early ordovician conodonts from tarutao island, southern peninsular thailand

EARLY ORDOVICIAN CONODONTS FROM TARUTAO

ISLAND, SOUTHERN PENINSULAR THAILAND

by SACHIKO AGEMATSU* , KATSUO SASHIDA* , SIROT SALYAPONGSE� and

APSORN SARDSUD�*Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan; e-mail: [email protected]

�Geological Survey Division, Department of Mineral Resources, Rama VI, Bangkok 10400, Thailand

Typescript received 5 March 2007; accepted in revised form 24 July 2008

Abstract: Early Ordovician conodont faunas of the Thung

Song Formation on Tarutao Island, southern peninsular

Thailand, consist of 14 known species belonging to 17 gen-

era, 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 inter-

val from the Rossodus manitouensis Zone to the Acodus delta-

tus-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 lithostrati-

graphically 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 deposi-

tional 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.

Key words: biostratigraphy, conodonts, Early Ordovician,

Tarutao Island, Thailand, Thung Song Formation.

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 Myan-

mar 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 Siebenhuner 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 (Tera-

oka 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 fau-

nas 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 (Buno-

pas 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 (Buno-

pas 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

[Palaeontology, Vol. 51, Part 6, 2008, pp. 1435–1453]

ª The Palaeontological Association doi: 10.1111/j.1475-4983.2008.00810.x 1435

locality of this group is in Nakhon Sri Thammarat prov-

ince, southern peninsular Thailand.

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 For-

mation is subdivided into lower, middle and upper mem-

bers. 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 charac-

terized 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 lime-

stones. In this study, we follow the nomenclature of Tera-

oka 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 con-

odonts, that have been reported from Tarutao Island.

They concluded that the Tarutao and Thung Song forma-

tions on Tarutao Island range approximately from Cam-

brian to Middle Ordovician (Arenig) in age, and that the

Cambrian–Ordovician boundary probably lies within the

middle member of the Tarutao Formation.

LITHOLOGY AND DEPOSITIONALENVIRONMENTS

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). Ter-

aoka 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

A B C

TEXT -F IG . 1 . 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.

1436 P A L A E O N T O L O G Y , V O L U M E 5 1

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).

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

TEXT -F IG . 2 . Lihological column with stratigraphic distributions of conodont species, conodont zones and sea-level change in

Section A.

A G E M A T S U E T A L . : E A R L Y O R D O V I C I A N C O N O D O N T S F R O M T A R U T A O I S L A N D , T H A I L A N D 1437

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 lime-

stones 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 lime-

stone 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 inter-

bedded with laminated limestones. One interval of mas-

sive 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 lime-

stone 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 terrige-

nous 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 fac-

tors may have been more or less influenced by trans-

gressions 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 con-

odonts. 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 man-

itouensis Repetski and Ethington, 1983. This zone yields

other species, including Variabiloconus bassleri (Furnish,

1938), Teridontus obesus Ji and Barnes, 1994, Utahconus

longipinnatus Ji and Barnes, 1994 and Acanthodus uncina-

tus Furnish, 1938. Acanthodus lineatus Furnish, 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 (Lindstrom, 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 pervetus Nowlan, 1985, Paroistodus num-

arcuatus (Lindstrom 1955) and Striatodontus prolificus Ji

and Barnes, 1994 are another species characteristic of this

zone (Text-fig. 2). This zone also includes U. longipinna-

tus, D. forceps, Protoprioniodus yapu Cooper, 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 obe-

sus, Variabiloconus bassleri and Acanthodus uncinatus

occur through the section. The lower and middle parts


of this section yields Coniform indet. E and Drepanodus

arcuatus Pander, 1856, respectively. Utahconus longipinn-

atus and Coniform indet. A are present in the upper

part of this section. Section B does not includes Rosso-

dus 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 Coni-

form 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 Ameri-

can 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.

TABLE 1 . List of conodont species from sections A and B. Samples 3–14 and 16–47 are collected from Section B and Section A,

respectively. About 1.5 kg of limestone was processed for each sample.

TEXT -F IG . 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 con-

odonts from the Setul Limestone in the Peris area of pen-

insular 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 bransoni Furnish, 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 con-

tain species in common with faunas of the present study.

On the basis of the occurrence of Serratognathus bilobatus

Lee, 1970, the fauna from locality 3 Arenig, and therefore

younger than the faunas from Tarutao Island.

Teraoka et al. (1982) first reported Ordovician cono-

dont faunas from Tarutao Island. They recovered Drepan-

odus arcuatus, Drepanoistodus basiovalis (Sergeeva, 1963),

Drepanoistodus forceps, Paroistodus parallelus (Pander,

1856), Paroistodus proteus (Lindstrom, 1955) and Proto-

panderodus rectus (Lindstrom, 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. Rossodus manitouensis, Variabiloconus bassleri,

Acanthodus lineatus and Acanthodus uncinatus, which are

representative species of the R. manitouensis Zone of Sec-

tion 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 Midconti-

nent, 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 (Lofgren 1997).

Therefore, the R. manitouensis Zone in this study is coeval

with the R. manitouensis Zone in the North American Mid-

continent. Based on the stratigraphic range of Pa. striatus

and P. numarcuatus, the Utahconus tarutaoensis and Fil-

odontus 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 zona-

tion of the Midcontinent realm for the Kechika Forma-

tion 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 Para-

panderodus 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 SONGFORMATION

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 lithostratigraphi-

cally 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 Ordo-

vician conodont faunas from the S1 and S3 members on

Tarutao Island, but several species were probably mis-

identified. 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 con-

trast with their faunas from the S1 and S3 members. The

presence of Chosonodina herfurthi Muller, 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 deltatus Lindstrom, 1955, and

Oepikodus evae (Lindstrom, 1955), which are early and

middle Arenig species. The ages of these faunas accords

with those in the present study.

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 mem-

ber, 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 sug-

gest 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 Bur-

rett, 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 litho-

stratigraphically. 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. manito-

uensis Zone of Section A. The zones of Section A are

comparable to the interval from the Rossodus manitouen-

sis Zone to the Acodus deltatus-Oneotodus costatus Zone

of the standard zonation in the North American Mid-

continent. 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 Para-

panderodus 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 pre-

viously reported from Tarutao Island, imply that the

ages of the S1, S2, S3 and S4 to S5 members of the

TEXT -F IG . 4 . 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.


Thung Song Formation are early Tremadocian, middle

Tremadocian, late Tremadocian to early Arenig, and

middle Arenig, respectively. The depositional environ-

ments of the S1–S3 members are inferred to be as fol-

lows. The lowermost limestone of the S1 member was

deposited on a shallow-water shelf in the early Trem-

adocian. 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 depos-

ited 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 Insti-

tute 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 uncinatus Furnish, 1938

Plate 2, figures 1–4

* 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).

EXPLANATION OF PLATE 1

Figs 1–4. Rossodus manitouensis Repetski 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 obesus Ji 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 longipinnatus Ji 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 (Lindstrom, 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 arcuatus Pander, 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 lm.


PLATE 1

AGEMATSU et al. Conodonts from Tarutao

1a

2a 3a

7a 7b

9b

10b 10a

13a

14a

14b

18b 17b 16a 15a

18a 17a

8a

6a

6b

5b

5a

11a

12a

11b

13b

16b 15b

8b

9a

4a 3b

4b 2b

1b

Remarks. Specimens in the Tarutao collection are identi-

fied 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 distin-

guished by having a straight and robust cusp and short

base. The e element of A. uncinatus in this study is a syn-

onym of ‘D. tortus’ of Furnish (1938). Mature elements of

this species are known to have serrations along the pos-

terior 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 speci-

mens 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 (Land-

ing et al. 1996). In this study, the species is from the

R. manitouensis Zone of the Thung Song Formation.

Genus DREPANOISTODUS Lindstrom, 1971

Type species. Oistodus forceps Lindstrom, 1955.

Drepanoistodus pervetus Nowlan, 1985

Plate 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 speci-

mens are comparable with nongeniculate elements

described by Nowlan (1985).

Occurrence. This species has been described from the

uppermost Cambrian to Lower Ordovician strata in Can-

ada (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.


Figs 1–4. Acanthodus uncinatus Furnish, 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 (Lindstrom, 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 pervetus Nowlan, 1985. Utahconus tarutaoensis Zone, Thung Song Formation. 8a–b, IGUT-ag2169, sample TAR-

38; lateral and aboral views; ·100.

Fig 9. Acanthodus lineatus Furnish, 1938. R. manitouensis Zone, Thung Song Formation. 9a–b, IGUT-ag2001, sample TAR-17; lateral

and aboral views; ·100.

Figs 10–12. Striatodontus prolificus Ji 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 yapu Cooper, 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.



PLATE 2


1a 1b

2b

2a

3a 4b

9b

9a

8a 7a

7b 6b

5b 5a

6a

10b

11b

11a

12a 13a

13b

17b

17a 16b

16a 15b 14b 15a 14a

10a

8b

12b

4a

3b

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.

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 ele-

ments, 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 circu-

lar 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 lm 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


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.



PLATE 3


1b 1a

2a

5a

5b

9b

10b

10a

16a

17a 17b

18b 16b

15a

15b 19a

18a

19b 21b 21a

20a

20b

11b

9a

11a

2b

3a

3b

4a

4b

8b

8a

7a 7b

6a

12b 13b

13a

12a

14a

14b

6b

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 mor-

photypes 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. ten-

uis 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)


* 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 appara-

tus of this species. Specimens of this study are in accor-

dance 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 occur-

rence 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 Midconti-

nent. The Filodontus tenuis Zone of the Thung Song

Formation in this study also yields this species.

Genus PAROISTODUS Lindstrom, 1971

Type species. Oistodus parallelus Pander, 1856.

Paroistodus numarcuatus (Lindstrom, 1955)


* 1955 Drepanodus numarcuatus Lindstrom, pp. 564–565,

pl. 2, figs 48–49, text-fig. 3I.

. 1997 Paroistodus numarcuatus (Lindstrom); Lofgren,

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

Lofgren (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 (oistodi-

form) 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

(Lofgren 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 manitouensis Repetski and Ethington, 1983


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.


Chen et al., pl. 1, fig. 2.



Ji and Barnes, figs. 13.13–13.20.


Landing et al. 1996, figs. 5.18, 5.20, 5.22–5.24.


Lofgren et al., pl. 3, figs. 13, 15, 21.


Pyle and Barnes, p. 100, pl. 13, figs. 1–6.


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 follow-

ing four types of element; acontiodiform a element, drep-

anodiform b element, suberect and symmetrical c element

and oistodiform e element (Ji and Barnes 1994). Speci-

mens 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, New-

foundland, New York, British Columbia, Quebec and South

China (Landing et al. 1986). Lofgren 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 prolificus Ji and Barnes, 1994


* 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 ele-

ments 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 ele-

ments 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 in-

constans-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 tarutao-

ensis 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 obesus Ji and Barnes, 1994


* 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 grad-

ually varies from symmetrical to subsymmetrical. A few

specimens bear faint groove on one lateral face, recog-

nized by Ji and Barnes (1994). These specimens are dis-

tinguished 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 manitouen-

sis-Polycostatus sulcatus Zone of the St. George Group in New-

foundland (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 appa-

ratus of Utahconus species contains three or four types of

element characterized by compression along the cusp and

rounded to triangular basal outline.

Utahconus longipinnatus Ji and Barnes, 1994

Plate 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 ele-

ments as constituting the apparatus of this species. Speci-

mens 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 taru-

taoensis zones in the Thung Song Formation, in this study,

include U. longipinnatus.

Utahconus tarutaoensis sp. nov.


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-lat-

eral 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 lat-

erally compressed. The cusp bends inwardly. The anterior mar-

gin 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 sub-

erectiform c element. The a element of U. tarutaoensis is

distinguished from that of U. utahensis and Utahconus

longipinnatus Ji and Barnes (1994) by the long cusp and

the slender base. The b element of this species is charac-

terized 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.


figs. 7.20–7.23, 7.25–7.33, 9.28–9.33.


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 strati-

graphic 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.

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 Mom-

busho International Scinetific Research (Number 11440148 to

K. Sashida).

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