Ocular Shell Structures in Some Loxoconchid Ostracoda

Abstract

Ocular structures are poorly known in some podocopid ostracodes, including the Family Loxoconchidae, Sars 1925, although the features are useful in taxonomy and seem to have potential in paleoenvironmental reconstruction. Here, 10 species of Loxoconcha and Loxocorniculum were studied with polished and thin sections, internal molds, and light and scanning electron microscopy. The ocular sinus may be well developed even in species that have indistinct external ocular structures. In general, the ocular sinus in adults of these loxoconchids is low, relatively wide, unstalked, and has a subcentral concavity and a distinct rim. In Loxoconcha, depending upon the species, either the anterior or posterior distal part of the sinus may be the higher, while in Loxocorniculum the anterior portion is higher. The sinuses in both genera are unlike the elongate, stalked, and distally bulbous structure in the trachyleberids. The ocular sinus in each species is diagnostic, thereby providing another character for taxonomic studies. Ocular structures also may provide a means of estimating paleodepths. INTRODUCTION Recently, ocular structures of some podocopid ostracode shells have received attention in detail (Andersson and Nilsson 1981; Myers and Kontrovitz 1988; Bonaduce and Danielopol 1988). The Family Trachyleberididae Sylvester-Bradley 1948, is best known in this regard, having been the subject of several previous studies (Kontrovitz and Myers 1984; Kontrovitz 1985; Kontrovitz 1987). Ocular structures remain poorly known in some other podocopid groups, although the features are useful in taxonomy and may have potential in paleoenvironmental reconstruction (Puckett 1991). The present study is the first to consider the ocular sinus beyond the ocular pit in some members of the ostracode Family Loxoconchidae, Sars 1925. In podocopids, the ocular sinus is the hollow structure in the shell between the ocular pit and the proximal surface of the eyespot that serves in accommodating the soft parts of the (Moore 1961). It encases the lateral eye cup and stalk, if the latter is present. MATERIAL AND METHODS It is not possible to study the ocular sinus by a simple examination with light microscopy because the sinus is formed within the shell and is therefore concealed from ordinary observation. Here, polished sections, thin-sections and internal molds (Kontrovitz 1982) were produced, then studied with transmitted and reflected light and scanning electron microscopy. All such terms as anterior, posterior, dorsal, and ventral are used to indicate directions or locations as they would be for the whole valve; e.g., the anterior of the sinus is toward the anterior of the valve. The dimension of the ocular sinus from its base to its most distal part is herein designated its height (H). Diameter (D) is defined as the maximum dimension across the sinus, perpendicular to its height. Species included in this study are Loxoconcha anderseni Puri 1953; Loxoconcha concentrica Krutak 1961; Loxoconcha creolensis Howe and Chambers 1935; Loxoconcha moralesi Kontrovitz 1976; Loxoconcha sarasotana Benson and Coleman 1963; Loxoconcha woodwardsensis Howe and Law 1936; Loxoconcha sp. Kilmartin 1982; Loxocorniculum fischeri (Brady 1869), as reported by Benson and Coleman (1963); Loxocorniculum postdorsolatum (Puri 1960); and Loxocorniculum wilberti (Puri 1953). It should be noted that concavities and other topographic features of the distal surface of the ocular sinus are the complement of the proximal surface of the eyespot (Kontrovitz 1982). See text-figure 1. All molds used in this study have been retained on SEM stubs, designated NLUSEM 500-506, in the Department of Geosciences, Northeast Louisiana University (NLU). They are available for study at NLU.