On the Cheek-Bones in Teleostome Fishes by T
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ON THE CHEEK-BONES IN TELEOSTOME FISHES BY T. S. WESTOLL, B.Sc., PH.D. Department of Zoology, University College, London THIS study was prompted by observations on the cheek-bones of certain Palaeoniscid and Holostean Fishes, which suggested that a more definite answer could now be given to the unsatisfactory problem of the homology of the bones of the cheek in certain Fishes. The examination of further material confirmed this view, and the body of evidence collected, though no doubt very incomplete, seems to be sufficient for a coherent account of the fate of certain cheek-bones during the long geological history of the Fishes. While gathering this evidence during the past year or two the writer has made observations on fossil Fishes belonging to many museums and institutes: in particular he desires to thank Mr C. Forster Cooper, M.A. (University Museum of Zoology, Cambridge), Mr T. Russell Goddard, M.Sc., F.L.S. (Hancock Museum, Newcastle-on-Tyne), Dr J. Piveteau (Paris), Dr A. C. Stephen (Royal Scottish Museum, Edinburgh), and Prof. D'Arcy W. Thompson (University Museum of Zoology, St Andrews) for allowing him to study and borrow so much material. Prof. E. A. Stensi6, Dr G. Save-Soderbergh, Dr H. Aldinger and Dr E. Nielsen were very kind in showing me much pertinent material, some of it yet unpublished, which forms part of the splendid collec- tions from East Greenland and Spitzbergen preserved in the Riksmuseum, Stockholm, and at Upsala, and I wish to thank them for these and many other courtesies. The work was begun while investigating the English Permian Palaeoniscids under the supervision of Prof. G. Hickling, F.G.S., to whom I am deeply indebted for constant kindness and encouragement. It has been continued in London, and I desire to thank Dr E. I. White, F.G.S., and the authorities of the Natural History Museum for allowing me the full use of the magnificent fossil fish collections there. I can only acknowledge in small part the great debt which I owe to Prof. D. M. S. Watson, F.R.S., who has continuously translated his interest in the writer's palaeontological research into active help and acute criticism. Finally, I wish to thank the Department of Scientific and Industrial Research for a Senior Research Award, without which this and other work could not have been attempted. STATEMENT OF PROBLEM The determination of the true homologue of the mammalian squamosal in the very primitive Tetrapods was for long a vexed question, and only in recent years has it been clearly demonstrated that the bone is represented in the cheek as a dermal plate associated, in the most primitive forms, with the On the Cheek-Bones in Teleostome Fishes 363 horizontal part of the jugal canal or its equivalent "slime-groove" (nomen- clature stabilised by Save-S6derbergh, 1933, p. 7). The equivalent of this bone in a Rhipidistian such as Osteolepis is easily distinguished, and has, in general, the same geographical and anatomical relationships. This provides us with a starting-point for a discussion of the piscine cheek. It will be unnecessary to discuss here the historical differences of opinion about the position of the squamosal in lower Tetrapods and primitive Fishes, as we are concerned with the fate of the squamosal, quadratojugal and pre- opercular, as defined, for example in Save-Soderbergh's account of Osteolepis (op. cit.). Since the recognition of the true squamosal in Fishes there have been few definite opinions expressed on the fate of that bone in the "higher" Fishes. Stensi6 (1921, p. 139), however, in a characteristically cautious passage, has stated most of the possibilities, according to his views on the nature of the Rhipidistian squamosal. He suggests that the squamosal in Crossopterygians may really be derived from a number of rudiments fused together, and goes on: " One may venture the supposition that all the homologues of the elements forming the squamosal in the Rhipidistids have never been fused with one another into one bone-plate in the Actinopterygians, but that they were either distributed among the surrounding bones, or with partial modifications had been fused into certain larger units which afterwards preserved their mutual independence or else were finally more and more reduced. In the Palaeoniscids, Platysomids and Catopterids it is conceivable that the original squamosal elements were divided up between the maxillary and the pre-opercular, and that they have even provided material for the so-called post-orbitals. In certain Protospondyli, such as the Semionotids and Eugnathids, they all probably provided material for the majority of the so-called post-orbitals (suborbitals); the ventral one, or some of them, can probably, as we have seen, be considered as corresponding to the quadratojugal: in the Teleosts the homologues of the squamosal elements usually seem to be reduced. Some- times, however, it seems possible that parts of them may persist in the plates of the infra-orbital chain, in the cases when these plates are much developed and cover the cheek between the orbit and the pre-opercular to any great extent." Save-Soderbergh (1934, p. 14), when comparing Crossopterygians with Actinopterygians, has adopted a very interesting point of view. "The dermal bones of the cheeks are certainly not simply homologous, or even easily com- parable in the two groups.... After all, it seems perhaps most probable that the pattern of dermal bones of the Choanata and that of the Actinopterygians have evolved quite independently from ancestors with either a very high number of bones-a mosaic-or with a more or less undivided dermal covering of the head." The writer's work on fossil Fishes has convinced him that the ancestors of the Choanata and Actinopterygians had, in times not much earlier than 364 T. S. Westoll the first known appearance of those groups, a mosaic of dermal bones covering the head, and that the sensory organs of the lateral line system were each related to a dermal bone. The examination of Dipnoans and Crossopterygians has made it clear to him, moreover, that many of the larger bones found in the skull of known Fishes are due to coalescence of the lateral line units, with growth of these elements at the expense of the "anamestic" bones between (Westoll, 1936), and not to indiscriminate fusion of plates of the original mosaic. The sensory organs of the lateral line system appear to the author to be of very great importance in determining the position of dermal bones in the primitive Fishes, and have allowed a new interpretation of the cheek to be made. In the following account the conditions in the various groups of Fishes will be described, followed by a discussion of the results in the light of what has been said above. CROSSOPTERYGII The cheek-bones of Osteolepis correspond so closely with those of an Ichthyostegid or Eogyrinus that there can be no doubt of individual homo- logies (fig. 1 A and B). The main morphological distinctions are: (i) the binding of the cheek-bones to the skull roof in Tetrapods, and (ii) the broad contact between squamosal and maxillary in Osteolepis (not found, however, in some Rhipidistia). The latter point is not of great importance, as it is probably due only to the excessive backward extension of the cheek in Osteolepis. The important anatomical similarity is the fact that the squamosal is pierced by the jugal canal of the lateral line system: in Osteolepis the squamosal is also associated with the dorsal part of the vertical cheek-line of pit-organs. The jugal canal passes backwards into the short vertical pre-opercular canal, and passes for- wards to anastomose with the infra-orbital canal in the jugal. The distribution of the other bones is well shown in fig. 1 B. This typical arrangement is only slightly modified in most other Rhipidistia (e.g. Diplopterax, Thursius, Mega- lichthys, Eusthenopteron, etc.'), but frequently a continuous sheet of cosmine covers squamosal and quadratojugal, although the bones are separate (cf. Westoll, 1936). The conditions in Holoptychius, however, are more complex (fig. 1 C). The structure can be made out admirably in H. flemingi from Dura Den, and the figure is derived from specimens at St Andrews. The cheek is comparatively much longer than in Osteolepis, and, probably because of the large size of the squamosal even in that fish, it appears that a single growth centre proved inadequate to cover a larger cheek. The consequence is that there are two squamosal bones, and two small bones have been necessary to fill space: these are of the type for which the author has proposed the name anamestic (Westoll, loc. cit.). The pre-opercular has been pushed backwards and has divided the bones of the opercular folds into two series. The structure will not be further 1 Author's observations. On the Cheek-Bones in Teleostome Fishes 365 c Fig. 1. Fig. 2. Fig. 1. A. Cheek of Ichthyostegid, mainly from Ichthyosteqopsis wimani S.-S. After Save- Soderbergh. B. Cheek of O8teolepis macrolepidotu8 Ag. After Save-Soderbergh. C. Cheek of Holoptychiusflemingi. Drawn from material at St Andrew's University Museum ofZoology. Fig. 2. A. Cheek of Whiteia (Lower Trias, Madagascar). After Moy-Thomas, additions by author. B. Cheek of Wimania sinuosa Stensio. After Stensio. C. Cheek of Axelia robusta Stensio. After Stensio. 366 T. S. Westoll described here, as Dr G. Save-Soderbergh is to deal with the structure of Holoptychius in detail. Turning now to the Coelacanths, we find more divergence. The cheek of the Devonian forms is, unfortunately, unknown. In most other Coelacanths from various geological formations the cheek has quite a characteristic aspect.