New Evidences of Silurian Phyllocarid Crustaceans from SW Sardinia
Total Page：16
File Type：pdf, Size：1020Kb
Bollettino della Società Paleontologica Italiana, 44 (3), 2005, 255-262. Modena, 30 novembre 2005255 New evidences of Silurian Phyllocarid Crustaceans from SW Sardinia Maurizio GNOLI & Paolo SERVENTI M. Gnoli, Dipartimento del Museo di Paleobiologia e dell’Orto Botanico, Università di Modena e Reggio Emilia, Via Università 4, I- 41100 Modena, Italy; [email protected] P. Serventi, Dipartimento del Museo di Paleobiologia e dell’Orto Botanico, Università di Modena e Reggio Emilia, Via Università 4, I- 41100 Modena, Italy; [email protected] KEY-WORDS - Crustacea, Phyllocarida, Silurian, Abdominal somites, Telson, Mandibles, SW Sardinia, Italy. ABSTRACT - Phyllocarid remains consisting of abdominal somites, caudal parts and secondarily phosphatized mandibles, from Silurian of SW Sardinia are described and illustrated. Some material described and left in open nomenclature by Gnoli & Serpagli (1984) is also reconsidered under Warneticaris cenomanensis (Tromelin, 1874). Other taxa like Ceratiocaris (Bohemicaris) bohemica (Barrande, 1872), C.? (B.) sp. ind. cf. bohemica Barrande, 1872, C. (C.?) cf. cornwallisensis damesi Chlupáè, 1963, and Warneticaris sp. ind. cf. W. cenomanensis (Tromelin, 1874) are also documented. RIASSUNTO - [Nuovi resti di fillocaridi (Crustacea, Artropoda) nel Siluriano della Sardegna sudoccidentale] - Dopo la prima descrizione e illustrazione di resti di fillocaridi provenienti dalla Sardegna sudoccidentale al limite Siluriano/Devoniano, avvenuta nella prima metà degli anni ottanta, ne viene presentata una ulteriore. Tutti gli esemplari esaminati provengono dalla Formazione di Fluminimaggiore e mostrano un eccellente stato di conservazione in quanto si presentano in tre dimensioni. Sulla base dei dati sedimentologici è possibile dedurre un ambiente deposizionale di mare poco profondo, normalmente ossigenato e sottoposto a moto ondoso nelle sue parti più elevate mentre era anossico nelle zone più profonde. Dallo studio dei nuovi campioni, costituiti da segmenti addominali, parti caudali ed altre mandibole fosfatizzate, vengono documentati i seguenti taxa, Ceratiocaris (Bohemicaris) bohemica (Barrande, 1872), C.? (B.) sp. ind. cf. bohemica Barrande, 1872, C. (C.?) cf. cornwallisensis damesi Chlupáè, 1963, e Warneticaris sp. ind. cf. W. cenomanensis (Tromelin, 1874). A questi si aggiunge il materiale già descritto e lasciato in nomenclatura aperta da Gnoli & Serpagli (1984), qui considerato appartenere a Warneticaris cenomanensis (Tromelin, 1874). INTRODUCTION Updated information on the lithology, palaeontology and environment of the Fluminimaggiore and Mason Porcus During the past three decades paleontological Formations in the locality Perd’e Fogu near investigations and accurate sampling of Silurian Fluminimaggiore and Argiola can be found in Serpagli sections, carried out by a team of the Dipartimento del (1998), Ferretti et al. (1998a, b), and Corradini et al. Museo di Paleobiologia e dell’Orto Botanico (Università (1998a, b). di Modena e Reggio Emilia), have supplied a rich The Sardinian localities where phyllocarid remains collection of fossils including phyllocarid remains. Some were found are summarised in Fig. 1. The Tab. 1 shows of these specimens were illustrated twenty years ago the phyllocarid remains recovered in Silurian sections by Gnoli & Serpagli (1984). However, the most or displaced blocks, with the age (biozones) and the important discovery on this topic is due to Hamman et lithology related to the reported samples. al. (1990) who demonstrated that the “phyllocarids” of the so called «phyllocarid beds of Taricco (1922)» actually belong to «an unusual trilobite-like arthropod» SOME REMARKS ON PHYLLOCARID named Tariccoia arrusensis, later regarded as a true PALEOECOLOGY AND BIOSTRATIGRAPHY trilobite of the Nectaspida Order (Hamman & Leone, 1997). The lithology of displaced blocks bearing phyllocarid The new phyllocarid remains, all collected from the remains, deduced from investigations on sedimentary Fluminimaggiore Formation, consist of three structures and associated fauna (Ferretti, 1989) fragmentary specimens with abdominal segments indicates a shallow sea shelf normally oxygenated in (preserved in three dimensions), a dozen caudal parts its upper parts and anoxic towards the bottom (Gnoli and several fragments and/or whole specimens of et al., 1980). secondarily phosphatized mandibles that commonly This shelf, reworked by wave motion, developed in occur in the acid-resistant residues of conodont a shallow basin, stirred by currents in its upper part samples. (Fluminimaggiore and Mason Porcus Formations (Gnoli Information on Lithostratigraphic units of SW et al., 1990)), where, during Pøídolí-early Devonian Sardinian Silurian-early Devonian rocks, including the time, pelagic-type sediments were formed (Gnoli, 1985). phyllocarids-bearing beds, and biostratigraphy of the Apparently, Silurian phyllocarids do not characterize key-section «Mason Porcus» can be found in Gnoli et a specific environment, being present either in the more al. (1988, 1990) and in Ferretti & Serpagli (1996). shallow marine facies (poorly washed biomicrite) or in ISSN 0375-7633 N8 255 09/01/06 15 55 256 Bollettino della Società Paleontologica Italiana, 44 (3), 2005 added to the good preservation of the majority of the fossils would suggest short and slight transport on a typically muddy bottom. The paleoecological analysis of the associated fauna does not help much in clarifing the paleoenvironmental setting because it is mainly represented by orthoconic nautiloids occurring only in some horizons of scattered sequences (i.e. «Argiola» ARG-BK 15). Phyllocarid life habits have been described in details (e.g. Chlupáè, 1994; Vannier et al., 1997) taking into account their adaptation to dysaerobic bottom conditions. The use of phyllocarids as stratigraphic tools is due to Chlupáè (1994) who carried out a revision and a general synthesis on the biostratigraphy of Bohemian phyllocarid. The age of phyllocarid remains in SW Fig. 1 - Location of the Sardinian fossiliferous localities in which Sardinia, has been mainly deduced by means of the phyllocarid remains have been collected. associated conodonts (Tab. 1) found in the acid- resistant residues of the blocks and/or sequence samples by several authors (Serpagli, 1971; Serpagli & the relatively deeper ones. Telson parts - sometimes Mastandrea, 1980; Olivieri & Serpagli, 1990; Olivieri with parallel orientation probably stirred up by current et al., 1981; Mastandrea, 1985; Gnoli et al., 1988; - are the most common phyllocarid fragments. On the Ferretti et al., 1998; Corradini et al., 1998; Corradini & contrary, abdominal somites are rare with only three Serpagli, 1998, 1999; Serpagli & Corradini, 1999). Also specimens recovered so far. This environmental setting other index fossils, like graptolites, have been sometime Tab. 1 - Distribution of taxa in the samples bearing phyllocarid remains: age (biozones) and lithology. Abbreviations: BK = displaced block; ARG = «Argiola» section, MP = «Mason Porcus» (House of pigs) section, SF = Fluminimaggiore path section, GALE = «Galemmu», PF = «Perd’e Fogu» (Fire Stone). Conodont biozonation according to Corradini & Serpagli (1998-99). * After Jaeger (pers. comm., letter of July 20th, 1987). N8 256 09/01/06 15 56 M. Gnoli, P. Serventi - Silurian Phyllocarids from SW Sardinia 257 used. The Sardinian conodont biozones are those can be seen in Fig. 2g, there are 9 longitudinal ridges defined by Corradini & Serpagli, (1998, 1999). that run along its whole length: one on the dorsal side, two orders of dorso-lateral, and two dorso-ventral between which there are slightly concave depressed SYSTEMATIC DESCRIPTIONS areas. Between the dorso-lateral ridges, several sub- elliptic alveoli are present for bristle insertion. The alveoli All the material studied is housed in Paleontological diameters are about 0.8 and 0.7 mm, the distance Collection of the Dipartimento del Museo di between them is quite regular and they are placed about Paleobiologia e dell’Orto Botanico under the Cat. nos. 2.2 mm apart. In the studied material the bristles are IPUM 19801, 19836, 24236-24246. never preserved. The furca rami are also polygonal- rounded in cross-section, depressed and bearing 10 Order PHYLLOCARIDA Packard, 1879 longitudinal ridges in their proximal part, whereas Suborder CERATIOCARINA Clarke, 1900 distally the ridges become rounded so they probably Family CERATIOCARIDIDAE Salter, 1860 terminate with a fairly sub-elliptic or lens-shaped cross- Genus Ceratiocaris Mc Coy, 1849 section. Another specimen (IPUM 24242) from the same displaced block (ARG-BK 15) bears three strongly Type-species - Ceratiocaris solenoides Mc Coy, damaged fragments of caudal parts: two of telson and 1849; Miller, 1889 by subsequent designation. a ramus of furca not preserved in anatomical position. These caudal parts are 59, 27 and 9 mm long Subgenus Ceratiocaris (Bohemicaris) Chlupáè, 1994 respectively and show the same features belonging to the same species as above. A further caudal fragment, Type-species - Ceratiocaris bohemica Barrande, 38 mm long in the counterpart, showing the same 1872, by original designation, Chlupáè, 1994, p. 14. morphology as above reported, is that from level 2b of the Mason Porcus section (IPUM 24244). Ceratiocaris (Bohemicaris) bohemica (Barrande, 1872) Remarks - The ornamentation of the fifth abdominal (Figs. 2a-i) somite previously described and, in particular, the presence of the very small depressions (Fig. 2b’) could 1984 Ceratiocaris