J. Paleont., 83(1), 2009, pp. 128–134 Copyright 䉷 2009, The Paleontological Society 0022-3360/09/0083-128$03.00
THE YOUNGEST CAMBROCLAVES: CAMBROCLAVUS ABSONUS FROM THE MIDDLE CAMBRIAN OF THE CANTABRIAN ZONE (NORTHWEST SPAIN) THOMAS WOTTE Geologisch-Pala¨ontologisches Institut, Westfa¨lische-Wilhelms Universita¨t Mu¨nster, Corrensstrasse 24, D-48149 Mu¨nster, Germany, ⬍[email protected]
Cambrian phosphatized problematica. The group is known from spine-like remains that are locally abundant on a number of Cambrian paleocontinents. Cambroclaves are known through a relatively short time interval (late Early Cambrian–early Middle Cambrian) mostly from tropical, carbonate platform successions on the Siberian Platform, East Gondwana (Australia, China, Kazakhstan), and West Gondwana. The West Gondwana occurrences include Cambroclavus ludwigsdorfensis Elicki, 1994 and Cambroclavus sp. from the late Early Cambrian of eastern Germany (Elicki, 1994, 2005; Elicki and Wotte, 2003), and Cambroclavus sp. from the early Middle Cambrian of Sardinia (Elicki and Wotte, 2003; Elicki et al., 2003; ´ lvaro (2006) reportElicki, 2005, 2006). Recently, Clausen and A ed the zhijinitid form Parazhijinites cf. guizhouensis Qian and Yin, 1984 from the early Middle Cambrian of the Cantabrian zone of northwest Spain. Other than the mentioned tropical occurrences, Landing (1991) published the obviously cool to cold water form Samsanoffoclavus matthewi Landing, 1991 from the middle Early Cambrian of Cape Breton Island (Avalonia). This report provides a description of cambroclavids from the Middle Cambrian of the Cantabrian zone (northwest Spain), the determination of their stratigraphic position, and a discussion of the consequences for the general stratigraphic range of cambroclaves. The Cambrian succession of the Cantabrian zone (Fig. 1) consists of the ?Neoproterozoic?–Early Cambrian siliciclastic Herrerı´a Formation, the Early–Middle Cambrian carbonate La´ncara Formation, and the Middle Cambrian–Tremadocian siliciclastic Oville Formation (Fig. 2). The La´ncara Formation is subdivided into a lower dolomitic member with limited fossils (algae, archaeocyaths, and some trilobites; cf. Debrenne and Zamarren˜o, 1970; Sdzuy, 1995; Perejo´n and Moreno-Eiris, 2003) and a fossiliferous upper carbonate member (Fig. 2). The latter is further subdivided into the Belen˜o facies, characterized by granular crystalline, grey to roan dolomitic limestones, showing a high content of glauconite and disarticulated echinoderms, and the overlying Barrios facies, represented in the main part by nodular limestones (Fig. 3). An abundant and very diverse fauna typifies this facies (e.g., Sdzuy, 1968; van den Boogaard, 1983; Geyer, 1986; Wotte ´ lvaro, 2006; Wotte et al., 2004; Wotte, 2005, 2006; Clausen and A and Mergl, 2007). Important, especially against the background of the stratigraphic position of the described cambroclaves, are the diachronous boundaries between at least the Belen˜o and the Barrios facies as well as between the La´ncara and the Oville Formation (Fig. 2; cf. Sdzuy, 1968; Zamarren˜o and Julivert, 1968; Zamarren˜o, 1972). Unfortunately, no data exist for the Herrerı´a Formation–La´ncara Formation boundary, nor for the contact between the two members of the La´ncara Formation, even if for the last one a similar diachronity could be guessed. AMBROCLAVES ARE
MATERIAL STUDIED AND LOCALITIES
With the exception of the zhijinitid sclerites of Parazhijinites cf. guizhouensis Qian and Yin, 1984 from the Belen˜o facies ´ lvaro, 2006) other occurrences of cambroclaves (Clausen and A
from the Iberian Peninsula were previously unknown. The cambroclavid sclerites described here (Fig. 3) were assigned to Cambroclavus absonus Conway Morris in Bengtson et al., 1990. They were discovered in the Barrios facies of the sections of Los Barrios de Luna (42⬚50⬘37.0⬙N, 005⬚52⬘00.0⬙W) and Genestosa (43⬚00⬘22.2⬙N, 006⬚00⬘41.1⬙W), both situated at the southwestern part (Somiedo-Correcilla subunit) of the Cantabrian zone (Fig. 1.2). Detailed information on these localities is published in Wotte (2005) and Wotte et al. (2007). At Los Barrios de Luna, the cambroclave-yielding sample BL q-2 is situated 4.2 m above the boundary between the lower and the upper member of the La´ncara Formation; at Genestosa the stratigraphic position of sample Ge 10E is located 4.9 m above this boundary (Fig. 3). STRATIGRAPHY
There has long been an ambitious effort to develop a chronostratigraphic scheme for the Early–Middle Cambrian of western ´ lvaro et al. (1999, 2000) the Early– Gondwana. According to A Middle Cambrian boundary of the Cantabrian zone is marked by ´ lvaro et al., 2000), which is situated a discontinuity (D2 sensu A between the lower and the upper member of the La´ncara Formation. Traditionally, the Iberian base of the Middle Cambrian, and therewith the base of the Leonian stage (early Toyonian of the Siberian nomenclature), is defined by the FAD of Paradoxides (Acadoparadoxides) mureroensis Sdzuy, 1958 (e.g., Lin˜a´n et al., 1993) situated just above D2. Comparing the Iberian nomenclature with the Early–Middle Cambrian transition of Morocco, which is suitable as the basis for a standard for West Gondwana (Geyer and Landing, 2004) and differs from the traditional, regional Cambrian series and stages used in Iberia, some significant changes in the subdivision of the Iberian stages must be considered, having also fundamental consequences for the stratigraphic range of the cambroclaves reported here (cf. Fig. 2). If the West Gondwana standard of Geyer and Landing (2004) is accepted, then the middle–late Bilbilian of the Iberian Peninsula correlates to the early Tissanfinian (Hupeolenus Zone) of the Moroccan nomenclature, which represents the earliest Agdzian of the West Gondwana chronostratigraphy (respectively the late Botoman– early Toyonian; Fig. 2). Thus, the Early–Middle Cambrian boundary of Iberia switches into a lower stratigraphic position, and the FAD of P. (A.) mureroensis would represent an early Middle Cambrian (early Agdzian) age (compare with Fig. 2). Consequently, the biostratigraphic boundary between the Early and the Middle Cambrian is not identical with the lithostratigraphic boundary between the lower and the upper member of the La´n´ lvaro et al., cara Formation, as assumed until recently (e.g., A ´ lvaro, 2006). Therefore, the Early– 1999, 2000; Clausen and A Middle Cambrian boundary must be situated beneath this lithostratigraphic boundary, but it must be also located above the archaeocyathan-level of the lower member of the La´ncara Formation (cf. Fig. 2), described by Debrenne and Zamarren˜o ´ lvaro et al. (2000), and Perejo´n (1970), Perejo´n (1986, 1994), A and Moreno-Eiris (2003). As mentioned above, also the diachronity of facies and formation boundaries is fundamental to the classification of the stratigraphic position of the described cambroclaves. At best this
WOTTE—CAMBRIAN PROBLEMATICA FROM SPAIN
FIGURE 1—1, Simplified geological map of the Iberian Peninsula; 2, Geological map of the Cantabrian zone (modified after Pe´rez-Estau´n et al., 1988) showing location of sections mentioned in the text; BL ⫽ Los Barrios de Luna section, Ge ⫽ Genestosa section; LL ⫽ La´ncara de Luna section.
diachronity is investigated at the boundary between the La´ncaraand the Oville Formation. This boundary becomes progressively younger from the northeast to the southwest of the Cantabrian zone (Sdzuy, 1974). So, in the La Sobia Bodon subunit (Fig. 1.2), it is situated within the Badulesia Zone (between the Acadolenus and the Pardailhania Zone), whereas the upper part of the La´ncara Formation of the Somiedo-Correcilla subunit (Figs. 1.2) lies within the Solenopleuropsis Zone (e.g., Zamarren˜o, 1972). Within the uppermost part of the La´ncara Formation of the La´ncara de Luna section (42⬚54⬘34.8⬙N, 005⬚55⬘26.5⬙W; Figs. 1.2, 3) Ctenocephalus (Ctenocephalus) coussesensis Thoral, 1946 occurs (Figs. 3, 4.19–4.20), which was previous known only from the late Caesaraugustan to early Languedocian of the Montagne Noire ´ lvaro and Vizcaı¨no, 2003). Consequently, C. (C.) coussesensis (A also offers a similar age for the uppermost La´ncara Formation of the southwestern part of the Cantabrian zone. Combining the biostratigraphic data with: 1) the geographic closeness of the Ctenocephalus-bearing section La´ncara de Luna and the cambroclavebearing localities Genestosa and Los Barrios de Luna within the southwestern part of the Cantabrian zone (Fig. 1.2) and 2), the localization of the observed sections at nearly the same Cambrian crop-out (Fig. 1.2), a Caesaraugustan–Languedocian age of the
boundary between the La´ncara Formation and the Oville Formation should be assumed for all the three investigated localities. Summarizing all these facts with the information that: 1) according to the Iberian nomenclature, the base of the Middle Cambrian corresponds with the base of the Leonian stage; 2) the boundary between the Belen˜o and Barrios facies is diachronous and becomes younger towards the southwest; and 3) C. absonus Conway Morris in Bengtson et al., 1990 occurs in Los Barrios de Luna 4.2 m and in Genestosa 4.9 m above the boundary between the lower and the upper member of the La´ncara Formation, a stratigraphic position of the reported cambroclaves of a middle–late Leonian age, respectively, a late Agdzian age (West Gondwana standard) is definite. CONCLUSIONS
The Cantabrian zone presents not only the youngest zhijinitids (Parazhijinites cf. guizhouensis Quian and Yin, 1984) as reported ´ lvaro (2006), the present report of C. absonus by Clausen and A Conway Morris in Bengtson et al., 1990 presents also the youngest cambroclavids and, overall, the youngest known stratigraphic occurrence of cambroclaves worldwide. Consequently, an expansion of the general stratigraphic range of cambroclaves up to the
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late Leonian (late Amgan of the Siberian nomenclature) is warranted. The relatively close occurrence of zhijinitid and cambroclavid morphotypes seems to support the hypothesis that both kinds of sclerites belong to one and the same organism (for discussion see Conway Morris in Bengtson et al., 1990; Conway Morris and Chen, 1991). Given that the local stratigraphic age determinations are based on different taxa and that there are fundamental problems in an intercontinental correlation of these stratigraphic schemes (especially for the Early–Middle Cambrian boundary interval), the reported stratigraphic positions of cambroclaves reflect only the recent state of knowledge in this correlation problem, but, also an artifact of sampling and preparation. So, maybe the huge stratigraphic gap between the first report of C. absonus from the Early Cambrian of Australia and the Iberian material becomes narrower than it seems at present. Indications are already given by Elicki and Wotte (2003), who distinguished different levels of cambroclave occurrences, ranging from the oldest one (early Ovetian/ early Isendalenian of the Doberlug Syncline (eastern Germany) up to the youngest report (earliest Leonian/early Agdzian) of Sardinia. SYSTEMATIC PALEONTOLOGY
Specimens are reposited in the collection of the Geological Institute of the Technische Universita¨t Bergakademie Freiberg. Cambroclaves are characterized by the identification FG 544/locality/microscopic/cambroclave/sample; trilobites are stored under FG 544/locality/macroscopic/trilobite/sample. Phylum uncertain Class CAMBROCLAVIDA Conway Morris and Chen, 1991 Family ZHIJINITIDAE Qian, 1978 Genus CAMBROCLAVUS Mambetov in Mambetov and Repina, 1979 Type species.⎯Cambroclavus antis Mambetov in Mambetov and Repina, 1979; Lower Cambrian (Atdabanian) of Kazakhstan (Talasskiy Alatau); Nr. 12/1; sample M36-72; figured on plate 14/ 1–2 of Mambetov in Mambetov and Repina (1979). Diagnosis.⎯See Mambetov in Mambetov and Repina (1979, pp. 124–125). CAMBROCLAVUS ABSONUS Conway Morris in Bengtson et al., 1990 Figure 4.1–4.18 Cambroclavus absonus CONWAY MORRIS in BENGTSON 104–119, figs. 64–69.
FIGURE 3—Facies distribution patterns of the upper member of the La´ncara Formation of the cited sections Genestosa (Ge), La´ncara de Luna (LL), and Los Barrios de Luna (BL). Columns start with the boundary between both members of the La´ncara Formation. Genestosa is the only locality were the Belen˜o facies is not developed (cf. Wotte and Mergl, 2007); the other sections show the separation into the both facies (Belen˜o facies and Barrios facies).
Description.⎯Elongate, hollow sclerites showing a thumb- to dumbbellshape (Fig. 4.1–4.18). Sclerites are nearly bilaterally symmetric. Occasionally, the anterior region is separated from the often elongated posterior region by a contraction of variable degree. The anterior region is mostly sub-circular. Sclerites often taper posteriorly (Fig. 4.10, 4.11, 4.18), but lateral margins could be also nearly sub-parallel (Fig. 4.4, 4.6, 4.8,4.9, 4.12–4.15) or become wider, resulting in a ‘‘duck-foot’’ shape (Fig. 4.1–4.5, 4.7, 4.16,4.17). The posterior margin of some sclerites is characterized by a slight groove (Fig. 4.7, 4.16,4.17). The upper surface of the sclerites is rounded, often shows a median ridge originated in the posterior field, and passes into the base of the anterior spine (Fig. 4.8, 4.9). Spines are broken; spine stumps are oval to circular in cross-section and mostly directed anteriorly. Material examined.⎯Five phosphatized sclerites from sample BL q-2 and eight additional sclerites from sample Ge 10E.
← FIGURE 2—Chronostratigraphic schemes of the Early–Middle Cambrian boundary interval of Siberia, Morocco, and Iberia in combination with the unified chronostratigraphy of Western Gondwana and the stratigraphic column of the Cantabrian zone (modified from Lin˜a´n et al., 1993; Geyer and Landing, 2004; Babcock and Peng, 2007). Marked is the stratigraphic position of the Early–Middle Cambrian boundary of the different chronostratigraphic schemes. Note further the boundary between the La´ncara- and the Oville Formation, which becomes progressively younger from the northeast to the southwest of the Cantabrian zone.
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WOTTE—CAMBRIAN PROBLEMATICA FROM SPAIN Discussion.⎯The absence of a lateral corrugation of the anterior spine rules out an affiliation of the described sclerites to Cambroclavus ludwigsdorfensis Elicki, 1994, C. clavus Mambetov in Mambetov and Repina, 1979, and C. undulatus Mambetov in Mambetov and Repina, 1979. Species such as C. olecranus (misspelled decranus by Conway Morris in Bengtson et al., 1990), C. clypeatus, C. hubeiensis, and C. dolicanthus reported by Duan (1984) from South China, which have been referred to C. fangxianensis by Quian and Zhang (1983), are most similar to C. bicornis Qian and Yin, 1984. As reported by Conway Morris et al. (1997), C. absonus is very similar to C. bicornis, which is synonymous with C. paradoxus Qian and Yin, 1984. In contrast to C. bicornis, sclerites of C. absonus Conway Morris in Bengtson et al., 1990 have more affinity to a thumb- to dumbbell-shape, which is a fundamental characteristic for the sclerites described here, and makes their affiliation to C. absonus definite. An affiliation to the type species C. antis Mambetov in Mambetov and Repina, 1979 can be also excluded. The herein reported sclerites show no distinct furrow around the anterior spine, as figured by Mambetov in Mambetov and Repina, 1979 (plate 14.1), and also the anterior directed spine of the Iberian material is contrary to the spine of C. antis, which is arranged nearly perpendicularly on the sclerite of this form. Stratigraphic and geographic range.⎯Early Cambrian Pararaita tatei Zone of the Parara Limestone, Horse Gully and Abadiella huoi Zone–Pararaita tatei Zone of the Ajax Limestone, Mt. Scott Range, South Australia (both occurrences correspond to the higher Atdabanian). Early Cambrian Todd River Dolomite, Ross River, Northern Territory, Australia. ACKNOWLEDGMENTS
For helpful discussions I am deeply indebted to O. Elicki (Technische Universita¨t Bergakademie Freiberg, Germany). Special thanks to A. Obst for SEM photography and I. Kogan (both Technische Universita¨t Bergakademie Freiberg, Germany) for his help with some Russian literature. For linguistic help I thank A. J. Kaufman (University of Maryland, U.S.A). Critical reviews by E. Landing (New York State Museum, Albany, U.S.A) and C. B. Skovsted (University of Uppsala, Sweden) significantly improved this paper. This work is part of the project EL 144/12 ‘‘The microfauna of the Early to Middle Cambrian La´ncara Formation (Cantabrian Mountains, NW Spain): micropalaeontology, palaeobiogeography, and process dynamics at the Early–Middle Cambrian transition of western Gondwana,’’ generously supported by the German Research Foundation. REFERENCES
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← FIGURE 4—1–18, Cambroclavus absonus Conway Morris in Bengtson et al., 1990, all scale-bars 100 m; 1–3, lower view; 1, FG 544/BL/mic/cam/q-2 (1); 2, FG 544/Ge/mic/cam/10E (1); 3, FG 544/BL/mic/cam/q-2 (2); 4–18, upper view; 4, FG 544/Ge/mic/cam/10E (2); 5, FG 544/Ge/mic/cam/10E (3); 6, FG 544/BL/mic/cam/q-2 (3); 7, FG 544/BL/mic/cam/q-2 (4); 8–9, FG 544/Ge/mic/cam/10E (4); 10–11, FG 544/Ge/mic/cam/10E (5); 12–13, FG 544/Ge/mic/cam/ 10E (6); 14–15, FG 544/Ge/mic/cam/10E (7); 16–17, FG 544/BL/mic/cam/q-2 (5); 18, FG 544/Ge/mic/cam/10E (8); 19–20, Ctenocephalus (Ctenocephalus) coussesensis Thoral, 1946, scale bars 2.5 mm; 19, FG 544/LL/mac/tril/12c T6, dorsal view of cephalon; 20, FG 544/LL/mac/tril/12c T12, dorsal view of cephalon.
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ACCEPTED 23 JULY 2008