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

The confirmation of definitive polypterid material at BDP 2000-19 supports the presence of Cladistia in the diverse assemblage of fishes of the Bahariya Formation. Extant polypterids are exclusively freshwater, and are known from coastal lowlands (Schaal, 1984; Werner and Gayet, 1997; Wiley, 1998; Daget et al., 2001). Fossil polypterids are known from Cenomanian and younger rocks in Africa and South America (Gayet and Meunier, 1996; Meunier and Gayet, 1996; Werner and Gayet, 1997; Dutheil, 1999; Werner, 1999; Cavin et al., 2001; Daget et al., 2001; Stewart, 2001; Gayet et al., 2002; Lopez-Arbarello, 2004). They are particularly diverse in fluvial deposits such as the Wadi Milk Formation of the Sudan (Werner and Gayet, 1997; Gayet et al., 2002). The presence of abundant, relatively unworn polypterid scales in the "Jon's Birthday" bonebed may mean that these remains are derived from nearby fluvial sources.

CONCLUSIONS

The Bahariya scales show thin alternating light and dark bands in the central area of the scale under cross-polarized light. This tissue has the optical properties and location expected for isopedine (elasmodin of Schultze, 1996), which lies between the dentine and bony base in the center of polypterid scales. Presence of this plywoodlike tissue distinguishes polypterid scales from the ganoid scales of other fishes (Meunier and Gayet, 1996; Schultze, 1996; Daget et al., 2001). The polypterids in this study differ from other polypterids in two ways. First, the ganoin coating is not continuous. second, the dentine adjacent to the ganoin ridges in the Bahariya scales has close-spaced vertical channels densely connected to cells by horizontal canals; similar vertical channels have not been reported in other polypterids for which scale histology is known. Cellular dentines are described by FrancillonVieillot et al. (1990), but differ from the tissue of the Bahariya polypterid in the concentration of cells. Additionally, the surface of the Bahariya ganoin, as preserved, is smooth. Ganoin surfaces of all living and fossil polypterids previously examined using SEM are ornamented with microtubercles. Microtubercles are typically found on the surfaces of ganoid scales (Richter and Smith, 1995). Differences in histology between the Bahariya polypterid and other polypterids suggest that the Bahariya form may belong to a new taxon. Definitive identification of this Bahariya fish must await the discovery of additional skeletal material, particularly the discovery of articulated specimens. It should be noted, however, that Schaal (1984) attributed a large ectopterygoid from Bahariya to his new species Polypterus? bartheli, which he believed probably belonged to the genus Polypterus, and certainly to a polypterid, based on the presence of a lateral process that would have articulated with the maxilla (Schaal, 1984, p. 51).

Our histologic study of the new Bahariya material, in light of recent work on polypterid scale histology (Sire, 1989; Meunier and Gayet, 1996; Daget et al., 2001), helps to confirm Stromer's (1936) identification of these scales as polypterid. Moreover, it supports previous conclusions that the Bahariya fossils represent a polypterid taxon that differs from Polypterus (Stromer, 1936; Schaal, 1984; Gayet et al., 2002). However, we are reluctant to name a new taxon based only on scale histology. It is probable in any case that these scales belong to the same polypterid taxon which Schaal (1984) referred to as Polypterus? bartheli. Further study of this taxon is needed to clarify its relationship to polypterid genera from other Cenomanian deposits in Africa.