' Problem ............................................. 78 Remarks on the Dorsal Vein Sinus in Acanthodes ............................. 81 Otico-Occipital Proportions ................................................. 81 Remarks on the Inner Ear in Chondrichthyans, Placoderms, and Agnathans ...... 81 Posterior Dorsal Fontanelle and Posterior Tectum ............................. 86 Lateral Otic Process ....................................................... 87 Hyomandibular Fossa ...................................................... 88 Occipital Arch, Otico-Occipital Fissure, and Hypotic Lamina ................... 89 What Kind of Shark was Cladodoides? ...................................... 92 Phylogenetic Patterns in Paleozoic Sharks .................................... 93 Conclusions ................................................................... 95 Acknowledgments ............................................................. 98 References .................................................................... 98 2005 MAISEY: CRANIAL MORPHOLOGY OF CLADODOIDES 3 ABSTRACT The braincase of an Upper Devonian shark (Cladodoides wildungensis) is investigated using high-resolution CT scanning, and its internal and external morphology is described from three- dimensional digital reconstructions. Many features are shared with modern elasmobranchs (neoselachians); these mostly represent conserved elasmobranch, chondrichthyan, or gnathos- tome characters. C. wildungensis resembles neoselachians in having a closed metotic ®ssure de®ning a glossopharyngeal canal, unlike some early chondrichthyans (e.g., Pucapampella, Orthacanthus) in which an open metotic ®ssure extends a considerable distance beneath the otic capsule. However, the glossopharyngeal canal in C. wildungensis is connected to a per- sistent dorsal ®ssure between the occipital arch and otic capsules, whereas this ®ssure becomes closed during neoselachian ontogeny. The embryonic polar cartilage apparently made a much greater contribution to the basicranium than in neoselachians and gnathostomes generally. This has affected the position of the efferent pseudobranchial foramen and the location of the orbital articulation in Cladodoides (both of which are located anteriorly in the orbit), as well as the size and extent of the dorsum sellae and the hypophyseal chamber inside the braincase. An- other unusual feature in Cladodoides is the postorbital position of its trigeminal and facial nerves; instead of being situated in a trigemino-facial fossa of the orbit as in neoselachians, their foramina are located behind the postorbital process, although the pituitary vein and the origin of the external rectus eye muscle (located within the trigemino-facial fossa in neose- lachians) both lie in the posterior part of the orbit. The oblique orientation of the postorbital arcade (derived from the embryonic lateral commissure) may be related to the exclusion of the trigeminal and facial nerves from the orbit. A laterally oriented canal through the process probably contained the trigeminal maxillary ramus (perhaps accompanied by the buccal ramus of the anterodorsal lateral line nerve). By contrast, in neoselachians these rami pass directly into the orbit (sometimes forming a buccal-maxillary complex), but they do not pass through the postorbital process. The Cladodoides braincase probably represents a juvenile or submature individual and shows indications of incomplete development (e.g., presence of a wide bucco- hypophyseal fenestra and absence of a precarotid commissure; incomplete enclosure of the internal carotids and orbital arteries by basicranial cartilage; incomplete closure of the brain- case roof along the dorsal midline). The braincase also lacks multiple-layered prismatic cal- ci®cation, possibly representing another juvenile feature. The phylogenetic relationship of Cla- dodoides wildungensis is discussed. Dental, postcranial, and cranial features lend some support to the old hypothesis that cladodont sharks form a monophyletic group. While such a view is at variance with several recently published phylogenetic analyses, many of the characters noted in the present work have yet to be tested within a rigorous cladistic framework, and their phylogenetic signi®cance is uncertain. INTRODUCTION drichthyans and elasmobranchs has helped Literature on the phylogeny of early gna- steer these hypotheses, whereas little atten- thostomes is dominated by descriptions and tion has been paid to extinct chondri- comparisons of osteichthyans, placoderms, chthyans. This stasis undoubtedly results and acanthodians, but the body of literature from the great rarity of morphologically in- concerned with fossil chondrichthyan anato- formative chondrichthyan fossils, as well as my is comparatively small. Despite this pau- from inadequate preservation and a limited city of reliable morphological data (or per- ability to observe crucial features even in haps because of it), chondrichthyans have better specimens. Whatever the reason, there sometimes been credited with considerable has never been a comprehensive survey of evolutionary importance (for example as pu- morphological diversity in early chondri- tative placoderm descendants; Holmgren, chthyans. 1942; StensioÈ, 1969), while at other times We need to look for more shark fossils, they have been treated rather summarily, as and the importance of new discoveries is am- a primitive sister group to osteichthyans (Ro- ply illustrated by recent discoveries of artic- sen et al., 1981; Gardiner, 1984a). Invariably, ulated and other well preserved remains of however, the morphology of modern chon- Lower and Middle Devonian chondri- 4 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 288 chthyans (e.g., Pucapampella, Doliodus, and