A new orthoceratoid cephalopod from the Ordovician (Caradoc) of east-central Iran

Geobios 39 (2006) 337–345 http://france.elsevier.com/direct/GEOBIO/

A new orthoceratoid cephalopod from the Ordovician (Caradoc) of east-central Iran Un nouveau céphalopode orthocératoïde de l’Ordovicien (Caradocien) d’Iran oriental-moyen Mohammad Dastanpour a, David H. Evans b, Michael G. Bassett c,* a

Department of Geology, University of Shahid Bahonar (Kerman), PO Box 76169-133 Kerman, Iran b English Nature, Northminster House, Peterborough PE1 1UA, England, United Kingdom c Department of Geology, National Museum of Wales, Cardiff CF10 3NP, Wales, United Kingdom Received 2 September 2004; accepted 29 November 2004 Available online 28 February 2006

Abstract Orthoconic cephalopods from the Ordovician Katkoyeh Formation at Banestan, Kerman Province, Iran, comprise a single species named as Sactorthoceras banestanensis Evans nov. sp. Associated faunas suggest close palaeobiogeographical linkage with other regions of north periGondwana, notably Morocco and Bohemia (Perunica), while Sactorthoceras itself is also indicative of relationships with Baltica and the SinoKorean Plate. © 2006 Elsevier SAS. All rights reserved. Résumé Les céphalopodes orthoconiques de l’Ordovicien de la Formation Katkoyeh à Banestan, Province de Kerman, Iran, comprennent une seule espèce nommée Sactorthoceras banestanensis Evans nov. sp. Les faunes associées suggèrent un lien paléobiogéographique étroit avec les autres régions du Péri-Gondwana nord, notamment le Maroc et la Bohême (Perunica), alors que Sactorthoceras lui-même témoigne également de relation avec la Baltique et le Plateau sino-coréen. © 2006 Elsevier SAS. All rights reserved. Keywords: Cephalopoda; Ordovician; Iran; New species Mots clés : Cephalopoda ; Ordovicien ; Iran ; Nouvelle espèce

1. Introduction Orthoconic cephalopods have been reported quite widely in Ordovician strata of various regions in Iran, but as yet none have been described systematically and the only illustrations of such material are those by Kalantari (1981: Pl. 2, Figs. 5–

* Corresponding

author. Tel.: +44 292 057 3212; fax: +44 292 066 7332. E-mail address: [email protected] (M.G. Bassett).

0016-6995/$ - see front matter © 2006 Elsevier SAS. All rights reserved. doi:10.1016/j.geobios.2004.11.008

14) from the Shirgesht Formation (?early-mid Ordovician) of the Tabas area in Yazd Province. This latter material was all named as Orthoceras vagans Salter, 1849, a species described originally from the middle-upper Ordovician (Caradoc–Ashgill) of Wales, England and Portugal (Salter, 1849) and which clearly needs re-evaluation at both generic and specific levels before the true identity of the Iranian material can be confirmed; unfortunately this material is not available for re-study as its present location is unknown. Otherwise, records are confined to faunal lists in stratigraphical papers, almost entirely at

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generic level only, and are too numerous to summarize here, but for general background data see Stöcklin and Setudehnia (1991) and Alavi-Naini (1993). Recent studies in the Zarand area of Kerman Province have confirmed the presence of a well exposed Ordovician sequence along the north-east side of the Kuh Banan Fault Complex (Vahdati Daneshmand et al., 1995), incorporating the Katkoyeh Formation for which an age of at least Arenig through Caradoc is now proved by the presence of graptolite, conodont and brachiopod faunas (Rickards et al., 1994; Hamedi et al., 1997; Bassett et al., 1999). These studies amplify the regional mapping and stratigraphy established by Huckriede et al. (1962). In the region between Dahu and Assiab, over a distance of some 15 km the upper Katkoyeh Formation (sensu Hamedi et al., 1997: Fig. 2) is well exposed in a number of sections, including both sides of a dry river bed (wadi) adjacent to the small farm 0.75 km north-north-west of Banestan village at 30° 51′ 59′′ N, 056° 39′ 12′′ E. Here the sequence comprises some 85 meters of buff and gray-green siltstones and silty calcareous mudstones and limestones, with an intervening red siltstone horizon, dipping at 45° NE from a strike of N55° W. Although reported originally by us (Bassett et al., 1999: Fig. 1) as being possibly of Llandeilo age at the base, further study now revises the dating and confirms that this upper division of the Katkoyeh Formation is entirely of Caradoc age; details of the stratigraphy are being published elsewhere, but we observe here that this sequence is in direct upward continuation of the section in the lower Katkoyeh Formation from which Rickards et al. (1994) obtained their Arenig graptolites. Locality maps covering the outcrop area of the Katkoyeh Formation along the Kuh Banan zone, including the Banestan area, are published by Huckriede et al. (1962: Fig. 8), Hamedi et al. (1997: Fig. 1), Ross et al. (2000: Fig. 1) and Niko et al. (1999: Fig. 1). The wider regional geology is illustrated in the 1:100,000 Zarand map sheet (Vahdati Daneshmand et al., 1995). From the lower marine unit within the upper Katkoyeh sequence, two of us (MD and MGB) obtained a sparse fauna of nautiloids, which have been studied taxonomically by DE. These may well be the same taxon as that reported by Huckriede et al. (1962: pp. 48) as Michelinoceras sp. from near Assiab (probably at Banestan). Our material is accompanied by a diverse fauna (though not abundant) of small rhynchonellid brachiopods, bivalves, bryozoans, echinoderms and ostracodes. Apart from the fact that the nautiloids comprise a new species, they are of particular interest in their biogeographical relationships as discussed below. We accept joint responsibility for the overall conclusions in this paper, but DE is solely responsible for the sections on preservation and systematics. 2. Material and stratigraphy At Banestan we divide the upper Katkoyeh Formation into five units, with the base defined at the base of a lowest division

(9.10 m thick) following conformably above a distinct sequence of non-marine red siltstones and mudstones that lack fossils. From the lowest marine unit in the measured section on the south-east bank of the river bed we collected five orthoceratoid specimens (Bassett field sample 02/11a). From the same unit on the north-west bank of the river bed we collected seven orthoceratoids (field sample 02/11c). In both collections all specimens are incomplete. Above this unit is a distinct green/gray unit (90 cm) of siltstones immediately below a central division of sandy micaceous, red non-marine siltstones that tongue into the middle part of the Katkoyeh Formation at this locality. No cephalopods have been collected from any of the marine units of the uppermost Kaykoyeh Formation above the central red beds. We have also collected co-eval faunas from the upper Katkoyeh Formation at its type locality near Dahu to the south-east of Banestan, where there is a rich fauna of brachiopods, bryozoans, gastropods and bivalves, but we have not found cephalopods there and we consider the Dahu sequence to be in a slightly further offshore setting. Ross et al. (2000) have described the bryozoan faunas from both localities, in amplification of the information on different faunal groups reported by Hamedi et al. (1997). All 12 specimens described here are housed in the Department of Geology, National Museum of Wales, UK; Accession Number 2003.8G. 2.1. Preservation Within the collected samples, the three phragmocones of greatest diameter are all deformed, having been crushed some time during burial. At least one of these fragments (2003.8G.3) [ca. 36 mm diameter]) is part of a body-chamber. Estimates of conch diameters were obtained using cord to measure their circumference. No portions of phragmocones with a diameter less than 12 mm are present in the collection (Table 1). It is unclear whether the lack of pieces of phragmocone less than 12 mm diameter is a consequence of collecting bias or reflects the removal of this fraction as part of the taphonomic process. Many of the phragmocones show signs of wear, most probably attributed to recent weathering. Only one specimen (2003.8G.3) retains any shell material belonging to the wall of the phragmocone. Whether or not they have been crushed or strongly weathered, all specimens display the septal sutures on the surface of the phragmocone. All the phragmocones are filled with sediment and sparite, recording a complex infill history. Some (2003.8G.10 and 2003.8G.6) largely contain sediment, whilst the connecting rings appear to be entirely missing, suggesting that sediment entered the camerae via the siphuncle, but it is not clear how the connecting rings came to be destroyed. It is likely that the phragmocone had been broken along its axis prior to the entry of sediment. This would have been necessary to generate a flow of water along the siphuncle in order that sediment could reach the camerae (Seilacher, 1969). Some parts of the phrag-

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Fig. 1. Graphs showing variation in a number of characters in Sactorthoceras banestanensis nov. sp. (open squares) and other species of Sactorthoceras described by Kobayashi (1927, 1934) and Yun (1999a, 1999b) (black squares). A. Rate of expansion of the shell. B. Depth of septa. C. Cameral depth. D. Position of siphuncle. E. Relative diameter of siphuncle at septal foramen. F. Maximum diameter of connecting ring.

mocone not filled with sediment remained as voids to be filled later with sparite. Other specimens of larger diameter (2003.8G.1-3) contain proportionally smaller volumes of sediment, either as a consequence of their greater volume or there being few points of entry for sediment, and sparite is the dominant component of the fill. In 2003.8G.2 and 2003.8G.3 the siphuncles are lined with sediment along their length, and this partly surrounds a sparite fill that also extends along the length of the siphuncle,

again suggesting that there was a flow of water through the siphuncle. All these specimens show evidence of damage in the form of crushing of the phragmocone, the presence of fractured and bent septa, as well as broken and displaced siphuncles. Specimen 2003.8G.2 provides evidence that sediment may have entered the phragmocone after fracturing had taken place, but it is not clear whether this is a consequence of the rearrangement of sediment within the phragmocone, or an influx of sediment caused by external disturbance. The fracturing

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Fig. 2. Details of siphuncles in Sactorthoceras banestanensis nov. sp. showing variation in the curvature of the septal necks and degree of inflation of the connecting rings. A. 2003.8G.2, showing suborthochoanitic septal necks and weakly inflated connecting rings. B. 2003.8G.6, suborthochoanitic septal necks and missing connecting rings. C. Holotype, 2003.8G.1, showing suborthochoanitic septal necks combined with more strongly inflated connecting rings. D. 2003.8G.3, septal necks change from suborthochoanitic to orthochoanitic adorally while the connecting rings become tubular. E. 2003.8G.5, suborthochoanitic connecting rings with inflated connecting rings adapically, the connecting rings have been deformed adorally. Table 1 Characters measured on the material described Specimen No. 2003.8G.8 2003.8G.9 2003.8G.10

A 66 – 44 48

B 31.2 – – 24.2

C 3.8 – – 3.6

2003.8G.11 2003.8G.12 2003.8G.1

14 32 74

– – 24.6

– – 7.0

2003.8G.2

73

28.6

6.5

2003.8G.3 2003.8G.4 2003.8G.6 2003.8G.7

47 30 34.5 21

– – – –

– – – –

D 33.4 29.0 23.5 25.0 23.3 12.0 27.4 26.7 22.4 31.5 25.6 35.8 17.7 14.0 15.7

E 19.5 13.7 12.7 11.8 13.7 25.0 13.1 13.1 17.8 10.5 15.2 7.5 21.5 22.8 14.6

F – – – 25.0 – 12.0 – 26.7 – – – – – – –

G – – – 14.0 – 15.0 – 14.6 – – – – – – –

H – – 38.7 42.0 – 42.0 – 37.0 – – – 38.0 – – –

I – – 15.9 13.6 – 21.2 – 13.3 – – – 9.7 – – –

J – – – – – 39.3 – 14.9 – – – 9.9 – – –

K – – – – – – – – – – – 7 – – –

A. Length (mm); B. Mean diameter (mm); C. Expansion rate (degrees); D. Phragmocone diameter (mm); E. Cameral depth as percentage of the conch diameter; F. Conch diameter (mm); G. Septal depth as percentage of the conch diameter; H. Siphuncle position as a percentage of the conch diameter; I. Siphuncle diameter at septal foramen as a percentage of the conch diameter; J. Siphuncle diameter at maximum inflation of the connecting ring as percentage of the conch diameter; K. Number of striae per mm.

of the septa and failure of the walls of the phragmocones were probably a consequence of the partial sediment infill of the camerae, and occurred when there was sufficient loading from the sediment above. Deposition of the sparite fill took place later. 3. Systematic palaeontology Order PSEUDORTHOCERIDA Flower and Caster, 1935 Family SACTORTHOCERATIDAE Flower, 1946 Sactorthoceras Kobayashi, 1934

Type species: Sactorthoceras gonioseptum Kobayashi, 1934, pp. 407, from the Middle Ordovician Jigunsan Formation of the Kangweondo area, South Korea. Remarks: as recognized by Sweet (1964: pp. K233), Sactorthoceras was then regarded as a straight or slightly curved orthocone possessing rather shallow camera and a sub-central siphuncle. The septal necks were regarded as suborthochoanitic, and the connecting rings were slightly expanded. No endosiphuncular or cameral deposits were known. The shell surface was regarded as smooth in the type species, and with fine transverse striae in S. wongiforme Kobayashi (1934: pp. 411).

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Kobayashi (1934) listed six species of Sactorthoceras, all from horizons in the Jigunsan Formation of the Joseon Supergroup in the Kangweondo area. Of these species, none appear to be represented by more than the holotype or lectotype and a few paratypes or paralectotypes. Bivariate plots (Fig. 1) of a number of characters based on the illustrations of specimens attributed to Sactorthoceras by Kobayashi (1934) and Yun (1999a) are here taken to suggest that a number of the species proposed by Kobayashi can be regarded probably as synonymous. With a few exceptions, all of the characters investigated either show discrete clustering or follow continuous trends. Significantly, such trends show a fairly discrete distribution of Kobayashi’s species along them, and this may suggest that size and ontogenetic changes in the population may have influenced Kobayashi in his taxonomic separation of this material. However, the relatively small size of the samples indicates that these conclusions should be regarded with some caution. Although Yun (1999a) synonymized several of Kobayashi’s species, he distinguished Sactorthoceras wongiforme on the basis of its broader siphuncle and deeper camerae, and S. tenuicurvatum Kobayashi, 1934 for its elliptical and compressed conch section. The data used here (Fig. 1) suggest that S. wongiforme is on a continuum plot with other previously described species of Sactorthoceras in these and other characters, and thus cannot be distinguished effectively as a separate species. Kobayashi’s illustrations (1934: Pl. 16, Fig. 2, Pl. 17, Fig. 10) of the conch cross-section of S. tenuicurvatum both show that the shell was only partially preserved and that the cross-section was reconstructed; this is not convincing evidence for an originally elliptical section. The only convincing distinctions between Kobayashi’s species of Sactorthoceras may lie in the nature of the shell sculpture of the type species and that of S. wongiforme (with transverse striae), and forms with ribs and fine transverse striae that were attributed by Yun (1999a) to S. makkolense Kobayashi, 1934. In his synonymy of Rhynchorthoceras coreanicum Kobayashi, 1927, Yun (1999a) included a syntype of Orthoceras makkolense, subsequently assigned to Sactorthoceras makkolense by Kobayashi (1934: pp. 409) who indicated that he considered this specimen (Kobayashi, 1927: Pl. 19, Fig. 2a–c; Kobayashi, 1934: Pl. 15, Fig. 9) to be the holotype. Kobayashi referred to the holotype of S. makkolense in the same text, and his synonymy included only the syntypes of O. makkolense. From this, it may seem reasonable to conclude that Kobayashi indicated his selection of one of the syntypes of O. makkolense as the holotype. As the selection of the holotype was subsequently to the original description of O. makkolense, this should be taken as the indication of the lectotype, whilst the remaining syntype (Kobayashi, 1927: Pl. 18, Fig. 5) is a paralectotype. Yun (1999a: p. 72) selected a lectotype of S. makkolense (and by implication, O. makkolense) represented by plate 18, Fig. 5 of Kobayashi (1927), or Kobayashi’s paralectotype. As Kobayashi’s indication of the lectotype has priority, Yun’s use of S. makkolense is not valid as the lectotype was assigned to

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Holmiceras coreanicum and the name cannot also be available for S. makkolense. The fact that Yun assigned Kobayashi’s lectotype Orthoceras makkolense to Holmiceras Hyatt, 1894 tends to highlight the rather nondescript characteristics of the genus Sactorthoceras. One of the main problems with Sactorthoceras is the lack of evidence for the presence of endosiphuncular and cameral deposits, and in particular, a determination as to whether this lack is real, or due to the absence of suitable material demonstrating the presence of these structures having never been available. Sactorthoceras banestanensis nov. sp. described here, based on several specimens, provides some additional features suggesting that Sactorthoceras may be placed within the Pseudorthocerida. The family Sactorthoceratidae was proposed by Flower (1946), and apart from the type genus, Flower included the poorly known Centroonoceras Kobayashi, 1927 and Sigmocycloceras Kobayashi, 1927 in the family. All three genera are known from the Jigunsan Formation, and only Sactorthoceras has been recorded from beyond Korea. It was considered that these three genera are united in the Sactorthoceratidae in possessing suborthochoanitic septal necks. The little additional data that can be derived from Kobayashi’s (1927,1934) descriptions indicate that they may be similar to Sactorthoceras in terms of their cameral depth and siphuncle diameter. However, without restudy of Centroonoceras and Sigmocycloceras, the legitimacy of grouping these genera together remains uncertain. Flower (1941: pp. 527, Fig. 1) considered that Sactorthoceras and Centroonoceras formed the early part of an ancestry that led ultimately to the Ascocerida. Flower regarded Centroonoceras as representing a stage between Sactorthoceras and Montyoceras Flower, 1941 wherein the curvature of the conch increased. However, Miller (1932) proposed an origin of the Ascocerida in the Oncocerida, and although not discussed further here, this is regarded (by DHE) as a more likely for the Ascocerida. Our specimens of Sactorthoceras banestanensis nov. sp. provide evidence that if endosiphuncular and cameral deposits were present in Sactorthoceras, they must have been restricted to the extreme adapical portion of the phragmocone. More significantly, this material also demonstrates that the structure of the siphuncle underwent changes during growth of the organism. Adapically, the septal necks are suborthochoanitic, whilst the connecting rings are inflated to a diameter roughly twice that of the diameter at the septal foramen. Adorally, the inflation of the connecting ring becomes less pronounced, until in the most mature part of the phragmocone the connecting ring is tubular, and the septal necks are orthochoanitic. Such a pattern of ontogenetic change is seen in pseudorthocerid families such as the Proteoceratidae (Flower, 1962). Sactorthoceras, at least, may represent part of a lineage that originated in the Proteoceratidae, or a closely related group that underwent suppression of the cameral and endosiphuncular deposits. Such a process may not be uncommon amongst orthoceroids, because the earliest members of the group possessed

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well developed cameral and endosiphuncular deposits, and these were later suppressed strongly in genera such as Orthoceras (see Flower, 1962: Pl. 4, Figs. 6 and 7), or lost entirely in Michelinoceras (see Serpagli and Gnoli, 1977: Pl. 2, Figs. 1–3). Sactorthoceras banestanensis Evans nov. sp. Diagnosis: Sactorthoceras with a sub-centrally positioned siphuncle; connecting rings markedly inflated in the adapical part of the phragmocone. Origin of name: from the type locality in the wadi adjacent to the village of Banestan in the Kerman area of East-Central Iran. Specimens 2003.8G.1-7 were collected from the lowest marine unit on the north-west bank of the wadi; specimens 2003.8G.8-12 are from the same unit on the south-east bank of the wadi. Holotype: NMW 2003.8G.1. Paratypes: NMW 2003.8G.8-12; NMW 2003.8G.2-7. Description: the holotype is part of a phragmocone, 74 mm long; it is worn, but has a circular cross-section and is estimated to be 27 mm in diameter. Sutures are straight, but inclined to the normal of the conch axis by about 5° (Fig. 2); the orientation of this inclination is not known, because in no specimen is it possible to determine which are the dorsal and ventral surfaces. Septa have a depth about 15% of that of the conch diameter, whilst the cameral depth varies from 13 to 18% of the conch diameter. The center of the septal foramen is positioned 37% of the way across the diameter of the phragmocone and has a diameter 13.3% of that of the phragmocone. Connecting rings are slightly inflated, reaching 14.9% of the diameter of the phragmocone. Septal necks (see Figs. 2c and 3a) are suborthochoanitic and about 1 mm long. In those specimens sufficiently well preserved for measurement of specific characters, such features appear to show consistent trends (Fig. 1) that seem to confirm that all the material belongs to the same taxon. Septal depth appears to remain at between 14 and 15% of the phragmocone diameter. Cameral depth decreases consistently with increasing conch diameter, whilst the position of the siphuncle remains constant. The relative diameter of the siphuncle increases steadily adapically and the connecting rings become increasingly inflated. In 2003.8G.3 (estimated diameter 35 mm) the septal necks are orthochoanitic, and the connecting rings tubular. In 2003.8G.11 (12 mm diameter) the septal necks are suborthochoanitic and the connecting rings are sufficiently inflated for them to take on a spherical shape. 2003.8G.3 is the only specimen in which the shell surface is preserved, consisting of adorally imbricated transverse striae at a density of 7 per mm. Remarks: Fig. 1 includes data obtained from illustrations of Sactorthoceras in Kobayashi (1927, 1934) and Yun (1999a). Comparison of these data with parameters for S. banestanensis suggests that the Iranian species differs from the South Korean species mainly in the more sub-central position of the siphuncle. There may also be some difference in the rate at which the relative diameters of the siphuncles change during growth. This rate appears to be higher in S. banestanensis. Whether the same

is true with regard to the inflation of the siphuncle is less clear, but S. tenuicurvatum (Kobayashi, 1934: Pl. 16, Fig. 1) may show connecting rings that become more inflated towards the adapical end of the preserved part of the phragmocone. Faunal affinities: the monospecific nature of the cephalopod fauna from Banestan makes it difficult to draw any firm conclusions with regard to its palaeobiogeographical affinities. This difficulty is compounded by the somewhat generalized nature of our current understanding of Sactorthoceras. The Taebaeksan basin, from which the type material of Sactorthoceras originated, forms part of the Sino-Korean Plate (Choi et al., 2001). It could be argued that this would indicate a North China affinity for our Iranian material. The Yongnam massif and associated Taebaekan basin are separated from the SinoKorean plate by the Kyonggi massif, regarded as part of the Yangtse Plate. Thus the configuration of the Yongnam massif and Taebaekan basin in relation to the rest of the Sino-Korean plate during the Ordovician may be uncertain. The Jigunsan Formation has been assigned a number of ages ranging from early Caradoc to Llanvirn Series (see Yun, 1999a: pp. 66–67 for references). The most recent dates (Lee and Lee, 1986) suggest a correlation with the lower part of the Upper Majiagou Formation of North China and the Llanvirn Series of Europe. However, this part of the Majiagou Formation is correlated with the Dawan Formation of Southern China (Lai, 1989: Table 1; Li, 1994: Table 4.1) and would thus correlate with part of the earlier Ordovician Arenig Series (Fortey et al., 2000: Fig. 34). Caradoc age nautiloids described recently from Yunnan (South China Plate; Zhang et al., 2002) appear to have little in common with the Sino-Korean faunas. The cephalopod fauna of the Jigunsan Formation has been revised partially (Yun, 1999a, 2002). It is clear that this fauna differs from other North China faunas in a number of aspects (Yun, 2002). Late Lower, and early Middle Ordovician cephalopod faunas of northern Chinese aspect are characterized by actinocerids (Lai, 1989: Table 1). Both the Maggol Formation, underlying the Jigunsan Formation, and the Dawibong Formation, resting upon the Jigunsan Formation yield actinocerids (Kobayashi, 1927; Yun, 1999b, 2002). By contrast, in the Jigunsan Formation, actinocerids are restricted to forms attributed to Ormoceras by Kobayashi. Apart from the demonstration of the presence of lituitids in the Jigunsan fauna, Yun (1999a, 2002) also reported Dideroceras Flower, 1950 and Troedsonella Kobayashi, 1935. Here it is considered that Geisonoceras abruptum Kobayashi, 1934 may belong in Rhynchorthoceras Remelé, 1881, whilst Robsonoceras (?) meridionale Kobayashi, 1934 might be assigned to Baltoceras Holm, 1897. Lituitids have also been reported in low latitude faunas from the Whiterock Series of Nevada, Oklahoma and Newfoundland (Flower, 1975), whilst Baltoceras has been described from the Whiterock of Nevada (Flower, 1964). Faunas rich in lituitids are restricted to southern China and the Baltic (Lai, 1989; King, 1999), as is Baltoceras. Their occurrence in the low latitude faunas of Laurentia and the Sino-Korean plate may reflect temporary migrations from intermediate latitudes, or a more permanent presence in deeper water, more marginal sites

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Fig. 3. Sactorthoceras banestanensis Evans nov. sp. A, B, K. Holotype 2003.8G.1, details of siphuncle in section, exterior of worn phragmocone showing septa, polished section of interior, × 3, × 1, × 1. C, F, G. Paratype 2003.8G.2, exterior of crushed phragmocone, details of siphuncle in section, polished section of interior, × 0.75, × 3, × 1. D, H, I, O, P. Paratype 2003.8G.3, exterior of partially crushed phragmocone, polished section of interior, detail of shell sculpture, septal surface in adapical view, details of siphuncle in section, × 1, × 1, × 2, × 1, × 2.5. E, J. Paratype 2003.8G.10, phragmocone showing inclined sutures, polished section of interior, × 1, × 1. L. Paratype 2003.8G.9, polished section of interior of worn phragmocone, × 1. M. Paratype 2003.8G.5, polished section with details of siphuncle, × 3. N, R. Paratype 2003.8G.4, exterior of phragmocone, polished section of interior, × 1.5, × 1. S, U. Paratype 2003.8G.11, polished section of interior, mould of phragmocone, × 2.5, × 2.5. Q, T. Paratype 2003.8G.6, polished section of interior, mould of phragmocone, × 1.75, × 1.5.

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on the continental shelves (e.g. Fortey and Owens, 1978: Fig. 7). Yun (2002) considered that the Baltic elements of the Jigunsan Formation indicated a temporary connection or migration route from this area at the time. All these data may be taken to suggest that whilst Sactorthoceras is reported from the Jigunsan Formation, it may belong to that part of a fauna associated with deeper water at the margins of the Sino-Korean plate, or migrated temporarily into these areas from higher latitudes. Thus the presence of Sactorthoceras in the Katkoyeh Formation of Iran does not necessarily imply a biogeographical origin for the genus within the Sino-Korean area. Despite making reference to the occurrence of Sactorthoceras in horizons belonging to the Whiterock and Mohawkian Series in the north-eastern United States and Canada, as well as from the “Orthoceras limestone of Sweden”, Flower (1946: pp. 251) and Flower (1976) never described or illustrated any of these taxa, and only ascribed Orthoceras vagum Ruedemann, 1906 from the Whiterock Series of New York to Sigmorthoceras Kobayashi, 1934. Sigmorthoceras was regarded as a probable synonym of Sactorthoceras by Sweet (1964: pp. K233). O. vagum (Ruedemann, 1906: pp. 435, Pl. 9, Fig. 9; Pl. 13, Figs. 1–3) is a very slender, slowly expanding shell with camera that retain their depth adorally. In this, O. vagum is unlike Sactorthoceras or Sigmorthoceras. Of the forms attributed to Sactorthoceras from the Middle Ordovician of the Oslo region (Sweet, 1958: pp. 60, Pl. 3, Fig. 12; Pl. 4, Figs. 1 and 7; only one specimen retained any internal structures. This specimen contained a siphuncle with orthochoanitic septal necks and tubular connecting rings at diameters much smaller than in the Iranian and Korean material. Two of the specimens illustrated (Sweet, 1958: Pl. 3, Fig. 12; Pl. 4, Fig. 1) are externally similar to other species of Sactorthoceras, but given the uncertainty regarding their internal morphology, they should be assigned to Sactorthoceras with some question. The most recent review of the Ordovician cephalopod faunas of the Prague Basin (Marek, 1999) indicates that although some of the taxa described by Barrande (1865–1877) have been assigned to more recently described genera, the majority remain in “Orthoceras”. On the basis of Barrande’s illustrations, much of this material appears to consist of fragmentary internal moulds wherein the septa and siphuncle have often been destroyed. Notwithstanding such factors, it is suspected that there may be substantial scope for the synonymy of many of the species described. Of those taxa described by Barrande, Orthoceras bisignatum (Barrande, 1874: pp. 177, Pl. 416, Figs. 6–19) shows some similarity to Sactorthoceras. Two individuals (Barrande, 1874: Pl. 416, Figs. 17 and 19) show transverse sections with siphuncles that are subcentral in position, whilst the connecting rings are slightly inflated, suggesting the presence of suborthochoanitic septal necks. However, the depth of the septa may be greater in some cases, while no specimen preserves a part of the phragmocone sufficiently to demonstrate the presence or absence of endosiphuncular or cameral deposits. The relatively low rate of expansion of the shell, combined with the position of the siphuncle, suggests that O. bisignatum and Sactorthoceras banestanensis may be

closely related, although such possible relationships require further investigation. O. bisignatum ranges from the Berounian (late Llandeilo-earliest Ashgill) into the early Kralodvorian (early Ashgill) (Marek, 1999: Fig. 1), encompassing the stratigraphic range of S. banestanensis. Potentially, forms such as Orthoceras audax Salter, 1866, from the Ashgill Series of England and Wales could also be closely related to Sactorthoceras. However, nothing is known of the internal morphology of this species. Thus, whilst Sactorthoceras may have an extensive range along peri-Gondwana, and possibly beyond into Baltica and Laurentia, much additional material awaits discovery if this is to be demonstrated with any degree of certainty. We emphasize, however, that other elements of the Katkoyeh invertebrate faunas give a clear biogeographical signal of close relationships between the Iranian plate and other regions of north and west peri-Gondwana. Ross et al. (2000) have emphasized such linkages for the bryozoans, and Bassett et al. (1999: pp. 485) have made similar observations with regard to the brachiopod faunas. The brachiopod Drabovia aff. crassior is common in parts of the Katkoyeh Formation, giving a close correlation with the late Caradoc Zahorany Formation of Bohemia (Perunica) where the species is also a common element (Havlíček, 1950). The genus is common throughout the Bohemian Caradoc and is equally well distributed in coeval sequences of Morocco (Havlíček, 1971). Unpublished data on Katkoyeh gastropod faunas give further strength to peri-Gondwanan relationships; all known genera and species are identified with taxa described previously from the Caradoc of Bohemia (Jan-Ove Ebbestad, Uppsala; personal communication 2004). Finally, it is relevant to comment here on another cephalopod fauna described recently from the vicinity of Banestan. Niko et al. (1999) reported six actinoceride and orthoceride taxa from “an unnamed formation near Banestan village”, vaguely of “late Ordovician or early Silurian” age. These authors favored a late Llandovery or early Wenlock date for the fauna. Their map of the localities (Niko et al., 1999: Fig. 1) indicates that this fauna is at least 1 km to the north-east of Banestan village. We have (MGB and MD) investigated this area in the steep gorges above Banestan, and identify the sequence from which the Niko et al. faunas were obtained as being from the Shabdjereh Formation of Hamedi et al. (1997: Fig. 1). Associated shelly faunas suggest to us that this sequence is probably of later Silurian age (Ludlow or even early Přídolí); as such the Niko et al. fauna has no relationship with our Katkoyeh samples. Niko et al. (1999: pp. 48) suggest a close affinity of their Silurian fauna with north-eastern Laurentia. Acknowledgements This work was undertaken within the framework of a Memorandum of Agreement for collaborative research between the University of Shahid Bahonar, Kerman, Iran and the National Museum of Wales, Cardiff. We are grateful to our colleagues Leonid Popov (Cardiff) and Mozaffar Eskandarizadeh (Kerman/Cardiff) for assistance in the field.

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