New evidences of Silurian Phyllocarid Crustaceans from SW Sardinia
Transcription
New evidences of Silurian Phyllocarid Crustaceans from SW Sardinia
255 Bollettino della Società Paleontologica Italiana, 44 (3), 2005, 255-262. Modena, 30 novembre 2005 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, I41100 Modena, Italy; gnolim@unimore.it P. Serventi, Dipartimento del Museo di Paleobiologia e dell’Orto Botanico, Università di Modena e Reggio Emilia, Via Università 4, I41100 Modena, Italy; pserventi@libero.it 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 During the past three decades paleontological investigations and accurate sampling of Silurian sections, carried out by a team of the Dipartimento del Museo di Paleobiologia e dell’Orto Botanico (Università di Modena e Reggio Emilia), have supplied a rich collection of fossils including phyllocarid remains. Some of these specimens were illustrated twenty years ago by Gnoli & Serpagli (1984). However, the most important discovery on this topic is due to Hamman et al. (1990) who demonstrated that the “phyllocarids” of the so called «phyllocarid beds of Taricco (1922)» actually belong to «an unusual trilobite-like arthropod» named Tariccoia arrusensis, later regarded as a true trilobite of the Nectaspida Order (Hamman & Leone, 1997). The new phyllocarid remains, all collected from the Fluminimaggiore Formation, consist of three fragmentary specimens with abdominal segments (preserved in three dimensions), a dozen caudal parts and several fragments and/or whole specimens of secondarily phosphatized mandibles that commonly occur in the acid-resistant residues of conodont samples. Information on Lithostratigraphic units of SW Sardinian Silurian-early Devonian rocks, including the phyllocarids-bearing beds, and biostratigraphy of the key-section «Mason Porcus» can be found in Gnoli et al. (1988, 1990) and in Ferretti & Serpagli (1996). ISSN 0375-7633 Updated information on the lithology, palaeontology and environment of the Fluminimaggiore and Mason Porcus Formations in the locality Perd’e Fogu near Fluminimaggiore and Argiola can be found in Serpagli (1998), Ferretti et al. (1998a, b), and Corradini et al. (1998a, b). The Sardinian localities where phyllocarid remains were found are summarised in Fig. 1. The Tab. 1 shows the phyllocarid remains recovered in Silurian sections or displaced blocks, with the age (biozones) and the lithology related to the reported samples. SOME REMARKS ON PHYLLOCARID PALEOECOLOGY AND BIOSTRATIGRAPHY The lithology of displaced blocks bearing phyllocarid remains, deduced from investigations on sedimentary structures and associated fauna (Ferretti, 1989) indicates a shallow sea shelf normally oxygenated in its upper parts and anoxic towards the bottom (Gnoli et al., 1980). This shelf, reworked by wave motion, developed in a shallow basin, stirred by currents in its upper part (Fluminimaggiore and Mason Porcus Formations (Gnoli et al., 1990)), where, during Pøídolí-early Devonian time, pelagic-type sediments were formed (Gnoli, 1985). Apparently, Silurian phyllocarids do not characterize a specific environment, being present either in the more shallow marine facies (poorly washed biomicrite) or in 256 Bollettino della Società Paleontologica Italiana, 44 (3), 2005 Fig. 1 - Location of the Sardinian fossiliferous localities in which phyllocarid remains have been collected. the relatively deeper ones. Telson parts - sometimes with parallel orientation probably stirred up by current - are the most common phyllocarid fragments. On the contrary, abdominal somites are rare with only three specimens recovered so far. This environmental setting 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 Sardinia, has been mainly deduced by means of the associated conodonts (Tab. 1) found in the acidresistant residues of the blocks and/or sequence samples by several authors (Serpagli, 1971; Serpagli & Mastandrea, 1980; Olivieri & Serpagli, 1990; Olivieri et al., 1981; Mastandrea, 1985; Gnoli et al., 1988; Ferretti et al., 1998; Corradini et al., 1998; Corradini & Serpagli, 1998, 1999; Serpagli & Corradini, 1999). Also 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). M. Gnoli, P. Serventi - Silurian Phyllocarids from SW Sardinia used. The Sardinian conodont biozones are those defined by Corradini & Serpagli, (1998, 1999). SYSTEMATIC DESCRIPTIONS All the material studied is housed in Paleontological Collection of the Dipartimento del Museo di Paleobiologia e dell’Orto Botanico under the Cat. nos. IPUM 19801, 19836, 24236-24246. Order PHYLLOCARIDA Packard, 1879 Suborder CERATIOCARINA Clarke, 1900 Family CERATIOCARIDIDAE Salter, 1860 Genus Ceratiocaris Mc Coy, 1849 Type-species - Ceratiocaris solenoides Mc Coy, 1849; Miller, 1889 by subsequent designation. Subgenus Ceratiocaris (Bohemicaris) Chlupáè, 1994 Type-species - Ceratiocaris bohemica Barrande, 1872, by original designation, Chlupáè, 1994, p. 14. Ceratiocaris (Bohemicaris) bohemica (Barrande, 1872) (Figs. 2a-i) 1984 Ceratiocaris cf. bohemica Barrande, 1872 - G NOLI & SERPAGLI, p. 258, figs. 1, 15. 1994 Ceratiocaris (Bohemicaris) bohemica CHLUPÁ È , pp. 5, 14; Pl. 1, figs. 1-6. 1994 Ceratiocaris (Bohemicaris) bohemica Chlupáè RACHEBOEUF, pp. 289-291, text-figs. 3B, 5. Material - Two specimens with fifth, sixth and seventh abdominal somites with poorly preserved caudal part (IPUM 24236 and unfigured 24246) plus various fragments of telsonal part with furcal rami (IPUM 24242, 24243, and unfigured 24244). Description - All the abdominal somites preserved were compacted; furthermore the fifth one preserves only its terminal part, but unlike the sixth and seventh ones that are apparently smooth, it bears an ornamentation consisting of longitudinal ribs that run roughly parallel to the axis of the segment and are distally anatomised (see Barrande, 1872, pl. 19, fig. 2 and present paper, Fig. 2a). In a small area of this ornamentation it is possible to distinguish very thin, small (0.27 to 0.12 mm in length) very characteristic, slit-like or long comma-like depressions (Fig. 2b’) slightly oblique to the ribs. Their length varies from corresponding to about 1/3 to 1/2 of the distance between ribs, that is 0.3 mm. The probable function of these depressions is discussed in the following remarks. The abdominal somites are 15, 26 and 34 mm long respectively. Caudal parts are very fragmented. One specimen (IPUM 24243) consists of a fragment of telson and two fragments of the furcal rami arranged sub-parallel to each other in the sediment. The telson shows a sub-polygonal cross section in its middle part where it corresponds to 5.5 mm in lateral width; as 257 can be seen in Fig. 2g, there are 9 longitudinal ridges 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 areas. Between the dorso-lateral ridges, several subelliptic alveoli are present for bristle insertion. The alveoli diameters are about 0.8 and 0.7 mm, the distance between them is quite regular and they are placed about 2.2 mm apart. In the studied material the bristles are never preserved. The furca rami are also polygonalrounded in cross-section, depressed and bearing 10 longitudinal ridges in their proximal part, whereas distally the ridges become rounded so they probably terminate with a fairly sub-elliptic or lens-shaped crosssection. Another specimen (IPUM 24242) from the same displaced block (ARG-BK 15) bears three strongly damaged fragments of caudal parts: two of telson and a ramus of furca not preserved in anatomical position. These caudal parts are 59, 27 and 9 mm long respectively and show the same features belonging to the same species as above. A further caudal fragment, 38 mm long in the counterpart, showing the same morphology as above reported, is that from level 2b of the Mason Porcus section (IPUM 24244). Remarks - The ornamentation of the fifth abdominal somite previously described and, in particular, the presence of the very small depressions (Fig. 2b’) could represent slits of probable sensory terminations below the exoskeleton for possibly sensing environmental variations (physical and/or biological). Their actual function remains unknown. These morphological peculiarities were never previously reported with the exception of Vannier et al. (1997), who figured something similar feature (Vannier et al., 1997, fig. 9B) from the right pleural fold of the Devonian archaeostracan phyllocarid Rhinocaris columbina Clarke in Hall & Clarke 1888. Ceratiocaris? (Bohemicaris) sp. ind. cf. bohemica (Barrande, 1872) (Figs. 3a-b) Material - Sixth, seventh abdominal segment exoskeletal parts and telson head (IPUM 24241). Abdominal somites are shifted over each other rather than squeezed and probably not anatomically jointed to a telson head for a total length of 53 mm in a little displaced block (probably coming from level 5 of the Mason Porcus section). Description - The specimen IPUM 24241 shows the sixth and seventh abdominal somites displaced from the anatomical position probably due to compaction of the muddy sediment. However, these preserve a peculiar ornamentation consisting of sub-parallel ribs oblique to the axis of the somites (about 45 degrees). This can be identified as the proximal part of the last abdominal somite where the ribs form a concentric botroidal pattern of ornamentation as shown in Fig. 3b. The telson head, which is the largest one among all 258 Bollettino della Società Paleontologica Italiana, 44 (3), 2005 specimens so far found in Sardinia (12.5 mm in width), bears as ornamentation fine oblique ridges disappearing posteriorly. Remarks - This poorly preserved material does not allow more detailed taxonomic studies. This sample represents the third abdominal segment exoskeletal part ever recovered from Sardinia. Fig. 2 - Ceratiocaris (Bohemicaris) bohemica (Barrande, 1872). a) Upper view of the whole specimen (No. 24236). Scale bar = 20 mm; b) fifth segment showing sub-parallel wrinkled pattern of ornamentation of the same specimen. Scale bar = 10 mm; b’) particular enlarged to show the very small slit-like structures between the ribs forming ornamentation of the same specimen. Scale bar = 5 mm (see text for interpretation); c) upper view of the sixth abdominal somite of the same specimen. Scale bar = 5 mm; d) upper view of the seventh abdominal somite of the same specimen. Scale bar = 20 mm; e) poorly preserved caudal part of the same specimen. Scale bar = 20 mm; f) upper view a couple of telsons (T) and a furcal ramous (Fr) of No. 24242 specimen. Scale bar = 10 mm; g, h) telson and furcal rami of another specimen (No. 24243) showing their polygonal cross-sections (pentagonal the telson, more complex and rounded the (h) furcal rami). Scale bar = 15 mm; i) the same specimen in perspective view (note the alveoli for bristle insertion). Scale bar = 5 mm; Note: the left furcal ramus cross-section of Fig. 3h is figured reflecting the right one horizontally because specimen No. 24243 is not so well preserved. M. Gnoli, P. Serventi - Silurian Phyllocarids from SW Sardinia 259 Remarks - Since the surface of the lower part of the telson, just after the telson head between the two ventro-lateral ridges, is moderately concave, (Fig. 4d), we prefer to leave this form in open nomenclature. This species is reported here for the first time from Sardinia. Genus Warneticaris Racheboeuf, 1994 Type species - Ceratiocaris cenomanense Tromelin, 1874 by subsequent designation, Racheboeuf, 1994. Warneticaris cenomanensis (Tromelin, 1874) (Figs. 5a-c) Fig. 3 - Ceratiocaris? (Bohemicaris) sp. ind. cf. bohemica (Barrande, 1872) (No. 24241). a) Lateral view of the whole specimen, scale bar = 10 mm; b) close-up of the same specimen to show ornamentation of the seventh somite, scale bar = 5 mm. Subgenus Ceratiocaris (Ceriatocaris) McCoy, 1849 1874 Ceratiocaris cenomanense T ROMELIN in Gullier & Tromelin, p. 590. 1876 Ceratiocaris cenomanensis Tromelin - T ROMELIN & LEBESCONTE, p. 651. 1886 Ceratiocaris cenomanensis Tromelin - GUILLIER, p. 54. 1935 Ceratiocaris cenomanense Tromelin - PÉNEAU, p. 551. 1984 Ceratiocaris sp., GNOLI & SERPAGLI, p. 260, figs. 2a-d. 1994 Warneticaris cenomanensis (Tromelin) - RACHEBOEUF, pp. 287-289; Pl. III, figs. 1-11; Pl. IV, figs. 1-2; text-figs. 3D, 4. Material - Left side of the last (seventh) abdominal somite and proximal part of telson (IPUM 19836) for a total length of 31 mm (already published by Gnoli & Serpagli (1984) and left in open nomenclature as Ceratiocaris sp.). Type-species - Ceriatocaris solenoides McCoy, 1849. Ceratiocaris (Ceriatocaris?) cf. cornwallisensis damesi Chlupáè, 1963 (Figs. 4a-d) 1886 Ceratiocaris damesi NOVÁK, p. 676 (nomen nudum). 1963 Ceratiocaris cornwallisensis damesi CHLUPÁè, pp. 104108; Pl. 12, figs. 9-10; Pl. 14, figs. 3-5; Pl. 15, figs. 1-4; text-figs. 3-5. Material - Telson (IPUM 24237) with 45 mm long broken tip (reaching 53 mm in the counterpart) from level 5 of the «Mason Porcus» section. Description - The well preserved telson (Figs. 4ad) shows two rows of alveoli for the insertion of bristles in the dorso-lateral ridge. Eleven symmetrical alveoli are preserved on both sides, showing an elliptical shape; they are regularly spaced 2.33 mm apart (Fig. 4c), the axes of which are 0.66 mm and 0.4 mm. In crosssection, towards the lower part after the ridges bearing alveoli there are two concave areas, followed by two ventro-lateral ridges ending with a ventral platform. The latter protects ventrally the articulation of the furca (Fig. 4b). The telson head is hemispherical in shape and bears an ornamentation consisting of fine radiating lirae. In its central part the cross-section is sub-pentagonal in outline and from each corner originates five longitudinal rounded ridges, which reduce to four towards its apical part (as reported by Chlupáè, 1963, p. 106, text-fig. 4; and Rolfe, 1963, p. 487, text-fig. 1; see also Fig. 4d in this paper). Fig. 4 - Ceratiocaris (Ceratiocaris?) cf. cornwallisensis damesi Chlupaè, 1963 (No. 24237). a) Dorsal view of the telson; b) lateral view of the telson showing the ventral recumbent protection to the articulation for the furcal rami movement; c) close-up of dorso-lateral view for bristle insertion; d) schematic proximal, distal, and terminal telson cross-sections. All scale bars = 10 mm. 260 Bollettino della Società Paleontologica Italiana, 44 (3), 2005 of this section had already been described by Gnoli et al. (1988). According to P. Racheboeuf (letter of February 25th, 2004), the specimen described by Gnoli & Serpagli (1984), «… could probably belong to a new genus because of the shape of the telson, with a sub halfrounded head in cross-section, becoming triangular posteriorly, and also because the morphology of the furca do not fit in with Ceratiocaris». Chlupáè (1985, personal comm.) suggested that the Sardinia Ceratiocaris sp. could be close to grata and perhaps classified as C. cf. grata. In addition, Racheboeuf (1994, p. 287) assigned the Bohemian Ceratiocaris grata Chlupáè, 1984, to his new genus Warneticaris suggesting that grata is the most closely allied to cenomanensis. On the basis of the aforementioned remarks and after careful restudy of the 1984 specimen (previously left in open nomenclature), we prefer to assign this form to Warneticaris cenomanensis (Tromelin, 1874). Warneticaris sp. ind. cf. cenomanensis (Tromelin, 1874) (Figs. 6a-e) Fig. 5 - Warneticaris cenomanensis (Tromelin, 1874) (No. 19836). a) Dorsal view of the whole specimen. Scale bar = 10 mm; b) enlargement of the last pleonite showing ornamentation by triangular scales and the telson head of the same specimen in dorso-lateral view with ornamentation represented by oblique fine V-shaped short striae. Scale bar = 5 mm; c) left lateral view of the telson of the same specimen showing oblique lateral ridge and the left row of alveoli for bristle insertion. Scale bar = 7.5 mm. Description - The last abdominal segment bears a characteristic ornamentation mainly consisting of triangular, apparently imbricate scales (see comparable features in Vannier et al., 1997, pp. 100-101, fig. 10) like for the Palaeozoic archaeostracan phyllocarids. The telson is triangular in cross-section and bears bilaterally rows of sub-elliptic alveoli, just above the dorso-lateral oblique ridges (about 0.2 mm wide, 0.6 mm long, and distant 2.2 mm from each other) for bristles insertion. Its ventral part shows longitudinal furrows, proximally semicircular to rounded-triangular distally. The telson head is sculptured by sub-triangular pattern of oblique short lines. Posteriorly to its head, which is semicircular in cross-section, the telson bears ventro-lateral apertures for the insertion and articulation of furcal rami which are flattened, shorter than the telson and lanceolate in shape. Unfortunately, only the left furcal ramus is preserved in the specimen studied here. Remarks - As for the abdominal segments (see introductory part) - these exoskeletal parts are rather rare, since only three have been so far recovered from the «Mason Porcus» section. The lithology and fauna Material - A triangular telson and furca, 73 mm long in cross-section (IPUM 24238), from level 5 of the Mason Porcus section. Fig. 6 - Warneticaris sp. ind. cf. cenomanensis (Tromelin, 1874) (No. 24238). Figs. a-c, scale bar = 15 mm. a) Dorsal view of the telson; a’) schematic middle telson sub-triangular cross sections; b) the same telson in lateral view; c) the same in ventral view; d) furcal rami of the same telson in upper view. Scale bar = 25 mm; e) close up of the left ventral ridge counterpart showing the row of very little alveoli close to each other for fine bristle insertion. Scale bar = 25 mm. M. Gnoli, P. Serventi - Silurian Phyllocarids from SW Sardinia Description - The 62 mm long telson (with incomplete tip), reaching 73 mm in the counterpart and 13 mm in width, shows the typical features of W. cenomanensis. However, in lateral view the telson is not straight, but gently curved upwards distally (Fig. 6b). Size and ornament are also completely different in the two forms. The dorso-lateral ridges are not clearly preserved as well as the ventral platform recumbent posteriorly to protect the furcal rami articulation. The alveoli for bristle insertion are clearly visible only in the counterpart of one of the two ventral ridges that run symmetrically along the whole ventral part of the telson The alveoli are very small (about 0.15 mm of diameter), close to each other and circular in shape (Fig. 6e). Remarks - The large size of the Sardinian telson implies that it probably belonged to either an adult or a gerontic specimen. This form is compared in its general shape to W. cenomanensis, but lateral and ventral ridges are sharp and not rounded as it appears in the reconstruction of Racheboeuf (1994, fig. 4B). Furthermore, the incomplete preservation of the material studied also lead us to leave this form in open nomenclature. ACKNOWLEDGEMENTS Thanks are due to the late Dr. Ivo Chlupá è (Charles University, Prague, Czech Republic), for useful discussion and suggestions on the first proofs of the paper. This work benefits also by from the expertise of Prof. Patrick R. Racheboeuf (Université de la Bretagne occidentale, Brest, France) and his profitable experience on the topic. Many thanks to Prof. E. Serpagli and Prof. C. Corradini for their help on updated conodont biostratigraphy. We thank also Prof. J. 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Manuscript received 18 March 2005 Revised manuscript accepted 10 November 2005