The Subocular Shelf of Fishes' C. LAVETT SMITH AND REEVE M. BAILEY Museum of Zoology, University of Michigan, Ann Arbor, Michigan The subocular shelf is a bony lamina cumorbital series. A further distinction is that extends inward from the suborbital necessary, however, since the circumor- bones of certain fishes and forms part of bitals are readily divisible into (1) a supra- the wall of the orbit. Its presence or ab- orbital series consisting of bones that lie sence has long been used as a taxonomic dorsal to the orbit and have no associated character, particularly at the family level. sensory canal and, (2) a suborbital series Boulenger (1895) was apparently the first of bones that completes the circumorbital to introduce the character, and later ('04) ring and is penetrated or traversed by the he recorded its occurrence in the families infraorbital branch of the sensory-canal of perciform fishes admitted by him. Smith system. The supraorbitals and suborbitals ('41) used the subocular shelf to separate have very different evolutionary destinies. some pentapodid and related fishes. Re- There has been some variation in the cently Akazaki ('58) has described varia- method of counting the bones of the sub- tions in the shelf among genera of sparoid orbital series. Most authors have included fishes of Japan, and Katayama ('59, '60) the first (anterior) bone, the lachrymal has employed the configuration of the shelf (preorbital), in the count, but some have to distinguish infrafamily groups of the excluded it and others have begun the Serranidae. It is apparent, however, that count from the posterior side of the orbit. few authors have properly appreciated the In the following discussion and descrip- utility of this character and it is cominonly tions the lachrymal bone, which in teleosts ignored. bears the anterior terminus of the infra- The function of the shelf seems to have orbital canal, is counted as the first mem- received little attention. Some authors dis- ber of the series. The count continues miss it with the statement that it supports backward to include the dermosphenotic. the eye, but the optical effectiveness of re- At least among teleosts the number of lated fishes that lack the shelf invites re- bones in the suborbital series is remark- consideration of this patent explanation. ably constant: usually there are 6, al- In an attempt to evaluate the taxonomic though in some fishes the bones have un- utility of the subocular shelf we have dergone reduction and in a few one or examined representatives of most of the more of them have divided, thus increasing currently recognized orders of fishes and the total number of elements. have given considerable attention to the distribution of the shelf within the Perci- Primitive fishes formes and, particularly, among the fam- ilies of the superfamily Percoidea. In this In such primitive fish groups as the study it has been necessary to consider paleoniscids, semionotids, and lepisosteids, all bones of the circumorbital ring and in the circumorbitals are connected to the sur- so doing we have developed what appears rounding dermal bones. The supraorbitals to be a satisfactory working hypothesis of are rigidly attached to the cranium, and the functional and phylogenetic signifi- the suborbitals may either be in contact cance of the subocular shelf. with the preopercle or separated from it by a series of postorbital bones. This Nomenclature closed bony sheathing of the circumorbital In primitive fishes the eye is completely area is relatively inflexible. In the Jurassic surrounded by a series of small bones that 1This study has been made possible by a research may conveniently be referred to as the cir- grant from the National Science Foundation (6-6368). 1 2 C. LAVETT SMITH AND REEVE M. BAILEY semionotid Dapedium the circumorbitals Lower teleosts approach uniformity in size and shape The primitive clupeifom Osteoglossum (fig. 1A). In Lepisosteus the anterior bones bicirrhosum (fig. 1C) has a suborbital se- are modified in conjunction with the elon- ries much like that of Amia, with the fourth gate snout (Gregory, ’33: 128-129) and and fifth bones greatly expanded to fill the several bones correspond to the single entire region between the orbit and the lachrymal of teleosts. The infraorbital preopercle and between the maxilla and canal extends forward through them and the pterotic. Here also the supraorbitals the series of fixed maxillary bones to join are wanting and the frontal and dermo- its fellow from the opposite side in a supra- sphenotic form the dorsal sector of the ethmoid (rostral) commissure, and to orbital rim. Gregory (’33: 163) identified unite with the supraorbital canal postero- the enlarged bone at the front of the orbit mediad of the posterior nostril. In Polyp- as a prefrontal. Since it is perforated by terus the solidification of the suborbitals the infraorbital canal, however, we think that it is at least in part the lachrymal and is extreme; the ventral elements behind that the suborbitals on Gregory’s figure the lachrymal are fused with the maxilla, should be renumbered. The prefrontal in and an anterior platelike extension of the Osteoglossum is unossified or is coos&ed preopercle covers the upper cheek and with the lachrymal. In the Salmonidae articulates with the dermosphenotic. the skull is more mobile and there are two In the course of fish evolution there supraorbital bones; the lachrymal is at- have been two prominent trends in the tached to the anterior of these rather than skull bones. The dermal bones have mi- to the prefrontal. All of the suborbitals grated inward so that they no longer pre- are thin and delicate, and the only contact sent sculptured external surfaces and there with the neurocranium is between the has been reduction in size and increased dermosphenotic and the sphenotic. In the mobility of the skull bones. The circum- Salmoninae the dermosphenotic is unossi- orbital bones have in general conformed to fied. The posterior suborbitals may be in these trends except where environmental contact with or remote from the pre- stresses have imposed additional demands opercle. that have been met by stiffening of the Some characids have a large supra- circumorbital ring. orbital that meets the dermosphenotic to In Amia calva (fig. 1B) the maxilla is exclude the frontal from the orbital rim, as in Hydrocyon and Brycon (Gregory, ‘33 : freely movable and bears no canal. Per- haps partly in response to the loss of sup- 184-185), or the supraorbital may be ab- sent, as in Erythrinus (Gregory, 182) port from the maxilla and partly as protec- ‘33: and Carnegiella (Weitzman, ’54: 249). In tion for the critical joints at the proximal the three genera first named the suborbitals end of that bone, the anteriormost suborbi- are large plating bones that fully sheath tal has become enlarged and modified as the cheek; in CarnegieZEa they are much the lachrymal. The infraorbital canal ex- reduced. The bone termed adnasal by tends forward from the lachrymal into the Weitzman seems to be the lachrymal; al- adnasal. The second and third suborbitals though not shown in his figure, it bears a are small and slender but the fourth and tube of the infraorbital canal. fifth are much enlarged and extend back- In most cyprinids (fig. 1D) and in the ward nearly to the preopercle; the upper more generalized genera of the Catos- edge of suborbital 5 contacts the pterotic as tomidae, the prefrontal provides the sole well as the dermosphenotic. Amia lacks osseus support of the anterior suborbitals, the supraorbital bones but this is probably and posteriorly only the dermosphenotic a specialization since some fishes higher in is in contact with the skull. There is a the evolutionary series retain them. Oc- single large supraorbital bone. In the casionally there are a few bony nodules, more advanced catostomids (Catostominae) “supplementary dermal bones,” that ap- the supraorbital is lacking, being fully pear to be vestigial postorbitals (Wood- coossified with the frontal (Weisel, ’60: ward, 1895: 369). 110). In the ictalurid catfishes the lachry- THE SUBOCULAR SHELF OF FISHES 3 mal is a small, Y-shaped bone and the re- suborbital series. This seems to be the maining 5 or 6 suborbitals are reduced to primitive condition but through selective slender tubular ossifications around the loss and retention of elements the shelf infraorbital canal. In the Ictaluridae there has assumed modified forms in various is no supraorbital. fish groups. In the beryciform Mono- Throughout the Clupeiformes and the centris japonicus (Starks, '04: 618), as in Cypriniformes modifications serve to most members of the perciform super- make the suborbital series more flexible. family Percoidea, the third bone alone con- The bones are reduced in size, the connec- tributes to the shelf. The third suborbital tions between them are ligamentous, and is nearly always involved and this is per- the diarthrosis between the lachrymal and haps presaged in Myripristis where the the prefrontal is developed to allow move- segment of the shelf derived from the third ment in any plane. The lachrymal has bone is the widest. In Channa lucius the been specialized to protect the anterior end third suborbital enters the shelf but the of the maxilla. At the same time the trend in the supraorbitals is toward reduction, fourth and fifth, which contribute equally, loss, or fusion so that the dorsal rim of the form most of the shelf, which consequently orbit is rigid. No malacopterygian is forms the posterior wall of the orbit. In known to have a subocular shelf. Channa striata only the fourth and fifth suborbitals participate in the shelf, but Development of the shelf the fifth is much the larger (fig. 3R). In In the Beryciformes there is a temporary Scorpaenidae and Mullidae the second and reversal of the trend toward increasing third bones contribute the shelf.