Actinocerid Cephalopods from the Ordovician of ...

Actinocerid Cephalopods from the Ordovician of Myanmar, and their Paleobiogeographic Implications for Northern Gondwana Author(s): Shuji Niko and Masatoshi Sone Source: Paleontological Research, 18(2):94-103. 2014. Published By: The Palaeontological Society of Japan DOI: http://dx.doi.org/10.2517/2014PR010 URL: http://www.bioone.org/doi/full/10.2517/2014PR010

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Paleontological Research, vol. 18, no. 2, pp. 94–103, April 1, 2014 © by the Palaeontological Society of Japan doi:10.2517/2014PR010

Actinocerid cephalopods from the Ordovician of Myanmar, and their paleobiogeographic implications for northern Gondwana SHUJI NIKO1

AND

MASATOSHI SONE2

1Department

of Environmental Studies, Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8521, Japan (e-mail: [email protected]) 2 Department of Geology, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia (e-mail: [email protected]) Received September 11, 2013; Revised manuscript accepted November 10, 2013

Abstract. This paper describes a new actinocerid cephalopod fauna from the western part of the Shan Plateau in Myanmar (Sibumasu Block), and discusses its significance. The cephalopod fossils are preserved in shallow marine limestones of the Wunbye Formation (Pindaya Group) and its equivalent strata. The fauna consists of Ordosoceras theini sp. nov. (Floian or Dapingian, late Early or early Middle Ordovician age), Armenoceras myanmarense sp. nov. (Darriwilian, late Middle Ordovician), Paratunkuskoceras sp. (Darriwilian), and Wutinoceras moeseini (Floian, late Early Ordovician). The presence of Ordosoceras, which was previously known only in the North China Block, and specific characters in the actinocerid fauna of Sibumasu indicate a strong linkage with that of North China during Early–Middle Ordovician time; in contrast, Sibumasu’s affinity to the coeval Australian fauna is less definable. It is possible that an actinocerid faunal exchange took place between North China and Sibumasu over the shelf sea environment open to the Prototethys Ocean but this did not happen to the inland seas of Australian Gondwana during the period. Key words: Actinocerida, Armenoceras, Ordosoceras, Paratunkuskoceras, Sibumasu Block, Wutinoceras

Introduction and material Since Noetling (1890) documented Orthoceras sp. indet. from the east of Mandalay, many Ordovician cephalopods of Myanmar (Burma) have been described in a series of paleontological works by Reed (1906, 1915, 1932, 1936) and in a paper of Thein (1968), or have been recorded in geologic reports, such as La Touche (1913), Brown (1932), Brown and Sondhi (1933a, b), Pascoe (1959), as well as Rietschel and Nitecki (1984, in a fossil list with receptaculitid algae). However, most of the reported taxa were listed in open nomenclature. The only three exceptions that were given species names are Actinoceras moeseini Thein, 1968, Michelinoceras burmese Thein, 1968, and M. kyaukse Thein, 1968. Nevertheless, our knowledge concerning the Ordovician cephalopods of Myanmar is still very limited, and Thein’s (1968) three species need re-examination with modern taxonomic treatments. Additional information on the fauna, therefore, is particularly important. We herein describe new material consisting of four

actinocerid cephalopod taxa from Ordovician limestones of Myanmar, namely, Ordosoceras theini sp. nov., Armenoceras myanmarense sp. nov., Paratunkuskoceras sp., and Wutinoceras moeseini (Thein, 1968), and discuss the significance of this material. All the specimens described in this paper are deposited in the Geology Museum, Department of Geology, Dagon University (prefixed DUGM) in Yangon, Myanmar. The cephalopods forming the basis of this study derive from collections made by Dr. Aye Ko Aung over a 40-year period. All sampled localities of the examined specimens are in the western part of Shan Plateau (Figure 1); that is, DUGM 3101 (A. myanmarense) and 3102 (W. moeseini) are from the vicinity of Kyaukchaw village in the Mandalay-Pyin U Lwin area, Mandalay Division; DUGM 3104 (Paratunkuskoceras sp.) is from nearby the village of Tha Yauk Myaung in the same area; the exact collection sites of DUGM 3108 (O. theini) and 3109 (A. myanmarense) are not known.

Ordovician cephalopods from Myanmar

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Figure 1. Geological map (A) of the western Shan Plateau in Myanmar showing distribution of Ordovician sedimentary rocks (the Pindaya Group), together with fossil sample localities in the Mandalay-Pyin U Lwin area. Abbreviations: Pȯ–ȯ, Precambrian–Cambrian; S, Silurian; P–Tr, Permian–Triassic. Geological map modified from Bender (1983). Inserted tectonic map (B) modified after Sone et al. (2012).

Geologic setting The most reliable stratigraphic division of Lower Paleozoic sequences in the Southern Shan State is that of Thein (1973). He named the Ordovician strata in the Southern Shan State as the Pindaya Group and subdivided it into the Lokepyin, Wunbye and Nao-on formations in ascending order. The present actinocerid fossils

were collected from limestones of the Wunbye Formation and its equivalent strata in the Mandalay-Pyin U Lwin area. The Wunbye Formation is thick (1,600í2,000 m) and composed of shelf-type sediments consisting of thick-bedded limestone and dolomite with intercalations of siltstone and sandstone (Bender, 1983). The partly oolithic lithology of the limestone (Thein, 1973) suggests that it was deposited under shallow tropical marine con-

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ditions. The type stratum of the Wunbye Formation is situated near Wunbye Hill in the Southern Shan State and the stratigraphic unit name was originally applied exclusively to Middle Ordovician sediments of the state. Its geographic distribution was extended by Rietschel and Nitecki (1984) to the adjoining Mandalay-Pyin U Lwin area. There is a consensus that macrofossils, such as cystoid echinoderms and brachiopods, from the formation range in age from Early to Middle Ordovician (Reed, 1936; Pascoe, 1959; Thein, 1973). However, more detailed age information on the formation is not available, except for its lower part, where Tremadoc to early Arenig (= Floian) conodonts were found by Stoppel (in Bender, 1983). Chhibber (1934) recognized two marked geologic provinces in Myanmar, namely the eastern ShanTenasserim unit and the western Arakan-Naga unit. To our current knowledge, these units are more or less equivalent to the Sibumasu (or Shan-Thai) Block and the West Burma Block, respectively (Metcalfe, 1984, 2006; Bunopas, 1992). It is generally accepted that the Sibumasu Block was located at the western margin of northern Gondwana (Australia) during the Ordovician (e.g. Scotese and McKerrow, 1991; Metcalfe, 2006; Cocks and Torsvick, 2013).

Age Known stratigraphic ranges of the four identified genera in this study are as follows: 1) Ordosoceras is restricted to the Maruyama stage (Kobayashi, 1940, 1977) indicating the Floian and Dapingian (= Arenig), 2) Armenoceras has a long range from the early Darriwilian to late Silurian (Flower, 1957; Teichert, 1964), 3) Paratunkuskoceras is recorded from the Darriwilian (Zhu and Li, 1996), and 4) Wutinoceras has a relatively long range from the Floian to Darriwilian (Flower, 1957; Kobayashi, 1977). As discussed in the systematic paleontology and indicated in the synonymy given there, Wutinoceras moeseini can also be recognized in the Lower Setul Limestone (= the Kaki Bukit Formation in Cocks et al., 2005) of the Langkawi Islands, northern Peninsular Malaysia (Stait and Burrett, 1982). Its occurrence in Langkawi is confined to Unit J of the limestone (see the lithostratigraphic subdivision of Wongwanich et al., 1983) and this species co-occurs with the conodonts Juanognathus variabilis, Protopanderodus nogamii, and Bergstroemognathus sp. (Laurie and Burrett, 1992). The associated conodont assemblage is indicative of a Floian age (e.g. Agematsu et al., 2008; Zhen et al., 2009). Although exact stratigraphic positions of the current four taxa are not determinable, they can be assigned into two (or three) chronostratigraphic levels, that is, the Floian (late

Early Ordovician) for Wutinoceras moeseini, the Floian or Dapingian (late Early or early Middle Ordovician) for Ordosoceras theini, and the Darriwilian (late Middle Ordovician) for both Armenoceras myanmarense and Paratunkuskoceras sp.

Significance of the actinocerid fauna Considering the generic composition of the present Myanmar actinocerid fauna, the presence of Ordosoceras is of particular importance. Occurrences of the genus have previously been restricted to Inner Mongolia (Chang, 1959, 1960; Chen and Zou, 1984), Shangdong (Chen, 1976), and Liaoning (Kobayashi, 1977) of the North China Block (Figure 2). Previous records of Paratunkuskoceras were also restricted to North China (Zhu and Li, 1996). The principal generic characters are given by Zhu and Li (1996); that is, Paratunkuskoceras has Ormoceras-like siphuncular segments but its septal necks are armenoceratoid. In fact, many nautiloid taxonomists often underestimated the importance of details of septal neck shapes in actinocerids before a clear definition was published in the Treatise (Teichert, 1964). Thus, older records of “Ormoceras,” which compositely show a cosmopolitan distribution, are likely a mixture of several different genera, possibly including Paratunkuskoceras. Armenoceras is a cosmopolitan genus; it is common in faunas of the Sibumasu Block and the North China Block. Wutinoceras is also common in the Sibumasu and North China blocks, and has a wide distribution to Tibet (Chen, 1975), Tasmania (Teichert and Glenister, 1953; Stait, 1984), and North America (Flower, 1968). It is noteworthy that the species most closely related to Ordosoceras theini, Armenoceras myanmarense, and Wutinoceras robustum are recognized in a fauna of the North China Block, as discussed in the following systematic paleontology. In addition, the other Ordovician actinocerids Georgina and Mesaktoceras are typical for faunas of inland seas (the Amadeus and Georgina basins) of Australian Gondwana (Wade, 1977) but are absent from the Sibumasu fauna (Figure 2). Although Stait and Burrett (1984) and Stait et al. (1987) reported some forms of these two genera from the Ordovician limestones near the border between Thailand and Malaysia, their generic assignments are questionable because of poor preservation of the examined material. In conclusion, the Floian to Dapingian actinocerid fauna of the Sibumasu Block shows a strong affinity with that of the North China Block; in contrast, such an affinity between Sibumasu and Australia is not obvious. Considering that Sibumasu is supposed to have adjoined Australian Gondwanaland during the period, these results are important. It is possible that an actinocerid

Ordovician cephalopods from Myanmar

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Figure 2. Distribution of Ordosoceras plotted on an Arenigian (= Floian to Dapingian; late Early to early Middle Ordovician) paleogeographic reconstruction map (base map modified from Scotese and McKerrow, 1991). Abbreviations: 1, Inner Mongolia; 2, Liaoning; 3, Shangdong; 4, Myanmar. Occurrences of Ordosoceras in North China are based on Chang (1959, 1960, 1976), Kobayashi (1977), and Chen and Zou (1984).

faunal exchange between Sibumasu and North China took place over the shelf sea environment open to the Prototethys Ocean but this did not happen to the inland seas of Australia during the Ordovician.

Systematic paleontology Order Actinocerida Teichert, 1933 Family Polydesmiidae Kobayashi, 1940 Genus Ordosoceras Chang, 1959 Type species.—Ordosoceras sphaeriforme Chang, 1959. Ordosoceras theini sp. nov. Figures 3.1í3.3, 4.10

Diagnosis.—Species of Ordosoceras with relatively large siphuncle and weakly recumbent brims; ratios of siphuncular segments (maximum diameter/length) large, attaining values of 3.1–3.4; central canal subdorsal; dorsal radial canals branching off with steep obliqueness for central canal; distal ends of radial canals indicating weak adoral curvature; perispatium wide with crescentic profile in longitudinal section. Description.—A single incomplete conch is available for study. It is an orthoconic phragmocone 55 mm in

length and probably has a circular cross section; dorsal shell partly missing and outer shell wall not preserved; reconstructed conch diameter (internal mold) near adoral end is approximately 32 mm. Septal curvature and cameral length moderate for the genus; sutures directly transverse and roughly straight; siphuncle relatively large, subcentral in position, siphuncular wall consists of cyrtochoanitic septal necks and thin connecting rings; diameters of septal foramina range from 4.3 to 5.1 mm; brims are weakly recumbent; length of brims and septal necks are 0.7–0.9 mm and 1.3–1.4 mm respectively; connecting rings weakly inflated in their apical 1/5 to 2/5 part, then exhibiting abrupt inflation; maximum diameters of connecting rings are 9.8–11.9 mm; adnation area narrow; each siphuncular segment discoid with ringlike projection consisting of abruptly inflated part of connecting ring in its midlength; form ratios of segments (maximum diameter/length) are large, attaining values of 3.1–3.4. Cameral deposits well developed as episeptalmural and hyposeptal deposits on ventral side and as episeptal ones on dorsal side; siphuncle filled with endosiphuncular deposits except for vascular system, latter is reticulate and consists of central canal, radial canals and perispatium; central canal subdorsal, located near the dorsal septal necks, having approximately 0.4 mm in maximum diameter; distal parts of ventral radial canals curved,

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Figure 3. 1í3, Ordosoceras theini sp. nov., holotype, DUGM 3108; 1, longitudinal polished section, dorsoventral view, venter on left, submerged in water; 2, partial enlargement of Figure 3.1 to show general siphuncular shape; 3, partial enlargement of Figure 3.1 to show details of septal necks, connecting rings, and endosiphuncular canals. 4í6, Armenoceras myanmarense sp. nov.: 4, 5, holotype, DUGM 3101; 4, longitudinal weathered section, side view, coated with ammonium chloride, arrow indicates position of cross sectional view in Figure 4.6; 5, partial enlargement of Figure 3.4 to show details of septal necks, connecting rings, and endosiphuncular canals; 6, paratype, DUGM 3109, longitudinal thin section, showing details of septal neck. 7, Paratunkuskoceras sp., partial enlargement of Figure 4.9 to show details of septal necks, connecting rings, and endosiphuncular canals of ventral siphuncular wall. Scale bar is 20 mm in Figure 3.1; 10 mm in Figure 3.2; 3.8 mm in Figure 3.3; 30 mm in Figure 3.4; 6 mm in Figure 3.5; 3 mm in Figures 3.6 and 3.7.

Ordovician cephalopods from Myanmar their proximal parts not preserved; dorsal radial canals adapically branching off from central canal with steep obliqueness, then turning outwards; distal ends of ventral and dorsal radial canals indicating weak adoral curvature and anastomose with perispatium near tips of brims; perispatium wide, having crescentic profile in longitudinal section, but almost occluded by perispatial deposits. Etymology.—The species name honors Dr. Myint Lwin Thein, in recognition of his works on the taxonomy of the Ordovician cephalopods and the stratigraphy of the lower Paleozoic in Myanmar. Type.—Holotype, DUGM 3108. Occurrence.—The Wunbye Formation (or equivalent strata, see the paragraphs above on the geologic setting) in the western part of Shan Plateau, Myanmar; Floian or Dapingian, late Early or early Middle Ordovician age. Discussion.—The presence of a ringlike projection in the midlength of each siphuncular segment of the Myanmar species is unique for actinocerids. Except for Ordosoceras, only Meitanoceras Chen and Liu (1974; type species, M. subglobosum Chen and Liu, 1974, from the Lower Ordovician of Guizhou, Southwest China) and Pomphoceras Zou and Shen in Chen and Zou (1984; type species, P. wuhaiense Zou and Shen in Chen and Zou, 1984, from the Lower Ordovician of Ordos, North China) share this character. Kobayashi (1977) distinguished Meitanoceras from polydesmiids on the basis of its considerably taller and narrower siphuncular segments and straight radial canals, and erected the new family, Meitanoceratidae; in this sense, Pomphoceras can also be included in this family. Large form ratios of the siphuncular segments (maximum diameter/length) and the steeply oblique radial canals for the central canal of Ordosoceras theini sp. nov. warrant its familial and generic placements. Ordosoceras theini from Myanmar represents the first record of the genus outside North China. Ordosoceras theini is most similar to O. jeholense Kobayashi (1977, p. 128–130, pl. 17, figs. 1, 2, pl. 18, figs. 2–4) from the Nanpiao area, Liaoning in its general siphuncular segment shape and subdorsal position of the central canal, but the oblique hornlike structure in the endosiphuncular deposits of O. jeholense is not developed in the present new species. Ordosoceras quasilineatum Chang (1959, p. 263, 264, 275, 276, pl. 6, figs. 3, 4) from the Ordos area, Inner Mongolia, is somewhat similar to O. theini, but it clearly differs in its subcentrally positioned central canal. Family Armenoceratidae Troedsson, 1926 Genus Armenoceras Foerste, 1924 Type species.—Actinoceras hearsti Parks in Tyrrell, 1913.

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Armenoceras myanmarense sp. nov. Figures 3.4í3.6, 4.6

Diagnosis.—Species of Armenoceras with dorsoventrally depressed cross section and deep septa indicating lateral diameter/depth ratios of approximately 4.3í5.3; siphuncle large with maximum diameter of connecting ring/corresponding conch diameter ratios of approximately 0.5; form ratios of siphuncular segments (maximum diameter of connecting ring/length) are 2.5í3.0; proximal portions of radial canals oblique and curved, then distally shifting straight. Description.—Longiconic orthocones with gradual shell expansion and dorsoventrally depressed cross section; holotype of imperfect phragmocone attaining 158 mm in length and approximately 46 mm in lateral diameter (internal mold) near adoral end; external shell wall and body chamber are not preserved in examined specimens. Septa deep for the genus; their form ratios (lateral diameter/depth) are approximately 4.3í5.3; cameral length moderate; there are 5í6 camerae in length of corresponding lateral conch diameter; siphuncle large, probably subcentral in position; siphuncular wall consisting of armenoceratoid septal necks and thin connecting rings; in the holotype, diameters of septal foramina are 7.1 and 10.5 mm at lateral conch diameters of 35 and 45 mm, respectively, and maximum diameters of connecting rings are 17.5 and 22.3 mm at lateral conch diameters of 35 and 45 mm, respectively; ratios of maximum diameter of connecting ring per corresponding lateral conch diameter are approximately 0.5; siphuncular segments thickly discoidal indicating form ratios (maximum diameter of connecting ring/length) of 2.5í3.0; brims strongly recumbent to form short septal necks; length of brims long, attaining 1.8 mm at septal foramen diameter of 7.5 mm in paratype; connecting rings indicating strong inflation, with moderately and very wide adnation areas respectively on apical and adoral surfaces of septum. Cameral deposits well developed in episeptal-mural and hyposeptal positions; siphuncle filled with endosiphuncular deposits except for vascular system, that is reticulate and differentiated into central canal, radial canals and perispatium; proximal portions of radial canals obliquely branching off from central canal and curved, then shifting straight with right angles for central canal; distal ends of radial canals anastomosing with perispatium near the most inflated portions of the connecting rings. Etymology.—The species name is derived from Myanmar. Types.—Holotype, DUGM 3101. Paratype, DUGM 3109. Occurrence.—The Wunbye Formation (or its equivalent strata, see discussion of geological setting above) in

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Figure 4. 1í5, 7, Wutinoceras moeseini (Thein, 1968), DUGM 3102; 1, longitudinal polished section, side view, submerged in water; 2, weathered cross section of adoral end, venter down, submerged in water; 3í5, partial enlargements of Figure 4.1 to show details of septal necks, connecting rings, endosiphuncular canals, and endosiphuncular deposits; 7, partial enlargement of Figure 4.1 to show general siphuncular shape. 6, Armenoceras myanmarense sp. nov., holotype, DUGM 3101, cross section, venter down, submerged in water. 8, 9, Paratunkuskoceras sp., DUGM 3104; 8, partial enlargement of Figure 4.9 to show details of septal necks and connecting rings of dorsal siphuncular wall; 9, longitudinal polished section, venter on left, submerged in water; 10, Ordosoceras theini sp. nov., holotype, DUGM 3108, weathered cross section of apical end, venter down, submerged in water. Scale bar is 30 mm in Figures 4.1 and 4.2; 3 mm in Figures 4.3í4.5 and 4.8; 20 mm in Figures 4.6, 4.9 and 4.10; 15 mm in Figure 4.7.

Ordovician cephalopods from Myanmar the western part of Shan Plateau, Myanmar; Darriwilian, late Middle Ordovician age. Discussion.—Armenoceras myanmarense sp. nov. shares several characteristics with some North Chinese species, namely, A. elongatum Endo (1932, p. 85, 86, pl. 17, figs. 1í4, pl. 18, fig. 8) and A. manchurense (Kobayashi, 1927, p. 194, pl. 20, fig. 10, pl. 21, fig. 7, pl. 22, figs. 2a, b), both from the Benxi area in Liaoning, as well as A. submarginale (Grabau, 1922, p. 86, 87, pl. 8, figs. 5a, b, pl. 9, fig. 3) from the Tangshan area in Hebei; for example, the dorsoventrally depressed cross section of the conchs and the large siphuncle in comparison with corresponding conch diameter are similar between these species. They may form a species-group complex within the genus. Among these Chinese species, A. submarginale is most similar to the new species, but it differs in having more shallowly concave septa (approximate diameter/depth ratios 8.4í9.1 versus 4.3í5.3 in A. myanmarense). Armenoceras elongatum and A. manchurense have more depressed siphuncular segments than those of A. myanmarense and in them the distal portions of the radial canals are weakly curved. Form ratios of their siphuncular segments are 3.7í4.8 and 3.6í4.3, respectively. Two named species of Armenoceras have been recorded in the Sibumasu Block. Armenoceras chediforme Kobayashi (1958, p. 229, 230, text-fig. 3, pl. 17, figs. 2a, b) was described by Kobayashi (1958) from the Thung Song Formation (Llanvirnian = Darriwilian, late Middle Ordovician) in the Thung Song area, southern Thailand. Burton (1974) ascribed other material from the same formation to A. coreanicum Kobayashi (1934, p. 449, pl. 30, figs. 1, 2). Subsequently, A. chediforme was also reported from northern Malaysia (Kobayashi, 1959; Stait et al., 1987), and some localities in northern to western Thailand (Kobayashi, 1961; Ingavat et al., 1975; Stait and Burrett, 1984). Armenoceras myanmarense can be easily distinguished from A. chediforme in having lower form ratios of siphuncular segments and from A. coreanicum in having larger siphuncular diameters for corresponding conch sizes and longer brims. Genus Paratunkuskoceras Zhu and Li, 1996 Type species.—Actinoceras suanpanoides Grabau, 1922. Paratunkuskoceras sp. Figures 3.7, 4.8, 4.9

Description.—A fragment of a longiconic phragmocone exhibits a faint exogastric curvature and is approximately 39 mm long. Septal curvature and spacing moderate for the genus; siphuncle subcentral in position;

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siphuncular segments subglobular to ellipsoid, with form ratios (maximum diameter of connecting ring/length) of 1.9í2.4; septal necks armenoceratoid; brims short and strongly recumbent, so that their adoral surface adheres to the septum. Cameral and endosiphuncular deposits well developed; endosiphuncular canals differentiated into central canal and nearly straight radial canals. Material examined.—DUGM 3104. Occurrence.—The Wunbye Formation (or its equivalent strata, see discussion above of geologic setting) in the western part of Shan Plateau, Myanmar; Darriwilian, late Middle Ordovician age. Discussion.—This species appears similar to the type species of the genus, Paratunkuskoceras suanpanoides (Grabau, 1922, p. 84í86, pl. 8, figs. 3a, b, 4a, b, pl. 9, figs. 1a, b), from the Taian area in Shangdong, North China, in its faint endogastric curvature of the conch, nature of the septa, siphuncular position and profiles of the siphuncular segments. Poor preservation of the single available specimen does not allow further comparisons. Paratunkuskoceras sp. from Myanmar represents the first record of the genus outside North China. Family Wutinoceratidae Shimizu and Obata, 1935 Genus Wutinoceras Shimizu and Obata, 1936 Type species.—Nybyoceras foerstei Endo, 1930. Wutinoceras moeseini (Thein, 1968) Figure 5.1í5.5, 5.7 Actinoceras moeseini Thein, 1968, p. 71, figs. 4í6, 11, 12. Wutinoceras robustum (Kobayashi and Matsumoto). Stait and Burrett, 1982, p. 194, figs. 2-AíL (not W. robustum (Kobayashi and Matsumoto) = Jeholoceras robustum Kobayashi and Matsumoto, 1942).

Description.—One large longiconic phragmocone is available for study. It is 65 mm long, with gradual expansion; conch cross section circular, approximately 70 mm in maximum diameter. Septa relatively deep for the genus; there are 3 camerae per half diameter of the corresponding conch; siphuncle subventral in position; siphuncular wall consisting of recumbent septal necks and relatively thick connecting rings; ratios of maximum diameter of connecting ring per corresponding conch diameter are approximately 0.3; siphuncular segments ellipsoid, indicating form ratios (maximum diameter of connecting ring/length) of 2.0í2.2; brims relatively long, 1.6í2.3 mm in length, having moderate curvature for the genus; septal necks 0.9í1.3 mm in length. Cameral deposits well developed at episeptal-mural and hyposeptal positions; endosiphuncular deposits filling siphuncle except for reticulate vascular system; central canal wide

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with subcircular to indistinct substellate cross sections; radial canal curved and frequently anastomosed; perispatium very wide, but partly occluded by perispatial deposits. Material examined.—DUGM 3102. Occurrence.—The Wunbye Formation (or its equivalent strata, see discussion above of geologic setting) in the western part of Shan Plateau, Myanmar and the Lower Setul Limestone of the Langkawi Islands, northern Peninsular Malaysia; Floian, late Early Ordovician age. Discussion.—Based on the two incomplete specimens from the Wunbye Formation in Phyauk Seik Pin, east of Kyaukse, Shan Plateau region, Thein (1968) erected the new species Actinoceras moeseini. Its recumbent septal necks with relatively long brims and reticulate vascular system indicate that this species should be placed in the genus Wutinoceras, that is, Wutinoceras moeseini comb. nov. The current specimen has no significant difference from the syntypes of W. moeseini, and they are here considered conspecific. Wutinoceras robustum (Kobayashi and Matsumoto, 1942, p. 316, 317, pl. 30, figs. 1í5, pl. 31, fig. 6) from the Nanpiao area in Liaoning, North China is closely related with W. moeseini. However, W. robustum differs from the Myanmar species in having larger form ratios of the siphuncular segments (maximum diameter of connecting ring/length), attaining more than 2.5. Stait and Burrett (1982) reported material from the Lower Setul Limestone of the Langkawi Islands, northern Peninsular Malaysia as W. robustum. However, because the siphuncular form ratios of the Langkawi specimens are approximately 2.0, they can be assigned to W. moeseini.

Acknowledgements All the examined fossils and information concerning their localities were provided by Aye Ko Aung (currently at the University of Malaya). The manuscript benefited from the constructive reviews of M. Ehiro (Tohoku University Museum) and C. Klug (University of Zurich). This research was supported by a High Impact Research grant UM.C/625/1/HIR/140 from the University of Malaya.

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