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

ABSTRACT-

Histology of newly discovered ganoid scales from the lower Bahariya Formation, in the Bahariya Oasis of western Egypt, confirms the presence of polypterid osteichthyans in this early Cenomanian locality. These fossils, occurring in the ~97 million-year-old lower Bahariya sequences, are among the earliest known polypterids. The Bahariya scales exhibit four tissues: ganoin, dentine, isopedine (elasmodin), and a basal plate of cellular bone, confirming their inclusion within the Polypteriformes. They have a discontinuous ganoin layer, present only as highly variable ridges and bosses. Dentine along the edges of the ganoin ridges appears to have undergone active remodeling, suggesting that the ganoin ridges represent the remnants of a continuous ganoin cover. Modern polypterids inhabit exclusively freshwater environments. Polypterids are not rare in the lower Bahariya Formation. Their presence in these coastal sediments suggests that freshwater habitats lay close to the site of deposition of this sequence during the early Cenomanian.

INTRODUCTION

THE DIVERSE fossil vertebrate assemblage from the lower Cenomuniun (Upper Cretaceous, ~97 Ma) Bahariya Formation of the Bahariya Oasis, Egypt (Fig. 1.1), includes fishes, turtles, plesiosaurs, squamates, crocodyliforms. and dinosaurs (e.g., Stromer, 1936). The known assemblage of fishes has been studied since the first paleontologieal explorations in the oasis and includes chondrichthyans, dipnoans, actinistians, enchodontids, and pycnodontids (e.g., see Stromer, 1925, 1936; Stromer and Weiler, 1930; Weiler, 1935: Slaughter and Thurmond, 1974; Martin, 1981; Schaal, 1984; Werner, 1989. 1990). Resumption of tetrapod-focused paleontology in the Bahariya Formation (J. B. Smith et al., 1999) has led to discoveries of new taxa (J. B. Smith et al., 2001), taxa not previously known from the unit, and new material of poorly represented groups, including fishes and invertebrates (e.g., Schweitzer et al., 2003). We report here on abundant, well-preserved scales of a polypterid osteichthyan fish from a site low in the portion of the Bahariya Formation sequence that is exposed in the oasis.

Stromer (1936) reidentified scales from Bahariya that Weiler (1935) attributed to Li-pidotus aft", souzai Woodward, 1908 as helonging to a new polypterid genus. His identification was based on both morphologic similarity to the scales of Polypterus SainlHilaire. 1802 and on a histologie section of one of the specimens. Stromer (1936) considered the histology to be different from that in extant Polypterus, but did not name the putative new taxon. Unfortunately, most of Simmer's Bahariya collection was destroyed during World War II (see J. B. Smith et al., 2006), making it difficult to assess his findings. Schaal (1984) described the gross appearance of polypterid scales with two different morphologies from Bahariya, but did not refer them to a genus. Like Simmer (1936), he fell that the fossils might belong to a new taxon distinct from Polypterus. Schaal (1984) addilionally described an eclopterygoid, which he assigned to the new form, Polypterus? bartheli. The material described by Stromer (1936) and Schaal (1984) probably belongs to a polypterid distinct from Polypterus (Gayet et al., 2002), but Schaal's material was, for the most part, poorly preserved and is not particularly helpful in resolving the issue. The new scales described here also support the idea that the Bahariya polypterid belongs to a genus, likely new, that is distinct from Polypterux.

AGE AND STRATIGRAPHIC CONTEXT

The new polypterid material was recovered in January and February 2000 and in January 2001 during the first two joint University of Pennsylvania-Egyptian Geological Museum expeditions to the oasis by the Bahariya Dinosaur Project (BDP). More than 300 well-preserved scales were excavated from the "Jon's Birthday" bonebed (BDP 2000-19, 28�20.107'N latitude, 28�59.047'E longitude), located at the base of Gebel el Fagga in the southeastern portion of the oasis depression (Fig. 1.2). They are catalogued under CGM (Egyptian Geological Museum) 81145 and 81153.

The fossils come from the Cenomanian Bahariya Formation, strata of which form the walls and floor of the depression (see El-Akkad and Issawi, 1963; Dominik. 1985; Ismail et al., 1989). The scales were recovered from low in the exposed section of the unit. Polypterid fossils have previously been reported from several localities in the northeastern portion of the oasis. Stromer (1936) reported multiple scales from his layer n near Gebel el Dist (associated with the type material of Mawsonia libyca Weiler, 1935), and also reported "ossa indet." of polypterids from the Gebel el Dist area. Six scales came from layer e near Ain Harra, and were associated with the type material of the osteichthyan Stronierichthyx Weiler, 1935. At least six scales came from the base of Gebel Maghrafa. The ectopterygoid that Schaal (1984) described as Polypterus? bartheli came from his (Schaal's) stratum 3, ~35-57 m above the exposed base of the Bahariya Formation. He also reported polypterid scales from his layers 3 and 6.