Upper Tremadoc–Lower Arenig? Anisograptid–Dichograptid fauna from the Cabitza Formation (Lower Ordovician, SW Sardinia, Italy)

Revue de micropaléontologie 51 (2008) 167–181

Original article

Upper Tremadoc–Lower Arenig? Anisograptid–Dichograptid fauna from the Cabitza Formation (Lower Ordovician, SW Sardinia, Italy) Faune a` Anisograptid´es–Dichograptid´es du Tr´emadocien sup´erieur–Ar´enigien inf´erieur ? de la Formation de Cabitza (Ordovicien inf´erieur, SW Sardaigne, Italie) Gian Luigi Pillola a,∗ , Sergio Piras a , Enrico Serpagli b b

a Dipartimento di Scienze della Terra, Universit` a di Cagliari, Via Trentino 51, 09127 Cagliari, Italy Dipartimento del Museo di Paleobiologia e dell’Orto Botanico, Universit`a di Modena e Reggio Emilia, Via Universit`a 4, 41100 Modena, Italy

Abstract The discovery of the anisograptid graptolite Araneograptus murrayi (Hall, 1865), the dichograptids Clonograptus (Clonograptus) cf. rigidus (Hall, 1858) and Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868) and of two unassigned species of Didymograptus in the Late Tremadoc–Early Arenig? sediments of the Fluminese area (SW Sardinia) adds new data on the biostratigraphy and palaeoecological setting of the Cabitza Formation, as well as on the paleobiogeographical distribution of these taxa. © 2007 Elsevier Masson SAS. All rights reserved. R´esum´e La d´ecouverte du graptolite Anisograptid´e Araneograptus murrayi (Hall, 1865), des Dichograptid´es Clonograptus (Clonograptus) cf. rigidus (Hall, 1858) et Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868) et de deux esp`eces ind´etermin´ees de Didymograptus dans les s´ediments du Tr´emadocien sup´erieur–Ar´enigien inf´erieur? du Fluminese (SW Sardaigne) fournit des nouvelles donn´ees sur le contexte pal´eo´ecologique et sur la biostratigraphie de la partie sup´erieure de la formation de Cabitza, ainsi que sur la distribution pal´eobiog´eographique de ces taxa. © 2007 Elsevier Masson SAS. All rights reserved. Keywords: Graptolites; Caryocarids; Early Ordovician; Cabitza Formation; Sardic phase; Sardinia Mots cl´es : Graptolites ; Caryocarid´es ; Ordovicien inf´erieur ; Formation de Cabitza ; Phase Sarde ; Sardaigne

1. Introduction Graptolites have been known in Lower Ordovician sediments of southwestern Sardinia since the first half of the 20th century. Novarese (1920) reported the occurrence of Dictyonema sp. on a loose rock specimen found, five years earlier, at Monte Cani (near Gonnesa). In the same year, Taricco (1920) described this material, consisting of several incomplete colonies and, despite their fragmentation, assigned them to “Dictyonema flabelliforme Eichwald, 1840”. The Dictyonema-bearing levels were regarded

∗

Corresponding author. E-mail address: [email protected] (G.L. Pillola).

0035-1598/$ – see front matter © 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.revmic.2007.08.002

by Taricco (in Novarese and Taricco, 1922: p. 320) as belonging to the Acadian (sic.) “Gruppo degli Scisti” (Cabitza Formation) and he ingenuously extended the range of “D. flabelliforme” into the Middle Cambrian. Subsequently, the presence of “Dictyonema” in Sardinia was usually rejected, disregarded or questioned (Lecompte, 1949) and the upper part of the Cabitza Formation continued to be considered Middle Cambrian in age (Cocozza, 1967, 1980; Rasetti, 1972). More recent research carried out in the Gonnesa region (Monte Cani) led to the discovery of the Rhabdinopora flabelliformis (Eichwald, 1840) bearing layers (Gandin and Pillola, 1985; Pillola, 1987; Barca et al., 1987). These in situ specimens were tentatively assigned to the morphotype “graptolithinum”

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or “forma typica” (Pillola and Gutierrez Marco, 1988: p. 559), which, in turn, was included in the subspecies R. f. flabelliformis (Cooper et al., 1998). Additional poorly preserved specimens were also collected at Case Lai–Case Massidda outcrops, close to the Cabitza Fm. type section (Loi et al., 1995, 1996) and new, well-preserved rhabdosomes were recovered in the Monte Cani area. The occurrence of acritarchs from similar levels close to Monte Lisau, SSE of Gonnesa, corroborated the Tremadocian age of the upper part of the Cabitza Formation (Barca et al., 1987); later on, the recovery of Upper Cambrian faunas, including trilobites, hyolithids, echinoderms, brachiopods (Loi et al., 1995, 1996; Pillola et al., 2002a and references therein) and conodonts (Serpagli et al., 1998), suggested the possibility of “continuous” sedimentation ranging from the Middle Cambrian up to the Early Tremadocian, including evidence of the Acerocare regressive event across the Cambrian/Ordovician boundary (Loi et al., 1995, 1996). A stratigraphic sketch of the Cabitza Fm. type section (close to Iglesias), with the position of the Rhabdinopora beds, was provided by Loi et al. (1995, 1996); further details on biostratigraphic framework have been provided by Pillola et al. (2002b and cited references). Such discoveries have also had important tectonic implications because the so-called “Sardic phase” of Stille (1939) was definitely considered as intra-Ordovician, located between early Tremadocian and Caradocian (Barca et al., 1987). In the present paper, we document the first occurrence of an Upper Tremadoc-basal Arenig? graptolitic assemblage including the Anisograptid Araneograptus murrayi (Hall, 1865) and the Dichograptids Clonograptus and Didymograptus, associated with caryocarids, in the Early Ordovician sediments of SW Sardinia. The aims of the study are to use the new graptolite data to revise and update the age of the upper part of the Cabitza Formation and to evaluate them from the biostratigraphic, palaeogeographic and palaecological point of view. 2. Geological setting, location, palaeontological content and preservation 2.1. Geological setting and location The Cabitza Formation occurs in a wide area in the Fluminese subregion, from the Gutturu Pala to the Monte Argentu–Piscina Morta area (Fig. 1). In the latter locality, the northern limb of the easternmost portion of the “Cabitzan anticline” is mostly unconformably flanked by the Upper Ordovician deposits of the Monte Argentu Fm., which are in turn overlain by the clastic rocks of the Caradocian and Ashgillian Monte Orri and Portixeddu Formations and younger deposits (Hammann and Leone, 1997). The deposits of the Cabitza Formation observed along the old road cut connecting the Riu Mannu valley to Buggerru show severe tectonic disturbances and, except for minor portions, appears rather monotonous. In spite of this, the four main lithofacies recognized in the uppermost Upper Cambrian and Tremadoc in the type area of the Cabitza Fm. (Loi et al., 1995, 1996) are

observed thorough the section. Locally, the widespread greygreen siltstones and shales “graded rhythmites” (Facies 4) and the rare hummocky cross stratification sandstones (Facies 3) (Plate 1, Figs. 1 and 2) are replaced, towards the south, by laminated shales and fine sandstones (Facies 2) and by more sandy siltstones with ripple-drift cross-lamination (Facies 1). Reoccurrences of rare sandstone beds, with HCS and planar to weakly inclined laminations and of Facies 4, have been observed; in addition, Facies 1 and 2 with red beds occur once along the transect, very close to the Piscina Morta pond. Thus, despite the structural complexity of the transect and the lack of the classical Middle Cambrian deposits, the facies succession of this portion of the Cabitza Fm. can be readily compared with many Upper Cambrian to Tremadocian successions of southwestern Sardinia. The fossiliferous beds crop out along a road cut, 1.25 km south of the junction with the Fluminimaggiore–Portixeddu road, not far from the locality “Piscina Morta” (Fig. 1). These are mainly composed of grey-green micaceous siltstones and shales “graded rhythmites” (above-mentioned Facies 4), with occasional thin sandy layers and rare, but more conspicuous, 5–30 cm-thick HCS sandstones beds (Plate 1, Fig. 1), suggesting an outer shelf sedimentary environment, not far from the storm wave-base. 2.2. Palaeontological content and preservation The grey-green siltstones of the Cabitza Formation have recently yielded several incomplete large rhabdosomes and immature colonies of a robust anisograptid graptolite assigned to A. murrayi (Hall, 1865), as well as a large incomplete specimen of Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868), large portion of Clonograptus (Clonograptus) cf. rigidus (Hall, 1858) and several didymograptid colonies. Caryocarid (Caryocaris cf. wrighti Salter, 1863) remains (Plate 1, Figs. 3 and 4) and few ichnofossils, which occur at different levels along the section, are being described in a separate paper. Rhabdosomes are slightly flattened and tectonically deformed and are of Fe-oxides and/or black carbon; rare distal twisted portions of stipes are preserved as a faint carbon film. Caryocarids occur as flattened, wrinkled carapaces and are mostly preserved as very thin grey shiny imprints on the bedding plane surfaces; occasionally, less deformed specimens show blackish carapaces (Plate 1, Fig. 3). Preparation of the specimens involved mechanical removal of overlying matrix, by means of a vibro-tool, gentle chemical dissolution with hydrofluoric acid and final impregnation with Paraloid resin. Photographs were taken under alcohol or water and drawings were made via camera lucida. 3. Systematic palaeontology All studied specimens are housed in the Department of Earth Sciences, Cagliari University, Italy and are labelled with the acronym: DSTC. For abbreviations, we use the 2TRD (two thecae repeat distance).

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Fig. 1. Geological sketch map of the Fluminimaggiore–Piscina Morta area. Fossiliferous locality described in this paper is indicated by asterisk (after Hammann and Leone, 1997, simplified). Fig. 1. Carte g´eologique sch´ematique de la r´egion de Fluminimaggiore–Piscina Morta. La localit´e fossilif`ere d´ecrite dans ce travail est indiqu´ee par un ast´erisque (d’apr`es Hammann et Leone, 1997, simplifi´ee). 169

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Plate 1. Fig. 1. Fossiliferous outcrop along the Piscina Morta (Fluminimaggiore) road cut section. The graptolite and caryocaridids level is located about 3.5 m towards the right from the sandstone bed (HCS) indicated by arrow. Fig. 2. Oblique view of the sample figured on Plate 2, Fig. 1, showing the most common features of the fossiliferous beds represented by graded rhythmites. Figs. 3 and 4. Caryocaris cf. wrighti. Late Tremadoc/? Earliest Arenig, Cabitza Fm., Piscina Morta (Fluminimaggiore) SW Sardinia. 3, DSTCP 23180 lateral view. 4, strongly compressed, cleaved and wrinkled randomly disposed carapaces, sample DSTCP 23181.

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Fig. 2. (A) Proximal part of A. murrayi (Hall, 1865), cleavage-trace is indicated by parallel black lines, sample DSTCP 23176; (B) detail of the apical part of A. murrayi, slab DSTCP 23177. Fig. 2. (A) Partie proximale de A. murrayi (Hall, 1865), la trace du clivage est indiqu´ee par les deux lignes parall`eles, e´ chantillon DSTCP 23176 ; (B) d´etail de la partie apicale de A. murrayi, e´ chantillon DSTCP 23177.

Order GRAPTOLOIDEA Lapworth, 1875 Family ANISOGRAPTIDAE Bulman, 1950 Genus Araneograptus Erdtmann and Vandenberg, 1985 Araneograptus murrayi (Hall, 1865) (Fig. 2A and B; Plate 2, Figs. 1–4 and Plate 3, Fig. 9) 1865. Dictyonema Murrayi Hall, p. 138, Pl. 20, Figs. 6 and 7. 1987. A. murrayi (J. Hall, 1865) – Guti´errez-Marco and Ace˜nolaza, p. 225, Pl. 1, Figs. 1–17 (with earlier synonymies). 1991. A. murrayi (J. Hall, 1865) – Lindholm, p. 294, Figs. 6 and 7,? 18c (with further synonymies). 1994.? Araneograptus cf. murrayi (J. Hall) – Heuse et al., p. 64, Pl. 9, Fig. 1. 1994. A. murrayi (J. Hall, 1865) – Ortega and Su´arez Soruco, 1994 p. 230, Text-Fig. 3i, Pl. 2, Figs. 1 and 6. 2001. A. murrayi (J. Hall, 1865) – Maletz and Egenhoff, p. 58, Fig. 8.10. v 2004. R. flabelliformis norvegica (Kjerulf, 1865) – Piras et al., 2004 p. 46.

Material: Three large incomplete rhabdosomes DSTCP 231161/62 and 23179, 10 fragments DSTCP 23163–23170, 23178 and two early stages of juvenile colonies provisionally assigned to this species DSTCP 23176/77; all specimens originate from Piscina Morta, Fluminimaggiore. Diagnosis: [after Guti´errez Marco and Ace˜nolaza (1987) and Lindholm (1991)] – Large conical pendent rhabdosome, with an angle of 60–75◦ when flattened, decreasing distally in big colonies. The rhabdosome may reach at least 30 cm in length. The shape of the meshes is variable from rectangular to oval. The stipes are generically 4–5 in 10 mm and the dissepiments are 3–4 in 10 mm; however, their density can be well outside these ranges (cf. Lindholm, 1991). Lateral width of the stipes range from 0.5 to 1.25 mm. The thickness of the dissepiments is variable. The sicula is 1.8–2.5 mm long, bearing a short nema. Cortical overgrowth is frequent in the proximal portion of the rhabdosome. Description: The largest rhabdosome (Plate 2, Figs. 1a–c), although incomplete, suggests that the total length exceeded at least 15 cm, confirming the usual large size of this anisograptid.

Figs. 5 and 6. R. f. flabelliformis (Eichwald, 1840), Monte Cani, Cabitza Fm., Gonnesa, Early Tremadoc. 5, Rhabdosomes at different astogenetic stages, sample DSTCP 21004a. 6, exceptionally well-preserved proximal portion of the rhabdosome DSTCP 23487a; note the full relief of the thecae and the occurrence of a robust nema. Planche 1 Fig. 1. Gisement fossilif`ere sur le talus de la route pour Piscina Morta (Fluminimaggiore). Le niveau fossilif`ere a` graptolites et caryocaridid´es est situ´e a` environs 3,5 m a` droite du banc de gr`es (HCS) indiqu´e par la fl`eche. Fig. 2. Coupe oblique de l’´echantillon figur´e sur la Planche 2, Fig. 1, montrant l’aspect le plus commun des niveaux fossilif`eres, repr´esent´es par les rythmites granoclass´es. Figs. 3 et 4. Caryocaris cf. wrighti Salter, 1863, Tr´emadocien sup´erieur/? Ar´enigien basal, Fm. de Cabitza, Piscina Morta (Fluminimaggiore) SW Sardaigne. 3, vue lat´erale, DSTCP 23180. 4, carapaces comprim´ees et fronc´ees, dispos´ees al´eatoirement, e´ chantillon DSTCP 23181. Figs. 5 et 6. R. f. flabelliformis (Eichwald, 1840), Monte Cani, Cabitza Fm., Gonnesa, Tr´emadocien inf´erieur. 5, rhabdosomes a` diff´erents stades astog´en´etiques, DSTCP 21004a. 6, partie proximale particuli`erement bien conserv´ee, rhabdosome DSTCP 23487a ; noter le relief des th`eques et la pr´esence d’un n´ema robuste.

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Plate 2. Figs. 1–4. A. murrayi (Hall, 1865), Late Tremadoc/? Earliest Arenig, Cabitza Fm., Piscina Morta (Fluminimaggiore), SW Sardinia. 1a, largest incomplete rhabdosome, associated with high order branches of Clonograptus (Clonograptus) cf. multiplex; DSTCP 23161b. 1b, counterpart, associated with Clonograptus (Clonograptus) cf. multiplex; DSTCP 23161a. 1c, detail of the mesh pattern, DSTCP 23161a. 2a, accumulation of rhabdosomes at different astogenetic stages and several fragments; note the juvenile stage at the lower right and the twisted stipes of the larger rhabdosome; sample DSTCP 23163a. 2b, detail showing the laterally

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The mesh pattern is characterised by a quite regular distribution of stipes and dissepiments giving rise to sub-rounded fenestrulae with length/width ratio mostly close to one. Meshwork generally possesses five stipes in 10 mm in the proximal part, four in the distal part of the rhabdosome. The dissepiments are constantly 3–4 in 10 mm. The lateral stipe width ranges from 0.5 to 1.25 mm and the thickness of the dissepiments varies between 0.5 and 1.4 mm. The thecae are 12 in 10 mm and seems to be simple straight tubes with dichograptid appearance. Sometimes, due in part to tectonic distortion, the apertural area can simulate a denticulate shape in lateral view (Plate 2, Figs. 2a and b). Bithecae or sicular bithecae have not been observed. Dissepiments that join the stipes to their dorsal side on large portions of the specimen (Plate 2, Figs. 4a and b) show the characteristic morphology that are reminiscent of the famous Gaud`ı balconies (pouch-like “balconies”, sensu Lindholm, 1991). Locally, these dissepiments appear darker in comparison with adjacent stipes which may reflect a primary structural/compositional differentiation. The oblique dissepiments are better preserved on coarse matrix, where flattening is less pronounced than in other colonies studied here. Finally, the observed siculae in the specimens assigned to juvenile forms of A. murrayi, are straight, of about 2.2 mm long, with a short thin nema (Fig. 2 and Plate 2, Fig. 3). Discussion: There is some doubt about the assignment of this taxon to Araneograptus, but derives in part from the suspected occurrence of bithecae and from the inferred bior triradiality of the rhabdosome (see Lindholm, 1991: p. 294, 295). The occurrence of three-vaned nemal structures in some colonies of A. murrayi, as observed by Ace˜nolaza et al. (1996), may support a possible synonymy between Araneograptus and Rhabdinopora. The early astogenetic stages studied here are suggestive of a primary biradiality (Fig. 2 and Plate 2, Figs. 2a and 3); however, the taphonomical effect of the mm thick accumulations as well as strong tectonic deformation, influence the preservation of the specimens. In absence of completely preserved examples, we are therefore obliged to maintain a doubt about the bi - or triradiality of this taxon. Despite this uncertainty, the Sardinian rhabdosomes do not differ significantly from those assigned to A. murrayi in the possession of short meshes and relatively broad dissepiments, such as the quadrangularis type specimen figured by Lindholm (1991: Fig. 6) from L´evis, Quebec, which represents the type area of this taxon (Hall, 1865: Pl. 20, Figs. 6 and 7). In some Scanian specimens, the density of stipes and dissepiments have a lower per unit length (Lindholm, 1991), while the fragmentary Thuringian specimen from the Schwarzburg anti-

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cline (Heuse et al., 1994), showing narrow dissepiments and robust stipes, are clearly affected by tectonical compression. Very similar measurements are shared by material from Sardinia, Huelva province of Spain and from South America (Argentina and Bolivia) (Guti´errez Marco and Ace˜nolaza, 1987: Pl. 1, Figs. 1–17; Ace˜nolaza et al., 1996: Pl. 1, Figs. 1–12; Maletz and Egenhoff, 2001: Fig. 8.10). Cortical overgrowth of the dissepiments, placed obliquely to the dorsal plane of the stipes (Gaud`ı balconies), is a noticeable feature in many specimens of A. murrayi. According to Ace˜nolaza et al. (1996), this was supposed to improve the water circulation through the rhabdosome for feeding and stabilization purposes. Distribution: A. murrayi has a worldwide distribution, including Europe (Scandinavia, Sweden, Great Britain, Germany, southern France, Spain and now Sardinia), North Africa (Morocco, Mauritania and? Algeria), eastern North America (Quebec, Canada), South America (Bolivia, Argentina) and NW China. In addition, its biogeographic distribution can be extended to Australasia and western Canada if A. pulchellus is considered a synonym of A. murrayi as suggested by Guti´errez Marco and Ace˜nolaza (1987). Age: A. murrayi range from the Late Tremadoc (A. murrayi zone and Hunnegraptus copiosus zone) to the base of the Arenig (Tetragraptus phyllograptoides/Tetragraptus approximatus zone), equivalent to the Australasian stage La2 and basal part of La3. Suborder DICHOGRAPTINA Lapworth, 1873 Family DICHOGRAPTIDAE Lapworth, 1873 Genus Clonograptus Nicholson, 1873 Subgenus Clonograptus (Clonograptus) Nicholson, 1873 Clonograptus (Clonograptus). cf. rigidus (Hall, 1858) (Fig. 3B and Plate 3, Figs. 1a and b) cf. 1858. Graptolithus rigidus n. sp. J. Hall, pp. 121–22. Pl. 11, Figs. 1–5. cf. 1989. C. rigidus (J. Hall) – Lindholm and Maletz, p. 720, Text-Figs. 2c–e; p. 721, Text-Figs. 3, 4g–j; p. 722, Text-figs. 5a and b. ? 2003. Clonograptus cf. C. rigidus (J. Hall) – Jackson and Lenz, p. 152, Fig. 12f. Material: One incomplete specimen on the slab DSTCP 23172a and counterpart DSTCP 23172b from Piscina Morta, Fluminimaggiore. Diagnosis: A narrow-stiped clonograptid, with several order of stipes, with somewhat equal thickness throughout of the

compressed stipes with evidence of the thecal morphology. 3, Early astogenetic stage, DSTCP 23176. 4a, large portion of a mature colony DSTCP 23162a. 4b, detail of the meshwork pattern with the peculiar “Gaud`ı balconies” dissepiments DSTCP 23162a. Planche 2. Figs. 1–4. A. murrayi (Hall, 1865), Tr´emadocien sup´erieur/? Ar´enigien basal, Fm. de Cabitza, Piscina Morta (Fluminimaggiore), SW Sardaigne. 1a, rhabdosome incomplet et de grande taille, associ´e avec des branches d’ordre e´ lev´e (? 4–6e ) de Clonograptus (Clonograptus) cf. multiplex ; DSTCP 23161b. 1b, contre-empreinte de la mˆeme colonie, associ´ee avec Clonograptus (Clonograptus) cf. multiplex ; DSTCP 23161a. 1c, d´etail de la maille tr`es r´eguli`ere, DSTCP 23161a. 2a, accumulation de rhabdosomes a` diff´erents stades astog´en´etiques et plusieurs fragments ; noter le stade juv´enile en bas a` droite et les stipes, comprim´ees lat´eralement, du rhabdosome de plus grande taille, DSTCP 23163a. 2b, stipes comprim´ees lat´eralement montrant le d´etail de la morphologie des th`eques. DSTCP 23163a. 3, stade astog´en´etique pr´ecoce, DSTCP 23176. 4a, vaste portion d’une colonie mature, DSTCP 23162a. 4b, d´etail de la morphologie de la maille avec les diss´epiments en « balcon de Gaud`ı » DSTCP 23162a.

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Plate 3. Graptolites of the Late Tremadoc/? Earliest Arenig, Cabitza Fm., Piscina Morta (Fluminimaggiore), SW Sardinia. Fig. 1. Clonograptus (Clonograptus) cf. rigidus (Hall, 1858). 1a, Rhabdosome showing overlapping branches and stipes DSTCP 23172a. 1b, counterpart DSTCP 23172b. Figs. 2 and 3. Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868). 2, fragment of the high order branches preserved on the slab 23161b. 3, incomplete rhabdosome associated with A. murrayi, DSTCP 23161a. Figs. 4 and 5 “Didymograptus” sp. 2. 4, cleaved rhabdosome, DSTCP 23173. 5, detail of thecal morphology, DSTCP 23175b.

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Fig. 3. (A) Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868), composite reconstruction after counterparts DSTCP 23171a-b; (B) Clonograptus (Clonograptus) cf. rigidus (Hall, 1865), drown after the sample DSTCP 23172a. Fig. 3. (A) Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868), reconstruction d’apr`es les contreparties de l’´echantillon DSTCP 23171a-b ; (B) Clonograptus (Clonograptus) cf. rigidus (Hall, 1865), dessin´e d’apr`es le rhabdosome DSTCP 23172a.

rhabdosome. Thecae are seldom visible, about 10–14 in 10 mm. Cortical thickening present. Description: Rhabdosome biradial, large, horizontal, with sixth order of branching and an extrapolated diameter of about 100 mm. The first order branches make a 6-mm-long funicle, but the sicula is not visible. The second order branches is 2.5–3.5mm-long and the third is about 6-mm-long. The stipes are thickened by cortical tissue and, in the proximal part, measure

about 1 mm, the thickness decreasing up to 0.5 mm in the higher order stipes. Thecae are rarely visible on the sixth order twisted stipes (Fig. 3B and Plate 3, Figs. 1a and b); their shape, in compressed lateral view, is that of straight tubes, with occasionally ventrally prolonged apertures. Thecal inclination to the axis of the stipes is 15–20◦ . Thecal length is 1.4–1.8 mm and the width varies between 0.6 and 0.9 mm. Overlap is one half or less and the 2TRD ranges from 1.25 to 1.4 mm (10–14 in 10 mm).

Figs. 6–8, 10. “Didymograptus” sp. 1. 6, poorly preserved accumulation of several rhabdosomes, slab 23175a. 7, Early astogenetic stage, slab 23175. 8, Early astogenetic stage, slab 23175. 10, strongly deformed rhabdosome, slab 23175a. Fig. 9. ? A. murrayi (Hall, 1865), early astogenetic stage, DSTCP 23177. Planche 3. Graptolites du Tr´emadocien sup´erieur/? Ar´enigien basal, Fm. de Cabitza, Piscina Morta (Fluminimaggiore), SW Sardaigne. Fig. 1. Clonograptus (Clonograptus). cf. rigidus (Hall, 1858). 1a, rhabdosome montrant la superposition de branches et de stipes, DSTCP 23172a. 1b, contre-empreinte DSTCP 23172b. Figs. 2 and 3. Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868). 2, fragment de branches d’ordre e´ lev´e (? 4–6e ), 23161b. 3, rhabdosome incomplet associ´e avec A. murrayi, DSTCP 23161a. Fig. 4 et 5 « Didymograptus » sp. 2. 4, rhabdosome endommag´e, DSTCP 23173. 5, d´etail de la morphologie des th`eques, e´ chantillon DSTCP 23175b. Figs. 6–8, 10. “Didymograptus” sp. 1. 6, accumulation de plusieurs rhabdosomes, mal conserv´es, 23175a. 7, stade astog´en´etique juv´enile, e´ chantillon 23175. 8, stade astog´en´etique juv´enile, e´ chantillon 23175. 10, rhabdosome tr`es d´eform´e, e´ chantillon 23175a. Fig. 9. ? A. murrayi (Hall, 1865), stade astog´en´etique juv´enile DSTCP 23177.

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Discussion: The aspect of the rhabdosome and the degree of branching of the specimen from Piscina Morta is comparable to those figured by Hall (1865) from L´evis (Quebec, Canada); in particular with the Figs. 1 and 2 of Pl. 11. The colonies of C. rigidus studied by Lindholm and Maletz (1989) from Quebec (including the material studied by Hall), do not differ significantly from the Sardinian specimen, except for its thicker stipes and slightly wider thecae. Clonograptus cf. C. rigidus from Yukon Territory of Canada, described by Jackson and Lenz (2003), differs only in having a shorter funicle. Although the differences with the type material are negligible, the lack of completely preserved Sardinian specimens prevents positive identification. Occurrence: Upper Cabitza Fm., Late Tremadoc–? basal Arenig. Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868) (Fig. 3A and Plate 3, Figs. 2 and 3) cf. 1989. C. multiplex (Nicholson) – Lindholm and Maletz, p. 727, Pl. 83, Figs. 1–6; p. 729, Text-figs. 7a and b. cf. 1997. C. multiplex (Nicholson) – Toro, p. 397, Pl. 1, Fig. 1. ? 2003. Clonograptus (Clonograptus) cf. multiplex (Nicholson) – Jackson and Lenz, p. 152, Figs. 10a, c, d. Material: One incomplete specimen on the slab DSTCP 23171a and some fragments in the counterpart DSTCP 23171b from Piscina Morta, Fluminimaggiore. Description: Rhabdosome large, horizontal, rigid, diameter of about 15 cm, at least four orders of branching. Funicle not visible, length of the second order stipes is variable between

17–20 mm, of the successive orders is 15–25 mm, 20–24 mm, 12–20 mm, respectively. The stipes are of normal dichograptid appearance and apparently without bithecae (Fig. 3A and Plate 3, Figs. 2 and 3). Stipe width ranges between 1.0 and 1.4 mm in the second order stipe, 0.5–0.6 mm or less in higher orders. The branching angle of the second dichotomy, is 80–90◦ , at the third order is 45–60◦ and for the fourth and the fifth orders is about 60◦ . Thecae is not visible, probably due to the cortical thickening. Discussion: The material from England and Scandinavia described by Lindholm and Maletz (1989) is closely similar to the Piscina Morta specimen, for example the same length and width of the stipes and angle of branching. The asymmetrical length of the stipes, also observed in the Sardinian rhabdosome (Fig. 3A and Plate 3, Figs. 2 and 3), is considered by Lindholm and Maletz (1989) as a peculiarity of Clonograptus multiplex. The specimen described and figured by Jackson and Lenz (2003) as Clonograptus cf. multiplex from Yukon Territories of Canada, has a similar dimension as our specimen, but the Sardinian colony possesses more slender stipes. C. multiplex figured by Toro (1997: Fig. 1.1) from Argentina has the same width of stipes and a similar angle of branching; however, the material from the Acoite Fm. shows a more regular length of stipes after each dichotomy. The absence of thecal parameters on the incomplete Sardinian specimen, do not permit any comparisons with other species. Occurrence: Upper Cabitza Fm., Late Tremadoc–? basal Arenig. Subfamily DICHOGRAPTINAE Lapworth, 1873 Genus Didymograptus McCoy, 1851

Fig. 4. (A) “Didymograptus” sp. 1, rhabdosome on the slab DSTCP 23175; (B) “Didymograptus” sp. 1, rhabdosome on the slab DSTCP 23175; (C) “Didymograptus” sp. 2, after the sample DSTCP 23173. Fig. 4. (A) « Didymograptus » sp. 1, rhabdosome sur l’´echantillon DSTCP 23175 ; (B) « Didymograptus » sp. 1, rhabdosome sur l’´echantillon DSTCP 23175 ; (C) « Didymograptus » sp. 2, d’apr`es l’´echantillon DSTCP 23173.

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Subgenus Didymograptus (Didymograptus) McCoy, 1851 “Didymograptus” sp. 1 (Fig. 4A and B and Plate 3, Figs. 6–8, 10) cf. 1988. Didymograptus cf. sinensis Lee and Chen – Molyneux and Rushton, p. 66, Figs. 9a and b. ? 1991. Didymograptus sp. 1 – Lindholm, p. 314, Text-figs. 14a and b. Material: Eight specimens from slab DSTCP 23175, Piscina Morta, Fluminimaggiore. Diagnosis: Rhabdosome of didymograptid type, with two declined or deflexed stipes, sicular length of about 1–1.25 mm. Thecae are straight tubes inclined of about 25–30◦ to the axis of the stipe. Thecae are 10–12 in 10 mm. Description: The rhabdosome is slightly declined or deflexed, with a stipe divergence angle of 100–145◦ . Due to the flattening of all the specimens, it is not possible to recognize detail of the proximal part of the rhabdosome, especially the possible presence of bithecae along the stipes. The stipes widen from about 0.53 proximally, reach 0.7 mm and remain costantly 0.7 mm in the distal part. The sicula is straight, about 1–1.25-mm-long and 0.3–0.35 wide at the aperture; presence of a little nema of about 0.35 mm (Fig. 4A and B and Plate 3, Figs. 6–8, 10). The thecae are straight tubes, inclined of 25–30◦ to the axis of the stipe. The overlapping is about 1/4. Thecae 10–12 in 10 mm. The 2TRD2 is 1.6 mm, 2TRD5 is 1.95 mm. Discussion: The general features of the studied rhabdosomes (including the Didymograptus sp. 2 described below) are common in didymograptids from the upper part of the Arenig and from the lower part of the Llanvirn, but are rare in the Upper Tremadoc, where there are few species comparable to the Sardinian material. The assignment of these specimens to Didymograptus is based, among others, on the apparent lack of bithecae along the stipes; in fact, the absence of bithecae is a distinctive feature of early Didymograptus in contrast to similar rhabdosomes retaining bithecae (e.g., Kiaerograptus Spjeldnaes, 1963 and allied taxa). The Sardinian species is very close to Didymograptus cf. sinensis, described by Molyneux and Rushton (1988) from England; a similar form described as Didymograptus sp. 1 by Lindholm (1991) has a larger sicula in comparison with the Sardinian specimens. Occurrence: Upper Cabitza Fm., Late Tremadoc–? basal Arenig. “Didymograptus” sp. 2 (Fig. 4C and Plate 3, Figs. 4 and 5) cf. 1988. Didymograptus sp. – Molyneux and Rushton, p. 66, Fig. 9c. Material: Two specimens from the slabs DSTCP 23173 and 23174, Piscina Morta, Fluminimaggiore. Diagnosis: Rhabdosome of didymograptid type, with two declined stipes, sicular length of 1.5–1.65 mm. Thecae are straight tubes inclined of about 30◦ to the axis of the stipe. Thecae are 14 in 10 mm.

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Description: The rhabdosome is declined, with a stipe divergence angle of 60◦ . Due to the flattening of the specimens, it is not possible to recognize details of the proximal part of the rhabdosome, nor the possible presence of bithecae along the stipes. The stipes widens from 0.8 mm proximally to 1 mm at the third thecae, then width remains constant in the distal part of the rhabdosome. The sicula (Fig. 4C and Plate 3, Figs. 4 and 5) is straight, 1.5–1.6 mm long and the aperture is 0.25–0.35 wide; a short nema about 0.17 mm long. The thecae are straight tubes ventrally prolonged, inclined of 25–30◦ , the overlapping is about 1/4. The thecae are 14 in 10 mm. The 2TRD2 is 1.4 mm, 2TRD5 is 1.6 mm, distally is about 1.6 mm. Discussion: The closest taxon comparable with the Sardinian Didymograptus sp. 2 is represented by Didymograptus sp. described by Molyneux and Rushton (1988) from the Late Tremadoc–Early Arenig Watch Hill Grits of Bitter Beck 3 locality (Lake District, England). The specimens studied here differ only in having a shorter sicula; however, the discrepancy observed in the sicular size may also be related to tectonic deformation. Occurrence: Upper Cabitza Fm., Late Tremadoc–? basal Arenig. 4. Age and palaeoenvironments of the upper Cabitza Formation 4.1. Age In the upper part of the Cabitza Fm., two biostratigraphically significant levels can be distinguished: R. f. flabelliformis and A. murrayi levels. The epipelagic graptolite R. f. flabelliformis is an Early Tremadoc taxon distributed in all biofacies, ranging from the base of Anisograptus matanensis zone, through the R. f. anglica zone and extending into the Adelograptus tenellus zone (see Fig. 5). Other fossil remains from these levels comprise acritarchs, rare Proteuloma geinitzi (Barrande, 1868) echinoderms plates, brachiopods (unassigned lingulates) and ichnofossils (Tomaculum problematicum Groom, 1902, Planolites s.s. pp., Cruziana cf. bagnolensis Mori`ere, 1878). Acritarchs from Monte Lisau (close to the R. f. flabelliformis level of Monte Cani, Gonnesa) include, among others, Acanthodiacrodium angustum (Downie) Combaz, 1967, Cymatiogalea cuvillieri (Deunff) Deunff, 1964, Dactylofusa squama (Deunff) Combaz et al., 1967 (Barca et al., 1987); this microflora is indicative of a general Early Tremadocian (zones d and e of Martin, 1982). The second level is characterised by A. murrayi, a taxon that appears in the Late Tremadoc (A. murrayi zone and Hunnegraptus copiosus zone) and ranges into the base of the Arenig (Tetragraptus phyllograptoides/Tetragraptus approximatus l.c. zones). The same range is demonstrated for C. multiplex (Nicholson, 1868) and C. rigidus (Hall, 1858) (Fig. 5). Thus, even if the Sardinian clonograptids, now with tentative nomenclature, are positively identified at the species level, a clear differentiation between Late Tremadoc and earliest Arenig, based on these three taxa, would not be possible. Finally, appar-

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Fig. 5. Correlation chart for the Tremadoc and Early Arenig of Scandinavia, Eastern North America South America (Bolivia) and Australasia (after Egenhoff et al., 2004; Cooper, 1999; Maletz and Egenhoff, 2001) and SW Sardinia (this paper). On right column is shown the range of the most significant taxa discussed in this paper; La = Lancefieldian; Be = Bendigonian. Fig. 5. Sch´ema de corr´elation pour le Tr´emadocien et l’Ar´enigien inf´erieur de Scandinavie, Nord-Ouest de l’Am´erique, Am´erique du Sud (Bolivie) Australasie (d’apr`es Egenhoff et al., 2004; Cooper, 1999; Maletz et Egenhoff, 2001) et SW de la Sardaigne (ce travail). La distribution verticale des taxa les plus significatifs cit´es dans ce travail est repr´esent´ee dans la colonne droite. La = Lancefieldien ; Be = Bendigonien.

ent didymograptids, present in the upper Cabitza Fm. biota, associated with Caryocaris cf. wrighti and unassigned ichnofossils, are of little use in resolving this problem. 4.2. Depositional environments The palaeoenvironments of the Cabitza Fm., formerly considered as shallow open-shelf or mainly distal turbidite deposits (Rasetti, 1972; Cocozza, 1980), have recently been more precisely defined following examination of a composite section cropping out in the type area (Loi et al., 1996) and in several areas within the Sulcis and Iglesiente subregions (SW Sardinia) (Fig. 6). It is now more clear that the Cabitzan succession began with deep shelf siliciclastics deposited during the Middle Cambrian (CAB 1–4a local informal zones) followed by rapid shallowing above the uppermost “Paradoxides beds” to the Upper Cambrian (CAB 4b, 5a and 5b), as displayed by diagnostic features of deposition on shoreface and delta tidedominated environments. A rapid deepening (mid-distal upper offshore) followed these shallow Furongian deposits and, immediately above, the HCS “Oryctoconus beds” accumulated on

the shoreface at the Cambrian–Ordovician boundary (peak of the ARE event). The lower part of the Cabitza Fm. reaching a thickness of up to 180 m, is overlain by the Early Ordovician “monotonous” upper portion representing the stratigraphic interval of this study. The upper part of the Cabitza Fm. has been poorly investigated, both from palaeontological and sedimentological point of view; the thickness is still unknown but, in all likelihood, exceeds 200 m. The succession is remarkably monotonously uniform and the structural setting adds further complications. In this context, the rare palaeobiological information assumes high values for age bracketing and for palaeoenvironmental interpretations. Laminated siltstones and shales and subordinate sandstones largely dominate the succession; sporadically more or less weathered ellipsoidal carbonatic Fe-rich concretions (up to 50 × 20 cm) and extremely rare thin calcareous beds have been found. Graded rhythmites and laminated sandy-micaceous siltstones and shales represent the overwhelming facies that is indicative of low energy environments below storm wave action. Palaeontological data supports the suggestion of the continuity of deep water environments through the upper Cabitza

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Fig. 6. Schematic log of the autochtonous late–Early Cambrian to Ordovician succession of SW Sardinia. CP1, CP2: informal biozonation of the Campo Pisano Fm.; CAB 1 to CAB 6: informal biozonation of the Cabitza Fm.(after Pillola et al., 2002a, 2002b, modified and updated). Fig. 6. Log sch´ematique de la succession autochtone du Cambrien inf´erieur sommital a` Ordovicien du SW de la Sardaigne. CP1, CP2 : biozonation informelle de la Formation de Campo Pisano ; CAB 1 a` CAB 6 : biozonation informelle de la Formation de Cabitza (d’apr`es Pillola et al., 2002a, 2002b, modifi´e et mis a` jour).

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Formation; furthermore, the environmental settings for the two graptolitic associations discussed above are in agreement with the ecostratigraphic model proposed by Cooper (1999). In fact, of the Sardinian outcrops of Early Tremadoc age, only three yield Rhabdinopora; two of them are composed of deposits belonging to the “Facies 4” described by Loi et al. (1996), indicating deposition in a distal upper offshore, while in the third (Case Lai outcrops, close to Cabitza), the R. flabelliformis-bearing level is represented by red beds with indications of possible shallow water deposition (this latter occurrence may represent a peculiar case in its usual distributional pattern). The present Piscina Morta findings also suggest an outer shelf depositional environment, dominated by A. murrayi and caryocarids, with subordinate clonograptids and didymograptids. Usually caryocarids are very common in outer shelf or slope deposits and are almost invariably associated with graptolites; their massive occurrence provides valuable indications of the depositional setting and bathymetry (Vannier et al., 2003). In all likelihood, the Sardinian Araneograptus levels accumulated during the transgression that followed the Ceratopyge regressive event (CRE), a time when graptolites became abundant and taxonomically diverse, especially in deep water deposits (Cooper, 1999). Similar graptolitic accumulations in outer shelf environments around Gondwana have been interpreted as a displacement of mesopelagic faunas outside their original biotope, triggered by upwelling phenomena and, in some cases, concomitant with massive death caused by an oxygen deficiency in the water masses (see Guti´errez Marco and Ace˜nolaza, 1987; Cooper, 1999 and references therein).

5. Conclusions • The discovery of the A. murrayi-dominated levels in the upper part of the Cabitza Fm. permits correlation with Hunnebergian (Late Tremadoc–earliest Arenig) and implies that there was “continuous” sedimentation through the Tremadocian of SW Sardinia. Structural complexity makes stratigraphical studies difficult and useful biostratigraphical data are very few; however, the complete Tremadocian is documented by: ◦ the occurrence of the “Oryctoconus beds” at the Cambrian/Ordovician boundary; ◦ the presence of trilobite P. geinitzi and of the graptolite R. f. flabelliformis in the lower part of the sequence; ◦ the presence of sparse but chronostratigraphically significant acritarchs communities indicating an undifferentiated Tremadocian age; ◦ the Late Tremadoc–Early Arenig? graptolitic association described herein in the upper Cabitza Fm. • Environmental conditions at that time in SW Sardinia seem to have been rather monotonous, oscillating between shoreface and mainly outer shelf siliciclastic platform. • Palaeoecological information indicated by the accumulation of an epipelagic-mesopelagic a caryocarid/graptolitic association, strongly supports the outer shelf depositional environments inferred for the bulk of the upper portion of the Cabitza Formation.

• Until now, there was no clear evidence of shallow water deposits prior to the emersion related to the so-called Sardic tectonic phase, nor of major eustatic events in the uppermost Cabitzan deposits. Acknowledgments We are deeply grateful to Mr. Luciano Menghi, Fluminimaggiore, who kindly supplied the specimens DSTCP 23161 and DSTCP 23172 and to Prof. Annalisa Ferretti (Modena University) for a critical reading of the manuscript. Prof. Alfredo Loi (Cagliari University) gave us useful suggestions. Prof. B.-D. Erdtmann (Technical University, Berlin) and J.C. Guti´errezMarco (University of Madrid) are deeply acknowledged for kindly supplying the papers on the topic. We are indebted to J. Maletz and A.C. Lenz, for the critical review of the manuscript and the improvement of the text. This research was supported by MIUR grants ex 60% G.L. Pillola “Paleobiodiversit`a: applicazioni nella ricerca di base e nella valorizzazione dei beni culturali geo-paleontologici” and PRIN A. Ferretti “L’inizio dopo la fine: comparse e riprese evolutive durante e dopo l’estinzione di massa del tardo Ordoviciano nel Nord-Gondwana”. This paper is a contribution to the IGC Project 503 “Ordovician palaeogeography and palaeoclimate”. References Ace˜nolaza, F.G., Ace˜nolaza, G.F., Esteban, S.B., Guti´errez Marco, J.C., 1996. Estructuras nemales de A. murrayi (J. Hall) (graptolito del Ordovicico Inferior) y actualizacion del registro perigondwanico de la especie. Memorias del xii Congreso Geologico de Bolivia – Tarija, Bolivia, pp. 681–689. Barca, S., Cocozza, T., Del Rio, M., Pillola, G.L., Pittau Demelia, P., 1987. Datation de l’Ordovicien inf´erieur par Dictyonema flabelliforme et Acritarches dans la partie sup´erieure de la formation « Cambrienne » de Cabitza (SW de la Sardaigne, Italie), cons´equences g´eodynamiques. Compte Rendu de l’Acad´emie des Sciences de Paris 305 (2), 1109–1113. Barrande, J., 1868. Syst`eme silurien du centre de la boh`eme, 1re Partie : Recherches Pal´eontologiques, v. 2, Classe des Mollusques, Ordre des C´ephalopodes, 3e S´erie, Prague, Paris. Bulman, O.M.B., 1950. Graptolites from the Dictyonema shales of Quebec. Quarterly Journal of the Geological Society of London 106, 63–99. Cocozza, T., 1967. Rapporti cambro-ordoviciani nella zona di Acquaresi. Resoconti Associazione Mineraria Sarda 72, 3–37. Cocozza, T., 1980. The Cambrian of Sardinia. Memorie della Societ`a Geologica Italiana 20 (1979), 163–187. Combaz, A., 1967. Un microbios du Tr´emadocien dans un sondage d’HassiMessaoud. Actes de la Soci´et´e linn´eenne de Bordeaux 104, 1–26. Combaz, A., Lange, F.W., Pansart, J., 1967. Les “Leiofusudae” Eisenack, 1938. Review of Paleobotany and Palynology 1, 291–307. Cooper, R.A., 1999. Ecostratigraphy, zonation and global correlation of earliest Ordovician planktic graptolites. Lethaia 31, 1–16. Cooper, R.A., Maletz, J., Wang, H., Erdtmann, B.-D., 1998. Taxonomy and evolution of earliest Ordovician graptoloids. Norsk Geologisk Tidsskrift 78, 3–32. Deunff, J., 1964. Syst´ematique du microplancton fossile a` Acritarches. Revue de Micropal´eontologie 7, 119–124. Egenhoff, S.O., Maletz, J., Erdtmann, B.-D., 2004. Lower Ordovician graptolite biozonation and lithofacies of southern Bolivia: relevance for palaeogeographic interpretations. Geological Magazine 141, 287–299. ¨ Eichwald, E.J., 1840. Uber das silurische Schichtensystem in Esthland. Academy Science St. Petersburg Bulletin 1 (2), 1–210.

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