RECONSTRUCTION OF AN APPARATUS OF NEOSTRACHANOGNATHUS TAHOENSIS FROM ORITATE, JAPAN AND SPECIES OF NEOSTRACHANOGNATHUS FROM OMAN

Abstract

Abstract: The apparatus of an Early Triassic conodont Neostrachanognathus tahoensisKoike, 1998 from Oritate, Kumamoto Prefecture, Japan, and a species of Neostrachanognathus from Oman were reconstructed. On the basis of five natural assemblages from the Oritate area, the three‐dimensional apparatus model of N. tahoensis is interpreted as bilaterally symmetrical and composed of 14 elements consisting of pairs of P₁, P₂, P₃, S₁, S₂, S₃, and S₄ elements. The P₁ and P₂ elements are coniform elements, the P₃ elements are digyrate forms, and the S elements are bipennate ramiforms. The S elements are arranged rostrally in the apparatus and the pairs of the P₁, P₂, and P₃ elements are subvertically arranged caudally and ventrally to the S array. One of the natural assemblages was formed by rostrocaudal collapse of the apparatus on the sea floor, whereas the other assemblages indicate that conodont animals came to rest nearly parallel with the substrate prior to burial. A collection of isolated elements from Jabal Safra, Oman, includes a second species of Neostrachanognathus with a comparable apparatus.

Neostrachanognathus tahoensis Koike, 1998 is an Early Triassic conodont having conical elements in its apparatus. Conodonts with coniform elements flourished mainly during the Early and Middle Palaeozoic, and decreased greatly in number after the Carboniferous. Although several conical elements have been described as isolated specimens from Triassic rocks (Ding 1983; Kozur and Mock 1991; Koike 1998), their multielement reconstructions have not yet been presented. We discovered several natural assemblages of Triassic conodonts in siliceous claystone of the Oritate area, Itsuki Village, Kumamoto Prefecture, Japan. Five of them are composed of conical and ramiform elements of which the conical forms are identified as N. tahoensis described by Koike (1998) from the Taho limestone of Ehime Prefecture, Japan. We also obtained well preserved discrete elements of species of NeostrachanognathusKoike, 1998 from Jabal Safra, Oman. On the basis of the natural assemblages from the Oritate area and elements from Oman, the present study reconstructs the apparatus of N. tahoensis, redefines the genus, and discusses its taphonomy and palaeoecology.

Geological setting and lithostratigraphy of oritate and oman

Sedimentary rocks of the Southern Sambosan Belt crop out in the Oritate Valley of Itsuki Village, Kumamoto Prefecture (Itsuki Village 1987). They consist mainly of siliceous claystone, chert, siliceous shale, and sandstone and make up a chert‐clastic sequence that is the representative lithology of the Jurassic accretionary complex of the Japanese Islands (e.g. Nakae 2000). Our study section is located about 1 km west of Oritate, Latitude 32°23′58′′N Longitude 130°46′11′′E (Text‐fig. 1A–B), where the siliceous claystone and chert strike N 60°E and dip 40 to 50°N. The thickness of the siliceous claystone is about four metres, and it is conformably overlain by black bedded chert about three metres thick (Text‐fig. 1C). This dark green to pale green siliceous claystone is very fissile and contains several intercalations of black chert less than three centimetres thick. The claystone contains conodonts and radiolarians in a matrix of clay minerals, fine quartz grains less than silt‐size, and fine‐grained opaque minerals. Alternations of siliceous claystone and black chert occupy the upper part of the siliceous claystone, and this gradually changes upward to black bedded chert.

Figure TEXT‐FIG. 1.

Index map and lithological column. A, map of Kyusyu Island. B, index map showing the study area in Oritate. C, lithological column with stratigraphical distributions of conodont and radiolarian horizons of the study section in the Oritate area.

Thirty‐three samples were collected from the sequence of siliceous claystone and chert and crushed into fragments for an optical microscopic observation. As a result, conodont elements were recovered from 17 samples (Text‐fig. 1C). Samples were also processed by hydrofluoric acid and although nine samples of chert yielded radiolarians no discrete conodont elements were recovered by this method. Samples 13s, 14, and 15s contain elements that we attribute to N. tahoensis. The natural assemblages came from Sample A, which consists of float that may be from levels that include samples 13s, 14, and 15s. The presence of P₁ elements of Triassospathodus symmetricus (Orchard, 1995), Neospathodus chionensis (Bender, 1967), and Cornudina? igoi (Koike, 1996), and elements of ‘Neohindeodella’? benderi (Kozur and Mostler, 1970) and ‘Ellisonia’dinodoides (Tatge, 1956) implies that this siliceous claystone is middle to late Spathian (late Olenekian) in age.

The Oman material was obtained from a red ‘Hallstatt’ limestone (Block 3 of Tozer and Calon 1990) collected from a 3.5 m thick, red and grey mottled olistolith in the Guwayza Formation of the Hamrat Duru Group. This formation is late Jurassic in age and contains both Permian and Lower Triassic olistoliths. It yielded the Spathian ammonoid Procarnites kokeni in its uppermost part (Tozer and Calon 1990, p. 205). Conodont samples collected at the base (103C) and at two metres above the base (103B) yielded Neostrachanognathus sp. as well as a mid Spathian conodont fauna consisting of Cratognathus sp., Novispathodus abruptus (Orchard, 1995), Triassospathodus symmetricus (Orchard, 1995), ‘Neospathodus’triangularis (Budurov, 1972), and Spathicuspus sp. The limestone probably accumulated slowly on a seamount on the southern margin of the Tethys ocean, far from a source of terrigenous sediment, and were subsequently transported downslope into deep water sediments.

Natural assemblage of Neostrachanognathus tahoensis from oritate

Apparatus model of N. tahoensis

Natural assemblages were recovered from siliceous claystone of sample A, which was crushed into fragments with maximum diameters of about 2–3 cm. These assemblages are not exposed on the surface of the rock fragments, but are entirely or partially buried in the rock. We attempted to expose them by acid etching of the sample surface and whittling around specimens with a needle, but failed owing to the fragility of conodont elements. The assemblage‐bearing fragments were immersed in water for photography with an optical microscope, or in the case of several well‐preserved specimens, with the Digital Microscope VHX of Keyence Corporation. We observed a total of 15 natural assemblages, which are mostly composed of ramiform elements that are difficult to assign to form‐species. However, at least five natural assemblages contain conical elements with a short base and stout cusp that we identify as Neostrachanognathus tahoensis. Koike (1998) described an apparatus consisting of four types of conical elements for this species, based on a statistic analysis of discrete elements obtained by acid dissolution. The specimens of this study show that of N. tahoensis has an apparatus, composed both of conical and ramiform elements, as discussed below.

The five natural assemblages form the basis of our reconstruction of the apparatus of N. tahoensis. We apply the notation of Purnell et al. (2000) to the elements, although the reconstructed apparatus is not entirely homologous to that of ozarkodinids (Purnell and Donoghue 1997, 1998). Preservation of the buried specimens restricts observations of the three‐dimensional morphology and spacial arrangement of each element in the natural assemblages. However, we determined that the apparatus of N. tahoensis is bilaterally symmetrical and composed of 14 elements: pairs of P₁, P₂, P₃, S₁, S₂, S₃, and S₄ elements (Text‐fig. 2). Eight bipennate ramiform elements in S positions form a rostral array with caudally oriented and distally elevated posterior processes. Based on the scheme of Purnell et al. (2000), the notation S₁, S₂, S₃, and S₄ are given to these four pairs of elements, which are numbered outward from the most interior pair. Rows of denticles on posterior processes of the S elements are inclined inwardly, but it is not possible to accurately estimate the angle of the inclination of each S element. The P1, P2, and P3 positions lie caudally to the S array and are numbered from the caudal to rostral pairs and also dorsal to ventral. Two forms of conical elements, one with a small denticle and the other without a denticle, are assigned to the P₁ and P₂ positions. These positions seem to be located adjacent to each other in the apparatus, but relative locations of the conical elements in the natural assemblages are unstable as mentioned below. Therefore, we temporarily assign the denticulated conical element to the P₁ position and the simpler cone to the P₂ position. The P₃ position comprizes digyrate elements, which lie with upwardly pointed cusps. Elements corresponding to the S₀ and M positions, as occur in typical 15‐element ozarkodinid apparatuses, are not recognized in the natural assemblage of N. tahoensis.

Figure TEXT‐FIG. 2.

Plans of the apparatus of Neosrachanognathus tahoensis. A, B, C, and D are rostral, caudal, dextral, and dorsal views, respectively.

Following an approach to three‐dimensional reconstruction of apparatuses (Purnell and Donoghue 1998), we made a simple model of the N. tahoensis apparatus and visually compared the juxtaposition of elements in the apparatus with the element arrangements observed in the five natural assemblages. After repetitions of this process, we arrived at a ‘best‐fit’ architecture for the N. tahoensis (Text‐fig. 2).

Orientations of collapse

Collapse of apparatuses was simulated by photographing the apparatus model from different directions, and a particular orientation of collapse was determined for each natural assemblage. Text‐figs 3–6 are the photographs, line drawings of the natural assemblages, and the apparatus models sketched in the orientation of collapse. In the apparatus models, the plane of the page is regarded as the bedding plane of the fossil, and the x, y, and z axes indicate the rostrocaudal, dorsoventral, and mediolateral axes, respectively (Text‐figs 3D, 4D, 5D, 6C, 6E). Superposed ramiform elements in S positions resemble each other in shape and cannot be differentiated as S₁ to S₄ elements due to poor preservation of the natural assemblages. Therefore, in the following interpretation, we use the term ‘S element (s)’ for elements in any S positions.

Figure TEXT‐FIG. 3.

Natural assemblage specimen 1. A, digital microscopic photograph of IGUT‐ag3468 (part). B, counterpart. C, drawing outline of part and counterpart of specimen 1. D, sketch of the apparatus model in the directions of collapse of specimen 1. The small cube indicates the axes of the apparatus: x = rostrocaudal axis, y = dorsoventral axis, z = mediolateral axis.

Figure TEXT‐FIG. 4.

Natural assemblage specimen 2. A, digital microscopic photograph of IGUT‐ag3469 (part). B, counterpart. C, drawing outline of part and counterpart of specimen 2. D, sketch of the apparatus model in the directions of collapse of specimen 2. The small cube indicates the axes of the apparatus: x = rostrocaudal axis, y = dorsoventral axis, z = mediolateral axis.

Figure TEXT‐FIG. 5.

Natural assemblage specimen 3. A, digital microscopic photograph of IGUT‐ag3470 (part). B, counterpart. C, drawing outline of part and counterpart of specimen 3. D, sketch of the apparatus model in the directions of collapse of specimen 3. The small cube indicates the axes of the apparatus: x = rostrocaudal axis, y = dorsoventral axis, z = mediolateral axis.

Figure TEXT‐FIG. 6.

Natural assemblages specimen 4 and 5. A, digital microscopic photograph of IGUT‐ag3573. B, drawing outline of specimen 4. C, sketch of the apparatus model in the directions of collapse of specimen 4. The elements of solid line are parts expected to preserved in the specimen 4. D, digital microscopic photograph of IGUT‐ag3574. E, sketch of the apparatus model in the directions of collapse of specimen 5. The elements of solid line are parts expected to preserved in the specimen 5. F, drawing outline of specimen 5. The small cubes of C and E indicates the axes of the apparatus: x = rostrocaudal axis, y = dorsoventral axis, z = mediolateral axis.

Natural assemblage of specimen 1 contains a total of 12 elements (Text‐fig. 3). Six S elements are piled together with similar orientations of their posterior processes. Two P₁ and two P₂ elements are located behind the S array. Two P₃ elements are sited also behind and below the S elements. This assemblage mainly has a component of rostrocaudal collapse (Text‐fig. 3D).

Specimen 2 is composed of two P₁, one P₂, two P₃, and eight S elements (Text‐fig. 4). The S elements form an array with their posterior processes approximately parallel. One of the P₁ elements is located on a distal part of posterior processes of the S elements, and the other lies below a stack of the S elements. The P₂ element is positioned in the upper part of the S element group. One of the P₃ elements is superimposed on a cusp of the lowermost S element. The other P₃ lies offset from the other elements. This assemblage represents a collapse with the nearly‐horizontal rostrocaudal axis and with the dorsoventral axis, rotated about 45 degrees from the bedding plane (Text‐fig. 4D).

Specimen 3 consists of 11 elements separated into dextral and sinistral components placed in opposition (Text‐fig. 5). The cusps and denticles of the S elements are directed inwardly. Two P₂ elements adjoin each other and are located behind the S element array of one lateral side. A P₃ element lies behind each lateral group of S elements. This specimen lacks a pair of P₁ elements. Based on the proposed apparatus architecture, this specimen appears to have collapsed dorsoventrally (Text‐fig. 5D).

The specimens 1, 2, and 3 are preserved as a part and a counterpart of natural assemblage, but only one part specimens 4 and 5 were recovered. Specimen 4 contains one P₁, one P₃, and four S elements (Text‐fig. 6A–B). The S elements entirely superposed and the P₁ and P₃ elements lie behind and below them, respectively. This specimen is a dorsoventrally collapsed assemblage, similar to specimen 2 (Text‐fig. 6C). A rotation of the dorsoventral axis on the rostrocaudal axis of specimen 4 is larger than that of specimen 2.

Specimen 5 includes a total of seven elements (Text‐fig. 6D, 6F). Three S elements are partially superposed with their posterior processes parallel. Four conical elements are placed above and behind the S elements. This specimen mainly has a dorsoventral component of collapse (Text‐fig. 6E).

The natural assemblages of this study, except for specimen 1, were formed by decay and collapse of the soft tissues of conodont animals that came to rest approximately parallel with the surface of the sediments. The rostrocaudally‐long body of the conodont animal represented by assemblage specimen 1 was subvertically thrust and buried into sediments on the sea floor.

In the natural assemblages, P₁ and P₂ elements are oriented in various directions. For example, in specimen 2, one P₁ and the P₂ element are both shifted anteriorly and displaced on the dorsoventral axis from positions we would expect based on the model (Text‐fig. 4). These displacements are considered to result from the small and simple shape of conical P₁ and P₂ elements, which are easy to move at the time of collapse of apparatuses.

The natural assemblage specimens 1, 2, and 3 each contain two P₃ elements. These elements are adjacent and located just below the S elements in specimen 1 (Text‐fig. 3). On the other hand, the P₃ elements lie some distance apart in specimens 2 and 3 (Text‐figs 4, 5). This difference in location of the P₃ elements appears to reflect a mobility of P₃ elements in an apparatus of a living conodont. Although an interpretation of a function of the N. tahoensis apparatus is difficult, it is probable that the pair of P₃ elements moved between the positions shown by the solid and dotted lines in Text‐figure 2.

Palaeoecology of Neostrachanognathus tahoensis

To date, Neostrachanognathus tahoensis has been reported from rocks in the Jurassic accretionary complex distributed in Japan and Russia (Buryi 1989; Koike 1998). Koike (1998) had firstly described N. tahoensis from the Triassic Taho limestone in Shikoku, which is thought to have been deposited on a sea mount in Panthalassa that consisted of MORB (Mid Oceanic Ridge Basalt). The Lower Triassic sequence with N. tahoensis consists of bedded limestone that was deposited in a shallow sea under low‐energy conditions (Koike 1998). The siliceous rock distributed in the Dal’negorsk area, Primorsky Kray, Russia, also yields this species as discrete elements (Buryi 1989). The Lower to Upper Triassic sequence in the Dal’negorsk area comprizes siliceous rocks including siliceous claystone (Buryi 1989), which are similar in composition to the sequence of this study. As mentioned above, the siliceous claystone in this study is thought to have been deposited in a deep, offshore environment that lacked coarse‐grained clastic rocks, and was rather an environment where bedded chert formed (e.g. Matsuda and Isozaki 1991). Specimens of the natural assemblages in this study prove that some conodonts of this species were directly buried and decayed in sediments on a sea floor after their death. Hence, N. tahoensis have dwelled in both shallow and deep water, far from land.

Species of Neostrachanognathus from oman

The collection from Oman includes several conical and ramiform specimens, which are identified as components of Neostrachanognathus assignable to S, P₁, P₂, and P₃ elements. The sketches of discrete specimens from Oman are shown in Text‐fig. 7 and compared with the apparatus model of Neostrachanognathus tahoensis from the Oritate area.

Figure TEXT‐FIG. 7.

Reconstructed components of apparatus of Neostrachanognathus tahoensis from Oritate and specimens from Oman.

Three types of conical elements of the Oman material are termed P elements. One P element, tentatively termed P₁, has an antero‐lateral process with 1–3 denticles. Its base bears one or more posterior denticles (Text‐fig. 8A–C). The P₁ element varies in the shape of the cusp and the length of lateral process. Some of them have a shorter and broader cusp and a shorter lateral process with a single denticle (Text‐fig. 8D–E). The second P element, termed P₂, has a relatively simple shape, having a single anterior denticle that is not strongly inturned (Text‐fig. 8F). The forms assigned to the P₃ position are digyrate coniform elements and have an arched cusp with antero‐lateral and postero‐lateral edges and an expanded base; the postero‐lateral projection of the base may have a single denticle (Text‐fig. 8G–H, 8L–M). The S elements are bipennate ramiform elements with a strongly down‐ curved and backward directed anterior process that may bear one or more denticles between the large cusp and the large and conspicuous anteriormost denticle; the long posterior process bears smaller but variably sized, inclined denticles (Text‐fig. 8I–K).

Figure TEXT‐FIG. 8.

SEM Photographs of conodonts from Oman. All figures magnified ×80. A–M, Neostrachanognathus sp. A, GSC‐131249, loc. C‐117673; P₁ element; lateral view. B–C, GSC‐131250, loc. C‐117673; P₁ element. B, lateral, and C, posterior views. D–E, GSC‐131251, loc. C‐117673; P₁ element. D, lateral, and E, posterior views. F, GSC‐131252, loc. C‐117674; P₂ element; lateral view. G, GSC‐131253, loc. C‐117674; P₃ element; lateral view. H, GSC‐131254, loc. C‐117674; P₃ element; lateral view. I, GSC‐131255, loc. C‐117673; S element; lateral view. J, GSC‐131256, loc. C‐117673; S element; lateral view. K, GSC‐131257, loc. C‐117673; S element; lateral view. L, GSC‐131258, loc. C‐117674; M element; lateral view. M, GSC‐131259, loc. C‐117674; M element; lateral view. All specimens from the Guwayza Formation, Jabal Safra, Oman.

The Oritate and Oman materials differ in the shape of each element. The P₁ element of N. tahoensis from the Oritate area shows much more simple shape compared with the lateral process and posterior denticles present in the Oman material. The P₂ element from Oman differs from that of N. tahoensis in having an anterior denticle. The P₃ elements of Oman are simpler than those of Oritate. The elements in the P3 positions have a variation from a digyrate coniform to digyrate in shape. The S elements of the Oman material differ from those of N. tahoensis in the following respects: in lateral view, the anterior process forms an more acute angle with the posterior processes than that of N. tahoensis; the conspicuous anterior denticle neither has additional denticles to its anterior, nor as many denticles between the conspicuous anterior denticle and cusp; and denticles on the posterior process of the Oman material are larger and discrete. These differences suggest that the Oman material represents a different species from N. tahoensis.

Conclusions

Five natural assemblages of Neostrachanognathus tahoensis were found in siliceous claystone in the Oritate area, Itsuki Village, Kumamoto Prefecture, Japan. The architecture and orientations of collapse of the N. tahoensis apparatus were reconstructed. As a result, the following emerged: the apparatus comprizes 14 symmetrically‐aligned pairs of P₁, P₂, P₃, S₁, S₂, S₃, and S₄ elements; the P₁ and P₂ elements are simple cones and the P₃ elements are digyrate; the S₁ to S₄ elements are bipennate ramiform; the S element array lies rostally in the apparatus and three pairs of P elements are subvertically arranged caudally and ventrally to the S elements; the apparatus lacks elements in the S₀ and M positions; one of the natural assemblages of this study resulted from rostrocaudal collapse and the other four formed by decay and lateral collapse. This study and previous reports of N. tahoensis indicate that this conodont lived in both shallow and deep water far from land. A collection of discrete elements from Jabal Safra, Oman, also includes the components of a Neostrachanognathus species in which close comparisons can be made with the Oritate assemblages.

Systematic Palaeontology

All natural assemblage specimens described here are deposited in the Graduate School of Life and Environmental Sciences, University of Tsukuba, with the prefix IGUT.

Genus NEOSTRACHANOGNATHUS Koike 1998

Type species. Neostrachanognathus tahoensis Koike 1998, from the Taho Limestone, Ehime, Japan.

Revised diagnosis. Conodonts of this genus have an apparatus consisting of 14 elements arranged in paired P₁, P₂, P₃, S₁, S₂, S₃, and S₄ positions. P₁ and P₂ are conical forms, which often have antero‐lateral and/or posterior projections with one or more denticles. The P₃ elements vary in shape from digyrate coniform to digyrate. The S elements are bipennate ramiform. The P₃ position is located ventrally and little rostally to the P₁ and P₂ positions, which are aligned just caudally to the S₁ to S₄ array.

Remarks. Most of Early Triassic conodonts belong to the superfamily Gondolelloidea and family Ellisoniidae. On the basis of natural assemblages of Neogondolella from the Middle Triassic of Switzerland (Orchard and Rieber 1999), Orchard (2005) reconstructed apparatuses of 26 species assigned to 26 genera belonging to the Gondolelloidea. These apparatuses contain 15 elements in P₁, P₂, M, S₀, S₁, S₂, S₃, and S₄ positions. Conodonts of Triassic Ellisoniidae have only been reported as isolated materials, but Koike (2004) described a natural assemblage from the uppermost Permian of Japan. They decided that an apparatus of the specimen, which is identified with Ellisonia sp. cf. E. triassicaMüller 1956, has a 15‐element plan similar to that of Gondolelloidea. The skeletal plan of neither the Gondolelloidea nor the Ellisoniidae accord with that of Neostrachanognathus.

The origin of Neostrachanognathus is problematic. Some elements resemble those of ellisonids and, apparently like the natural assemblage of Koike (2004), the Neostrachanognathus apparatus does not include elements in the M position. However, we cannot be sure whether E. sp. cf. E. triassica originally had a M element in its apparatus or not based on the single specimen. Furthermore, the natural assemblage of E. sp. cf. E. triassica includes a S₀ element, which is not contained in the Neostrachanognathus apparatus.

Materials from both Japan and Oman show morphological variations of the P₁, P₂, and P₃ elements. The P₁ and P₂ forms range from a simple cone without a denticle to a unit with a denticulated antero‐lateral process and with posterior denticles; the P₃ elements vary in shapes from digyrate coniform without a denticle to digyrate. It is probable that the other ‘coniform’ elements, which have been described as isolated specimens from the Triassic rocks (Ding 1983; Kozur and Mock 1991), also represent components in apparatuses with complex multidenticulate elements.

Neostrachanognathus tahoensisKoike 1998;
Text‐figs 3A–C, 4A–C, 5A–C, 6A–B, 6D, 6F.

1989 Cratognathodus sp. Buryi; pl. 2, fig. 6.

1998 Neostrachanognathus tahoensisKoike; p. 127, 128, figs. 4, 9:1–19, 22.

Material. Five natural assemblages: IGUT‐ag3468 to 3470, 3573, 3574.

Description. Bilaterally symmetrical apparatus comprizes 14 elements: pairs of P₁, P₂, P₃, S₁, S₂, S₃, and S₄. The S elements form a rostal array and their posterior processes are caudally oriented and slightly lifted distally. These elements have inwardly inclined cusps and denticles. The P₁ and P₂ positions are located caudally to the S array. The P₃ position lies ventrally and little rostally to the P₁ and P₂ positions.

The P₁ element has a conical form with 270 and 500 μm in maximum and minimum length from tip of the cusp to antero‐basal corner of the base, respectively. Proclined cusp has arched anterior and straight posterior margins in lateral view. Upper margin of short base is 30–60 μm in length. The basal margin is approximately straight and 100–170 μm long in lateral view. The base bears a thin, discrete antero‐basal denticle, which is a half length of the cusp.

The P₂ element is an adenticulated cone with proclined cusp and short base. Length and width of the cusp and base are similar to those of the P1 element.

The P₃ element is breviform digyrate with two lateral processes forming an angle of 90–120 degrees in lateral view. Each process has three or four discrete, short denticles. Cusp is long, straight and proclined.

Four pairs of bipennate ramiform elements are termed the S₁ to S₄ elements. Anterior process is turned downwardly at an angle of approximately 120 degrees with the posterior process. Large cusp is reclined to recurved. Anterior process bears 7–9 denticles, which are fused at the bottom and discrete at the top. Third or fourth denticle from the anterior end is prominently long and wide, and the other denticles are short and delicate. Posterior process has more than 10 delicate denticles, which are also fused at the bottom and discrete at the top. The denticles on the posterior process are generally shorter than those on the anterior process.

Remarks. Koike (1998) originally described the apparatus of this species as ‘quadrimembrate’, consisting of four types of ‘coniform’ elements. His Sa, Sb, and a part of Sc elements, having single antero‐basal or antero‐lateral denticle, are identified as the P₁ element redefined in this study. The M element of Koike (1998) is a simple form without denticles or processes and is termed P₂ element here. Koike (1998) described specimens both with antero‐lateral and posterior denticles as a part of his Sc element. However, the elements with posterior denticles are not recognized in the Oritate materials and are components of different species from Neostrachanognathus tahoensis.

Occurrence. This species have been reported from the Spathian strata in the Kumamoto Prefecture, Japan, Ehime Prefecture, Japan (Koike 1998), and Primorsky Kray, Russia (Buryi 1989).

Acknowledgments

Acknowledgements. E.T. Tozer is thanked for collecting the samples from Oman. We are also grateful to the reviewer, M. A. Purnell and an anonymous reviewer for offering many useful comments and suggestions.