J. Paleont., 79(5), 2005, pp. 1031–1035 Copyright q 2005, The Paleontological Society 0022-3360/05/0079-1031$03.00
PALAEGA RUGOSA, A NEW SPECIES OF FOSSIL ISOPOD (CRUSTACEA) FROM MAASTRICHTIAN ROCKS OF TUNISIA RODNEY M. FELDMANN1 1
AND
STIJN GOOLAERTS2
Department of Geology, Kent State University, Kent, Ohio 44242 ,
[email protected]. and 2Department of Geology-Geography, KU Leuven, Redigenstraat 16, B-3000 Leuven, Belgium ,
[email protected].
INTRODUCTION ECENT, INTENSIVE
collecting for Maastrichtian ammonites in Tunisia has yielded a single small isopod specimen from the Kalaat Senaan region. The isopod is a new species of Palaega Woodward, 1870 and appears to be the first representative of the genus to be recorded from the Maastrichtian of northern Africa.
R
GEOGRAPHIC AND STRATIGRAPHIC SETTING
A 5,000 m thick sedimentary record, ranging in age from the Aptian to the middle Ypresian, crops out in the west-central part of Tunisia, close to the Algerian border, between Kalaat Senan, Tajerouine, and Kalaa Khasba (see Robaszynski et al., 1990, 1993a, 1993b, 1993c, 2000; Hardenbol et al., 1993; Dupuis et al., 2000; and Steurbaut et al., 2000 for detailed litho- and biostratigraphy) (Fig. 1). An almost 600 m thick marly sequence of the El Haria Formation forms a low, gently undulose landscape, situated between a cliff of the 200 m thick upper limestone bar of the underlying Abiod Formation (which ranges in age from late Campanian to early Maastrichtian) and a cliff of the Eocene nummulitic limestone bar in the Metlaoui Formation. Numerous gullies deeply eroded into the Maastrichtian El Haria Formation between these two cliffs provide excellent exposures of the 600 m thick sequence. Several lithostratigraphic units are distinguishable within the El Haria Formation, two of which crop out in the Aı¨n Settara section: the lowermost Aı¨n Settara Member, consisting of dark grey jarositic marls with alternating 0.2–0.8 m thick, whitish, carbonate-rich beds, and the uppermost Sidi Nasseur Member, consisting of alternating blue grey marls and thin whitish limestone beds. The base of the Sidi Nasseur marls is defined by a 60 cm thick dark clay, called the ‘Dark Boundary Clay.’ A few centimeters above its base is an orange-colored layer formed of platy jarosite nodules. It is within these nodules that an enrichment of Ir- and Ni-rich spinels is found in association with an abundance of planktic foraminifera and calcareous nannoplankton, defining the Cretaceous/Paleogene (K/P) boundary. The Aı¨n Settara K/P boundary section has been studied over the past years in detail and has become one of the important K/P boundary sections in Tunisia in addition to those from El Kef, Elles, El Melah, and Seldja. During one of the field campaigns to describe the detailed history of Late Maastrichtian ammonites in the Tunisian basin, a newly discovered K/P boundary section close to the K/P boundary section of Aı¨n Settara yielded a single isopod specimen about 7 m below the K/P boundary. This new section is called the Tabet Zaara section, and it is exposed in a tributary of the Oued Settara about 500 m west of the Aı¨n Settara section. The Tabet Zaara section will be described in detail in the near future, but is almost identical to the late Maastrichtian Aı¨n Settara section. The only difference is that only the uppermost 30 m of the Maastrichtian deposits are well exposed in contrast to the uppermost 80 m of the Maastrichtian rocks in the Aı¨n Settara section. The late Maastrichtian macrofauna associated with the isopod
specimen mainly consists of small pyritic or pyritized fossils, including ammonite nuclei (typically ,30 mm), small solitary scleractinian corals, gastropods, bivalves, brachiopods, bryozoans, rudists, and echinoids. The depositional environment, based on benthic foraminifera, was probably an outer shelf to upper bathyal environment (Peryt et al., 2002). From a biostratigraphic point of view, the isopod specimen was found within the Plummerita hantkeninoides Foraminiferal Biozone and the CC26 Nannofossil association (Robaszynski et al., 2000), both indicating a late late Maastrichtian age for the isopod specimen. The cyclostratigraphic framework of Hennebert and Dupuis (2003) places the 7 m below the K/P boundary, within which the isopod was found, at about 90 Ka before the K/P boundary. SYSTEMATIC PALEONTOLOGY
Order ISOPODA Latreille, 1817 Suborder FLABELLIFERA G. O. Sars, 1882 Family CIROLANIDAE Dana, 1852 Genus PALAEGA Woodward, 1870 sensu lato Palaega WOODWARD, 1870, p. 496, pl. 22, figs. 3–6; HESSLER, 1969, p. R380, fig. 198. Aegites VON AMMON, 1882, p. 537.
Type species.Palaega carteri Woodward, 1870, by original designation. Putatively included fossil species.Palaega acuticauda Secretan, 1975, 1976; Palaega anconitana Andrussow, 1887; Palaega catulloi de Zigno, 1880; Palaega carteri; Palaega collinsi Quayle, 1982; Palaega danica Van Straelen, 1928; Palaega gastaldi Sismonda, 1846; Palaega goedertorum Wieder and Feldmann, 1989; Palaega guadalupensis Rathbun, 1935; Palaega huetteri Fraaye and Summesberger, 1999; Palaega ilerdensis Calzada and Go´mez Pallerola, 1994; Palaega jurassica Stolley, 1910; Palaega kessleri Reiff, 1936; Palaega kunthi Von Ammon, 1882; ?Palaega lamnae Bowman, 1971; Palaega mccoyi Carter, 1889; Palaega pumila Gall and Grauvogel, 1971; Palaega scrobiculata Von Ammon, 1882; Palaega sismondae Ristori, 1891; ?Palaega stemmerbergensis Malzahn, 1968; Palaega suevica Reiff, 1936; Palaega taurica Spassky and Kravtsov, 1976; Palaega tremembeensis Martins-Neto, 2001; Palaega undecimspinosus Karasawa, Nobuhara, and Matsuoka, 1992, as Bathynomus undecimspinosa, by Karasawa, Suzuki, and Kato, 1995 and Karasawa, 1997, and as Bathynomus sp., by Imaizumi, 1953; Palaega williamsonensis Rathbun, 1935; ?Palaega sp. Feldmann, Jagt, and Tshudy, 1990; Palaega sp. aff. P. doederleini (Ortmann, 1894), by Karasawa, Nobuhara, and Matsuoka, 1992, and as Bathynomus sp. aff. B. doederleini, by Karasawa, 1997. PALAEGA
new species Figure 2 Diagnosis.Pleon with coarsely wrinkled surface formed of anastomosing ridges; pleotelson slightly longer than wide. Description.Moderate-sized for genus, specimen 37.8 mm long, exhibits four moderately complete pleonites and at least one
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FIGURE 1—Map and stratigraphic section showing the geographic and geologic position of the Tabet Zaara Section from which Palaega rugosa n. sp. was collected.
poorly preserved pleonite anterior to others. Pleonites equally long, 3.1 mm, and approximately equally wide; maximum width 15.3 mm. Articulations weakly convex forward. Lateral margins crushed but pleurae appear to be posteriorly directed. Pleotelson subrectangular, tapering posteriorly; width 14.4 mm, slightly less than inferred length of 16.3 mm; however, posterior portion of pleotelson poorly preserved. Axis of pleotelson with narrow ridge becoming obscure posteriorly. Margins of pleotelson not well enough preserved to describe. Uropodal exopod? large, flattened, keeled longitudinally, extending laterally from beneath pleotelson, margins not well preserved. Surface of pleon uniformly coarsely wrinkled with anastomosing ridges surrounding smooth concavities. Ridges with steep posterior sides and gentle anterior sides resulting in terracelike structures. Etymology.The trivial name is derived from the Latin word, ruga, meaning wrinkled or shriveled, in reference to the wrinkled appearance of the pleon surface. Type.The holotype, IRSNB TC MI 10998, is deposited in the Koninklijk Belgisch Instituut voor Natuurwetenschappen (KBINIRSNB, Brussels, Belgium). Occurrence.The holotype was collected from the Late Cretaceous (latest Maastrichtian Plummerita hantkeninoides Foraminiferal Biozone and the CC26 Nannofossil association), part of the El Haria Formation (Aı¨n Settara Member, 7 m below the K/ P boundary) in the Tabet Zaara section (WGS84: 35847.41N, 8827.20E, M. Hennebert, personal commun., January, 2004), which is situated between the villages Kalaat Senan, Tajerouine, and Khalaa Khasba (Tunisia, northern Africa).
Discussion.Twenty-seven species-level taxa have been assigned to Palaega. However, examination of the original literature and much of the subsequent work describing these species confirms that the genus contains an array of taxa whose morphologies are so different that there is no possibility that they can all be assigned to the same genus. The list of putatively included fossil species includes at least three different groups based upon the morphology of the head region. One group, characterized by Palaega scrobiculata, possesses a cephalon surrounded posteriorly and posterolaterally by the first pereonite and bearing eyes that are dorsally placed (Von Ammon, 1882; Hessler, 1969). A second group, characterized by Palaega kessleri, exhibits a cephalon not surrounded posterolaterally by the first pereonite and with eyes that are laterally placed (Reiff, 1936; Hessler, 1969). The third group, documented by Palaega goedertorum (Wieder and Feldmann, 1989, 1992), bears a cephalon surrounded posteriorly and posterolaterally by the first pereonite but on which the eyes are not visible in dorsal aspect and are presumed to be sessile and ventral. Based upon the characters of the dorsal surface, including the cephalon, that are useful in distinguishing living taxa of isopods, it is untenable to argue that all of the fossil species could be included within a single genus, let alone family. Isopods are difficult to classify and interpret in the fossil record for a variety of reasons. Notable among the reasons is the observation that the group is as variable as any group of crustaceans (Schram, 1986) and the style of differentiation of body regions is different, but vexatiously familiar. For example, the cephalon consists of the typical crustacean preoral and oral somites but also includes the first and sometimes the second thoracic segment (Schram, 1986). The trunk section, the perion, consists of the
PALEONTOLOGICAL NOTES
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FIGURE 2—Illustrations of two species of cirolanid isopods illustrating the nature of the identifiable regions. 1, Palaega rugosa n. sp. Dorsal view of the pleon. Scale bar equals 1 cm. 2, Palaega goedertorum Wieder and Feldmann, 1989. Line drawing of the entire animal, modified from Wieder and Feldmann, 1989, fig. 3. Scale bar equals 1 cm.
remainder of the typical thoracic somites. Finally, the abdominal region is typically comprised of the first five abdominal somites, the pleon; whereas the terminal feature, the pleotelson, represents the fusion of the sixth abdominal somite and the telson. This pattern is illustrated in Figure 2.2. As a result, the superficial similarity of this tripartite subdivision to the cephalon, thorax, and abdomen of the stylized crustacean is highly misleading. To further complicate the matter, the perion and pleon of different taxa may be quite similar but the cephalon may differ in such essential ways that the taxa are considered quite unrelated. Additionally, the classification of isopods is based heavily upon characters of the venter and appendages which are almost never preserved in the fossil record. The molting style of isopods provides the final problem for the interpretation of fossil isopods.
The problem of identification and classification of fossil isopods is exacerbated by the observation that many of the taxa assigned to Palaega are represented only by the posterior part of the skeleton. Nearly all isopods molt in a two-stage process, called biphasic molting (George, 1972). As molting begins, the animal extricates itself from the posterior part of the exoskeleton, which includes the last three pereonites and the entire pleon, including the pleotelson. This portion of the molt process usually results in an articulated unit that is preservable and recognizable in the fossil record. The second stage typically involves the fragmentation and disarticulation of the cephalon and the anterior pereonites. This part of the exuvium would be difficult to recognize and has not been reported in the fossil record to our knowledge. Thus, the entire exoskeleton would be anticipated if a corpse were
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preserved and only the posterior portion would be anticipated if an exuvium were preserved, in which case none of the features of the cephalon could be described. The type species of Palaega, P. carteri consists solely of molted remnants so that it is impossible to determine the configuration of the cephalon and the placement of the eyes. Therefore, although the morphology of the pleon and pleotelson of P. carteri closely resemble those of the living cirolanid genus Bathynomus A. Milne Edwards, 1879, the synonymy is highly subjective. The question of synonymy was raised by Wieder and Feldmann (1989) who took the position that Bathynomus was the junior subjective synonym of Palaega. That decision subsequently was placed before the International Commission on Zoological Nomenclature. They ruled that, when questions of synonymy of the two genera arise, Bathynomus A. Milne Edwards, 1879 will have precedence over Palaega Woodward, 1870 (ICZN, 1992). As a result of this decision, and because Palaega Woodward remains a valid genus, both the systematics and the taxonomy of the included species is complex. Although the list of putatively included fossil species, given above, is as complete as possible, the types of many of the taxa have not been examined and, therefore, constructive revision of the genus cannot be undertaken at this time. Despite the absence of the anterior part of the pereon, it is clear that the morphology of Palaega rugosa lies within the range of variation of the genus, as currently defined, but is clearly different from that of any of the other species referred to the genus. Its integrity as a distinct species is assured. The pleotelson is subtriangular, slightly tapered in the posterior direction, and slightly longer than wide. The medial ridge on the pleotelson is strong anteriorly and becomes obscure and disappears posteriorly. The most distinctive character is that the surface of the entire pleon and the preserved part of the pereon is ornamented by anastomosing ridges forming terraced structures. This feature is unique and distinguishes Palaega rugosa from all other species that have been referred to the genus. The genus Palaega s.l. has a geologic record extending from the Late Triassic to Pliocene and been reported from Europe, North America, Australia, and Japan (Hessler, 1969; Gall and Grauvogel, 1971; Karasawa, 1997). Cretaceous records, in addition to P. rugosa, include the type species P. carteri, from the Upper Cretaceous of Great Britain and P. sp. from the Maastrichtian of Belgium (Feldmann, Jagt, and Tshudy, 1990); P. guadalupensis, from the Upper Cretaceous of Texas, USA (Rathbun, 1935); P. huetteri, from the Cretaceous of Austria (Fraaye and Summesberger, 1999); P. ilerdensis, from the Lower Cretaceous of Spain (Calzada and Go´mez Pallerola, 1994); P.? lamnae, from the Upper Cretaceous of Texas (Bowman, 1971); P.? stemmerbergensis, from the Hauterivian of Germany (Malzahn, 1968); P. taurica, from the Cenomanian of Crimea (Spassky and Kravtsov, 1976); and P. williamsonensis, from the Maastrichtian of Texas (Rathbun, 1935). Species of Palaega are also well represented in Cenozoic sediments so that there is no demonstrable effect of the Cretaceous/Paleogene extinction event. The occurrence of the new species in Tunisia represents the first occurrence of the genus in Africa. ACKNOWLEDGMENTS
We are grateful to the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT Vlaanderen) for financial support. We are also grateful to M. Hennebert, C. Dupuis, E. Steurbaut, and especially M. Yahia for their guidance and assistance during the field campaigns. A special thanks to J. W. M. Jagt, who was the first to recognize the ‘strange looking fossil’ as an isopod, probably a Palaega sp., and brought SG in contact with the first author on the field trip of the Second Symposium on Mesozoic and Cenozoic Decapod
Crustaceans (Boxtel-Maastricht, The Netherlands, 3–6 September 2003). The manuscript was carefully read by C. Schweitzer, Kent State University Stark Campus. Finally, the reviews of H. Karasawa, Mizunami Fossil Museum, and an anonymous reviewer substantially improved the final draft. The errors remain ours. REFERENCES
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