MICROSTRUCTURE OF POLYPTERID SCALES (OSTEICHTHYES: ACTINOPTERYGII: POLYPTERIDAE) FROM THE UPPER CRETACEOUS BAHARIYA FORMATION, BAHARIYA OASIS, EGYPT

Journal of Paleontology, Nov 2006 by Smith, Joshua B, Grandstaff, Barbara S, Abdel-Ghani, Medhat Said

Histology.-Scales of Polypterus are histologically distinct from those of other ganoid fish (Sire, 1989; Schultze, 1996; Daget et al., 2001; Gayet et al., 2002). They possess four tissue layers: ganoin, dentine, plywoodlike isopedine (elasmodin of Sehultze, 1996), and pseudolamellar bone of the basal plate (see Daget et al., 2001). Isopedine (elasmodin) is a distinctive, finely laminate juvenile tissue lying between the dentine and basal bone plate of the scale (Sire and Huysseune, 2003). In adults, it is sometimes preserved in a small region at the center of the scale (Sire, 1989). It is found both in the living genera (Polypterus and Erpetoichthys J. A. Smith, 1865) and in the scales of all fossil polypterids that have been examined (Meunier and Gayet, 1996; Dagel et al., 2001). Isopedine comprises thin lamellae. The collagen fibers in adjacent lamellae run at angles of ~90� to each other. Because of this plywoodlike fiber orientation, isopedine is characterized by the alternating light and dark bands under polarized light (Sire, 1989).

Two of the scales (CGM 81145a and b) were impregnated with epoxy and sectioned on an Isomet 11-1180 low speed saw from Buehler. Multiple cranial to caudal thin sections of each scale were made. These were examined using transmission light microscopy with natural and polarized light on a Nikon Eclipse E600 P02 light microscope with a Nikon H-III adapter and Nikon FDX-35 camera attachment. Although both scales showed surface ridges, neither possessed clearly visible ganoin when viewed under low magnification in reflected light. Thin sections showed that the surface ridges of both scales do preserve layered ganoin. Surface features of a third scale with visible ganoin on its ridges (COM 81145c) were examined using scanning electron microscopy (see below).

Cross sections of CGM 81145a and b show well-preserved microstructure (Figs. 3, 4). Both scales have vascular bone at the base and layered ganoin on their surfaces. As in most polyptcrids the ganoin is crossed by vertical vascular channels, some of which are visible in histologie sections. Under polarized light COM 81145b shows a zone of alternating light and dark bands between the dentine and bone near the center of the scale (arrows in Fig. 3.2). The bands correspond in position to the isopedine layers preserved in other fossil polypterids (Meunier and Gayet, 1996; Daget et al., 2001). and transmit polarized light in the same manner as the isopedine of fossil polypterids. Presence of isopedine has not previously been reported in polypterids from Bahariya (Stromer, 1936: Meunier and Gayet, 1996). Histologie descriptions by Weiler (1935) and Stromer (1936) predated Sire's (1989) recognition of isopedine in polypterid scales.

The CGM 81145 and 81153 specimens differ from most polypterid scales in having an interrupted surface layer of ganoin, In most polypterid taxa the free scale surface is covered with a continuous layer of ganoin, and the covered portion of the scale lacks ganoin (Meunier and Gayet, 1996). Ganoin ridges on the new Bahariya fossils are extremely variable between scales. In both CGM 81145a and b the regions between ganoin ridges are occupied by a densely cellular tissue (Fig. 4.1, 4.2). The cells are concentrated around vertical channels, to which they are connected in a dendritic pattern by radial canals. Concentrations of cells (osteocytes) occur around vascular channels in the dentine of Polypterus (Aldinger, 1937; Schultze, 1968; Daget et al., 2001). Concentration of the vertical channels and cells in the Bahariya scales is greatest close to the edges of the ganoin, suggesting that the ganoin has been resorbed and replaced by an unornamented dentine surface. Resorption and incomplete replacement of the ganoin by dentine has been reported in Latinopollia suarezi Meunier and Gayet, 1998 (Daget et al., 2001, fig. 5b).