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# New ichthyofaunal microfossils from the Late Permian at Chaotian, Sichuan, Southwest China

Li Jiachun, Sun Zuoyu, Gilles Cuny, Zhao Siyan, Yao Mingtao, Song Lijun

► To cite this version:

Li Jiachun, Sun Zuoyu, Gilles Cuny, Zhao Siyan, Yao Mingtao, et al. New ichthyofaunal microfossils from the Late Permian at Chaotian, Sichuan, Southwest China. Swiss Journal of Palaeontology, 2025, 144 (1), pp.29. 10.1186/s13358-025-00377-2. hal-05131538

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# New ichthyofaunal microfossils from the Late Permian at Chaotian, Sichuan, Southwest China

Li Jiachun<fnref n="1" />*, Sun Zuoyu<fnref n="2" />*, Gilles Cuny<fnref n="3" />, Zhao Siyan<fnref n="2" />, Yao Mingtao<fnref n="2" /> and Song Lijun<fnref n="1" />

**Abstract**

Chondrichthyan remains are reported for the first time from the Late Permian of the Sichuan Province, Southwest China. They include four tooth genera, comprising Eugeneodontiformes (aff. *Agassizodus* sp. and Eugeneodontiformes indet.) and Euselachii (*Favusodus* cf. *orientalis* and Neoselachii gen. et sp. indet.), as well as two morphotypes of dermal denticles. Eugeneodontiforms and euselachians are represented by durophagous sharks, which are the most common component of the described assemblage. Aff. *Agassizodus* sp. probably represents a new helicoprionid shark that features a trilobate tooth root with two distinct lingual foramina. The Neoselachii gen. et sp. indet. probably also represents a new taxon and could be the first Palaeozoic neoselachian shark with a holaulacorhize root vascularization. Moreover, high-resolution micro-computed tomography (micro-CT) images were used to investigate the 3D structural characteristics of their dental histology. The aff. *Agassizodus* sp. displays isolated branching pulp canals connecting with pulp cavities, whereas the Neoselachii gen. et sp. indet. shows networked pulp canals without a pulp cavity.

**Keywords** Chondrichthyans, Eugeneodontiformes, Euselachii, Tomographic analyses, Histology

**Introduction**

Chondrichthyans from the Late Permian remain lower taxonomic diversity (Schnetz et al., 2024). The generic diversity of chondrichthyans declined during the Guadalupian (Middle Permian), leading to the end-Guadalupian extinction event, followed by an intensified extinction rate through the following Lopingian (Late Permian), which culminated in the Late Permian extinction event

(Ivanov et al., 2020; Koot, 2013). To date, Eugeneodontiformes, Petalodontiformes, Cochliodontiformes, Ctenan-canthiformes and Euselachii (Hybodontiformes) have been reported from Greenland (Nielsen, 1932), West USA (Branson, 1933), UK (Hampe, 2012), Germany (Die-drich, 2009; Hampe, 2012), Poland (Dankina et al., 2021), Russia (Minikh & Minikh, 2001; Obruchev, 1964; Sen-nikov & Golubev, 2006), Iran (Golshani & Janvier, 1974; Hampe et al., 2013), Pakistan (Ginter et al., 2010), India (Kapoor & Sahni, 1971), Japan (Goto, 1994) and Australia (Leu, 1989). In China, the Lopingian chondrichthyans are quite poorly known (Wang, 2006; see Supp. Table 1). Young (1950) firstly reported one isolated tooth of *'Pet-alodus' shingkuoi* from Chongqing. According to Hansen (1978) and Gai et al. (2021), this species is better attrib-uted to the genus *Megactenopetalus* (Chondrichthyes, Pristodontidae). Later on, three partly preserved euge-neodontiform tooth whorls, including *Sinohelicoprion changhsingensis* from Changxing, Zhejiang Province (Liu & Wang, 1994; Liu & Zhang, 1963) and *S. macrodentus* from Jiahe, Hunan Province (Lei, 1983), were reported.

Handling editor: Jürgen Kriwet

*Correspondence:

Li Jiachun
lijiachun@xsyu.edu.cn
Sun Zuoyu
sunzuoyu@pku.edu.cn

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<footnote n="1">¹ Shaanxi Key Laboratory of Petroleum Accumulation Geology, School of Earth Sciences and Engineering, Xi'an Shiyou University, Xi'an 710065, China</footnote>

<footnote n="2">² Laboratory of Orogenic Belt and Crustal Evolution, Ministry of Education, Department of Geology and Geological Museum, Peking University, Yiheyuan Street 5, Beijing 100871, China</footnote>

<footnote n="3">³ Université Claude Bernard Lyon 1, LEHNA UMR 5023, CNRS, ENTPE, 69622 Villeurbanne, France</footnote>
Wang et al. (2007) assigned tooth crown from Meishan, Zhejiang Province to neoselachian tooth type-1, and two fragmentary teeth from Xiushui, Jiangxi Province to the hybodonts *Lissodus xiushuiensis* and *'Polyacrodus jiangxiensis*. In addition, several dermal denticles from these two sites were considered closely allied to Hybodontiformes, Neoselachii and Ctenacanthiformes (Wang et al., 2007). More recently, a complete hybodontiform skeleton, named *Gansuselache tungseni*, was recovered from the Mazongshan Mountain, Gansu Province (Wang et al., 2009). However, the Late Permian age of its fossil-bearing dark shales is now considered to be Early Triassic instead (Gao et al., 2010; Xu et al., 2015). The current study reports the first record of a chondrichthyan assemblage from a new palaeoichthyological locality—the Chaotian section of the Mingyuexia Gorge in southwest China.

## Geological setting

From the Permian to the Early Triassic, South China was located on the most eastern part of the Pangea at a low latitude around the equator. Thick shallow-marine limestones and terrigenous detrital sediments were broadly deposited on the carbonate platform of the Yangtze Block (Yang et al., 1987; Zhao et al., 1981). Relatively deep-water facies with muddy limestones were particularly accumulated on a slope/basin along the northwestern edge of South China (Wang & Jin, 2000; Zhu et al., 1999). The Chaotian section lies in such a deep-water environment in northern Sichuan on the upper Yangtze platform (Fig. 1B). It has been extensively studied around the Late Permian double mass extinction events (end-Guadalupian and end-Changhsingian), with special emphasis on biostratigraphy (fusulines, ammonoids, radiolarians, conodonts, brachiopods and corals; Isozaki et al., 2004, 2007, 2008; Ji et al., 2007; Kuwahara et al., 2007, 2008; Lai et al., 2008; Xu, 2006; Yang et al., 1987; Zhao et al., 1978), chemostratigraphy (carbon isotopes, Lai et al., 2008; Saitoh et al., 2015; Saitoh & Isozaki, 2021; nitrogen isotopes, Saitoh et al., 2014; sulfur isotopes, Saitoh et al., 2017; Saitoh, 2021; Saitoh et al., 2021) and radiometric ages (U–Pb zircon ages, He et al., 2007). Geographically, the Chaotian section is ca. 20 km to the north of Guangyuan City, northern Sichuan Province, and about 60 km to the west of Shangsi section (Li et al., 1989) that was once proposed as a GSSP candidate for the Permian–Triassic boundary and belongs to the same geotectonic domain in a deep-water basin (Fig. 1B). The Chaotian section ranging from the Middle Permian to the Lower Triassic with good outcrops along the Mingyuexia Gorge of Jialingjiang River. It presents four stratigraphic units around the P–T, i.e. the Guadalupian Maokou Formation, the Lopingian Wujiaping and Dalong Formations, and the lowermost Triassic

Feixianguan Formation (Fig. 1C) (Isozaki et al., 2004, 2007). Detailed information concerning the lithology of the Chaotian section was provided by Ji et al. (2007) and Saitoh and Isozaki (2021). We obtained several chondrichthyan and actinopterygian micro-remains from the Dalong Formation (Changhsingian, Late Permian) at the Chaotian section, which is next to the south gate of Mingyuexia scenic area and belongs to the southeastern flank of the anticlinal fold of Mingyuexia.

## Materials and methods

The studied material was collected in the Chaotian Sect. (32°36′54′′N, 105°50′55′′E) of the Mingyuexia Gorge, Guangyuan City, Sichuan Province, southwest China (Fig. 1A). The Dalong Formation outcrops therein and consists of grey limestones intercalated with dark shale and chert layers. A total of 66 bags (with a total weight of approximately 840 kg) of sediments were processed in search of chondrichthyan remains for this study. In 2021, 27 small samples of approximately 10 kg each and five large samples of 25 kg each were collected from the Maokou and Dalong Formations in Sichuan. Then, 27 small and seven large samples from shark-teeth-bearing layers (Dalong Formation) were processed in 2022 from the same section. Each sample was broken into ~2×2 cm pieces, then dissolved in 10% buffered acetic acid (Jeppsson et al., 1999) three times, and thereafter a final treatment with 3% formic acid (Jeppsson & Anehus, 1995) was applied. After sieving with water through 74 µm and 2 mm meshes and air-drying, six shark teeth and two dermal denticles were handpicked under a binocular microscope. These remains were photographed using a JEOL JCM-6000 Plus scanning electron microscope (SEM) at an acceleration voltage of 10 kV, as well as a KEYENCE VHX-7000 Digital Microscope. In addition, two teeth (GMPKU-P-3804, 3805) and dermal denticle (GMPKU-P-3803) were scanned using a SkyScan1172 micro-CT scanner that provides a pixel size of 0.56 µm. 961 projections were performed with a 180° rotation and a rotation step of 0.2°. The source voltage and current were set to 48 kV and 200 mA, respectively. The generated slice files were processed with Avizo 2022.2 to reconstruct the internal vascularization system of the examined materials. SEM photos were taken at the Laboratory of Orogenic Belt and Crustal Evolution, School of Earth and Space Sciences, Peking University, China. The KEYENCE VHX-7000 Digital Microscope was operated at the University Claude Bernard Lyon 1, LEHNA, France. Micro-CT scans were conducted at the State Key Laboratory of Systematic and Evolutionary Botany, Institute of Botany, the Chinese Academy of Sciences. The specimens illustrated in the paper are stored in the collections of the Geological Museum of Peking University, Beijing, China (GMPKU). The systematic framework of shark tooth and
<figure><img src="image_5.png" /><figcaption>**Fig. 1** A, map of the north Sichuan showing the location of the studied Chaotian section; B, Palaeo-geographic map in the late Changhsingian (Late Permian) in South China; C, Stratigraphic column of the Chaotian section (modified from Saitoh et al., 2021)</figcaption></figure>

dermal denticle terminologies follows Johns et al. (1997), Ginter et al. (2010) and Cappetta (2012). The terminology of dental histology follows Jambura et al. (2020) and Ivanov and Duffin (2024). The use of open nomenclature follows

Bengtson's (1988) recommendations. The use of the term 'root' in the following descriptions does not imply direct homology with the root of the teeth of other gnathostomes (Underwood et al., 2015).
## Systematic palaeontology

Class Chondrichthyes Huxley, 1880

Subclass Euchondrocephali Lund & Grogan, 1997

Order Eugeneodontiformes Zangerl, 1981

Family Helicoprionidae Karpinsky, 1911

Genus *Agassizodus* St. John & Worthen, 1875.

Type species—*Lophodus variabilis* Newberry & Worthen, 1870

aff. *Agassizodus* sp.

<figure><img src="image_6.png" /><figcaption>**Fig. 2** A–J, aff. *Agassizodus* sp., A–E, GMPKU-P-3805; F–J, GMPKU-P-3807; K–T, Eugeneodontiformes indet., K–O, GMPKU-P-3802; P–T, GMPKU-P-3806. A, F, K and P in labial views; B, G, L and Q in lingual views; C, H, M and R in apical views; D, I, N and S in basal views; E and T in lateral views; J and O in mesio/disto-apical views. All scales equal 200 µm. Black and white images (letter followed by 1) were taken with a JEOL JCM-6000 Plus SEM, whereas corresponding colour images (letter followed by 2) were taken with a KEYENCE VHX-7000 Digital Microscope</figcaption></figure>
Figures 2A–J, 3

*Material*—Two lateral teeth: one is complete (GMPKU-P-3805) and the other is incomplete (GMPKU-P-3807).

*Description*—the complete tooth, GMPKU-P-3805, measures 1.6 mm mesio-distally, 0.9 mm labio-lingually and 1.2 mm in height. The flattened crown lacks cusplets. There is a moderately sharp, quite irregular longitudinal crest that is curved lingually at one extremity. It separates the crown surface into labial and lingual parts (Fig. 2C). The labial crown surface displays four pronounced buttresses. They are dumbbell-shaped, labially protruding, and formed by both crown and root tissues (Fig. 2A). The adjacent buttresses are separated by a deep notch. Oblique and vertical ridges are irregularly distributed on the labial crown surface. They originate from the longitudinal crest and extend into the upper part of the labial buttresses and notches. The ridges on the lingual crown surface are arranged in a way like those on the labial crown surface. Some of them bifurcate into two in the middle part of the crown and then reach the crown/root boundary (Fig. 2B). A deep groove separates the crown from the root lingually and labially, but the labial one is partially covered by the four strong buttresses. In comparison, the other specimens, GMPKU-P-3807 (Fig. 2F-J), is preserved as a fragmentary crown with two labial buttresses.

The root equals the crown in height. Its outline is slightly convex labially, trilobed lingually, and rounded mesially and distally (Fig. 2D). There are two large foramina that open at the baso-lingual edge of the root (Fig. 2B). They form two apico-basal crevices situated between the lobes (Fig. 2B, D). The labial root surface is vertically flattened, but its upper part is occupied by the four labial buttresses mentioned above (Fig. 2A, E). Foramina are absent on both labial and basal root surfaces.

*Dentinal histology*—The inner core of dentine displays many vascular canals (Fig. 3A–R). It comprises three main pulp cavities, nine pulp canals, seven ascending canals and two transverse canals. Two larger pulp cavities (pca1 and pca2) run into two prominent transverse canals (tc1 and tc2), and open at the lingual rim of the root (Fig. 3G, J). Seven ascending canals originate from both the lingual and labial parts of the base and rise to the middle level of the root where they are connected to the pulp cavities (Fig. 3J, P). These canals stretch downwards but do not open at the root surface (Fig. 3O). There

are six slender and three thick pulp canals projecting upwards from the pulp cavities. The slender pulp canals bifurcate into smaller branching tubules (Fig. 3J, L, P) and reach the middle level of the crown (Fig. 3G, I, M). The thick pulp canals terminate near the crown-root junction and one of them opens as a foramen at the distal or mesial extremity (Fig. 3M, N). The upper pulp canals are located on the lingual side of the pulp cavities, whereas the lower ascending canals are mostly located on the labial side (Fig. 3H, K, N, Q).

Remarks—The relatively low crown with strong labial buttresses is reminiscent of the lateral teeth of several eugeneodontid sharks (Ginter et al., 2010; Zangerl, 1981): e.g., *Agassizodus*, *Arpagodus*, *Bobbodus*, *Caseodus* and *Eugeneodus*. The material described herein shares the following characters with *Agassizodus*, *Arpagodus* and *Caseodus*: a deep root, moderately sharp longitudinal crest, both labial and lingual ridges on crown surfaces and the absence of lingual buttresses. It is more closely allied to *Agassizodus* than to *Arpagodus* and *Caseodus* due to its labial buttresses extending over both crown and root. However, the root of GMPKU-P-3805 has a trilobed outline, two large lingual foramina and smooth labial and basal surfaces, which differs from the root of *Agassizodus* that is moderately pitted, roughened and sometimes ornamented with a lamellar layer on its bottom (Newberry & Worthen, 1870, pl. 4, figs. 4, 5, 11; St-John & Worthen, 1875, pl. 8, figs. 2–21; Zangerl, 1981, fig. 85; Ginter et al., 2010, fig. 119). The unusual trilobed root makes the material described herein difficult to attribute with confidence to *Agassizodus* and other eugeneodontid taxa. Furthermore, a complicated controversy developed over the relationship between *Agassizodus* and *Campodus* (see Supp. Table 2). In 1875, St-John and Worthen designated *Lophodus variabilis* from the Coal Measures (Pennsylvanian) of La Salle, Illinois, USA, described by Newberry and Worthen (1870), as the holotype specimens of the genus *Agassizodus* they proposed. They also added a mandibular dentition from the Coal Measures (Pennsylvanian) of Osage County, Kansas, USA (St. John and Worthen, 1875 pl.8:1) into *Agassizodus variabilis*. Afterwards, Eastman (1902a, 1902b, 1903) associated his newly founded symphyseal dentition from the Coal Measures of Cedar Creek, Nebraska with the mandibular dentition from Osage and placed them in a new taxonomic combination '*Campodus variabilis*. The genus *Agassizodus* was thus considered as a synonym of

(See figure on next page.)

**Fig. 3** Tomographic images of aff. *Agassizodus* sp., GMPKU-P-3805. **A–C**, surface rendering; **D–F**, virtual sections in **D**, frontal view (line b); **E**, sagittal view (line a); **F**, axial view (line c). **G–I, M–O**, transparent tooth with vascularization system; **J–L, P–R**, reconstruction of vascular canals. **pc**: pulp canal; **pca**: pulp cavity; **tc**: transversal canal; **ac**: ascending canal; **uc**: unidentified canal. **A, G and J** in lingual views; **C, I and L** in apical views; **M and P** in labial views; **B, H, K, N and Q** in lateral views; **O** and **R** in basal views. All scales equal 200 µm
<figure><img src="image_7.png" /><figcaption>**Fig. 3** (See legend on previous page.)</figcaption></figure>
*Campodus* (Eastman, 1902a, 1902b, 1903; Eaton, 1962; Lohest, 1884). However, Zangerl (1981), Ginter et al. (2010) and Ginter (2018) disagreed because they thought St. John and Worthen (1875) incorrectly connected the mandible from Osage with the type specimens of *Lophodus variabilis* Newberry & Worthen, 1870, and it should be excluded from *Agassizodus variabilis* (Ginter, 2018; Ginter et al., 2010). In that case, *Agassizodus* is not con-specific with *Campodus*. In fact, lateral teeth of *Agassizodus* display strong buttresses on the labial crown surface and the lingual side is almost flat, which distinguishes it from those of *Campodus* that bear both labial and lingual transverse ridges symmetrically separated by a longitudinal crest (Ginter, 2018). We follow the view of Zangerl (1981), Ginter et al. (2010) and Ginter (2018) and tentatively attribute the herein material to aff. *Agassizodus* based on their crown similarities, pending better material and further taxonomic investigation.

Eugeneodontiformes indet.

Figure 2K-T

*Material*—Two incomplete teeth, GMPKU-P-3802, 3806.

*Description*—The fragmentary teeth measure 0.4–0.5 mm labio-lingually, 0.8–1.3 mm in height and tooth (GMPKU-P-3802) is 1.5 mm long mesio-distally, whereas the other (GMPKU-P-3806) measures 3.1 mm mesio-distally. The teeth are flat and mesio-distally elongated, lacking cusplets on the crown. An irregular crest traverses the crown mesio-distally and asymmetrically separates the relatively wider lingual face from the narrower labial face. The bar-shaped crown is ornamented by vertical ridges that originate from the longitudinal crest and reach to the base of the crown. Ridges may sometimes bifurcate in the middle part of the crown. The ridges are sharp, parallel to each other, and are densely distributed on the crown surface (Fig. 2K). The ornamentation of the labial crown surface is less developed than that on the lingual face. A shallow groove is present at the crown/root junction lingually, whereas such a groove is absent labially. The specimen GMPKU-P-3806 displays a fragmentary crown and root. There is a small cusp-like protrusion on the longitudinal crest (Fig. 2P). It shares a similar crown ornamentation pattern with the specimen

GMPKU-P-3802, but the ridges ornamenting the crown are more oblique.

The root of the specimen GMPKU-P-3802 is partially preserved. Two rounded foramina are present on the labial face: they differ in size and the larger one lies above the small one (Fig. 2K). There is a row of shallow indentations below the labial crown/root junction. A crevice-like foramen extends apico-basally on the lingual face of the root (Fig. 2L).

*Remarks*—The material described herein closely resembles the posterior teeth of *Caseodus varidentis* recovered from the Early Triassic of Western Canada (Mutter & Neuman, 2008). They share a bar-shaped slender crown with a conspicuous longitudinal crest. The material also resembles the posterior teeth of Caseodontidae gen. et sp. indet., which was associated with *Caseodus varidentis* in the same site (Mutter & Neuman, 2008, fig. 18). Besides, the material differs from the above-described aff. *Agassizodus* sp. in having a rather slender crown and the absence of a pronounced labial buttress. There is, however, a possibility that these differences may be caused by the heterodonty pattern observed in most eugeneodontiform sharks (Ginter et al., 2010; Mutter & Neuman, 2008). In addition, the lateral and posterior teeth were usually considered questionable, unknown or reportedly equivocal for taxonomic identification (Mutter & Neuman, 2008). For example, the lateral teeth pavement of some eugeneodontids, such as *Agassizodus*, *Caseodus* and *Bobbodus*, are generally very similar in morphology (see Zangerl, 1981). It further blurs a possible link between the lateral teeth of aff. *Agassizodus* described above and the potential posterior teeth described herein. We therefore tentatively assign the material to Eugeneodontiformes indet.

Subclass Elasmobranchii Bonaparte, 1838

Cohort Euselachii Hay, 1902

Order incertae sedis.

Genus *Favusodus* Li et al., 2022

Type species—*Favusodus orientalis* Li et al., 2022

*Favusodus* cf. *orientalis*

Figure 4A–E

*Material*—incomplete tooth, GMPKU-P-3808.

*Description*—The fragmentary tooth measures 1 mm labio-lingually, 1.1 mm in height and more than 3 mm mesio-distally. There is a low main cusp and no lateral

(See figure on next page.)

**Fig. 4** A–E, *Favusodus* cf. *orientalis*, GMPKU-P-3808; F–J, Neoselachii gen. et sp. indet., GMPKU-P-3803; K–O, dermal denticle of morphotype 1, GMPKU-P-3804; P–S, dermal denticle of morphotype 2, GMPKU-P-3815. A and F in labial views; B and G in lingual views; C, H and M in apical views; D, I, N and R in basal views; E, O and S in lateral views; J in mesio/disto-apical views; P in anterior view; Q in posterior view; K in anterior? view; L in posterior? view. All scales equal 200 µm. Black and white images (letter followed by 1) were taken with a JEOL JCM-6000 Plus SEM, whereas corresponding colour images (letter followed by 2) were taken with a KEYENCE VHX-7000 Digital Microscope
<figure><img src="image_8.png" /><figcaption>**Fig. 4** (See legend on previous page.)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 5** Tomographic images of Neoselachii gen. et sp. indet. (one root lobe was broken in the process of scanning), GMPKU-P-3803. **A–C**, surface rendering; **D–F**, virtual sections; **G–I, M–O**, transparent tooth with vascularization system; **J–L, P–R**, reconstruction of vascular canals. **pc**: pulp canal; **ng**: nutrition groove; **fo**: foramina; **uc**: unidentified canal. **A, G and J** in labial views; **C, I and L** in apical views; **M** and **P** in lingual views; **B, H, K, N** and **Q** in lateral views; **O** and **R** in basal views. All scales equal 200 µm</figcaption></figure>
cusplets on the crown. The crown is flat and slightly arched, which causes the height of the crown to decline from the main cusp to the mesial and distal extremities. The apex of the main cusp is domed and rounded (Fig. 4E). A blunt mesio-distal crest separates the rather larger labial crown surface from the lingual one. Ridges ornament the crown surface between the occlusal crest and the crown shoulder on both the labial and lingual faces. They are parallel to each other and sometimes fade in the middle part of the crown surface. The lingual ridges are sparser than the labial ones. Seven ridges converge into the apex of the main cusp labially, whereas there is only one vertical ridge that runs across an overhanging peg on its lingual surface (Fig. 4B). Adjacent ridges anastomose with each other and form a horizontal ridge along the crown shoulder. There are a few small honeycombed pits that surround the horizontal ridge on the labial crown surface (Fig. 4A). The crown/root junction is marked by a groove both labially and lingually.

The mesial or distal side of the root is partially damaged. Foramina appear to be randomly distributed and vary in size on the labial face of the root, of which some small foramina are close to the crown/root boundary and other larger foramina are located on the lower part of the root. The lingual face of the root shows a row of foramina at its mid-height (Fig. 4B).

*Remarks*—The following features: a rather flat crown with a reduced main cusp, an overhanging lingual peg, parallel ridges originating from a blunt longitudinal crest, reduced ornamentation on the lingual crown surface and a groove between both labial and lingual crown/root junctions, make the material herein described similar to the teeth of *Favusodus orientalis* from the Early Triassic of Guangxi, South China (Li et al., 2023), the Middle Triassic of Guizhou, South China (Li et al., 2022) and Ehime Prefecture, Japan (Yamagishi, 2004). The honeycombed pits of the Permian specimens described herein are less-developed than those in the Middle Triassic *Favusodus orientalis* (Li et al., 2022; Yamagishi, 2004) and resemble those in Early Triassic specimens more closely (Li et al., 2023). Such a difference is here considered to be due to intraspecific (Li et al., 2023) and could indicate that a honeycombed ornamentation became better developed through time. Considering that the incomplete root makes the material difficult to attribute to *Favusodus orientalis* with certainty, we herein ascribe it to *Favusodus cf. orientalis* mainly based on its crown characteristics.

Neoselachii gen. et sp. indet.

Figures 4F–J; 5.

*Material*—complete tooth, GMPKU-P-3803.

*Description*—The tooth measures 1.4 mm mesio-distally, 0.3 mm labio-lingually and 0.7 mm in height. The crown is labio-lingually compressed and lacks a main

cusp and lateral cusplets. It is oblong, and three times wider than its height. A straight occlusal crest is located at the lingual edge of the crown surface. The crown ornamentation is well-differentiated. Labially, there are ten coarse ridges on the middle part of the crown surface. The mesial or distal edge of the crown displays a row of small serrations with acute apices oriented upward (Fig. 4F). Lingually, the crown is ornamented by two large overhanging processes, one small accessory cusplet, and a few ridges and nodes. The apices of the overhanging processes may reach the occlusal crest (Fig. 4G). A small accessory cusplet is located between two larger processes and its apex points upwards. Several weak nodes are randomly scattered on the crown surface. The crown/root junction is incised labially, whereas it is marked by a deep groove with a short uvula lingually (Fig. 4G). The uvula lies below a larger petaloid process and extends on the upper part of the root.

The root is mesio-distally longer and slightly higher than the crown. Two asymmetric elongated root lobes are separated by a well-developed nutritive groove (Fig. 4I). These two lobes are labio-lingually flattened with a trapezoidal shape and rounded at their mesial and distal extremities. One is twice as long as the other. The larger lobe is smooth lingually but displays three circular foramina labially. Among them, the outer foramen is larger than the other two foramina, and the inner one is located on the groove separating the crown from the root. The smaller lobe is smooth and devoid of foramina on both its labial and lingual surfaces. This lobe is attached to the crown at an acute angle. In basal view, the two lobes are botuliform and slightly recurved labially. A foramen opens basally in the nutritive groove. The vascularization is evocative of a holaulacorhize type.

*Dentinal histology*—There are four pulp canals inside the tooth dentine (Fig. 5G). They are nearly vertical and restricted between the crown/root boundary and the middle level of the crown (Fig. 5G, M, H, N). Three of them connect together at their middle or upper parts (Fig. 5J, P), whereas the last one is isolated. This canal opens in the nutritive groove separating the two lobes of the root (Fig. 5G, O).

*Remarks*—The short lingual uvula at the crown/root boundary, elongated bilobed root with two asymmetrical lobes and a complete nutritive groove with an open foramen are reminiscent of the anterolateral/lateral tooth of Jurassic neoselachians like *Paracestracion bellis*, *P. falcifer* and *Protospinax bilobatus*, as well as the Late Triassic neoselachian *Vallisodus coppi* known from southern England (Duffin, 1982, 2021; Ronan et al., 2020; Underwood, 2002; Underwood & Ward, 2004). However, the oblong and labio-lingually flattened crown and root, multiple labial vertical ridges on the labial crown surface and
the two large lingual processes separate GMPKU-P-3803 from the crown of these neoselachians. Its crown ornamentation pattern is, to the best of the authors' knowledge, unknown in any other chondrichthyan teeth. The holaulacorhize root vascularization, with a complete vascular canal dividing the basal face of the root into two parts (Casier, 1947a, b, c; Cappetta, 1987, 2012), is present in many extant sharks (Heterodontiformes, Carcharhiniformes and Lamniformes), batomorphs (Torpediniformes, Mylobatiformes and Rajiformes) and fossil taxon (Protospinacidae). The difference is that the holaulacorhize root of living neoselachians is not as asymmetrically separated as those of the Permian specimens described herein. Anyway, this type of root has not been recorded before the Late Triassic, but such a typical character supports the idea that it belongs to a Palaeozoic neoselachian taxon. Therefore, we propose here that the tooth is closely affiliated to Neoselachii based on its holaulacorhize-like root vascularization.

## Chondrichthyan dermal denticles

The chondrichthyan dermal denticles herein described are attributed into two morphotypes based on differences in crown ornamentation and pedicle structure. The dermal denticle of morphotype 1 (complete dermal denticle, GMPKU-P-3804, Fig. 4K–O) is 1 mm long, 0.6 mm wide and 0.7 mm high. The crown displays eighteen blade-like ridges that extend apico-basally and protrude laterally. These ridges are separated by deep notches and are polycusped with no more than 4 small cusplets. In apical view, a zigzag-shaped occlusal crest runs across the entire crown and connects to each ridge. There is no groove between the crown and pedicle. The crown overhangs the pedicle. The pedicle is as high as the crown. On one side (anterior? Fig. 4K), the pedicle surface is curved, convex laterally and concave centrally, forming a shallow and wide incision. On the other side (posterior? Fig. 4L), the pedicle displays an oval depression with one large central foramen that crosses the pedicle antero-posteriorly. The basal surface of the pedicle is smooth. In lateral view, there is a foramen on one extremity of the pedicle (Figs. 4O, 6H, K). The extensively developed blade-like ridges on the entire crown surface and the foramen crossing the pedicle antero-posteriorly remind of *'Listracanthus'* denticles reported from the Early Permian of Urals, Russia (Ivanov, 2005). However, the *'Listracanthus'* denticle exhibits a relatively high crown and an apico-basally flattened base, which differ from the lower crown and antero-posteriorly flattened pedicle of the dermal denticle described here. The inner vascular system is made of only one pulp cavity and a few branching pulp canals on top of it. The pulp cavity connects a transversal canal that crosses the central part of the pedicle (Fig. 6G–R).

It is similar to the vascularization pattern of the teeth of aff. *Agassizodus* sp. mentioned above. Nevertheless, this dermal denticle is difficult to attribute to a eugeneodontid because no correlation between dermal denticle vascularization and tooth vascularization has been demonstrated so far.

The dermal denticle of morphotype 2 (incomplete dermal denticle, GMPKU-P-3815, Fig. 4P–S) is 0.3 mm long, 0.6 mm wide and 0.7 mm high. The crown has three well-separated cusps. The central cusp is slightly higher than the two lateral cusps. All cusps are inclined posteriorly, point upwards and display a prominent ridge anteriorly. Each ridge originates from the apex of the cusp and extends to the pedicle, giving the pedicle a wrinkled surface. There is no neck between the crown/pedicle boundary. The pedicle is low, oval and anteriorly protruding. The basal face of the pedicle is concave and displays many opening canals. Characters such as a multi-cusped and ridged crown, undeveloped neck and concave pedicle basal face with opening canals, remind of the dermal denticle of *Sinacrodus donglingensis* reported from the Upper Permian of Xiushui and Xinfeng, Jiangxi Province and Changxing, Zhejiang Province in South China (Wang et al., 2007, figs. 8b-f).

## Actinopterygian remains

There are three kinds of actinopterygian remains found in the Chaotian section. The first one is a tooth-like structure (Fig. 7A–B), measuring 0.52 mm in width and 2.18 mm in height. It is ornamented apico-basally by dense ridges on the entire outer surface. The mesial surface is convex and the distal one is concave. However, to the best of the authors' knowledge, such a distal concave surface is unknown in any actinopterygian teeth. The second specimen is a claw-like scale with three separated cusps. The central cusp is slightly longer than the two lateral cusps (Fig. 7C–J). All cusps are slender, thin, divergent laterally and recurved posteriorly. The base of the crown is straight, narrow and hollow. This scale resembles one specimen found from the Middle Permian of West Texas, USA (Ivanov et al., 2020), in their slender cusps and hollow base. Ivanov et al. (2020) considered it to be a possible gill-raker denticle or pharyngeal tooth of an actinopterygian fish. The third one is a flattened and rhomboidal scale, possibly belonging to *Alilepis* sp. (Ivanov et al., 2020). The external surface is ganoine-covered, with five slender tubercles that extend antero-posteriorly. They are parallel to each other and separated by narrow grooves. The tubercles are longer in the middle part of the scale than near the extremities (Fig. 7K).
<figure><img src="image_10.png" /><figcaption>**Fig. 6** Tomographic images of dermal denticle of morphotype 1, GMPKU-P-3804. **A–C**, surface rendering; **D–F**, virtual sections in **D**, frontal view (line b); **E**, sagittal view (line a); **F**, axial view (line c). **G–I, M–O**, transparent tooth with vascularization system; **J–L, P–R**, reconstruction of vascular canals. **pc**: pulp canal; **pca**: pulp cavity; **cc**: central canal; **uc**: unidentified canal. **A, G and J** in posterior? views; **C, I and L** in apical views; **M** and **P** in anterior? views; **B, H, K, N** and **Q** in lateral views; **O** and **R** in basal views. All scales equal 200 µm</figcaption></figure>

# Discussion

## Fish assemblage and taxa distribution

The material from the Chaotian section includes teeth belonging to eugeneodontiforms (aff. *Agassizodus* sp. and

Eugeneodontiformes indet.) and euselachians (*Favusodus* cf. *orientalis* and Neoselachii gen. et sp. indet.) as well as two dermal denticle morphotypes of unknown affinities. Besides, actinopterygian scales of *Alilepis* sp. and
<figure><img src="image_11.png" /><figcaption>**Fig. 7** A–K, Actinopterygian remains. A–B, indeterminate tooth-like structure, GMPKU-P-3814. C–J, Possible gill-raker denticles or pharyngeal teeth, C–E, GMPKU-P-3812; F–J, GMPKU-P-3811. K, scales of *Alilepis* sp., GMPKU-P-3813. A, C and F in mesial views; B, D and G in distal views; H in apical view; I in basal view; E and J in lateral views; K in external view. All scales equal 200 µm. Black and white images were taken with a JEOL JCM-6000 Plus SEM, whereas corresponding colour images (letter followed by 2) were taken with a KEYENCE VHX-7000 Digital Microscope</figcaption></figure>

a tooth-like and claw-like actinopterygian fish remains are also recorded. Aff. *Agassizodus* sp., Eugeneodontiformes indet., Neoselachii gen. et sp. indet. and dermal denticle of morphotype 1 are endemic taxa that are so far restricted to the Chaotian area of southwest China. *Favusodus* cf. *orientalis* has a strong affinity with *Favusodus orientalis* recovered from the Early-Middle Triassic of South China (Li et al., 2022, 2023) and Japan (Yamagishi, 2004). The dermal denticle of morphotype 2 is close to *Sinacrodus donglingensis* from the Late Permian of Zhejiang and Jiangxi Provinces (Wang et al., 2007), whereas the actinopterygian *Alilepis* sp. and claw-like scales have previously been reported from the Middle Permian of West Texas, USA (Ivanov et al., 2020).

Durophagous eugeneodontiforms and euselachians are common components of chondrichthyan assemblages in Germany, USA and Iran during the Middle-Late Permian (Hampe, 2012; Hampe et al., 2013; Ivanov et al., 2020). In China, durophagous sharks are rather represented by the hybodontiforms *Lissodus xiushuiensis* and *'Polyacrodus' jiangxiensis* in Jiangxi (Wang et al., 2007) and the petalodontiform *Megactenopetalus shingkuoi* from Chongqing (Young, 1950). The eugeneodontiform *Sinohelicoprion changhsingensis* and *S. macrodontus* (Lei, 1983; Liu & Wang, 1994; Liu & Zhang, 1963) have specialized symphyseal tooth whorls for capturing, processing, and transporting soft-bodied prey (Ramsay et al., 2015), lacking in durophagous forms. Herein, all taxa from Chaotian

have crushing teeth with low and ornamented crown, indicating that a durophagous diet prevailed in these Late Permian chondrichthyan assemblages, although we need to acknowledge that the number of teeth recovered is limited. Friedman and Sallan (2012) speculated a selective loss of durophagous lineages as a result of the end-Permian mass extinction (EPME), but the biodiversity distribution in the fossil record around the P–T boundary suggests proportional losses and no selective loss of specific feeding habits (Koot, 2013). The teeth of *Favusodus* described here are known from the Triassic of south China (Li et al., 2022, 2023) and Japan (Yamagishi, 2004) have survived the EPME. *Omanoselache*, a genus closely allied to *Favusodus*, was also considered to pass through the EPME and then spread worldwide in the Triassic (Koot et al., 2013, 2015). Meanwhile, the Eugeneodontiformes indet. described herein, reminding somewhat of *Caseodus varidentis*, may potentially be another survivor re-appearing in the Early Triassic of western Canada (Mutter & Neuman, 2008). These durophagous eugeneodontiforms and euselachians seem, therefore, to have been weakly affected by the Late Permian extinction.

## Root structure and dental histology

The root structure and dental histology could only be investigated in aff. *Agassizodus* sp. and Neoselachii gen. et sp. indet., the rest of the teeth not being sufficiently well-preserved.
Aff. *Agassizodus* sp. displays a complete root with two large foramina on the lingual side, which differs from other helicoprionid genera such as *Agassizodus* and *Arpagodus* that display numerous foramina on both labial and lingual root surfaces (Ginter et al., 2010). As for dental histology, aff. *Agassizodus* sp. shows three pulp cavities with branching pulp canals, resembling *Helicoprion ferrieri* (Bendix-Almgreen, 1966, fig. 22). Thus, the root structure of aff. *Agassizodus* sp. has a unique root structure but shares with other eugeneodontiformes (*Helicoprion*) a similar dental histology made of osteodentine, which might suggest that aff. *Agassizodus* sp. represents a yet undescribed taxon of helicoprionid shark among Eugeneodontiformes.

The holaulacorhize-like root vascularization of Neoselachii gen. et sp. indet. appears to be the first record of such a vascularization in the Palaeozoic. Its bilobate root separated by a complete canal displaying a basal foramen, is reminiscent of rhinobatoid-type vascularization (foramen set in the center of the groove, (Casier, 1947a, 1947b, 1947c) of batomorphs (Duffin, 1982). Other Palaeozoic teeth assigned to neoselachians also show similarities with the root characters present in modern sharks. For example, anachronist sharks (*Amaradontus; Cooleyella; Ginteria* and *Gunnellia*) display a median internal vascular canal connecting a medio-internal foramen and a medio-external foramen (Duffin & Ward, 1983; Duffin et al., 1996; Gunnell, 1933; Hodnett & Elliott, 2018; Ivanov, 2005; Ivanov & Duffin, 2024; Ivanov et al., 2020, 2021; Koot et al., 2013), which resembles the hemiaulacorhize vascularization in more advanced forms of sharks and rays from the Jurassic to the present. *Mcmurdodus* possess a cancellous, bone-like root with a distinctive central groove on its basal face. It existed briefly only during the Devonian and is similar to the anaulacorhize-like root of extant hexanchid shark (Burrow et al., 2008; Goto, 1994; Turner & Young, 1987; White, 1968). The phylogenetic relationships of these Palaeozoic tooth genera are still controversial, but they may represent early stages of the evolution of neoselachian sharks. On the other hand, the internal dental structure of the Neoselachii gen. et sp. indet. described herein is characterized by a single pulp canal, a network of primary pulp canals, and is devoid of a pulp cavity. It differs from that of other Palaeozoic neoselachian teeth (*Amaradontus, Cooleyella, Ginteria, Gunnellia, Turovella* and *Reifalle*) that display several isolated transversal, longitudinal and pulp canals, as well as a central pulp cavity (Ivanov & Duffin, 2024; Ivanov et al., 2020).

## Conclusion

The shark assemblage from the Chaotian section comprises four chondrichthyan tooth genera: aff. *Agassizodus* sp., Eugeneodontiformes indet., *Favusodus* cf. *orientalis* and Neoselachii gen. et sp. indet., as well as two morphotypes of dermal denticles. Besides, scales of *Alilepis* sp., gill-raker denticles or pharyngeal teeth and an undetermined tooth-like actinopterygian fish remains are also recognized. The durophagous eugeneodontiformes and euselachians are common components in shark assemblage during the Late Permian. Aff. *Agassizodus* sp. displays a trilobate root with two lingual foramina and branching pulp canals connecting with corresponding pulp cavities. The Neoselachii gen. et sp. indet. displays a complete canal connected with a basal foramen and networked pulp canals without a pulp cavity. It represents a new taxon of Palaeozoic neoselachian and the earliest fossil record of a chondrichthyan tooth with a holaulacorhize-like root.

## Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s13358-025-00377-2.

Additional file 1 (DOCX 24 KB)

### Acknowledgements

The authors would like to thank Wu Guichun and Ji Zhansheng (researchers at the Institute of Geology, Chinese Academy of Geological Sciences) for providing stratigraphic information about the material described herein; Chang Jiashuai and his wife (inhabitants of Mingyuexia village) for collecting rock samples in the field; Colin Issartel (engineer at University Claude Bernard Lyon 1) who helped with the use of the Keyence VHX-7000 Digital Microscope, and Dr. Xu Xiuping (staff of the Plant Science Facility of the Institute of Botany, Chinese Academy of Sciences) for her help during the scan session; also Wang Mingcui for dissolving the rock samples and picking up shark remains in the laboratory. Two reviewers (Chris Duffin and Darja Dankina) and the editor Jürgen Kriwet provided very useful remarks and comments that improved the quality of the manuscript.

### Author contributions

ZS and JL organized and designed of the study, JL, MY and ZS conducted field work and collected samples, JL and GC identified the fish remains and wrote the manuscript; SZ analysed CT data and reconstructed dental histology, All authors revised the paper.

### Funding

This study was financially supported by the National Key Research and Development Program of China (2022YFF0800200) and the National Natural Science Foundation of China (No. 42172009, 41920104001, 41876124), China Geological Survey (No. 121201102000150012-09), State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, CAS (No. 123107, 143108, 173123, 20182107) and China Scholarship Council (No. 202206010129). Natural Science Basic Research Program of Shaanxi (No. 2025JC-YBQN-409).

### Availability of data and materials

No datasets were generated or analysed during the current study.
# Declarations

## Ethics approval and consent to participate

Not applicable.

## Consent for publication

Not applicable.

## Competing interests

The authors declare no competing interests.

Received: 8 January 2025 Accepted: 19 May 2025

Published online: 18 June 2025

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