EARLY ORDOVICIAN CONODONTS FROM TARUTAO ISLAND, SOUTHERN PENINSULAR THAILAND
Abstract
Abstract: Early Ordovician conodont faunas of the Thung Song Formation on Tarutao Island, southern peninsular Thailand, consist of 14 known species belonging to 17 genera, and eight undescribed species. Utahconus tarutaoensis and Filodontus tenuis are new species. Three conodont zones: the Rossodus manitouensis Zone, the Utahconus tarutaoensis Zone and the Filodontus tenuis Zone, in ascending order, are defined in the study sections. These are coeval with the interval from the Rossodus manitouensis Zone to the Acodus deltatus‐Oneotodus costatus Zone of the standard zonation in the North American Midcontinent. Based on the conodonts studied here and fossils previously reported from Tarutao Island, the Thung Song Formation is early Tremadocian to middle Arenig (Ibexian) in age. This formation is lithostratigraphically subdivided into the S1 to S5 members, and our study sections consist of the S1 to S3 members. These strata accumulated on a shelf in the Early Ordovician. The depositional environments of the limestones making up the S1 and S3 members were in deeper‐shelf conditions. Limestone and shale of the S2 member formed in a shallow‐water, high‐energy environment.
The Sibumasu Block is one of the smaller continental blocks that existed in northern Gondwana during the early and middle Palaeozoic. It extends today from Myanmar and southwestern China in the north, to Sumatora Island in the south (Metcalfe 1999). Lower Palaeozoic sedimentary rocks are widely distributed on this block and contain macro‐ and microfossils (Kobayashi 1964; Kobayashi and Hamada 1964a, b; Igo and Koike 1967, 1968; Hamada et al. 1975; Hahn and Siebenhüner 1982; Stait and Burrett 1984; Boucot et al. 1999; Fortey and Cocks 1998; Agematsu et al. 2006a–c, 2007). Southern peninsular Thailand is one of the principal areas for palaeontological and stratigraphic studies of the Lower Palaeozoic of the Sibumasu Block (Wongwanich et al. 1990). In particular, Cambrian and Lower Ordovician rocks are well exposed on Tarutao Island, which is near the west coast of peninsular Thailand (Cocks et al. 2005). Several faunas with trilobites, nautiloids, brachiopods and conodonts have been reported from Tarutao Island (Teraoka et al. 1982; Fortey and Cocks 1998; Cocks et al. 2005). These fossil collections added materially to the detailed biostratigraphy of Lower Palaeozoic sequences in Southeast Asia. We investigated Ordovician rocks and collected limestone samples on Tarutao Island in 2004, and recovered abundant Early Ordovician conodonts. The present study describes and correlates these conodont faunas and discusses the stratigraphy of the Ordovician sequence on Tarutao Island.
Geological setting
It is the general consensus that the geology of Thailand comprises two principal continental blocks, the western Sibumasu Block and the eastern Indochina Block (Bunopas 1981; Metcalfe 1999). According to Bunopas (1981), these two blocks have a suture zone between them that is bordered by the western Sukhothai and eastern Loei‐Petchabun Fold belts. The area of the suture zone is referred to as Nan‐Uttradit in the north and Sra Kaeo‐Chanthaburi in the south (Text‐fig. 1A). The Cambrian and Ordovician rocks in Thailand have been referred to the Tarutao and Thung Song groups, respectively (Bunopas 1981). The Tarutao Group, which comprises clastic rocks, is typically exposed on Tarutao Island and is also present in the Kanchanaburi and Chiang Mai areas of western and northwestern Thailand respectively (Bunopas 1992). The Thung Song Group is mainly composed of carbonate rocks and widely distributed in northwestern, western and southern Thailand (Bunopas 1992). The type locality of this group is in Nakhon Sri Thammarat province, southern peninsular Thailand.
A, index map showing the study area. B, map of Tarutao Island and a distribution of the Thung Song Formation. C, geological map of southern coast of the island and sampling points in Section A.
Tarutao Island, which extends 12 km north to south and 5 km east to west, lies about 25 km west of Satun in southern peninsular Thailand (Text‐fig. 1B). Cocks et al. (2005) correlated the Lower Palaeozoic sequences of the Sibumasu Block and redefined the Tarutao and Thung Song Groups on Tarutao Island as the Tarutao Formation and the Thung Song Formation (Thung Song Limestone), respectively. According to these workers, the Tarutao Formation is subdivided into lower, middle and upper members. The lower member mainly comprises thickly bedded mudstones, and the middle member consists of sandstone strata. The upper member is composed of sandstone beds with limestone lenses. Although Cocks et al. (2005) did not subdivide the Thung Song Formation, Teraoka et al. (1982) classified limestones of this formation into five members, which are, in ascending order, the S1, S2, S3, S4 and S5 members. According to Teraoka et al. (1982), the S1 member is composed of thinly bedded limestones interlaminated with shales, and the S2 member is characterized by alternations of limestones and red, grey and green shales. The S3 member is divided into a lower part, which is similar to the S1 member, and an upper part of thickly bedded limestone. The lithology of the S4 member is almost the same as that of the S2 member. The S5 member is composed of thinly and thickly bedded limestones. In this study, we follow the nomenclature of Teraoka et al. (1982) and Cocks et al. (2005) and use the Tarutao and Thung Song Formations and the S1 to S5 members.
Cocks et al. (2005) reviewed occurrences of fossils, including trilobites, nautiloids, brachiopods and conodonts, that have been reported from Tarutao Island. They concluded that the Tarutao and Thung Song formations on Tarutao Island range approximately from Cambrian to Middle Ordovician (Arenig) in age, and that the Cambrian–Ordovician boundary probably lies within the middle member of the Tarutao Formation.
Lithology and depositional environments
Conodonts were recovered from sections A and B on Tarutao Island (Text‐fig. 1B–C). Section A lies along the southeastern coast of the island, and Section B is situated along a path to a hilltop in at the northwestern part of the island. Section A mainly comprises limestones that are partially intercalated with shale beds. The rocks strike N 10° to 20°E and dip 30° to 50° east (Text‐fig. 1C). Teraoka et al. (1982) showed the distribution of the Tarutao and Thung Song formations on this island and presumed that the boundary of these formations could be found on the southeastern coast between the sample localities TAR‐23 and TAR‐24 of the present study. However, the boundary of the Tarutao and Thung Song formations on the southeastern coast is actually to the west of sample locality TAR‐16 (Text‐fig. 1C). The sequence measured in this section is about 1000 m thick, and is classified into three members as follows, in ascending order: the S1 member of limestones with thin shale beds (about 600 m); the S2 member of alternating limestone and shale (about 200 m); the lower part of the S3 member of grey, bedded limestones (about 200 m). These three members are conformable (Text‐fig. 2).
Lihological column with stratigraphic distributions of conodont species, conodont zones and sea‐level change in Section A.
Limestones in the S1 member are subdivided into a lowermost part (about 30 m thick) and a main part (about 570 m thick) (Text‐fig. 2). Rocks in the lowest part of the S1 member consists of dark grey, argillaceous limestone alternated with black shale beds, and are 1–30 cm in thickness. The main part of the S1 member comprises thickly bedded, grey limestones. These limestones are micrite or pelmicrite. The S2 member consists of interbeds of grey limestones, 0.1–1 m thick, and red shales (Text‐fig. 2). Seen under a microscope, the limestone is micrite or biomicrite with abundant quartz grains, which frequently form cross‐laminations. The shale beds, mostly red in colour, are composed of silt‐size quartz and calcite grains and contain some fine sand‐size grains. Grey, bedded limestone in the lower part of the S3 member has beds 1–30 cm thick (Text‐fig. 2) and are micrites or pelmicrites similar to those of the S1 member.
Section B is made up of 40 m of limestone beds, which strike N 40°W to N 70°W and dip 30° to the northeast. This sequence comprises grey, massive limestones interbedded with laminated limestones. One interval of massive limestone is more than 50 cm thick. The limestones of this section correspond to member S1.
Depositional environments
The depositional environments of Section A are inferred from field observations and thin section analyses. The reconstructed paleoenvironments and sea‐level changes within Section A are shown in Text‐fig. 2. The limestone facies in the lowermost part of the S1 member implies that its depositional environment was a shelf with high‐energy conditions with a continuous influx of terrigenous grains. The environment became a deeper‐water shelf owing to transgression, and persisted during a limestone deposition of the S1 member. Interbeds of limestone and shale of the S2 member represent the existence of a low‐sea‐level period. This interval was characterized by the inflow of large amounts of terrigenous material and strong currents, which formed cross laminations. The alternation of limestone and shale implies cyclical changes of environmental factors, such as depth, salinity, temperature and turbidity. These factors may have been more or less influenced by transgressions and regressions. The lower part of the S3 member is thought to have been deposited under deeper water conditions, similar to the environment of the S1 member.
Conodont biostratigraphy
Most samples from each stratigraphic section yielded conodonts. Seventeen species belonging to 14 genera, and eight unidentified species, are identified (Table 1). The following three conodont zones are established in Section A, in ascending order: the Lower Ordovician Rossodus manitouensis Zone, the Utahconus tarutaoensis Zone and the Filodontus tenuis Zone (Text‐fig. 2).
Rossodus manitouensis Range Zone
This zone covers the S1 member in Section A (Text‐fig. 2). The lower and upper boundaries of this zone are marked by the first and last occurrences of Rossodus manitouensisRepetski and Ethington, 1983. This zone yields other species, including Variabiloconus bassleri (Furnish, 1938), Teridontus obesusJi and Barnes, 1994, Utahconus longipinnatusJi and Barnes, 1994 and Acanthodus uncinatusFurnish, 1938. Acanthodus lineatusFurnish, 1938 and Scolopodus sp. are present in the lower part of the zone. Coniform indet. A, B and C are included in the upper part. Drepanoistodus forceps (Lindström, 1955) appears in the uppermost part of this zone and ranges into younger strata of the section (Text‐fig. 2).
Utahconus tarutaoensis Range Zone
This zone ranges through the upper 100 m of the S2 member in Section A (Text‐fig. 2), and coincides with the stratigraphical range of Utahconus tarutaoensis sp. nov. Drepanoistodus pervetusNowlan, 1985, Paroistodus numarcuatus (Lindström 1955) and Striatodontus prolificusJi and Barnes, 1994 are another species characteristic of this zone (Text‐fig. 2). This zone also includes U. longipinnatus, D. forceps, Protoprioniodus yapuCooper, 1981 and Coniform indet. H.
Filodontus tenuis Range Zone
This is the uppermost zone of Section A. The base and top of the zone, about 100 m and 30 m below the top of the section, respectively, are defined by the appearance and disappearance of Filodontus tenuis sp. nov. Other representative species of this zone are D. forceps and Parapanderodus striatus (Graves and Ellison, 1941) (Text‐fig. 2). S. prolificus, Scolopodus sp., P. yapu, Acodus sp., Coniform indet. D and Coniform indet. F also occur in this zone.
Comparison between sections A and B on Tarutao Island
Section B includes five species belonging to five genera and two unidentified species (Table 1). Teridontus obesus, Variabiloconus bassleri and Acanthodus uncinatus occur through the section. The lower and middle parts of this section yields Coniform indet. E and Drepanodus arcuatusPander, 1856, respectively. Utahconus longipinnatus and Coniform indet. A are present in the upper part of this section. Section B does not includes Rossodus manitouensis. However, this section is inferred to be comparable to the R. manitouensis Zone of Section A, because Section B shares all species, except for Coniform indet. E, with the zone of Section A. This fact supports the above‐mentioned correlation based on lithostratigraphy.
Correlation
Correlations of the study sections with other Ordovician sequences worldwide are possible. First, we will compare Section A to Lower Ordovician successions in adjacent areas. Next, the zonation established in Section A will be correlated to the standard zonation of the North American Midcontinent (Ross et al. 1997; Webby et al. 2004), to zones of the Midcontinent realm in British Columbia (Pyle and Barnes 2002) and to shallow‐water zones in Newfoundland (Ji and Barnes 1994). These correlations are shown in Text‐fig. 3.
Correlation of the Ordovician conodont zones of the study section with zonations of the North American Midcontinent, North Atlantic, British Columbia and Newfoundland.
Comparison with conodont faunas in Thailand and Malaysia
Metcalfe (1980) and Agematsu et al. (2006c) studied Early Ordovician conodonts from Malaysia and Thailand, respectively. Metcalfe (1980) recovered Ordovician conodonts from the Setul Limestone in the Peris area of peninsular Malaysia. He presented a list of the Early and Middle Ordovician conodont faunas, which occurred at localities 1, 2 and 3 in (Metcalfe 1980). According to him, the ages of the faunas from localities 1 and 3 are Early Ordovician and the fauna from locality 2 is Middle Ordovician. The fauna from locality 1 is Tremadocian in age, because it includes Loxodus bransoniFurnish, 1938, which has been described from strata corresponding to the Rossodus manitouensis Zone of the standard zonation of the North American Midcontinent (Ethington and Clark 1982). However, the fauna he studied does not contain species in common with faunas of the present study. On the basis of the occurrence of Serratognathus bilobatusLee, 1970, the fauna from locality 3 Arenig, and therefore younger than the faunas from Tarutao Island.
Teraoka et al. (1982) first reported Ordovician conodont faunas from Tarutao Island. They recovered Drepanodus arcuatus, Drepanoistodus basiovalis (Sergeeva, 1963), Drepanoistodus forceps, Paroistodus parallelus (Pander, 1856), Paroistodus proteus (Lindström, 1955) and Protopanderodus rectus (Lindström, 1955) from limestones of the S1 member and the lower part of the S3 member. However, P. parallelus and P. proteus clearly indicate a younger age than the faunas in our study. Teraoka et al. (1982) photographically illustrated only D. arcuatus, D. basiobalis and D. forceps without description, so it is difficult to compare the details of their specimens with conodonts in this study. Taking into account the poor preservation of specimens in Teraoka et al. (1982), their identification may be questionable.
Comparison with the zonation of the North American Midcontinent
The standard zonation for Early Ordovician conodonts in the North American Midcontinent, summarized by Ross et al. (1997) and Webby et al. (2004), is shown in Text‐fig. 3. Rossodusmanitouensis, Variabiloconus bassleri, Acanthodus lineatus and Acanthodus uncinatus, which are representative species of the R. manitouensis Zone of Section A in this study, are common to the R. manitouensis Zone in the Midcontinent area (Ethington and Clark 1981; Repetski and Ethington 1983; Landing et al. 1996; Sweet and Tolbert 1997). D. forceps and Parapanderodus striatus occur from the upper part of Section A in this study and are relatively long‐ranging species. The oldest occurrence of P. striatus is in the Low‐diversity interval of the Midcontinent (Ethington and Clark 1981). Paroistodus numarcuatus has not been described from the Midcontinent, but it is a representative species of the North Atlantic region and has been reported from the upper Paltodus deltifer Zone to the lower P. proteus Zone (Löfgren 1997). Therefore, the R. manitouensis Zone in this study is coeval with the R. manitouensis Zone in the North American Midcontinent. Based on the stratigraphic range of Pa. striatus and P. numarcuatus, the Utahconus tarutaoensis and Filodontus tenuis zones of the study section are comparable to the Low‐diversity interval to the Acodus deltatus‐Oneotodus costatus Zone in the Midcontinent (Text‐fig. 3).
Comparison with zonation in British Columbia
Pyle and Barnes (2002) used the Lower Ordovician zonation of the Midcontinent realm for the Kechika Formation in British Columbia (Text‐fig. 3). Because the R. manitouensis Zone in the present study shares several species, including R. manitouensis, V. bassleri, Utahconus longipinnatus, A. lineatus and A. uncinatus, with the R. manitouensis Zone of the Kechika Formation, these zones are correlated with each other. T. obesus has been described from the Iapetognathus Zone and D. pervetus and S. prolificus from the R. manitouensis Zone of the Kechika Formation (Pyle and Barnes 2002). The oldest known Pa. striatus in British Columbia has been reported from the Scolopodus subrex Zone. As mentioned above, the upper limit of the occurrence of P. numarcuatus is the P. proteus Zone in the North Atlantic region. These lines of biostratigraphic evidence indicate that the U. tarutaoensis and F. tenuis zones of the study section are comparable to the S. subrex Zone and to the Acodus kechikaensis Zone in British Columbia.
Comparison with zonation in Newfoundland
The Lower Ordovician shallow‐water zones of the St. George Group in Newfoundland, proposed by Ji and Barnes (1994), are shown in Text‐fig. 3. Several representative Tarutao Island species are known from Newfoundland. T. obesus, V. bassleri, R. manitouensis, A. lineatus, A. uncinatus, U. longipinnatus and D. pervetus occur from the R. manitouensis‐Plycostatus sulcatus Zone to the Colaptoconus floweri‐Colaptoconus bolites Zone of the St. George Group. S. prolificus ranges from the C. floweri‐C. bolites to S. prolificus‐Striatodontus lanceolatus zones in Newfoundland. Pa. striatus is one of the youngest species in the St. George Group and occurs in the Parapanderodus carlae‐Paraserratognathus ovatus Zone and overlying zones. Based on the stratigraphical range of these species and above‐mentioned P. numarcuatus, zonal correlation between the study section and the St. George Group is as follows: the R. manitouensis Zone in the study section is coeval with the R. manitouensis‐P. sulcatus and C. floweri‐C. bolites zones; the U. tarutaoensis and F. tenuis zones in this study are compared to the C. floweri‐C. bolites to S. prolificus‐S. lanceolatus zones and the P. carlae‐S. ovatus Zone, respectively. The stratigraphic ranges of S. prolificus and D. pervetus on Tarutao Island are thought to extend into younger strata than do those in other areas.
Age of the Thung Song Formation
Text‐figure 4 shows the stratigraphic sequence of the Tarutao and Thung Song Formations and their ages, based on conodonts in this study and previously reported fossils. Section A in this study correlates lithostratigraphically with the interval from the S1 member to the lower part of the S3 member in the Thung Song Formation. As mentioned above, Teraoka et al. (1982) reported Ordovician conodont faunas from the S1 and S3 members on Tarutao Island, but several species were probably misidentified. Teraoka et al. (1982) also illustrated conodonts from the uppermost part of the Tarutao Formation and the S4 and S5 members of the Thung Song Formation. These include abundant well‐preserved specimens, in contrast with their faunas from the S1 and S3 members. The presence of Chosonodina herfurthiMüller, 1964 indicates that the uppermost strata of the Tarutao Formation also correlate with the R. manitouensis Zone in the North American Midcontinent. The faunas from the S4 and S5 members contain Acodus deltatusLindström, 1955, and Oepikodus evae (Lindström, 1955), which are early and middle Arenig species. The ages of these faunas accords with those in the present study.
Column showing the stratigraphic sequences from the Late Cambrian to the Early Ordovician on Tarutao Island and relative positions of some of faunas reported from the island.
Several macrofossils faunas have been reported from the Thung Song Formation of Tarutao Island (Text‐fig. 4). Tremadocian brachiopods have been illustrated by Fortey and Cocks (1998) from limestone in the S1 member, which is exposed along the northwestern coast of the island. Although Cocks et al. (2005) revised the age of this fauna to Arenig, conodonts in the present study suggest that the S1 member is Tremadocian in age. Stait and Burrett (1984) also described nautiloids from the S1 member, which crops out along the northwestern coast of the island. The sum of evidence is that the S1 member is a Tremadocian sequence. Limestone in the S4 member, which is distributed on Lae Tong Island, southeastern Tarutao Island, contains Arenig nautiloids (Stait and Burrett, 1984). This age coincides with that of conodonts reported by Teraoka et al. (1982) from the S4 member. To sum up the faunal evidence given above, the ages of the S1, S2, S3 and S4 to S5 members of the Thung Song Formation are early Tremadocian, middle Tremadocian, late Tremadocian to early Arenig and middle Arenig, respectively (Text‐fig. 4).
Conclusion
The Thung Song Formation of Tarutao Island, southern peninsular Thailand, was investigated bio‐ and lithostratigraphically. Early Ordovician conodonts occur from study sections A and B and three conodont zones are established in Section A. These zones are the Rossodus manitouensis Range Zone, the Utahconus tarutaoensis Range Zone and the Filodontus tenuis Range Zone, in ascending order. Section B is equivalent to the R. manitouensis Zone of Section A. The zones of Section A are comparable to the interval from the Rossodus manitouensis Zone to the Acodus deltatus‐Oneotodus costatus Zone of the standard zonation in the North American Midcontinent. The zones in the present study also correlate with the intervals from the R. manitouensis Zone to the Acodus kechikaensis Zone in British Columbia and the R. manitouensis‐Polycostatus sulcatus Zone to the Parapanderodus carlae‐Paraserratognathus ovatus Zone in Newfoundland.
The Thung Song Formation is subdivided into S1–S5 members, and Section A of this study consists of the S1–S3 members. Our data of conodonts, and fossils previously reported from Tarutao Island, imply that the ages of the S1, S2, S3 and S4 to S5 members of the Thung Song Formation are early Tremadocian, middle Tremadocian, late Tremadocian to early Arenig, and middle Arenig, respectively. The depositional environments of the S1–S3 members are inferred to be as follows. The lowermost limestone of the S1 member was deposited on a shallow‐water shelf in the early Tremadocian. The environment changed to a deeper‐water shelf owing to transgression, and this condition persisted during deposition of the limestones of the S1 member. The middle Tremadocian was a low‐sea‐level interval. The limestone and shale of the S2 member were deposited in a shallow‐water setting, which experienced deeper water due to a transgression before the late Tremadocian. Limestone in the S3 member accumulated on a deeper shelf.
Systematic Palaeontology
Conodonts from the study section are shown in Plates 1–4. The illustrated specimens are deposited in the Institute of Geoscience, University of Tsukuba, with the prefix IGUT. Our collection includes several taxa represented by quite a few specimens. Therefore, 11 taxa, including stratigraphically important species and new species, are described here. Conodonts are only classified to genus and species, because a suprageneric classification is still uncertain. Element terminology essentially follows that of Barnes et al. (1979).
Genus ACANTHODUS Furnish, 1938
Type species. Acanthodus uncinatus Furnish, 1938.
Remarks. Landing et al. (1996) revised the diagnosis of Acanthodus as consisting of three elemental morphologies, drepanodiform, scandodiform and suberectiform. The apparatus of A. uncinatus in the Tarutao material is composed of a (drepanodiform), c (suberectiform) and e (scandodiform) elements, which is in accordance with the definition of Landing et al. (1996).
Acanthodus uncinatusFurnish, 1938Plate 2, figures 1–4
Figs 1–4. Acanthodus uncinatusFurnish, 1938. Rossodus manitouensis Zone, Thung Song Formaiton. 1a–b, IGUT‐ag2946, sample TAR‐31; lateral and aboral views of c element; ×50. 2a–b, IGUT‐ag2898, sample TAR‐3; lateral and aboral views of a element; ×60. 3a–b, IGUT‐ag2167, sample TAR‐13; lateral and aboral views of e element; ×50. 4a–b, IGUT‐ag2166, sample TAR‐13; lateral and aboral views of e element; ×50.Figs 5–7. Drepanoistodus forceps (Lindström, 1955). R. manitouensis Zone to Filodontus tenuis Zone, Thung Song Formation. 5a–b, IGUT‐ag2066, sample TAR‐46; lateral and aboral views of a element; ×100. 6a–b, IGUT‐ag3115, sample TAR‐46; lateral and aboral views of e element; ×100. 7a–b, IGUT‐ag2005, sample TAR‐46; lateral and aboral views of c element; ×100.Fig 8. Drepanoistodus pervetusNowlan, 1985. Utahconus tarutaoensis Zone, Thung Song Formation. 8a–b, IGUT‐ag2169, sample TAR‐38; lateral and aboral views; ×100.Fig 9. Acanthodus lineatusFurnish, 1938. R. manitouensis Zone, Thung Song Formation. 9a–b, IGUT‐ag2001, sample TAR‐17; lateral and aboral views; ×100.Figs 10–12. Striatodontus prolificusJi and Barnes, 1994. U. tarutaoensis Zone to F. tenuis Zone, Thung Song Formation. 10a–b, IGUT‐ag2170, sample TAR‐41; lateral and aboral views of e element; ×100. 11a–b, IGUT‐ag3257, sample TAR‐46; lateral and aboral views of b element; ×100. 12a–b, IGUT‐ag3256, sample TAR‐46; lateral and aboral views of b element; ×150.Fig 13. Protoprioniodus yapuCooper, 1981. U. tarutaoensis Zone to F. tenuis Zone, Thung Song Formation. 13a–b, IGUT‐ag3202, sample TAR‐42; lateral and aboral views of b element; ×80.Figs 14–15. Acodus sp. F. tenuis Zone, Thung Song Formation. 14a–b, IGUT‐ag2000, sample TAR‐46; lateral and aboral views of b element; ×50. 15a–b, IGUT‐ag2949, sample TAR‐46; lateral and aboral views of g element; ×100.Figs 16–17. Scolopodus sp. R. manitouensis Zone to F. tenuis Zone, Thung Song Formation. 16a–b, IGUT‐ag1966, sample TAR‐18; lateral and aboral views; ×80. 17a–b, IGUT‐ag1968, sample TAR‐46; lateral and aboral views; ×100.Scale bars indicate 100 μm.
* 1938 Acanthodus uncinatus Furnish, p. 337–338, pl. 42, fig. 30, text‐fig. 2B.
1996 Acanthodus uncinatus Furnish; Ji and Barnes, figs. 12.7–12.11.
. 1996 Acanthodus uncinatus Furnish; Landing et al., p. 669, 671, figs. 8.1–8.9, 9.13, 9.5–17.
2002 Acanthodus uncinatus Furnish; Pyle and Barnes, p. 59, pl. 1, figs. 1–2.
2003 Acanthodus uncinatus Furnish; Landing et al. fig. 10.7.
Material. Fifty‐one specimens; 36 a, 4 c and 11 e elements (IGUT‐ag2166–2167, 2898–2946).
Remarks. Specimens in the Tarutao collection are identified with these forms. The a element of this study is a synonym of ‘A. uncinatus’ and ‘D. suberectus’ of Furnish (1938). The form of the c element described here equals with ‘O. erectus’ of Furnish (1938). This is similar to the c element of species of Drepanoistodus, but it is distinguished by having a straight and robust cusp and short base. The e element of A. uncinatus in this study is a synonym of ‘D. tortus’ of Furnish (1938). Mature elements of this species are known to have serrations along the posterior margin. Although specimens of this study are poorly preserved and lack the distal part of cusp, some elements show some serrations (Pl. 2, fig. 4). Landing et al. (1996) also described a drepanodiform element, which bears an antero‐lateral costa, but the Tarutao specimens do not contain this element. A larger collection is necessary to more precise discussion.
Occurrence. A. uncinatus is widespread in the Rossodus manitouensis Zone in North America and its equivalents in North China, Siberia, Australia, Korea and Iran (Landing et al. 1996). In this study, the species is from the R. manitouensis Zone of the Thung Song Formation.
Genus DREPANOISTODUS Lindström, 1971
Type species. Oistodus forceps Lindström, 1955.
Drepanoistodus pervetusNowlan, 1985Plate 2, figure 8
* 1985 Drepanoistodus? pervetus Nowlan, p. 112–113, text‐figs 5.53–5.55, 6.1–6.3.
1981 Drepanoistodus? n. sp.; Landing and Barnes, p. 1615–1616, pl. 2, figs. 1–2, 5, 7–8.
1994 Drepanoistodus pervetus Nowlan; Ji and Barnes, p. 35–36, pl. 7, figs. 21–27, text‐fig. 24B.
1996 Drepanoistodus pervetus Nowlan; Ji and Barnes, fig. 11.12–11.19.
2002 Drepanoistodus pervetus Nowlan; Pyle and Barnes, p. 64, pl. 6, figs. 16–18.
Material. Four specimens (IGUT‐ag2169, 2965, 3002–3003).
Remarks. Laterally compressed elements of this study have a cusp with keeled anterior and posterior margins and an antero‐posteriorly expanded base. These specimens are comparable with nongeniculate elements described by Nowlan (1985).
Occurrence. This species has been described from the uppermost Cambrian to Lower Ordovician strata in Canada (Landing and Barnes 1981; Nowlan 1985; Ji and Barnes 1994). The Utahconus tarutaoensis Zone of the Thung Song Formation also yields D. pervetus on Tarutao Island.
Genus FILODONTUS Pyle, Barnes and Ji, 2003
Type species. Scolopodus filosus Ethington and Clark, 1964.
Remarks. Pyle et al. (2003) erected the genus Filodontus and assigned a single species, Filodontus filosus, to this genus. They considered Filodontus as a bi‐membrate apparatus. Elements of this genus are characterized by bearing many extremely fine longitudinal costae from the tip of the cusp to above the basal margin.
Filodontus tenuis sp. nov.Plate 3, figures 5–8.
Figs 1–4. Parapanderodus striatus (Graves and Ellison, 1941). Filodontus tenuis Zone, Thung Song Formation. 1a–b, IGUT‐ag2010, sample TAR‐46; lateral and aboral views of a element; ×100. 2a–b, IGUT‐ag2012, sample TAR‐46; lateral and aboral views of b element; ×200. 3a–b, IGUT‐ag3167, sample TAR‐46; posterior and aboral views of c element; ×100. 4a–b, IGUT‐ag3182, sample TAR‐46; lateral and aboral views of e element; ×200.Figs 5–8. Filodontus tenuis n. sp. F. tenuis Zone, Thung Song Formation. 5a–b, paratype; IGUT‐ag3135, sample TAR‐46; lateral and aboral views of a element; ×150. 6a–b, holotype; IGUT‐ag1987, sample TAR‐46; lateral and aboral views of a element; ×200. 7a–b, paratype; IGUT‐ag2018, sample TAR‐46; lateral and aboral views of e element; ×150. 8a–b, paratype; IGUT‐ag2017, sample TAR‐46; lateral and aboral views of e element; ×150.Fig 9. Coniform indet. E. R. manitouensis Zone, Section B, Thung Song Formation. 9a–b, IGUT‐ag3463, sample TAR‐4; posterior and aboral views; ×100.Fig 10. Coniform indet. H. Utahconus tarutaoensis Zone to F. tenuis Zone, Thung Song Formation. 10a–b, IGUT‐ag1964, sample TAR‐46; lateral and aboral views; ×100.Figs 11–14. Utahconus tarutaoensis sp. nov. U. tarutaoensis Zone, Thung Song Formation. 11a–b, paratype; IGUT‐ag3357, sample TAR‐42; lateral and aboral views of e element; ×100. 12a–b, paratype; IGUT‐ag1991, sample TAR‐42; lateral and aboral views of b element; ×80. 13a–b, holotype; IGUT‐ag1988, sample TAR‐39; lateral and aboral views of a element; ×100. 14a–b, paratype; IGUT‐ag3326, sample TAR‐39; lateral and aboral views of a element; ×100.Fig 15. Coniform indet. A. R. manitouensis Zone, Thung Song Formation. 15a–b, IGUT‐ag3449, sample TAR‐30; lateral and aboral views; ×80.Fig 16. Coniform indet. B. R. manitouensis Zone, Thung Song Formation. 16a–b, IGUT‐ag1982, sample TAR‐32; lateral and aboral views; ×60.Figs 17–18. Coniform indet. C. R. manitouensis Zone, Thung Song Formation. 17a–b, IGUT‐ag1975, sample TAR‐27; lateral and aboral views; ×100. 18a–b, GUT‐ag1974, sample TAR‐27; lateral and aboral views; ×100.Fig 19. Coniform indet. D. F. tenuis Zone, Thung Song Formation. 19a–b, IGUT‐ag3463, sample TAR‐46; lateral and aboral views; ×200.Fig 20. Coniform indet. F. F. tenuis Zone, Thung Song Formation. 20a–b, IGUT‐ag1969, sample TAR‐46; lateral and aboral views; ×60.Fig 21. Coniform indet. G. Thung Song Formation. 21a–b, IGUT‐ag1985, sample TAR‐36; lateral and aboral views; ×100.Scale bars indicate 100 μm.
Derivation of name. From the Latin, slim.
Holotype. a element IGUT‐ag1987, Thung Song Formation, Tarutao Island, Thailand; Early Ordovician; Plate 3, figure 6.
Material. Twenty‐two specimens; 17 a and 5 e elements (IGUT‐g1987, 2017–2018, 3133–3151).
Diagnosis. Apparatus comprises coniform a and e elements, with a proclined, slender cusp and an unexpanded, cylindrical base. The cusp and the upper part of the base bear many extremely fine longitudinal costae. The cross section of the a element is circular through the unit. The posterior margin of the cusp of the e element is laterally compressed and sharply edged.
Description. a elements are simple and coniform with a circular cross‐section throughout their length. The unit slightly curves posteriorly at one‐third to one‐half length from the basal margin. The cusp gradually tapers toward the tip. The base is unexpanded and cylindrical. All sides of the surface are ornamented by fine longitudinal costae that extend from the tip of the cusp to the upper part of the base. The costae of a distal part are thicker than those of a basal part of elements and are 24 μm in maximum width. The surface is smooth at the lower part of the base. The height of the basal cavity is 1.5 times of the diameter of the basal margin. The lower part of the base of some specimens twists anteriorly. The basal margin is straight in lateral view and circular in aboral view.
e elements are similar to the a elements in the curvature of the unit and ornamentation of the surface. However, the e element differs from the a element in that the posterior margin is keeled at the cusp and that the height of the basal cavity is twice of the diameter of the basal margin. The lower part of the base twists anteriorly.
Remarks. Pyle et al. (2003) described two element morphotypes of F. filosus. One is a slender form, and the other is stout and squat. Both types have a reclined cusp and a more or less expanded base. All specimens of F. tenuis in this study are more slender, and have a proclined cusp and a shorter base than elements of F. filosus of Pyle et al. (2003). Repetski (1982) reported proclined coniform elements as S. filosus, which are similar to the a element of F. tenuis, but his specimens bear thinner and weaker costae on the surface than those of F. tenuis.
Occurrence. This species occurs in the F. tenuis Zone of the Thung Song Formation on Tarutao Island.
Genus PARAPANDERODUS Stouge, 1984
Type species. Parapanderodus arcuatus Stouge, 1984.
Parapanderodus striatus (Graves and Ellison, 1941)Plate 3, figures 1–4
* 1941 Drepanodus striatus n. sp. Graves and Ellison, p. 11, pl. 1, figs. 3, 12.
. 2000 Parapanderodus striatus (Graves and Ellison); Johnston and Barnes, p. 30, pl. 12, figs. 11, 13–15, 17–19.
2003 Parapanderodus striatus (Graves and Ellison); Pyle and Barnes, figs. 12.18–12.19.
2003 Parapanderodus striatus (Graves and Ellison); Pyle et al., figs.8.9–8.10.
Material. Forty‐eight specimens; 15 a, 30 b, 1 c and 2 e elements (IGUT‐ag2010, 2012, 3152–3197).
Remarks. Four types of coniform elements, a, b, c and e, are included in the apparatus. All specimens have a long cusp and a slender base, and are finely striated. Ji and Barnes (1994) defined a, b, c and e elements as an apparatus of this species. Specimens of this study are in accordance with the description of these elements.
Occurrence. P. striatus has been described from the Lower and Middle Ordovician strata in North America and Argentina (Johnston and Barnes 2000). The oldest occurrence of this species is known from sequences in Utah (Ethington and Clark 1981) and Argentina (Albanesi et al. 1998), corresponding to the Low diversity interval of the standard zonation in North American Midcontinent. The Filodontus tenuis Zone of the Thung Song Formation in this study also yields this species.
Genus PAROISTODUS Lindström, 1971
Type species. Oistodus parallelus Pander, 1856.
Paroistodus numarcuatus (Lindström, 1955) Plate 1, figures 13–14
Figs 1–4. Rossodus manitouensisRepetski and Ethington, 1983. R. manitouensis Zone, Thung Song Formation. 1a–b, IGUT‐ag3216, sample TAR‐21; posterior and aboral views of c element; ×150. 2a–b, IGUT‐ag3076, sample TAR‐33; lateral and aboral views of e element; ×100. 3a–b, IGUT‐ag3213, sample TAR‐17; lateral and aboral views of a element ×100. 4a–b, IGUT‐ag1978, sample TAR‐16; lateral and aboral views of a element; ×100.Figs 5–7. Teridontus obesusJi and Barnes, 1994. R. manitouensis Zone, Thung Song Formation. 5a–b, IGUT‐ag3308, sample TAR‐20; lateral and aboral views of a/b element; ×150. 6a–b, IGUT‐ag1941, sample TAR‐6; lateral and aboral views of e element; ×100. 7a–b, IGUT‐ag3283, sample TAR‐8; posterior and aboral views of c element; ×150.Figs 8–10. Utahconus longipinnatusJi and Barnes, 1994. R. manitouensis and U. tarutaoensis zones, Thung Song Formation. 8a–ab, IGUT‐ag2951, sample TAR‐17; posterior and aboral views of c element; ×100. 9a–b, IGUT‐ag2959, sample TAR‐32; lateral and aboral views of e element; ×80. 10a–b, IGUT‐ag3577, sample TAR‐36; lateral and aboral views of a element; ×100.Figs 11–12, 15–16. Variabiloconus bassleri (Furnish, 1938). R. manitouensis Zone, Thung Song Formation. 11a–b, IGUT‐ag3388, sample TAR‐6; lateral and aboral views of e element; ×100. 12a, IGUT‐ag3434, sample TAR‐14; posterior view of c element; ×100. 15a–b, IGUT‐ag1935, sample TAR‐8, lateral and aboral views of a element; ×100. 16a–b, IGUT‐ag1936, sample TAR‐4; lateral and aboral views of b element; ×100.Figs 13–14. Paroistodus numarcuatus (Lindström, 1955). U. tarutaoensis Zone to Filodontus tenuis Zone, Thung Song Formation. 13a–b, IGUT‐ag3018, sample TAR‐38; lateral and aboral views of r element; ×100. 14a–b, IGUT‐ag3013, sample TAR‐38; lateral and aboral views of q element; ×100.Figs 17–18. Drepanodus arcuatusPander, 1856. R. manitouensis Zone, Thung Song Formation. 17a–b, IGUT‐ag1951, sample TAR‐6; lateral and aboral views of q element; ×60. 18a–b, IGUT‐ag3008, sample TAR‐36; lateral and aboral views of q element; ×80.Scale bars indicate 100 μm.
* 1955 Drepanodus numarcuatus Lindström, pp. 564–565, pl. 2, figs 48–49, text‐fig. 3I.
. 1997 Paroistodus numarcuatus (Lindström); Löfgren, pp. 921–922, text‐figs 2O–U, 3A–G, 4A–K.
Material. Sixteen specimens; 15 q and 1 r elements (IGUT‐ag3010–3025).
Remarks. Elements of this species are distinguished by Löfgren (1997) into seven types, a, b, c, d, e, f and g. Johnston and Barnes (2000) redescribed a Paroistodus apparatus consisting of two element forms, r (oistodiform) and q (drepanodiform) elements. We follow the concept of Johnston and Barnes (2000).
Occurrence. This species is widely known from the uppermost Tremadocian strata in the North Atlantic areas (Löfgren 1997). In this study, the species is present in the Utahconus tarutaoensis and Filodontus tenuis zones of the Thung Song Formation on Tarutao Island.
Genus ROSSODUS Repetski and Ethington, 1983
Type species. Rossodus manitouensis Repetski and Ethington, 1983.
Rossodus manitouensisRepetski and Ethington, 1983Plate 1, figures 1–4
1982 New genus A, n. sp. A, Repetski, p. 56–57, pl. 28, figs. 1–4.
* 1983 Rossodus manitouensis Repetski and Ethington, p. 293–300, figs. 1A–V, 2A–T, 3A–R, 4A–D.
1993 Rossodus manitouensis Repetski and Ethington; Wang, p. 199, pl. 7, figs. 15–17, 25.
. 1994 Rossodus manitouensis Repetski and Ethington; Ji and Barnes, p. 56, pl. 17, figs. 1–9.
1994 Rossodus manitouensis Repetski and Ethington; Seo et al., figs. 7.23–7.29.
1995 Rossodus manitouensis Repetski and Ethington; Chen et al., pl. 1, fig. 2.
1996 Rossodus manitouensis Repetski and Ethington; Ji and Barnes, figs. 13.13–13.20.
1996 Rossodus manitouensis Repetski and Ethington; Landing et al. 1996, figs. 5.18, 5.20, 5.22–5.24.
1999 Rossodus manitouensis Repetski and Ethington; Löfgren et al., pl. 3, figs. 13, 15, 21.
2002 Rossodus manitouensis Repetski and Ethington; Pyle and Barnes, p. 100, pl. 13, figs. 1–6.
2005 Rossodus manitouensis Repetski and Ethington; Wu et al., pl. 1, fig. 5.
Material. Thirteen specimen; 4 a, 7 c and 2 e elements (IGUT‐ag1978, 1980, 2966, 3076, 3209–3217).
Remarks. The apparatus of this genus consists of following four types of element; acontiodiform a element, drepanodiform b element, suberect and symmetrical c element and oistodiform e element (Ji and Barnes 1994). Specimens of this study are classified into asymmetrical a, symmetrical c and oistdontiform e elements.
Occurrence. This is a representative species of the R. manitouensis zone and its equivalents in North American Midcontinent, Newfoundland, New York, British Columbia, Quebec and South China (Landing et al. 1986). Löfgren et al. (1999) reported this species from the middle Tremadocian strata in the Baltica area. On Tarutao Island, this species range through the R. manitouensis Zone of the Thung Song Formation.
Genus STRIATODONTUS Ji and Barnes, 1994
Type species. Striatodontus prolificus Ji and Barnes, 1994.
Striatodontus prolificusJi and Barnes, 1994Plate 2, figures 10–12
* 1994 Striatodontus prolificus Ji and Barnes, p. 61, pl. 20, figs. 1–26, text‐fig. 36A.
1998 Striatodontus prolificus Ji and Barnes; Lehnert et al., p. 57, pl. 1, figs. 1–4, 7, 8.
non 2002 Striatodontus prolificus Ji and Barnes; Pyle and Barnes, p. 83, pl. 15, figs. 1–4.
2003 Striatodontus prolificus Ji and Barnes; Landing et al., figs. 11.11–11.15.
Materials. Nine specimens; 7 b and 2 e elements (IGUT‐ag2170, 3254–3258, 3260–3262).
Description. Specimens in the Tarutao collection are slender coniform and distinguished into b and e elements. b elements are transitional triangulariform and have a proclined cusp and a short base. Two types of the b element are recognized in this study. One is a symmetrical form having smooth lateral faces and a wide longitudinal depression on the posterior face. The other is asymmetrical and characterized by deep antero‐ and postero‐lateral grooves on the inner face. These grooves are bounded by sharply developed anterior, inner lateral and posterior costae. The grooves and costae are carried from the tip of the cusp to the mid base. The outer face is rounded. The base slightly expands posteriorly and is a circle in aboral view.
e elements are symmetrical with an erect cusp. The anterior margin is narrowly rounded. A median groove on the posterior face ranges from the tip of the cusp to above the basal margin. The base slightly flares antero‐posteriorly. The basal margin is elliptical in aboral view.
Remarks. Ji and Barnes (1994) included a, b, c and e elements in the apparatus of this species. The material of this study does not contain the a and c elements. The b elements show two forms, as mentioned above, without a transitional form. It may be necessary to classify the b elements in more detail. Figured specimens of Pyle and Barnes (2002) seem to differ from the type specimens of Ji and Barnes (1994).
Occurrence. S. prolificus has been reported from the Colaptoconus floweri‐Colaptoconus bolites to Protopanderodus inconstans‐Scolopodus subrex zones of the St. George Group in Newfoundland (Ji and Barnes 1994) and from the lower part of the Ponon Trehue Formation in Argentina (Lehnert et al. 1998). This species is also included in the Utahconus tarutaoensis Zone and the Filodontus tenuis Zone of the Thung Song Formation on Tarutao Island.
Genus TERIDONTUS Miller, 1980
Type species. Oneotodus nakamurai Nogami, 1967.
Remarks. Ji and Barnes (1994) re‐defined the apparatus of this genus to comprise four types of element: a, b, c and e. The e elements are partly identified with species of Monocostodus described by Miller (1980). On the other hand, Nicoll (1994) recognized six forms, a, b, c, d, f and g elements, in an apparatus. We follow the definition of Ji and Barnes (1994), because the Tarutao material contains laterally compressed e elements.
Teridontus obesusJi and Barnes, 1994Plate 1, figures 5–7
* 1994 Teridontus obesus Ji and Barnes, p. 65, 66, pl. 24, figs. 10–17, text‐fig. 37B.
1998 Teridontus obesus Ji and Barnes; Rao and Tortello, p. 37, 38, pl. 2, figs. 1–7.
2002 Teridontus obesus Ji and Barnes; Pyle and Barnes, p. 71, pl. 15, figs. 20–22.
Materials. Fifty‐six specimens; 38 a/b, 4 c and 14 e elements (IGUT‐ag1941, 3263–3317).
Remarks. Ji and Barnes (1994) described four types of element as components of the apparatus of this species. The Tarutao collection contains a/b, c and e elements. a/b elements are compared with the a and b elements of Ji and Barnes (1994). The form of a/b elements gradually varies from symmetrical to subsymmetrical. A few specimens bear faint groove on one lateral face, recognized by Ji and Barnes (1994). These specimens are distinguished from elements of Teridontus nakamurai (Nogami 1967) by the a/b element with a short, expanded base and the c element with a straight, slender cusp and a flared base.
Occurrence. This species has been reported from the Polycostatus falsioneotensis‐Rossodus tenuis Zone to the Rossodus manitouensis‐Polycostatus sulcatus Zone of the St. George Group in Newfoundland (Ji and Barnes 1994), the lower Tremadocian Cardonal Formation in Argentina (Rao and Tortello 1998), the lowest Tremadocian strata of the Kechika Formation in British Columbia (Pyle and Barnes 2002) and the R. manitouensis Zone of the Thung Song Formation in this study.
Genus UTAHCONUS Miller, 1980
Type species. Paltodus utahensis Miller, 1969.
Remarks. According to Pyle and Barnes (2002), an apparatus of Utahconus species contains three or four types of element characterized by compression along the cusp and rounded to triangular basal outline.
Utahconus longipinnatusJi and Barnes, 1994Plate 1, figures 8–10.
* 1994 Utahconus longipinnatus n. sp. Ji and Barnes, p. 66, 67, pl. 25, figs. 1–8, text‐fig. 38A.
1985 Utahconus aff. U. utahensis (Miller, 1969); Nowlan, p. 117, figs. 5.45–5:52.
p 1996 Scalpellodus longipinnatus (Ji and Barnes, 1994); Landing et al., p. 675, 676, figs. 7.9–7.10, 7.12–19, 7.24, 9.22, 9.23 [non figs. 7.8, 7.11 (= indet.)].
2002 Utahconus longipinnatus Ji and Barnes; Pyle and Barnes, p. 72, pl. 17, figs. 1–3.
Material. Twenty specimens; 3a, 8c and 9e elements (IGUT‐ag1959, 1960, 2951–2957, 2959–2964, 3000, 3575–3578).
Remarks. Ji and Barnes (1994) recognized a, c and e elements as constituting the apparatus of this species. Specimens in this study are symmetrical to asymmetrical coniform, which are also distinguished into a, c and e elements, and characteristics of the element conform to the description of Ji and Barnes (1994). Cusps of these elements are longer than U. utahensis specimens.
Occurrence. The Lower Ordovician strata in North America (Nowlan 1985; Ji and Barnes 1994; Landing et al. 1996; Pyle and Barnes 2002) and the Rossodus manitouensis and Utahconus tarutaoensis zones in the Thung Song Formation, in this study, include U. longipinnatus.
Utahconus tarutaoensis sp. nov. Plate 3, figures 11–14
1982 Protopanderodus? n. sp. 1 s.f.; Repetski, p. 41, pl. 18, figs. 4, 6.
Derivation of name. Refers to Tarutao Island where the study sections crop out.
Holotype. A element IGUT‐ag1988; Plate 3, figure 13; Thung Song Formation, Tarutao Island, southern peninsular Thailand; Early Ordovician.
Material. Thirty‐six specimens; 18 a, 7 b and 11 e elements (IGUT‐ag1988, 1991, 3323–3351, 3353–3357).
Diagnosis. Specimens are simple, coniform and classified into a, b and e elements. All the elements have a proclined to erect cusp and an edged posterior margin. a elements are subsymmetrical and bear a rounded anterior margin with antero‐lateral costae. Subsymmetrical to asymmetrical b elements are laterally compressed. The anterior margin is narrowly rounded and bounded by antero‐lateral grooves, developed on the both or inner side of the unit. e elements are symmetrical and laterally compressed strongly.
Description. Elements are a long, slender unit with a proclined to erect cusp and a posteriorly expanded base. Subsymmetrical a elements have a cusp that inclines inwardly. The anterior margin is broadly convex and rounded costae are carried on antero‐lateral faces. The posterior margin is sharply keeled from the tip of the cusp to the basal margin. The basal cavity points toward the anterior face and its height is 1–1.5 times of the antero‐posterior width of the basal margin. Outline of the basal margin is expanded laterally and tapers to the posterior end.
b elements are a subsymmetrical to asymmetrical unit and laterally compressed. The cusp bends inwardly. The anterior margin is narrowly rounded and bounded by antero‐lateral grooves on the both or inner side of the unit. The basal cavity opens narrowly and its height is one‐third to one half of the antero‐posterior width of the basal margin. The postero‐basal corner twists outwardly.
Laterally compressed e elements are a symmetrical acostate unit. The anterior margin is edged or narrowly rounded. The form of the basal cavity resembles that of the b element, but the basal opening is straight in aboral view.
Remarks. This species does not bear a symmetrical suberectiform c element. The a element of U. tarutaoensis is distinguished from that of U. utahensis and Utahconus longipinnatusJi and Barnes (1994) by the long cusp and the slender base. The b element of this species is characterized by the antero‐lateral grooves along the anterior margin and the twisted postero‐basal corner of the base. The e element of U. tarutaoensis differs from that of the other species in a lateral compression of the element and the unexpanded base.
Occurrence. This species is included in the lower part of the El Paso Group in Texas (Repetski 1982) and the U. tarutaoensis zone of the Thung Song Formation on Tarutao Island.
Genus VARIABILOCONUS Landing, Barnes and Stevens, 1986
Type species. Paltodus bassleri Furnish, 1938.
Variabiloconus bassleri (Furnish, 1938)Plate 1, figures 11–12, 15–16
* 1938 Paltodus bassleri Furnish, p. 331, pl. 42, fig. 1.
1986 Variabiloconus bassleri (Furnish); Landing et al., p. 1946, 1947, pl. 3, figs. 1–7, 9.
1994 Variabiloconus bassleri (Furnish); Ji and Barnes, p. 67, 68, pl. 25, figs. 14–26, text‐fig. 38D.
1994 Variabiloconus bassleri (Furnish); Seo et al., figs.10.25–10.26.
1996 Variabiloconus bassleri (Furnish); Ji and Barnes, figs. 14.19–14.25.
1996 Variabiloconus bassleri (Furnish); Landing et al., figs. 7.20–7.23, 7.25–7.33, 9.28–9.33.
2003 Variabiloconus bassleri (Furnish); Landing et al., fig. 8.2–8.5.
Material. One hundred and twenty‐two specimens; 75 a, 38 b, 6 c and 3 e elements (IGUT‐ag1935, 1936, 3218–3253, 3358–3447).
Remarks. Ji and Barnes (1994) re‐defined this species as a conodont bearing a, b, c and e elements. Specimens in the Tarutao collection are classified into four elements, a, b, c and e. The characteristics of these specimens accord with the description of Ji and Barnes (1994).
Occurrence. V. bassleri is one of the representative species of the Early Ordovician in North America, Australia and North China (Wang 1984; Landing et al. 1986). The upper limit of its stratigraphic range is the Rossodus manitouensis zone and the Low diversity interval of the standard zonation in North American Midcontinent (Sweet and Tolbert 1997). This species occurs from the Rossodus muskwaensis to Acanthodus uncinatus zones of the Thung Song Formation, in this study.
Acknowledgments
Acknowledgements. We are indebted to the referees and the editorial board, and to J. Repetski, for reading the manuscript and offering many useful comments and suggestions. We would like to thank the Department of Mineral Resources of Thailand for providing facilities for our research in Tarutao Island. The fieldwork was funded by a grant from the Mombusho International Scinetific Research (Number 11440148 to K. Sashida).
