Northern Gondwanan Siluro-Devonian Palaeogeography Assessed by Cephalopods
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Palaeontologia Electronica http://palaeo-electronica.org NORTHERN GONDWANAN SILURO-DEVONIAN PALAEOGEOGRAPHY ASSESSED BY CEPHALOPODS Maurizio Gnoli Department of the Museum of Paleobiology and Botanical Garden (Palaeontology), Modena and Reggio Emilia University, Italy, e-mail: [email protected] ABSTRACT Worldwide distribution of cephalopod limestone biofacies distribution is used herein to assess the reconstruction of the North Gondwana margin, with implications for southern hemisphere palaeobiogeography during the Silurian-Early Devonian. Three areas are of concern: the Tinduf Basin (Northwest Sahara, Morocco), the Uppony Mountains (Northeast Hungary), and perhaps the Ukrainian continental plat- form, which may have been part of northern Gondwana. Key words – Palaeobiogeography, cephalopod biofacies, faunal similarity Gondwana. Copyright: Palaeontological Association. 14 March 2003 Submission: 28 March 2003 Acceptance: 24 October 2002 INTRODUCTION persal patterns in the Ludford-Prídolí interval and Silurian-Devonian boundary (Richardson et al. During the Silurian-Early Devonian, cephalo- 2001), Silurian stromatoporoids (Nestor 1990), pod limestone biofacies represent one of the best Devonian stromatoporoids (Stock 1990), rugose documented sedimentary deposits in the strati- corals (Pedder and Oliver 1990), algae (Poncet graphic column. Reconstruction of the northern 1990), and Early-Middle Devonian gastropods margin of Gondwana by means of cephalopod (Blodgett et al. 1990). Nevertheless, in his discus- limestone biofacies also represents one of the sion of Silurian biogeography Boucot (1990, pp. most interesting topics of Early Palaeozoic palaeo- 191-196) noted that, “A number of potentially use- biogeography. ful groups such as the nautiloids and stony bryozo- Several authors have investigated the geo- ans remain largely unstudied.” As reported graphical position of various fossil groups during previously by Holland (1971, p. 70) “…The pre- the Silurian-Early Devonian, including: nautiloid dominant impression is of cosmopolitan faunas, cephalopods (Crick 1990), bryozoans (Tuckey but this may be because outcrops of effectively 1990), brachiopods (Cocks and McKerrow 1973; observable Silurian rocks broadly follow the sup- Boucot and Blodgett 2001), phytoplankton (Col- posed Silurian equator. Platforms were apparently bath 1990), ostracodes (Berdan 1990), higher land exceptionally large and geosynclines limited in the plants (Edwards 1990), palynology and plant dis- Silurian; the platforms comprise two "magnafa- Gnoli, Maurizio, 2003. Title. Palaeontologia Electronica 5(2):19pp, 917KB; http://palaeo-electronica.org/paleo/2002_2/gondwona/issue2_02.htm GNOLI.: GONDWANA CEPHALOPOD PALEOGEOGRAPHY Figure 1. Map showing modern locations of the localities in which nautiloid cephalopod biofacies during the Silurian- Early Devonian (see text for detailed locality descriptions). cies," characterised by (a) shelly faunas and (b) shallow shelves or platforms (see Crick 1990 and graptolite-bivalve-orthocone faunas.” (see also Kríz 1999). Cocks 2001). This latter represents the subject of Another important environmental factor is that this investigation. “The persistence in space and recurrence in time of the “Orthoceras” limestone facies are probably THE CEPHALOPOD LIMESTONE BIOFACIES IN linked to both eustatic-climatic sea-level changes THE GONDWANA NORTHERN BORDERLAND affecting the CCD, and periodic reactivation of the rifting system.” (Vai 1991, pp. 234-237). According Boucot et al. (1968) report the first citation of a to the same author, the effect of a very shallow car- micritic orthoceratid limestone in the entire circum- bonate compensation depth (CCD), reinforced by Mediterranean Silurian, later followed by Vai high non-calcareous productivity (e.g., graptolites, (1978). According to Kríz (1984), Ferretti and Kríz acritarchs, and chitinozoans), constituted a crucial (1995) and Kríz and Serpagli (1993) the cephalo- event in the Palaeozoic. “Eustatic sea-level falls pod limestone biofacies in the Prague Basin, may explain synchronous recurrence of wide- southwestern Sardinia, Carnic Alps, and Montagne spread “Orthoceras”-limestone horizons, which Noire represents a useful hard substratum for the can be traced as key beds for more than 1200 kilo- settlement of the Cardiola Community Group. This meters from Mali to Algeria, as observed by the Sil- faunal assemblage is mostly composed of epibys- urian-Devonian Boundary Subcommission during sate forms. Kríz (1984, pp. 191-192) writes, “The their 1971 field-trip” (see Kríz 1984). According to accumulation of empty cephalopod shells on the Vai (1991) this is attributable to differing regimes of sea floor created good conditions for the attach- oceanic and atmospheric circulation due to a ment of organisms in this environment, with con- warmer, more uniform climate than at present. stant water flow providing a sufficient food supply. Frakes (1979, p. 107), who provides an exhaustive The origin and distribution of this biofacies was study of climates throughout geologic time, writes: closely related to surface currents carrying empty “A widespread marine transgression in Early Sil- floating shells of cephalopods before they were urian reflects the melting of polar ice in Africa and deposited on the relatively shallow sea floor.” Many South America, and several features combine to bivalve community groups are linked to nautiloid indicate generally warm and possibly dry climates cephalopod tapho-communities that represent the during the Silurian and Devonian periods. Carbon- remnant of living cephalopods inhabiting relatively ates in general and reefs in particular seem to be more abundant than in the earlier Palaeozoic.” 2 GNOLI.: GONDWANA CEPHALOPOD PALEOGEOGRAPHY Frakes continues (p. 106) “In summary, the pale- TAXONOMIC NOMENCLATURE olatitudinal distributions of several climatic indica- Because the nautiloid taxa reported here from tors fall generally within the latitudinal ranges for various localities have been referred to in publica- their modern counterparts. This, while establishing tions by authors from the nineteenth century the credibility of the paleomagnetic reconstruction onwards, a revised taxonomic nomenclature was for the Silurian and Devonian, also suggests that employed whenever possible. For example, early Paleozoic temperature gradients and humid- according to Serpagli and Gnoli (1977) Ortho- ity patterns did not differ greatly from those of ceras bohemicum Barrande, 1866 is considered today. There is some faunal indication of warming the senior synonym of Orthoceras fluminese in the early Silurian and gentle cooling until the Meneghini, 1857. Likewise, Serpagli and Gnoli middle Devonian, followed by a warm late Devo- (1977) referred to Meneghini’s species as Ortho- nian. Overall, aridity seems to have typified Silurian cycloceras? fluminese (Meneghini 1857). Thus, and Devonian climates, except for the late Silurian any occurrence of Orthoceras bohemicum Bar- and early late Devonian.” rande, 1866 is here reported as Orthocycloceras? Before entering into any palaeobiogeographic fluminese (Meneghini 1857). Similarly, in its origi- and palaeoecological speculations based on nauti- nal binomen Orthoceratites subannularis Mün- loid cephalopod assemblages, Crick’s (1990, p. ster, 1840 was first changed to Orthoceras 147) comment on nautiloid paleoecology is worth subannulare by Barrande (1866) and remained so noting; “The distribution of fossil nautiloids illus- for the following 112 years, before being assigned trated here (Late Cambrian to end of Devonian), to Metaspyroceras Foerste by Zhuravleva (1978). and those of Flower (1976) and Crick (1980), indi- Accordingly, all occurrences of this species are cates that these nautiloids were not truly part of the herein reported as Metaspyroceras subannulare nekton capable of oceanic dispersal, but were (Münster 1840). members of the shallow-shelf vagrant benthos and were thus capable of dispersal only along continu- NEW DATA ON CEPHALOPOD LIMESTONE ous shelves or over shallow stretches of open BIOFACIES FROM NORTHEAST HUNGARY ocean. For these reasons, simple distance and the depth of water separating shallow shelf seas were Kovács (1989) reported olistoliths from the capable of restricting the dispersal of nautiloid Strázsahegyi Formation in the Uppony Mountains cephalopods until such time as the physical envi- (northeastern Hungary) bearing nautiloid orthoc- ronment removed these barriers.” According to ones in profusion («orthoceratid lumachella») Crick, taking into account various magnitude within the Ludlow Series (Gorstian) belonging to events controlling in general the biogeography of the Ozarkodina s. sagitta - Ancoradella ploeck- nautiloid cephalopods and in particular those of the ensis conodont biozones. Gnoli (in Gnoli and first magnitude such as geotectonic, it is possible Kovács, 1992) described and illustrated this to separate nautiloid biogeography into four epi- faunule and, despite the poor preservation of the sodes: (1) Late Cambrian; (2) Ordovician; (3) Sil- inner features in the available material due to slight urian through Early Devonian; and (4) Middle metamorphism, recognized Michelinoceras mich- through Late Devonian. The third episode is the elini (Barrande 1866), Mimogeisonoceras? cf. concern of this article. liberum (Barrande 1870), Kopaninoceras sp. Holland et al. (1994) documented the com- sensu Gnoli in Gnoli and Kovács, 1992, Kionoc- monly high concentration of specimens, character- eras? cf.