Additional thylacocephalans (Arthropoda) from the Lower Triassic (upper Olenekian) Osawa Formation of the South Kitakami Belt, Northeast Japan

Accepted Manuscript Title: Additional thylacocephalans (Arthropoda) from the Lower Triassic (upper Olenekian) Osawa Formation of the South Kitakami Belt, Northeast Japan Authors: Masayuki Ehiro, Osamu Sasaki, Harumasa Kano, Toshiro Nagase PII: DOI: Reference:

S1871-174X(18)30161-6 https://doi.org/10.1016/j.palwor.2019.03.001 PALWOR 495

To appear in:

Palaeoworld

Received date: Revised date: Accepted date:

9 November 2018 17 January 2019 13 March 2019

Please cite this article as: Ehiro, Masayuki, Sasaki, Osamu, Kano, Harumasa, Nagase, Toshiro, Additional thylacocephalans (Arthropoda) from the Lower Triassic (upper Olenekian) Osawa Formation of the South Kitakami Belt, Northeast Japan.Palaeoworld https://doi.org/10.1016/j.palwor.2019.03.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Additional thylacocephalans (Arthropoda) from the Lower Triassic (upper Olenekian) Osawa Formation of the South Kitakami Belt, Northeast Japan

Masayuki Ehiro *, Osamu Sasaki, Harumasa Kano, Toshiro Nagase

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The Tohoku University Museum, Sendai 980-8578, Japan

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* Corresponding author. E-mail adrress: [email protected]

Abstract

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The Lower Triassic Osawa Formation in the South Kitakami Belt, Northeast Japan,

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consisting mostly of mudstone of shallow-marine environment, was deposited during

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the late Olenekian (ca. 250 Ma), and is an important unit through which to examine the biotic recovery process after the end-Permian mass extinction. The Osawa Formation is

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the only unit in Japan that yields thylacocephalans (Arthropoda). Three species

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belonging to three genera have been reported before: Ankitokazocaris bandoi, Kitakamicaris utatsuensis and Ostenocaris sp. In addition to the known species, some

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thylacocephalans, including one new genus and three new species, are described in the present paper: Ankitokazocaris tatensis n. sp., Concavicaris parva n. sp., Miyagicaris

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costata n. gen. n. sp. and Ostenocaris? sp. Although Thylacocephala have a rather long stratigraphic range (from Silurian to Cretaceous) and are known from a wide

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geographical region, there are only about thirty genera in this group. The Osawa thylacocephalan fauna comprises at least five genera, making it one of the most diverse in the world at the generic level. During the Triassic Period, the Thylacocephala diversified and spread widely throughout low-latitude regions.

Keywords: biotic recovery; Early Triassic; Osawa Formation; South Kitakami Belt; 1

Thylacocephala

1. Introduction Ehiro et al. (2015) reported three species of Thylacocephala (Crustacea) from Japan for the first time, namely Ankitokazocaris bandoi Ehiro and Kato in Ehiro et al., 2015,

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Kitakamicaris utatsuensis Ehiro and Kato in Ehiro et al., 2015, and Ostenocaris sp., all of which were collected from the Lower Triassic Osawa Formation in the South

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Kitakami Belt, Northeast Japan. The Osawa Formation yields a diverse late Olenekian (= Spathian of Tozer, 1967) ammonoid fauna, consisting of twenty-five genera (Ehiro

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et al., 2016). As pointed out by Nakajima and Izumi (2014) and Takahashi et al. (2014),

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this rich ammonoid fauna suggested that the nektonic to nektobenthic faunas had

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already recovered after the end-Permian (ca. 252.2 Ma) mass extinction by the beginning of the late Olenekian in the South Kitakami Belt.

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Although the class Thylacocephala has a long stratigraphic record, ranging from

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Silurian (or Cambrian) to Cretaceous, and a wide geographic distribution, only about thirty genera are known (Schram, 2014; Ehiro et al., 2015). Recently, four additional

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species of Thylacocephala were collected from the Osawa Formation. The rich thylacocephalan fauna of the formation is very important for demonstrating

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thylacocephalan diversity at the beginning of the Mesozoic. This paper describes these new materials as Ankitokazocaris tatensis n. sp., Concavicaris parva n. sp.,

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Miyagicaris costata n. gen. n. sp. and Ostenocaris? sp., and discusses their significance.

2. Geological setting The Osawa Formation belongs to the Lower–Middle Triassic Inai Group, which is widely distributed in the southern part of the Southern Kitakami Massif (the South 2

Kitakami Belt), Northeast Japan (Fig. 1). The Inai Group is one of the most important reference sequences of the Lower–Middle Triassic of Japan, because it contains a continuous shallow marine (partly alluvial–nearshore marine) clastic sequence and yields rather rich marine fossils. It exceeds 2000 m in total thickness, and is divided, in ascending order, into the Hiraiso, Osawa, Fukkoshi, and Isatomae formations (Onuki

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and Bando, 1959). The Hiraiso Formation is 200–250 m thick. The lower part of the formation consists of thin basal conglomerate, which unconformably covers Late

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Permian mudstone-dominated strata, and coarse- to medium-grained calcareous

sandstone with thin mudstone. The upper part of the formation is composed of

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alternating beds of medium- to fine-grained calcareous sandstone and mudstone. The

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Osawa Formation is 250–350 m thick. The lowermost part of it consists of thin

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alternating beds of mudstone and sandstone to laminated mudstone, which comprises thick (2–20 cm in thickness) mudstone beds and thin sandstone beds (mostly 1–2 cm,

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often 5 cm) or laminae (around 0.5 cm). The main part of the Osawa Formation is

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mostly composed of laminated mudstone, which is often intercalated with sandstones. The sandstone laminae in the main part are mostly less than 2 mm in thickness. The

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Fukkoshi Formation is composed of thick sandstone and alternating beds of sandstone and mudstone, with total thickness of 200–300 m. The Isatomae Formation is more than

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1000 m thick and consists of sandy laminated mudstone often with thick sandstones or alternating beds of sandstone and mudstone.

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From the middle part of the Hiraiso Formation in the Motoyoshi area (Fig. 1),

Shigeta and Nakajima (2017) reported the occurrence of Tirolites cf. ussuriensis Zharnikova in Buryi and Zharnikova, 1981, which is the only ammonoid species described from the formation. T. ussuriensis occurs in the upper part of the Tirolites-Amphistephanites Zone in the South Primorye area, Far East Russia (Zakharov and Popov, 2014; Fig. 2). The Osawa Formation is famous for yielding one 3

of the oldest ichthyopterygian, Utatsusaurus hataii Shikama, Kamei and Murata, 1978. It also yields a rich ammonoid fauna, including Columbites and Subcolumbites. Bando and Shimoyama (1974) divided the Osawa Formation biostratigraphically into two ammonoid zones: the Subcolumbites Zone (occupying the lowermost to upper part of the formation) and the Arnautoceltites Zone (uppermost part) (Fig. 2). The former zone

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is subdivided into the Columbites parisianus Subzone (lowermost part of the zone) and

Subcolumbites perrinismithi Subzone (main part). C. parisianus Hyatt and Smith

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ranges over the Subcolumbites Zone and co-occurs with S. perrinismithi (Arthaber)

throughout the S. perrinismithi Subzone. They are associated with ammonoids such as

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Hemilecanites discus (Arthaber), Albanites sheldoni (Kummel), Tardicolumbites aff.

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tardicolumbus Guex et al., Yvesgalleticeras sp., Hellenites elegans Guex et al., and

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Nordophiceratoides bartolinae Guex et al. (Ehiro et al., 2016). The uppermost part of the Osawa Formation (uppermost part of the Arnautoceltites Zone) yields Procarnites

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kokeni (Arthaber), one of the typical late Olenekian ammonoids, and Eosturia

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towaensis Bando and Ehiro (Bando and Ehiro, 1982). The Fukkoshi Formation is very poor in ammonoids. Only Gymnites cf. watanabei (Mojsisovics), Hollandites sp. and

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Balatonites cf. kitakamicus (Diener), which indicate an early Anisian age, are considered to come from the Fukkoshi Formation (Onuki and Bando, 1959), although

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there is debate about the stratigraphic position of these ammonoids (Ishibashi, 2006). Based on these ammonoids, the Hiraiso and Osawa formations are roughly

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correlated with the lower part of the upper Olenekian (Tirolites harti beds of Guex et al., 2010; Tirolites-Amphistephanites Zone of Zakharov and Popov, 2014; Fig. 2) and middle-upper part of the upper Olenekian (Columbites parisianus Zone to Subrobustus Zone of Guex et al., 2010; Neocolumbites insigens Zone to Subfengshanites multiformis Zone of Zakharov and Popov, 2014), respectively. In South China, the upper Olenekian is divided only into two ammonoid zones (Guo et al., 1982; Tong et al., 4

2001; Fig. 2). Galfetti et al. (2007) showed a more detailed ammonoid zoning of the Olenekian of South China (Fig. 2), but without stratigraphic data. There are some problems in precisely correlating the Osawa ammonoid zones (and horizons of thylacocephalans with associated ichthyopterygians) with those of the upper Olenekian of outside Japan. Columbites and Subcolumbites are zonal fossils of the middle

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Olenekian (C. parisianus Zone, N. insigens Zone, etc.) and upper Olenekian

(Subcolumbites Zone), respectively. For instance, in North America and South China,

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Subcolumbites occurs always above the horizon of Columbites (Guo et al., 1982; Tong et al., 2001; Guex et al., 2010; Fig. 2). However, they co-occur in the Osawa Formation.

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Arnautoceltites co-occurs with Subcolumbites in the Subcolumbites Zone in North

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America (Guex et al., 2010) and South China (Guo et al., 1982; Tong et al., 2001). In

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the Osawa Formation, on the other hand, these two genera co-occur only in the basal part of the Arnautoceltites Zone, and no Subcolumbites has been known from the main

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part of the zone (Fig. 2). Therefore, further work is needed on the precise stratigraphic

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correlation of the Osawa Formation.

More information about the Osawa Formation and its fossil assemblages can be

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found in Ehiro et al. (2015, 2016).

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3. Materials and methods Specimens described here were collected from two localities in the Tatezaki area in

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Utatsu, Minamisanriku Town, Miyagi Prefecture (Fig. 3). One site, Tatezaki A Locality (Fig. 3, Locality A, 38°42′34″N, 141°32′06″E), is located on the western coast of Cape Tatezaki, where the top part of the lowermost part and the lower part of the main part of the Osawa Formation are exposed (Fig. 4, Locality A). The lowermost part consists of alternating beds of mudstone and thin sandstone. The main part is composed of laminated mudstone, and is intercalated with sandstone-rich alternating beds of 5

sandstone and mudstone with a total thickness of about 12 m in the middle part. This site is the type locality of U. hataii, and specimens of Utatsusaurus have been collected from the laminated mudstone both occupying the lower and upper horizons of the intercalated sandstone-rich alternating beds (Shikama et al., 1978). From the lower part of this sequence, just above the boundary between the lowermost part and the main part

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(Fig. 4, Locality A), we collected some ammonoids fossils, identifiable as

Subcolumbites perrinismithi (Fig. 5). A thylacocephalan specimen, Ostenocaris? sp.

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described in this paper, was collected from the mudstone intercalated in the uppermost

part of the sandstone-rich alternating beds of sandstone and mudstone (Fig. 4, Locality

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A).

The other site, Tatezaki B Locality (Fig. 3, Locality B, which coincides with the

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Tatezaki site of Ehiro et al., 2015, 38°42′50″N, 141°32′07″E), is located about 900 m north of Cape Tatezaki. At this site, there is an outcrop of laminated mudstones, the

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total thickness of which is about 14 m. The horizon of the Tatezaki B Locality is

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considered to be in the middle part of the Osawa Formation. More than 300 specimens, belonging to the three thylacocephalan species reported by Ehiro et al. (2015), have

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been collected from an interval from 2–4 m above the bottom part of the sequence (Fig. 4, Locality B). In this thylacocephalan fauna, Kitakamicaris utatsuensis is dominant,

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comprising more than 90% of the collection, whereas Ankitokazocaris bandoi and particularly Ostenocaris sp. are rare. All specimens described in this paper, except for

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one specimen described as Ostenocaris? sp., were collected from this interval. The ammonoid fossils reported by Ehiro et al. (2016) were collected from higher horizons, although many of them were collected from floats. The ammonoids originated from an interval (about 1 m thick) just above the thylacocephalan horizon were Columbites parisianus,

Tardicolumbites

aff.

tardicolumbus,

Nordophiceratoides bartolinae. 6

Yvesgalleticeras

sp.,

and

Locality B is considered stratigraphically higher than Locality A, because the sequence of the Locality A, the total thickness of which is about 35 m, includes the boundary between the lowermost and main parts of the formation (Fig. 4) and is positioned at the basal part of the main part. However, the detailed stratigraphic relationship between these two localities is uncertain because of the complicated

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geologic structure and poor exposure.

Microscopic texture observations and chemical analyses of samples were

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performed using a field-emission scanning electron microscope (FE-SEM, JSM-7001, JEOL, Tokyo Japan), with the samples coated with platinum and carbon. In SEM

Electron

microprobe

analyses

(EPMAs)

were

performed

using

an

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45°.

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observations, the accelerating voltage was 5 kV and the angle of sample stage was 0° or

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energy-dispersive X-ray spectrometer (EDS; X-Max system, Oxford Instruments, Oxford, UK) together with the FE-SEM. In the EPMAs, the accelerating voltage was

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15 kV and a carbon-coated sample was used. Back-scattered electron (BSE) images

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were also obtained by the FE-SEM.

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4. Systematic description

The specimens described here are held in the Tohoku University Museum (Institute

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of Geology and Paleontology, Tohoku University: IGPS) and in the Utatsu Ichthyosaur Museum (Educational Committee of Minamisanriku Town: UIM). The taxonomic

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classification above family level follows Schram (2014).

Class Thylacocephala Pinna, Arduini, Pesarini and Teruzzi, 1982 Order Concavicarida Briggs and Rolfe, 1983 (sensu Schram, 2014) Family Concavicarididae Schram, 2014

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Genus Ankitokazocaris Arduini, 1990 Type species: Ankitokazocaris acutirostris Arduini, 1990. Discussion: The genus Ankitokazocaris includes four species: A. acutirostris, A. bandoi Ehiro and Kato in Ehiro et al., 2015, A. chaohuensis Ji et al., 2017 and A. tatensis n. sp. The diagnostic characters of the genus are sub-rectangular carapace with

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anterior rostrum but no posterior spine (Arduini, 1990). In addition, the type specimen of A. acutirostris has a dorsal “carina” running parallel to the dorsal margin on the

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carapace (Arduini, 1990, pp. 200–201, fig. 2). Similar structures are also observed in A.

bandoi (lateral ridge and furrow) and A. tatensis n. sp. (lateral ridge), but not in A.

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chaohuensis. In this respect, there remain some doubts about the generic attribution of

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A. chaohuensis.

Ankitokazocaris tatensis n. sp.

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(Fig. 6)

Materials examined: Five specimens; holotype: IGPS coll. cat. no. 111873; paratypes:

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IGPS coll. cat. nos. 111874–111875 and UIM 30625–30626. Etymology: The specific epithet refers to “Tatehama (Tate Beach)”, a place name near

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the type locality.

Diagnosis: Species of Ankitokazocaris that has a slender, trapezoidal carapace. The

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concave anterior margin is connected to the ventral margin with a narrowly rounded, nearly right-angled corner. The rostrum is distinct with lateral carinae, which extend to the anterior part of the carapace body. Description: The carapace is trapezoidal in lateral view, with a broadly convex dorsal margin, concave and broad anterior (optic) margin, mound-shaped ventral margin, and nearly straight, narrow posterior margin. The anterior half to two-thirds of the ventral 8

margin is broadly convex, but the posterior half to one-third, extending to an upward bend, is nearly straight. The anterior-ventral corner is nearly right-angled and very narrowly rounded. The dorsal margin extends into a distinct and straight to gently curved rostrum. The rostrum bears a remarkable lateral carina that passes into the anterior part of the carapace body. In some specimens (holotype: IGPS coll. cat. no.

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111873 and paratype: IGPS coll. cat. no. 111875), there is a fine and straight, but short or intermittent?, longitudinal ridge (carina) on the dorsal side, running nearly parallel to

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the dorsal margin (Fig. 6A–D). The surface of the carapace seems to be smooth, except for the aforementioned ridge.

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The maximum height of the carapace is at its middle part where the ventral margin

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is bent. The specimens have a high length to height ratio. Although the exact lengths of

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the carapace (including the rostrum) are unknown because of poor preservation, in the largest specimen (IGPS coll. cat. no. 111875), which has a height of ca. 16 mm, the

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length exceeds 32 mm. From the carapace shape, its full length is considered to be

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33–35 mm, and, if it is an appropriate value, the ratio of the carapace height to the length (H/L) is 0.46–0.48. In another paratype (IGPS coll. cat. no. 111874), the height

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of which is ca. 15.5 mm, the length is estimated to be ca. 30 mm (H/L = ca. 0.52). Some structures, probably emerging under the carapace, are observed near the

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anterior-ventral corner and the central part of the carapace, although they could not be identified with absolute certainty. Near the anterior-ventral corner, there are some

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irregularly shaped tubercle-like structures (Fig. 6A–D). In the central part, some (more than seven) vertically long structures are present (Fig. 6A, B), which look like a part of body segments. Discussion: The carapace outline of Ankitokazocaris tatensis n. sp. (late Olenekian) is similar to that of A. acutirostris (Arduini, 1990, p. 199, figs. 2, 3) from the Lower Triassic of Madagascar. But the latter has a broader outline (H/L = ca. 0.56), a more 9

broadly rounded anterior-ventral process, and lacks lateral carina on the lateral sides of the rostrum. The present new species also somewhat resembles A. chaohuensis (Ji et al., 2017, p. 175, figs. 3–8) from the upper Olenekian of Chaohu, Anhui Province, China. The Chaohu species is distinguished from the Utatsu species by having a triangular shaped lateral outline of the carapace, with an angular anterior-ventral corner, and by

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lacking the longitudinal ridge on the lateral side. A. bandoi (Ehiro and Kato in Ehiro et

al., 2015) from the same locality and horizon of the present species is easily

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distinguished by having a broad based rostrum.

Occurrence: From the middle part of the Osawa Formation (upper Olenekian), which

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is distributed in a region to the north of Cape Tatezaki (Fig. 3, Tatezaki B Locality),

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Utatsu, Minamisanriku Town, Miyagi Prefecture, Northeast Japan.

Genus Concavicaris Rolfe, 1961

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Type species: Ceratiocaris (Colpocaris) fredleyi Meek, 1872.

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Discussion: Many species of the genus Concavicaris have been described from the Devonian of the Czech Republic (Chlupáč, 1963), Poland (Zatoń and Rakociński,

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2014; Zatoń et al., 2014; Broda et al., 2015; Broda and Zatoń, 2017) and Australia (Briggs and Rolfe, 1983), and from the Carboniferous of USA (Meek and Worthen,

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1868; Meek, 1872; Briggs and Rolfe, 1983; Schram, 1990) and the Czech Republic (Rak et al., 2018). Schram (2014) tentatively grouped the species of the genus into four

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species-groups (group 1 to 4). The carapace shape of the present new species resembles some species in the Schram’s species-group 1 and 2. And, therefore, the new species is considered to belong to this genus, although there remains some doubts about the generic attribution because no species of this genus has hitherto been reported from the post-Carboniferous strata. Thus, this species represents an over forty million years range extension for the genus. 10

Concavicaris parva n. sp. (Figs. 7–9)

IGPS coll. cat. nos. 111877–111882 and UIM 30627–30628.

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Materials examined: Nine specimens; holotype, IGPS coll. cat. no. 111876; paratypes:

Etymology: The specific epithet refers to the small carapace size of the species.

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Diagnosis: Small Concavicaris having a semi-oval outline with a broadly rounded

venter and downwardly bending rostrum. The carapace bears densely distributed small

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pits on the whole carapace surface.

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Description: The small carapace is sub-oval in outline with a slightly convex dorsal

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margin, broadly rounded ventral margin without remarkable corner, and a slightly convex to flat, narrow posterior margin (Figs. 7, 8). The anterior margin (optic notch) is

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semi-circular with a short, downwardly curved rostrum and a pointed anterior-ventral

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process. The angles of the latter process are slightly larger than 90°. The carapace is 9.4 to 14.8 mm in length (average 12.3 mm, n = 8), 5.9 to 9.7 mm in height (average 7.4

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mm, n = 8), and about 1.5 to 2.5 mm in posterior height (average 1.9 mm, n = 8). The average ratio of the height to the length of the carapace is about 0.6, and that of the

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posterior height to the height of the carapace is 0.2 to 0.3. A semicircular impression (muscle scar?) is observed in the lower part of the anterior third in a specimen, IGPS

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coll. cat. no. 111877 (Fig. 7D, E). There are many small pits, distributed across almost the entire carapace surface,

but without a systematic arrangement (Fig. 7C, H). About 200 pits can be observed in 1 mm2. The BSE images (Fig. 9A, C) and EPMA results (Fig. 9E, F) indicate that the carapace body (bright regions in Fig. 9A, C) is composed of apatite (dominantly fluorapatite) and some pits are filled with clay minerals (dark regions). The FE-SEM 11

and BSE images show that these pits have rather simple structures. The pits in which clay minerals (argillaceous sediments) are completely distributed (pits a and d in Fig. 9C, D) look slightly shallower than those in which clay minerals are lacking or only partly distributed (pits b and c). Therefore, it is considered that the shapes of the pits b and c are close to their original shape. They are semi-circular, with diameters of 30 to

such as micro-pores, trenches, or ridges in and around the pits.

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40 µm, and are shallow and basin-like (Fig. 9B, D). There are no concomitant structures

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Discussion: The general carapace outline of the new species is somewhat similar to

Concavicaris sinuata (Meek and Worthen) (Meek and Worthen, 1868, p. 22 as

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Ceratiocaris? sinuatus; Rolfe, 1969, p. R317, fig. 140.1a; Briggs and Rolfe, 1983, pl.

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35, fig. 19) from the Upper Carboniferous of USA and C. cf. sinuata (Meek and

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Worthen) (Briggs and Rolfe, 1983, pl. 36, fig. 14) from the Lower Carboniferous of USA, but differs from the latter by having a broadly rounded venter and downwardly

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bending rostrum. C. viktoryni Rak, Broda and Kumpan, 2018 from the Lower

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Carboniferous of the Czech Republic has similar carapace outline to the present new species, but the former differs from the latter by having more slender form and by

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having vertical falciform lirae on the carapace surface. C. incola Chulpáč (1963, p. 112, pl. 12, figs. 1, 2, pl. 16, figs. 1, 2) from the Upper Devonian of Moravia, the Czech

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Republic also resembles C. parva n. sp. in the general carapace form, but the former has an acute posterior margin.

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Concavicaris parva n. sp. is also distinguished from all other species of the genus

Concavicaris by having many small pits over the entire surface of the carapace. Although the pits are very small (30–40 µm in diameter), the presence of them can be observed by high magnification loupe or binocular stereo microscope. To date, no such structure has been observed in existing species of Concavicaris, except for C. aff. bradleyi (Meek) from the Upper Devonian of Poland (Broda and Zatoń, 2017). Two 12

types of circular structure are observed in C. aff. bradleyi (Broda et al., 2015; Broda and Zatoń, 2017). One consists of circular depressions, each located in the central area of polygonal structures (ca. 50 μm). These polygonal structures cover almost the entire surface of the carapace, but not along their marginal parts. The circular depression, the diameter of which is 15–20 µm, is considered to be originally hollow pores and sealed

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during phosphatisation processes (Broda and Zatoń, 2017). Another type is intracuticular microstructures occur as elongated tubular structures, circular to oval in

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cross section (less than 15 µm in diameter). They are distributed along the dorsal and

ventral margins forming thin belts. The circular structures in C. parva n. sp. differs

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from those in C. aff. bradleyi by having larger diameters (30–40 µm), by having simple,

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shallow and basin-like pit structures without concomitant structures, and by their

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distribution pattern.

C. parva n. sp., the carapace of which is less than 15 mm in length, is one of the

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smallest thylacocephalans comparable to Microcaris minuta Pinna, 1974, Concavicaris

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remipes Schram, 1990, Thylacocephalus cymolopos Lange et al., 2001, and Victoriacaris muhiensis Hegna et al., 2014.

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Occurrence: From the middle part of the Osawa Formation (upper Olenekian), which is distributed in a region to the north of Cape Tatezaki (Fig. 3, Tatezaki B Locality),

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Utatsu, Minamisanriku Town, Miyagi Prefecture, Northeast Japan.

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Family Microcarididae Schram, 2014

Genus Miyagicaris n. gen. Type species: Miyagicaris costata n. sp. Etymology: Named after the Miyagi Prefecture where the fossil-bearing Osawa Formation is distributed. 13

Diagnosis: Large thylacocephalan with a trapezoidal carapace, on which vertical ribs develop. The concave anterior margin is connected at about 70° to the ventral margin. The vertical ribs run parallel to each other in the posterior two-thirds of the carapace, but dendritically branch upward and downward several times in the anterior one-third. Discussion: Although the type specimen of the present new genus lacks its rostrum, its

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general carapace shape and remarkable vertical ribs on the carapace are characteristic

for the genus. Until now, four genera belonging to the family Microcarididae have been

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known from the Triassic strata: Microcaris Pinna, 1974; Atropicaris Arduini and Brasca, 1984; Ferrecaris Calzada and Mané, 1993; and Kitakamicaris Ehiro and Kato

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in Ehiro et al., 2015. They have a pointed rostrum and vertical ribs on the carapace, and

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somewhat resemble to the present new genus. The new genus Miyagicaris, however,

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can be clearly distinguished from other Triassic microcarid genera by having some attributes as follows: 1) the anterior-ventral process of Miyagicaris is acute (ca. 70°),

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whereas that of Kitakamicaris is ca. 90° and those of Microcaris, Atropicaris and

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Ferrecaris are obtuse angles; 2) the vertical ribs on the carapace of the new genus are dendritically branched in the anterior one-third, while ribs of the others are not; 3) the

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new genus has rather large carapace reaching more than 70 mm in length, while the

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other are smaller than 40 mm.

Miyagicaris costata n. sp.

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(Fig. 10A–D)

Materials examined: Two specimens; holotype: IGPS coll. cat. no. 111883; paratype: UIM 30629. Etymology: The specific epithet is derived from its remarkable surface ornamentation. Diagnosis: Same as for the genus. 14

Description: Two fragmental carapaces are at hand. One specimen (holotype: IGPS coll. cat. no. 111883) is slightly broken and the rostrum is missing. Another specimen (UIM 30629) is a part of the anterior-ventral part of the carapace. Although the rostrum is missing, the carapace is rather large, reaching more than 70 mm in length. The carapace is trapezoidal in lateral view, with a broadly convex dorsal margin, concave

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and broad anterior (optic) margin, and mound-shaped ventral margin, with a vertex in its central part. The narrow posterior margin is not well preserved but seems to be

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straight. The lower half of the anterior margin is nearly straight and is connected to the

nearly straight anterior half of the ventral margin by a narrowly rounded process, with

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an angle of approximately 70°. The folding angle of the central part of the venter is

N

about 130°.

A

The whole carapace bears vertical ribs that run from the dorsal margin to ventral margin and taper toward both ends. The ribs are wider than the spaces between them,

M

but their internal casts are narrower. In the central to posterior part of the carapace, they

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are inclined slightly forward and run parallel to each other, although they occasionally branch off and/or join together. They dendritically branch upward several times near

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the anterior margin and dendritically branch downward several times in the next lower half of the anterior part. In the anterior third, the ribs bend remarkably backward near

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the ventral margin. Some short and narrow rib-like ridges, which run parallel to the venter, are observed near the ventral margin.

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Occurrence: From the middle part of the Osawa Formation (upper Olenekian) to the north of Cape Tatezaki (Fig. 3, Tatezaki B Locality), Utatsu, Minamisanriku Town, Miyagi Prefecture, Northeast Japan.

Order Conchyliocarida Secrétan, 1983 (sensu Schram, 2014) Family Ostenocarididae Schram, 2014 15

Genus Ostenocaris Arduini, Pinna and Teruzzi, 1984 Type species: Ostenia cypriformis Arduini, Pinna and Teruzzi, 1980.

Ostenocaris? sp.

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(Fig. 10E, F)

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Material examined: One specimen: UIM 30624.

Description: Only one specimen, in which the dorsal margin and posterior margin

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were not well preserved, was examined. The large carapace is trapezoidal in lateral

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view, with a wide and broadly concave anterior (optic) margin and bending ventral

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margin. The upper two-thirds of the anterior margin, continuing from a broad-based rostrum, is semicircular, while the lower third is nearly straight. The anterior two-thirds

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of the ventral margin is nearly straight and connected to the lower part of the anterior

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margin with an acute process (ca. 73°). The posterior third of the ventral margin is convex, connecting the anterior two-thirds with a broadly rounded flexure. The

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carapace length exceeds 77 mm, and the height exceeds 36 mm. There is a narrow (1–2 mm wide) border zone along the anterior and ventral margins. The carapace surface

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seems to be smooth.

Remarks: The general carapace outline and the smooth surface of the present

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specimen are similar to those of the genus Ostenocaris. Nevertheless, there remains some doubt about the generic identification, because the dorsal part of the carapace was missing and we could not observe the posterior part bending down the dorsal margin, which is characteristic of the genus Ostenocaris. The present species differs from the other Lower Triassic species of Ostenocaris, O. ambatolokobensis Arduini from Madagascar (Arduini, 1990, p. 202, fig. 4) and 16

Ostenocaris sp. from the Osawa Formation (Ehiro et al., 2016, p. 279, fig. 10) due to its large size and having broadly rounded ventral flexure. Occurrence: From the lower part of the Osawa Formation (upper Olenekian), which is distributed on the west coast of Cape Tatezaki (Fig. 3, Tatezaki A Locality), Utatsu,

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Minamisanriku Town, Miyagi Prefecture, Northeast Japan.

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5. Discussion

5.1. Carapace structure of Concavicaris parva

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Concavicaris parva n. sp. is characterized by densely distributed small pits over the

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whole carapace surface. Similar pits or circular structures are known in some other

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Mesozoic thylacocephalans, for example, Paraostenia voultensis Secrétan from the Callovian of France (Secrétan, 1985), Paradollocaris vannieri Charbonnier and

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Thylacocaris schrami Audo and Charbonnier from the Cenomanian of Lebanon

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(Charbonnier et al., 2017), Protozoea hilgendorfi Dames, Hamaticaris damesi (Roger), Pseuderichthus cretaceus Dames and Thylacocephalus cymolopos Lange et al. from

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the Santonian of Lebanon (Schram et al., 1999; Lange et al., 2001; Charbonnier et al., 2017). Recently, related circular structures have also been reported in the Late

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Devonian species Concavicaris aff. bradleyi from Poland (Broda et al., 2015; Broda and Zatoń, 2017).

A

These circular structures are considered pressure-receptors (Rolfe, 1985) or some

other kind of sensory organs (Secrétan, 1985; Schram et al., 1999; Broda and Zatoń, 2017). Lange and Schram (2002) attempted to compare the grid-like pattern or longitudinal rows of circular punctuations observed on the carapace surface of some Cretaceous thylacocephalans, such as P. hilgendorfi, H. damesi, P. cretaceus and T. cymolopos, with the lattice organs of some thecostracan crustaceans. Broda and Zatoń 17

(2017) stated that tubular structures (with circular cross section) in the carapace of the Devonian C. aff. bradleyi are very similar to crustacean sensory organs. Circular structures in the Jurassic–Cretaceous species have a grid-like pattern (P. hilgendorfi, P. cretaceous and T. cymolopos) or are positioned in longitudinal rows (P. voultensis and P. damesi). They are larger in diameter by at least one order of

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magnitude than those of C. parva (30–40 µm in diameter). Devonian species C. aff. bradleyi has two types of circular structures: sealed hollow pores having circular cross

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section (15–20 µm in diameter), located in the central area of polygonal structures (ca. 50 μm in diameter), and intracuticular tubular structures perpendicular to the carapace

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surface with circular cross section (less than 15 µm in diameter), distributed along the

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dorsal and ventral margins (Broda and Zatoń, 2017). The circular structures in C. parva

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have different shape and size, and different patterns of distribution. From the FE-SEM and BSE images (Fig. 9A–D), they are simple pits with shallow and basin-like

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structures, with no concomitant structures, such as micropores, trenches or ridges in

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and around the pits. They are distributed almost on the entire carapace surface but without systematic arrangement (Fig. 7C, H). This simple structure and distribution

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pattern (without systematic arrangement) are quite different from the ‘pore field’ and ‘keel in a trough’ types of lattice organs belonging to some thecostracan crustaceans

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(Høeg and Kolbasov, 2002), also different from some ‘keel in a trough’ type-like structures observed in P. hilgendorfi (Lange and Schram, 2002), and the hollow pores

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and tubular structures (Broda and Zatoń, 2017) in C. aff. bradleyi. Based on their simple, shallow pits structure and pattern of distribution without a

systematic arrangement on the carapace, it is unlikely that the circular structures of C. parva are some kind of sensory organs, and therefore their function remains unresolved.

18

5.2. Diversity and paleobiogeography of the Triassic Thylacocephala Although the stratigraphic range of Thylacocephala is rather long (from at least the Silurian to Cretaceous) and they are distributed over a wide geographical region, there are only about thirty genera. The Osawa thylacocephalan fauna consists of at least five genera, and therefore is one of the most diverse thylacocephalan faunas in the world at

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the generic level, with the other being the Late Cretaceous (Santonian) Sahel Alma fauna of Lebanon (Charbonnier et al., 2017).

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Recently Ji et al. (2017) described a new species of Thylacocephala,

Ankitokazocaris chaohuensis, from the upper Olenekian (Lower Triassic) of Chaohu,

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Anhui Province, China. In addition, some new occurrences of Triassic Thylacocephala,

N

not yet described, have been reported from Western Australia (Haig et al., 2015: lower

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Olenekian), Yunnan Province, China (Feldmann et al., 2015: Anisian), and Idaho, USA (Brayard et al., 2017: upper Olenekian). The Australian species probably belongs to the

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family Austriocarididae (Haig et al., 2015). The Yunnan species is associated with the

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Luoping Biota and its carapace has some similarities with genera in the family Microcarididae Schram, 2014. The two species from Idaho probably belong to the

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Microcarididae and Concavicarididae, respectively. As discussed by Ehiro et al. (2015), all nine existing Triassic thylacocephalan

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genera are known from the Tethyan province and the low-latitude area of Tethys-Panthalassa border. Chaohu and two of three new localities reported above

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(Western Australia and Yunnan Province, China) are in the same area, while Idaho, USA, is located in a low-latitude area of eastern Panthalassa. Thus, the paleogeographic distribution of the Triassic Thylacocephala became very wide, although limited to the low-latitude area (Fig. 11). The number of Triassic thylacocephalan genera reached eleven following the present discoveries from the Osawa Formation, even when excluding the four undescribed species mentioned above, and the Triassic is the most 19

diversified period for the Thylacocephala at the generic level. In addition, it should be noted that the thylacocephalan species were diversified and widely distributed in the Olenekian (Idaho, Western Australia, Madagascar, South China and South Kitakami Belt, Japan), although this was shortly after the Permian/Triassic mass extinction.

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Acknowledgments

The authors wish to express their gratitude to the Educational Committee of

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Minamisanriku Town and Abei-Gumi Co. Ltd. for their support during field work. Appreciation is also due to Katsuhiko Sato for providing a specimen. The manuscript

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was greatly improved by the constructive reviews of Thomas A. Hegna and an

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anonymous reviewer.

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Figure captions

Fig. 1. Index map showing the study area (Tatezaki area) in the southern part of the

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Southern Kitakami Massif (South Kitakami Belt), Northeast Japan.

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Fig. 2. The upper Olenekian (Spathian) ammonoid zonations from some low-latitude localities with ranges of some index ammonoid genera and species. Guex et al. (2010)

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treated Arnautoceltites as a synonym of Paragoceras. The monospecific genus Subfengshanites was elected by Zakharov et al. (2008) based on Subcolumbites

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multiformis Kiparisova. Sz.: Subzone.

27

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Fig. 3. Geologic map of the Tatezaki area, Utatsu, Minamisanriku Town, Miyagi

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Prefecture, showing the sites where fossils were collected: Tatezaki A and Tatezaki B

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localities.

Fig. 4. Generalized columnar section of the Osawa Formation in the Utatsu area (left),

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and detailed columnar sections of the Locality A (right) and Locality B (center), showing ammonoid and thylacocephalan horizons. Cp. Sz.: Columbites parisianus Subzone.

28

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Fig. 5. Subcolumbites perrinismithi (Arthaber) from the lower part of the Osawa

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Formation on the western coast of Cape Tatezaki, Utatsu (Locality A).

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Fig. 6. Ankitokazocaris tatensis n. sp. from the Osawa Formation. (A, B) IGPS coll. cat. no. 111873 (holotype), left lateral view of the carapace (A) and its interpretive drawing (B), anterior to left, posterior margin missing. (C, D) IGPS coll. cat. no. 111875, left

A

lateral view of the carapace (outer mold) (C) and its interpretive drawing (D), anterior to right, posterior margin missing. (E–G) UIM 30626, right lateral view of the carapace (outer mold) (E) and its interpretive drawing (F), anterior to left, posterior half missing, and right lateral view of the anterior part of the carapace (inner mold) (G). (H, I) IGS coll. cat. no. 111874, right lateral view of the carapace (inner mold) (H) and its

29

interpretive drawing (I), anterior to right, rostrum missing. lr: lateral ridge, rc: lateral

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carina on the rostrum, so?: segmental organ?.

Fig. 7. Concavicaris parva n. sp. from the Osawa Formation. (A–C) IGPS coll. cat. no. 111876 (holotype); right lateral view of the carapace with a part of the left side (A) and

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its interpretive drawing (B), anterior to right; (C) close up of the carapace surface, showing the small pits. (D, E) IGPS coll. cat. no. 111877, right lateral view (D) and its interpretive drawing (E), anterior to right. (F–H) IGPS coll. cat. no. 111878; left lateral view of the carapace (F) and its interpretive drawing (G), anterior to left; (H) close up of the carapace surface, showing the small pits. 30

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Fig. 8. Concavicaris parva n. sp. from the Osawa Formation. (A, B) UIM 30628, left

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lateral view of the carapace and its inner mold (A) and its interpretive drawing (B), anterior to left. (C, D) IGPS coll. cat. no. 111879, left lateral view of the carapace (C)

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and its interpretive drawing (D), anterior to left. (E, F) IGPS coll. cat. no. 111880, right lateral view of the carapace (E) and its interpretive drawing (F), anterior to right,

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scanning electron microscope (SEM) images and back-scattered electron (BSE) images

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of the carapace surface (position is shown in a square frame) are in Fig. 9.

Fig. 9. Scanning electron microscope (SEM) images, back-scattered electron (BSE)

31

images, and the electron microprobe analyses of the carapace surface of Concavicaris parva n. sp. (IGPS coll. cat. no. 111880). (A, C) BSE images. (B, D) SEM images; (B) the angle of sample stage is 45°; (D) the angle of sample stage is 0° (light from upper side). (E, F) Electron microprobe analyses of the bright and dark regions of (C),

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respectively. Small pits a-d in (C) correspond respectively to those in (D).

Fig. 10. Miyagicaris costata n. gen. n. sp. and Ostenocaris? sp. from the Osawa Formation. (A–D) Miyagicaris costata n. gen. n. sp.; (A, B) IGPS coll. cat. no. 111883

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(holotype), left lateral view of the carapace (A) and its interpretive drawing (B), anterior to left, rostrum is missing; (C, D) UIM 30629, left lateral view of the carapace fragment (ventral part) (C) and its interpretive drawing (D), anterior to left. (E, F) Ostenocaris? sp., UIM 30624, left lateral view of the carapace (E) and its interpretive drawing (F), anterior to left, dorsal and posterior parts missing.

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Fig. 11. Distribution map of the Triassic Thylacocephala. 1. South Kitakami (Osawa

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Formation), Japan; 2. Chaohu, South China; 3. Western Australia; 4. Madagascar; 5.

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Idaho, USA; 6. Longtan, South China; 7. Hebei, South China; 8. Luoping, South China; 9. Slovenia; 10. Spain; 11. Austria; 12. Udine, Northeast Italy; 13. Bergamo, North

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Italy; 14. Lazio, Central Italy.

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