J. Paleont., 80(6), 2006, pp. 1179–1185 Copyright 䉷 2006, The Paleontological Society 0022-3360/06/0080-1179$03.00
MICROSTRUCTURE OF POLYPTERID SCALES (OSTEICHTHYES: ACTINOPTERYGII: POLYPTERIDAE) FROM THE UPPER CRETACEOUS BAHARIYA FORMATION, BAHARIYA OASIS, EGYPT JOSHUA B. SMITH,1 BARBARA S. GRANDSTAFF,2
AND
MEDHAT SAID ABDEL-GHANI3
Department of Earth and Planetary Sciences, Washington University, 1 Brookings Drive, Campus Box 1169, St. Louis, Missouri 63130-4899, ⬍
[email protected]⬎, 2Department of Earth and Environmental Science, University of Pennsylvania, 240 South 33rd Street, Philadelphia 191046316, and 3Egyptian Geological Museum, Egyptian Geological Survey and Mining Authority, Athar El Nabi, Maadi, Cairo 1
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-yearold 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
T
fossil vertebrate assemblage from the lower Cenomanian (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 paleontological 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. HE DIVERSE
Stromer (1936) reidentified scales from Bahariya that Weiler (1935) attributed to Lepidotus aff. souzai Woodward, 1908 as belonging to a new polypterid genus. His identification was based on both morphologic similarity to the scales of Polypterus SaintHilaire, 1802 and on a histologic 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 Stromer’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 Stromer (1936), he felt that the fossils might belong to a new taxon distinct from Polypterus. Schaal (1984) additionally described an ectopterygoid, 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.
FIGURE 1—1, Location of the Bahariya Oasis in Egypt, ⬃320 km southwest of Cairo. 2, Locations of landforms in the northeastern area of the oasis where BDP material has been found. Dashed line in 2 indicates the road to Cairo; black dots indicate the town of Bawiti and BDP 2000–19, the discovery location of the materials described here (modified from J. B. Smith et al., 2006).
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JOURNAL OF PALEONTOLOGY, V. 80, NO. 6, 2006 TABLE 1—Measurements of the polypterid scales illustrated in Figure 2. Scales are measured in directions parallel to their edges. The d-v width is parallel to the ventral articular peg. The a-pt length is perpendicular to the ventral articular peg, and runs from the unornamented cranial sector to the thin caudal edge. Thickness measured is maximum thickness, which lies at the cranial end of the beveled caudal portion of the scale. Specimen CGM CGM CGM CGM
FIGURE 2—Polypterid scales (CGM 81153) from BDP 2000–19 in external (1) and internal (2) views. Cranial (covered) margins of the scales are toward the top of the figure; dorsal and ventral margins are indicated beside each scale. Scales are, from the top, A, B, C, and D, respectively. Scale bars ⫽ 1 cm. Dimensions of these scales are provided in Table 1.
The new scales described here also support the idea that the Bahariya polypterid belongs to a genus, likely new, that is distinct from Polypterus.
81153a 81153b 81153c 81153d
thickness (mm) 2.51 4.57 4.31 3.09
d-v width (mm) a-pt length (mm) 11.33 18.01 13.39 9.68
14.30 18.16 14.54 9.98
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 Stromerichthys 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. The Bahariya Formation has been considered to be Cenomanian in age since the earliest studies of oasis geology (Ball and Beadnell, 1903; Stromer, 1914). However, there has been no uniform agreement as to whether the age is early or late Cenomanian. The vertebrates and oysters of the unit within the oasis exhibit late Cenomanian affinities (see Dominik, 1985; Hantar, 1990). Dinoflagellates, foraminifera, ostracods, and palynomorphs, both from outcrops in the oasis and from subsurface wells north of the basin, suggest an early Cenomanian age (see Ismail et al., 1989; Barakat et al., 1993; El Beialy, 1994, 1995; Nabil and Hussein, 1994; Ismail and Soliman, 2001; Ibrahim, 2002). Lejal-Nicol and Dominik (1990) and Ismail and Soliman (2001) have argued that the Bahariya Formation is time-transgressive, with the lower portion of the unit being early Cenomanian in age. The general consensus is that the base of the sequence exposed in the oasis basin is low in the unit, and there is mounting evidence that the lower portion of the formation was deposited fairly quickly (Lacovara et al., 2001, 2002). The balance of information appears to favor an early Cenomanian age, which places the lower portion of the Bahariya Formation exposed in the oasis at ⬃97 Ma (Gradstein et al., 2005). Previous occurrences of Cenomanian polypterids have been placed at ⬃95 million years (Werner and Gayet, 1997) in the Wadi Milk beds of Sudan and ⬃93 million years from the Kem Kem beds of Morocco (Sereno et al., 1996). The Bahariya fossils thus appear to be slightly older than any previously reported polypterids.
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
DESCRIPTIONS
General morphology.⎯The new scales are square to rectangular, with peg-and-socket articulations visible at their edges (Fig. 2). The craniodorsal corner of each scale is drawn out into a process of varying size. This process articulates with a notch near the middle of the ventral edge of the next-dorsal scale. Many
SMITH ET AL.—POLYPTERID SCALES FROM THE LATE CRETACEOUS OF EGYPT
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FIGURE 3—Cranial to caudal thin section of CGM 81145b in transmitted natural (1) and polarized (2) light. In transmitted light (1) the scale shows a thick base of vascular bone overlain by richly cellular dentine with vertical channels. Layered ganoin is present over part of the scale, and stands up as ridges. The central area of the scale lacks ganoin. Faint horizontal lineations are visible below the smaller ganoin ridge at the right end of the section. In polarized light (2) the ganoin is strongly birefringent. Underneath the ganoin and dentine in the central part of the scale is a zone with alternating dark and light bands visible under polarized light (running from right to left just above the arrows in 2). The differential birefringence is due to differences in collagen fibril orientation in successive layers of isopedine (Daget et al., 2001). Some of the cellular channels continue across this layered tissue, suggesting the isopedine may also be undergoing remodeling, as reported by Sire (1989). Dimensions of 1 ⫽ 2.1 ⫻ 3.4 mm; dimensions of 2 ⫽ 1.1 ⫻ 1.9 mm.
scales have a bump on their ventral edge which apparently fits into a socketlike hollow just caudal to the craniodorsal process of the next-ventral scale. Some scales also possess a dorsal peg just caudal to the craniodorsal process, forming a complex interlocking articulation between adjacent scales. The peg-and-socket articulations are well preserved on many of the scales from BDP 2000–19. Like the craniodorsal processes, the peg-and-socket articulations vary in size between scales. None of the scales have a continuous ganoin surface layer. Dimensions of the scales illustrated in Figure 2 are given in Table 1. The cranial edge of each scale has a narrow unornamented facet on its outer surface, while its caudal area is beveled on the inner surface. The thin caudal edges apparently overlapped the cranial edges of the next-caudal scales. This indicates that the scales were imbricated in vivo, as in living Polypterus (Gemballa and Bartsch, 2002). True scale rows in extant polypterids spiral from craniodorsal to caudoventral (Gemballa and Bartsch, 2002). Each scale overlaps the cranial edge of the one caudal to it, and meets the ventral edge of the one craniodorsal to it. Ornamentation on the uncovered (free) portion of the scale of the Bahariya polypterid consists of highly variable short ganoin ridges and bosses. 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 Schultze, 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; Daget 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 (CGM 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 polypterids the ganoin is crossed by vertical vascular channels, some of which are visible in histologic sections. Under polarized light CGM 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). Histologic 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.
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FIGURE 4—Cranial to caudal thin section in normal transmitted light of CGM 81145a showing dentine channels and cells preserved near the edge of the ganoin ornament. Like CGM 81145b, this scale has a large vascular zone in the center of the scale within the bony basal layer. The dentine adjacent to the ganoin in this part of the scale shows a very dense network of channels and cell bodies suggestive of active remodeling. 1, Low magnification view of the full thickness of CGM 81145. 2, Higher magnification of the central-upper part of 1. Dimensions of 1 ⫽ 2.1 ⫻ 3.4 mm; dimensions of 2 ⫽ 1.1 ⫻ 1.9 mm.
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). The edges of ganoin coverings on scales of fishes typically show progressive thinning, with each layer of ganoin extending farther than the one under it as the scale grows. The ganoin adjacent to densely cellular regions in the Bahariya scales terminates in a nearly vertical edge, unlike the progressively off-lapped edge associated with centrifugal scale growth. Progressively thinned ganoin edges of this type are found in areas of the Bahariya scales that do not lie adjacent to zones of remodeling. While Daget et al. (2001) reported partial replacement of ganoin in Latinopollia
suarezi scales, the ganoin layer is never completely breached or removed by remodeling. The process of ganoin replacement seems to be much more extensive in the Bahariya polypterid than in most other polypterids. Scanning electron microscopy shows that the ganoin ridges of the Bahariya scales are smooth (Fig. 5.1). There is no indication of the microtubercles that are typically found on the surfaces of ganoid scales (Richter and Smith, 1995; Schultze, 1996), including those of living and fossil polypterids (Meunier and Gayet, 1996; Daget et al., 2001). No microtubercles were found on the edges of the ganoin ridges (Fig. 5.1, 5.2). As these areas are undergoing active remodeling, any surface microstructure could have been removed by the remodeling process. However, there also are no microtubercles evident in the central regions of ganoin elevations (Fig. 5.3), which have not been subject to remodeling. The ganoin of the Bahariya polypterid may have lacked microtubercles. Absence of microtubercles would make the Bahariya scales different from those of other polypterids that have been examined under SEM. Postmortem transport abrasion might have removed these structures, but no trace of even abraded microtubercles is evident on CGM 81145c. Other fossil polypterid scales preserve microtubercles despite taphonomic processes (Meunier and Gayet, 1996; Daget et al., 2001). It seems unlikely that abrasion would have completely removed these structures on wellpreserved scales, especially in the very low energy conditions
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FIGURE 5—Scanning electron photomicrographs of the caudodorsal portion of CGM 81145c. The ridges visible in 1 are covered by ganoin; adjacent scale areas lack a ganoin coating. Note that the ganoin surface of the ridges does not show microtubercles. Vertical canals through the ganoin are visible in cross section (Fig. 4.1, upper right), but are not evident in these surface pictures. We suspect the absence of visible vascular canal openings under scanning electron microscopy is a taphonomic artifact. No microtubercles were found using scanning electron microscopy. Scale bar in 1 is 500 m; scale bars in 2 and 3 are 5 m.
recorded in the Bahariya Formation (Lacovara et al., 2003), where there does not appear to have been significant transport of even small fossils (J. B. Smith et al., 2001). DISCUSSION
The new Bahariya polypterid scales differ in gross appearance from those reported in most polypterids because they possess a discontinuous ganoin layer. The ganoin present forms an ornament of ridges on the scale surface. Ornamented polypterid scales with isolated ganoin ridges from the Eocene of the Fayum were described by Stromer (1905, 1936) and figured by Schaal (1984). Both Stromer (1936) and Schaal (1984) figured polypterid scales from the Bahariya Formation that are ornamented with ganoin ridges. Schaal states that these scales are different from the ornamented scales of Polypterus bichir ornatus Arambourg, 1947 (Arambourg, 1947). In the Bahariya scales, the ganoid ridge ornamentation appears to result from removal of ganoin from the scale surface, producing ganoin-free areas. Resorption and replacement of ganoin by dentine has been reported in a fossil polypterid (Daget et al., 2001). Histologic sections of ornamented polypterid scales from other localities, including those from the Fayum (Stromer, 1905, 1936; Schaal, 1984), may help determine whether their ornament was developed by ganoin removal or by deposition of an incomplete ganoin layer. 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; Lo´pez-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
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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. The presence of abundant polypterid scales, and the ectopterygoid described by Schaal (1984), in the lower Bahariya Formation suggests that there were freshwater habitats proximal to the coastline during the time that the Bahariya sediments were being laid down. Previous workers have speculated that in the Cenomanian, Bahariya was located some 500 km to the north of the closest major freshwater source (Said, 1990), and there is little evidence of freshwater inputs to the Bahariya sequences at the stratigraphic level of BDP 2000–19 (K. J. Lacovara, personal commun., 2002). However, the occurrence of relatively unworn polypterid scales in the tidal inlet preserved at BDP 2000–19 argues for a freshwater input in the general area at the specific time the inlet sediments were deposited. That this is a valid hypothesis is supported by occurrences of polypterid fossils from other sites distributed across the oasis basin (Stromer, 1936; Schaal, 1984). ACKNOWLEDGMENTS
We thank M. Lamanna, P. Dodson, J. Poole, R. Giegengack, A. Swedan, K. Soleiman, Y. Attia, Y. Abdelrazik, K. Lacovara, J. R. Smith, J. Caton, A. Tumarkin-Deratzian, J. Losos, the Egyptian Geological Survey and Mining Authority, and the El Beshmo Lodge for invaluable assistance both in and out of the field. The authors are indebted to C. Suarez, M. Suarez, D. Terry, and D. Grandstaff for support and assistance in thin-sectioning the scales, to D. B. Dutheil for helpful discussion, and to M. Veith for SEM work. G. Arratia, W. E. Bemis, S. Novak, and A. Tumarkin-Deratzian provided valuable critical reviews of this manuscript. The Bahariya Dinosaur Project (BDP) has been supported by grants to JBS from Cosmos Studios, MPH Entertainment, the University of Pennsylvania, the Andrew W. Mellon Foundation, and the Delaware Valley Paleontological Society, and to P. Dodson (Penn) from the late E. de Hellebranth and the University of Pennsylvania. This paper is BDP contribution 6. REFERENCES
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