# New insights into the paleobiogeography of teleosauroid crocodylomorphs in Southeast Asia

Supanut Bhuttarach, Uthumporn Deesri, Jeremy Martin, Sita Manitkoon, Jirasak Charoenmit, Komsorn Lauprasert

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Supanut Bhuttarach, Uthumporn Deesri, Jeremy Martin, Sita Manitkoon, Jirasak Charoenmit, et al.. New insights into the paleobiogeography of teleosauroid crocodylomorphs in Southeast Asia. Palaeoworld, 2025, 228, pp.200981. 10.1016/j.palwor.2025.200981. hal-05166144

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New insights into the paleobiogeography of teleosauroid crocodylomorphs in Southeast Asia

Supanut Bhuttarach, Uthumporn Deesri, Jeremy E. Martin, Sita Manitkoon, Jirasak Charoenmit, Komsorn Lauprasert

PII: S1871-174X(25)00074-5

DOI: https://doi.org/10.1016/j.palwor.2025.200981

Reference: PALWOR 200981

To appear in: *Palaeoworld*

Received Date: 6 April 2025

Revised Date: 29 May 2025

Accepted Date: 25 June 2025

Please cite this article as: S. Bhuttarach, U. Deesri, J.E. Martin, S. Manitkoon, J. Charoenmit, K. Lauprasert, New insights into the paleobiogeography of teleosauroid crocodylomorphs in Southeast Asia, *Palaeoworld* (2025), doi: https://doi.org/10.1016/j.palwor.2025.200981

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© 2025 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS

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New insights into the paleobiogeography of teleosauroid crocodylomorphs in Southeast Asia

Supanut Bhuttarach<sup>a</sup>, Uthumporn Deesri<sup>a, b</sup>, Jeremy E. Martin<sup>c</sup>, Sita Manitkoon<sup>b, d</sup>, Jirasak Charoenmit<sup>e</sup>, Komsorn Lauprasert<sup>a, b *</sup>

**Abstract**

Teleosauroids are poorly known from Southeast Asia, with limited records from the Indochina Terrane of Thailand. To better understand their distribution and diversity, we describe new teleosauroid remains from two distinct geological terranes in Thailand. A nearly complete periorbital region and isolated teeth found in association from the Phu Kradung Formation (Upper Jurassic–Lower Cretaceous) of the Indochina Terrane are identified as cf. *Indosinosuchus* sp. The cranial specimen exhibits periorbital dimensions significantly larger than those of previously described *Indosinosuchus* specimens, likely representing the largest known individual of the genus and expanding our understanding of morphological diversity within this taxon. The discovery confirms the wide distribution of teleosauroids within the Phu Kradung Formation of the Khorat Plateau during the Late Jurassic to Early Cretaceous. Additionally, a frontal bone, left postorbital, and isolated teeth from the Klong Min

<footnote><sup>a</sup> Department of Biology, Faculty of Science, Mahasarakham University, Maha Sarakham, Thailand</footnote>

<footnote><sup>b</sup> Excellence Center in Evolution of Life, Basin Studies and Applied Palaeontology, Mahasarakham University, Maha Sarakham, Thailand</footnote>

<footnote><sup>c</sup> Laboratoire de Géologie de Lyon: Terre, Planète, Environnement, UMR CNRS 5276 (CNRS, ENS, Université Lyon 1), Ecole Normale Supérieure de Lyon, 69364 Lyon Cedex 07, France</footnote>

<footnote><sup>d</sup> Palaeontological Research and Education Centre, Mahasarakham University, Maha Sarakham, Thailand</footnote>

<footnote><sup>e</sup> Phunphin Geological Museum, Phun Phin District, Surat Thani Province, Thailand</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: komsorn.l@msu.ac.th (K. Lauprasert)</footnote>
Formation (upper Middle–Upper Jurassic) of the Sibumasu Terrane are assigned to Teleosauroidea indet. These remains, morphologically more primitive than *Indosinosuchus*, represent the first evidence of teleosauroids from southern peninsular Thailand and extend their known temporal range to the Middle Jurassic. Our findings demonstrate that teleosauroids were widely distributed across Thailand’s freshwater environments from the Middle Jurassic through Early Cretaceous, significantly expanding our understanding of their biogeographic distribution in Southeast Asia.

**Keywords:** Teleosauroidea; Indochina Terrane; Sibumasu Terrane; Jurassic; *Indosinosuchus*

# 1. Introduction

Thalattosuchia is a diverse group of aquatic crocodylomorphs that lived from the Early Jurassic to Early Cretaceous. To date, two genera of teleosauroids have been discovered in Asia: *Peipehsuchus* and *Indosinosuchus* (Young, 1948; Li, 1993; Buffetaut et al., 1994b; Martin et al., 2019). Only *Indosinosuchus* has been reported from Thailand. A teleosauroid in Thailand was first reported by Buffetaut et al. (1994b), who mentioned an elongated lower jaw (the current whereabouts of this specimen is unknown) and a slender curved femur from the Upper Jurassic at Khao Nin Phu Sri in theikhao District, Trang Province, southern Thailand. However, these teleosauroid remains were not thoroughly described, and their taxonomic identification was left uncertain, casting doubt on the possible existence of teleosauroids in the Sibumasu Terrane. In 2006, additional evidence was noted based on characteristics of isolated teeth from the Phu Kradung Formation (Upper Jurassic–Lower Cretaceous) at Khok Sanam, Kham Muang District, Kalasin Province, northeastern Thailand (Lauprasert, 2006). Two years later, Thai-French paleontologists discovered the Phu Noi locality and continuously excavated it for a decade. From this site, Martin et al. (2019) erected the first Thai teleosaurid, *Indosinosuchus potamosiamensis*. Almost a dozen teleosaurid skulls were discovered in the Upper Jurassic to Lower Cretaceous continental deposits at Phu Noi.

The discovery of *Indosinosuchus* at the Phu Noi locality raises intriguing questions about the geographic distribution of teleosauroids in Asia. Previously, teleosauroid remains were primarily documented in Europe (e.g., Westphal, 1962), Madagascar (Newton, 1893), Morocco (Benani et al., 2023), Tunisia (Dridi and Johnson, 2019), Central Asia (Storrs and Efimov, 2000), China (Young, 1948; Li, 1993), and Thailand (Buffetaut et al., 1994b; Martin et al., 2019).

Initially, a single teleosauroid species *Indosinosuchus potamosiamensis* was reported from the Phu Noi locality (Martin et al., 2019). Subsequently, Johnson et al. (2020) proposed an updated phylogenetic data matrix and established a new species *Indosinosuchus kalasinensis* based on a single skull (PRC-239). However, Johnson et al. (2020)'s taxonomic assessment relied on first-hand comparisons conducted on three specimens only (PRC-11, PRC-238, and PRC-239), and did not incorporate a further 10 cranial specimens studied by Martin et al. (2019). Therefore, the taxonomic validity of *I. kalasinensis* will have to be carefully reassessed in a separate study.
Here, we thoroughly describe two recently discovered cranial bones and some isolated teeth attributable to teleosauroids, expanding the record of this group in both major terranes in Thailand and enhancing our comprehension of the distribution of this family in Southeast Asia.

## 2. Geological setting

Thailand comprises two major terranes: the Sibumasu Terrane and the Indochina Terrane, delineated by the Inthanon zone and the Sukhothai Fold Belt (Fig. 1). These terranes were sequentially rifted from peri-Gondwana from the middle to late Paleozoic and then collided progressively during the late Triassic (Sone and Metcalfe, 2008; Gardiner et al., 2016). The collision of the Sibumasu and Indochina terranes, known as the Indosinian Orogeny, is associated with the closure of the Paleo-Tethys in Southeast Asia and the disappearance of marine deposits in the Indochina Terrane during the Late Triassic (Sone and Metcalfe, 2008). Conversely, the Triassic-Middle Jurassic maritime environment emerged in extensive regions of the Sibumasu Terrane (Chonglakmani, 2011; Meesook and Saengsrichan, 2011).

In the Indochina Terrane, the Phu Kradung Formation is the lowermost unit of the Khorat Group, which outcrops on the Khorat Plateau in northeastern Thailand (Racey, 2009). This formation is primarily composed of fluvial sandstones, siltstones, and mudstones. Its age is debated, with fossil vertebrate suggesting a late Jurassic age (Buffetaut et al., 2001; Buffetaut and Suteethorn, 2007; Tong et al., 2009; Cuny et al., 2014; Martin et al., 2016, 2019; Manitkoon et al., 2023a, 2023b), whereas palynological data indicate an early Cretaceous age (Racey et al., 1996; Racey and Goodall, 2009). The upper part of this formation is characterized by a higher prevalence of sandstones, representing a meandering river system under a semi-arid to humid bi-seasonal climate, while the lower section corresponds to a lake-margin environment (Sattayarak, 1983; Mouret, 1994; Racey et al., 1996; Racey, 2009).

In this study, we describe teleosauroid specimens newly discovered from the Indochina Terrane in northern Thailand at Ban Wang Din So (Wang Thong District, Phitsanulok Province), located west of the Khorat Plateau on the side of the main road (mile 24+100, Road No. 12). This locality most likely corresponds to the upper part of the Phu Kradung Formation (possibly Lower Cretaceous). Strata in this area are primarily composed of coarse grey sandstone intercalated with siltstones. Several vertebrate fossils have been found in the coarse basal sandstone, including those of hybodont sharks, semionotiform fish, lungfish, turtles, and crocodylomorph remains (Department of Mineral Resources, 2007; Cuny et al., 2014; Srisuk, 2002).

In the Sibumasu Terrane, the Khlong Min Formation is the basal unit of the Mesozoic Thung Yai Group in southern Thailand. It primarily comprises non-marine mudstone, siltstone, sandstone, and limestone (Meesook et al., 2005). A diverse array of fossils has been discovered in the Khlong Min Formation, encompassing bony fish (Pycnodontiformes) (Cavin et al., 2009), hybodont sharks (Fontaine, 1990; Cuny et al., 2005, 2014), lungfish (Martin and Ingavat, 1982), turtles (Buffetaut et al., 1994b), crocodylomorph remains (Buffetaut et al., 1994b), Mamenchisauridae (Buffetaut et al., 2005; Suteethorn et al., 2012), and amphibians (Buffetaut et al., 1994a, 1994b; Nonsrirach et al., 2021), palynomorphs (Lei and Thanasuthipitak, 1993), charophytes
(Buffetaut et al., 1994b), and bivalves (Saengsrichan et al., 2011), suggesting a late
Middle to Late Jurassic age.

Teleosauroid specimens were discovered at Ban Mab Ching (Thung Song District, Nakhon Si Thammarat Province) and Ban Khuan Kun (Trang Province), located in the southern peninsula of Thailand. Ban Khuan Kun is located 60 km southwest of Ban Map Ching. Both localities were interpreted as non-marine sedimentary deposits within the Klong Min Formation (Buffetaut et al., 1994b, 2005; Teerarungsigul, 1999; Meesook et al., 2005; Meesook and Saengsrichan, 2011; Saengsrichan et al., 2011; Cuny et al., 2009). Both outcrops comprise calcareous siltstones, sandstones, and gray and brown claystones (Saengsrichan et al., 2009). According to Tong et al. (2002), the fossil assemblage at the Mab Ching locality suggests a lacustrine environment.

<Figure 1>

**3. Material and methods**

Two skull fragments (PRC-390 and PRC-391) and seven isolated teeth (PRC-208, PRC-209, and PRC-385 to PRC-389), which come from three different localities, are housed in the Palaeontological Research and Education Centre (PRC) at Mahasarakham University. To eliminate the overlying matrix, PRC-390 and PRC-391 were manually processed with a pneumatic air pen. However, the sutures between the bones on PRC-390 are obscure. Consequently, computed tomography (CT) scanning was considered a suitable alternative for detailed examination. The skull fragment from Ban Wang Din So (PRC-390) was scanned using a Canon Prime SP Aquilion (Faculty of Medicine, Mahasarakham University) at an acceleration voltage of 120-kilovolt peak (kVp) and 250 milliamperes (mA). The slice thickness was set at 0.5 mm. The generated raw data file has the following voxel dimensions: x = 228, y = 228, and z = 308. After converting into the DICOM format, the output data were loaded into the 3D Slicer software 5.6.2 (version 32448) for additional processing (refer to the 3D Slicer official website: https://download.slicer.org/). Interesting bone features were identified and digitally extracted through the 3D Slicer section editor for visualization, which included thresholds and paint tools. Finally, a three-dimensional model of the skull fragment was reconstructed.

**Anatomical abbreviations:** ca: carina; cc: crista cranii frontalis; f: frontal; itf: infratemporal fenestra; n: nasal, or: orbit; pf: prefrontal; po: postorbital; pob: postorbital bar; pof: postorbital foramen; r: ridge; stf: supratemporal fenestra.

**4. Systematic paleontology**

**4.1. Cranial materials**
Clade Crocodylomorpha Hay, 1930

Suborder Thalattosuchia Fraas, 1901

Superfamily Teleosauroidea Geoffroy Saint-Hilaire, 1831

Family Teleosauridae Geoffroy Saint-Hilaire, 1831

Genus *Indosinosuchus* Martin et al., 2019

cf. *Indosinosuchus* sp.

(Fig. 2A, B)

**Horizon and locality:** Phu Kradung Formation (Upper Jurassic–Lower Cretaceous), Ban Wang Din So, Wang Thong District, Phitsanulok Province.

**Material:** PRC-390, an almost complete periorbital area including the nasals, prefrontals, and frontal (PRC-390; former no. TF2348) (Fig. 2), along with five isolated teeth (PRC-385, PRC-386, PRC-387, PRC-388, and PRC-389) (Fig. 6A–C) were discovered within the collections of PRC, Mahasarakham University. These specimens were collected in 1978 by Paladej Srisuk, a curator of Srisuk’s House Museum. PRC-390 was initially misidentified as a crocodylian osteoderm due to the thick matrix surrounding it, making both preparation and observation difficult.

<Figure 2>

**Description:** The bone sutures of the skull fragment are clearly distinguishable in the three-dimensional model of PRC-390 (Fig. 2B). This specimen is composed of the prefrontals, frontal, nasals, and orbits. The total length of PRC-390 is 125.5 mm, the maximum width is 80.2 mm, and the thickness is 25.5 mm. This specimen is flattened dorsoventrally and lacks an orbital ridge. The frontal surface is notably ornamented with ellipsoid or circular pits, particularly in the posterior region. Still, the frontal, nasal, and prefrontal processes exhibit a lower density of ornamentation. While the posterior part of the frontal is damaged, the broad triangular anterior process is intact and divides the posteriormost region of the nasals. Dorsally, the posterior tip of the nasal gradually decreases in size. Although the prefrontals are preserved on both sides, their anterior portions are missing. The medial part of the prefrontal contacts the nasal anteriorly and the frontal posteriorly. The left prefrontal seems longer than wide, with the orbital boundary curve visible along the bone's lateral edge. The ventral view shows the crista cranii frontalis, which defines a midline groove for the olfactory tract. Conversely, the anteromedial corner of the orbit is identified by the projection between the frontal and prefrontal bones on the left side.
**Comparison** (Fig. 5A–C): The Ban Wang Din So specimen is identified as a teleosauroid thalattosuchian based on the following characteristics (Fanti et al., 2016; Martin et al., 2019; Johnson et al., 2020): frontal ornamentation extends from the center to the anterior-most areas; the frontal projection is mediolaterally thin and extends anteriorly past the anterior orbital rim into the nasals; there are no anterolateral projections between the nasals and the prefrontals; the orbital dorsal margins are flush with the skull dorsal surface; and the periorbital area of skull is flattened dorsoventrally. Further taxonomic identification of PRC-390 is challenging due to poor preservation. However, the CT scan of PRC-390 shows an unmistakable resemblance to the autapomorphic characters of the Thai teleosaurid genus *Indosinosuchus*, in which the elongated posterior nasal processes reach toward the orbits but do not entirely limit contact between the prefrontal and frontal bones. Except for *Indosinosuchus*, no other teleosaurid species shows this character (Martin et al., 2019; Johnson et al., 2020).

<Figure 3>

<Table 1>

Comparison between PRC-390 and *Indosinosuchus* specimens from the Phu Noi locality reveals a noticeable difference in the size of the frontal bone. According to the available records, PRC-239 possesses the most extended skull of the genus *Indosinosuchus*, measuring 57.5 cm from the tip of the premaxilla to the supraoccipital (see table 1 in Martin et al., 2019). In the Ban Wang Din So specimen, the width between its posterior nasal tips measures 50.36 mm, compared to 24.2 mm in PRC-239. Based on the size of its frontal bone, the Ban Wang Din So specimen seems likely to be twice as big as the largest known *Indosinosuchus* specimen from the Phu Noi locality.

The width between the posterior nasal tips (WNT) serves as a parameter for estimating the skull length (Fig. 3) of the Ban Wang Din So specimen (PRC-390). The regression analysis using the four complete specimens of *Indosinosuchus* (Table 1) suggests that the PRC-390 would have an estimated skull length of 80.51 cm (y = 9.770x + 28.148, R² = 0.407). Furthermore, the least-squares regression analysis of Young et al. (2016) provides an estimated skull length and corresponding total body length (y = 4.016x + 92.317, R² = 0.9315; see fig. 3.1 in Young et al., 2016). Following this analysis, the skull of cf. *Indosinosuchus* sp. from the Ban Wang Din So locality is estimated to correspond to a total body length of approximately 417 cm. However, this estimate should be interpreted with caution, as the applied equations do not account for variation in longirostry, which may affect body length projections.

Teleosauroidea indet.

(Fig. 4A–E)
**Horizon and locality:** Klong Min Formation (upper Lower-lower Middle Jurassic), Ban Mab Ching, Thung Song District, Nakhon Si Thammarat Province.

**Material:** PRC-391 (Fig. 4) includes two skull bone fragments, consisting of a nearly complete frontal and an associated left postorbital bone. This specimen was discovered among numerous turtle shells during our field trip in July 2024.

<Figure 4>

**Description:** PRC-391 is a cranial fragment, including a nearly complete frontal and a left postorbital (Fig. 4A-E). The frontal bone measures 72.1 mm long, 48.6 mm wide, and 11.8 mm thick. The postorbital bone measures 77.6 mm long, 19.15 mm wide, and 17.35 mm thick. The frontal is longer than wide and has a densely ornamented dorsal surface with ellipsoid to circular pits. It is dorsoventrally compressed and devoid of orbital ridges. The anterior process of the frontal is slender and extended. The posteromedial corner of the orbital area can be observed at the lateral edge of the frontal. In contrast, the anteromedial corner of the supratemporal fenestra has a slightly curved angle and a deep groove along its edge. The ventral surface of the bone is smooth and plain, with the crista cranii frontalis visible.

The left postorbital is primarily complete, featuring several deep subcircular and elongated pits randomly scattered across the external surface. The postorbital structure is T-shaped, with one branch extending toward the midline, another branch directed toward the back of the skull, and the postorbital bar extending ventrally. The medial process has been damaged approximately at its middle point but keeps a slightly forward orientation. In lateral view, the posterior process is elongated and slender, extending posteriorly at a perpendicular angle to the postorbital bar. Despite the anterior process being nearly absent, the remaining traces indicate that it extended to the posteroventral boundary of the orbit, forming part of the orbital margin. The anterolateral corner of the left supratemporal fenestra is formed by combining the medial and posterior processes; its anterior margin is inclined anteriorly and laterally. The postorbital bar defines the anteriormost edge of the infratemporal fenestra, forming a nearly straight angle that also includes the postorbital foramen.

<Figure 5>

**Comparison** (Fig. 5D): The reconstruction of PRC-391 exhibits a large sub-rectangular supratemporal fenestra, a characteristic shared among most teleosauroid taxa. The frontal bone shows the diagnostic characteristics of a teleosauroid as follows: the orbital dorsal margins align with the dorsal surface of the skull (Fanti et al., 2016), and the ornamentation extends from the center of the frontal to the anterior or lateral edges (Johnson et al., 2020). Apart from *Mystriosaurus laurillardi*,
*Mycterosuchus nasutus*, and PRC-391, a well-developed and heavily ornamented postorbital region has not been observed in these taxa (Johnson et al., 2020). However, the ornamented pits on the postorbital of *Mys. laurillardi* exhibits a grooved pattern (Sachs et al., 2019), while the ornamentation pattern on the prefrontal is not described in *Myc. nasutus*. Nevertheless, based on these characteristics, we suggest that the Mab Ching specimens should be placed within the superfamily Teleosauroidea, which contains members of Teleosauridae and Machimosauridae (Johnson et al., 2020).

Two characters of PRC-391 are comparable to the holotype of *Indosinosuchus*. First, the anterior margin of the left supratemporal fossa is inclined anteriorly and laterally. Second, even though the anterior process of the postorbital in PRC-391 is nearly absent, the remaining traces suggest that it extends to the posteroventral boundary of the orbit and forms the orbital margin. Nevertheless, some characteristics that differ from *Indosinosuchus* are also recognized. In size comparison, the frontal width of PRC-391 is narrower than that of *Indosinosuchus*, while its posterior process is broader. The postorbital bar of PRC-391 is distinctly oriented — nearly perpendicular to the posterior process of the postorbital — unlike that of *Indosinosuchus*, which exhibits a pronounced bend in the posteroventral direction. Furthermore, PRC-391 exhibits prominent ornamentation on the dorsal surface of the postorbital, a feature absent in all known *Indosinosuchus* specimens preserving this element, including those ranging in skull length from 44 cm (PRC-10) to 57.5 cm (PRC-239) (Martin et al., 2019; Johnson et al., 2020). According to the comparison above, the teleosauroid specimen from the Sibumasu Terrane (PRC-391) does not appear to be assignable to *Indosinosuchus*.

## 4.2. Dentition

- Clade Crocodylomorpha Hay, 1930

- Suborder Thalattosuchia Fraas, 1901

- Superfamily Teleosauroidea Geoffroy Saint-Hilaire, 1831

- Teleosauroidea indet.

(Fig. 6A–E)

**Material:** Ban Wang Din So: Five isolated teeth (PRC-385, PRC-386 (Fig. 6A), PRC-387 (Fig. 6B), PRC-388, and PRC-389 (Fig. 6C)). Ban Khuan Kun: Two isolated teeth (PRC-208 (Fig. 6D) and PRC-209 (Fig. 6E)) were discovered from a local pond at Pitak Palm Oil Company Limited, Sikhao District, Trang Province.
<Figure 6>

The Ban Wang Din So collection includes five isolated teeth (PRC-385 to PRC-389) of varying sizes. Despite being partially preserved, they have a slender, conical shape with minor labiolingually compression. PRC-388 and PRC-389 preserve their pointed crown apices. The carinae shown in PRC-389 extend along the entire apicobasal length of the tooth crown in mesial view. PRC-387 and PRC-389 show fine enamel ridges. Particularly, PRC-389 displays these ridges, which converge apically toward the mesial carinae on the labial side, while the ridges remain parallel on the lingual side and are visible through the apex. The enamel on the apical surface of PRC-389 is relatively smooth. Although the crown tips of PRC-385 and PRC-386 are broken and the enamel on both specimens is worn, thin striations can still be seen at the base of the crowns. PRC-386 is the largest tooth in the collection, measuring 10.10 mm in diameter and 30.25 mm in height (excluding the tip). In contrast, the smallest tooth, PRC-388, is 4.4 mm in diameter and 7 mm in height from root to apex.

Two isolated teeth from the Ban Khuan Kun locality (PRC-208 and PRC-209) are robust with minor labiolingually compression and exhibit a subcircular outline in basal view; however, the bases of tooth are not preserved. Although the crown apices are not complete, PRC-209 appears to be blunt or round at the apex of the tooth crown. The carinae of the tooth run along the entire apicobasal length of the tooth crown in both mesial and distal views. Both specimens exhibit numerous fine enamel ridges along the entire apicobasal length of the tooth crown. The ridges near the carinae gradually converge towards the mesial and distal carinae, while those on the remaining surfaces remain parallel towards the apices. The enamel surface at the apex of the crown on PRC-209 is relatively smooth. PRC-208 and PRC-209 have 31 and 37 ridges on the lingual side and 47 and 49 ridges on the labial side, respectively. PRC-208 exhibits an apicobasal length of 16.90 mm and a diameter of 8.61 mm. In contrast, PRC-209 is more curved, measuring 13 mm in length with a diameter of 10.31 mm.

**Comparison:** Johnson et al. (2020) proposed an updated phylogenetic data matrix classifying teleosauroids based on dentition characteristics as follows: In most teleosauroids, the carinae extend along the entire apicobasal length of the tooth crown, and enamel ridges cover the entire crown, including the apex. While most teleosauroids have elongated, slender teeth with pointed apices, members of the Machimosaurini (Jouve et al., 2016) possess conical teeth with blunt and rounded apices. Additionally, the apical enamel surface in Teleosauridae is rather smooth and mostly unornamented, whereas Machimosaurini developed a more complex ornamentation pattern.

Teleosauroid teeth in Thailand have been discovered and reported by Lauprasert (2006) and Martin et al. (2019). They are elongated with a robust appearance and a conical shape. In the mesial and lateral views, their labial ridges gradually bend and contact the carinae, while the lingual ridges align themselves parallel to the apex. Teeth with these characteristics can be found in *Peipehsuchus teleorhinus* from the upper part of the Lower Jurassic Maanshan Formation, China (Young, 1948), and *Indosinosuchus potamosiamensis* represented by PRC-13 (fig. 10 in Martin et al., 2019) from the Jurassic part of the Phu Kradung Formation, Thailand.
The specimens from Ban Wang Din So and Ban Khuan Kun exhibit a slender to conical shape, with carinae extending along the entire apicobasal length on the mesial and distal sides. Enamel ridges cover the entire crown. These morphological characteristics are reminiscent of teeth found among teleosauroids. Furthermore, the apical enamel surface of specimens from both localities (PRC-209 and PRC-389) is relatively smooth and mostly unornamented, a characteristic of Teleosauridae. On the other hand, the shape of the crown apices differs between the two localities. The specimens from Ban Wang Din So (PRC-389) have pointed apices, characteristic of Teleosauridae, whereas the specimens from Ban Khuan Kun (PRC-209) have blunt and rounded apices, resembling those of Machimosaurini. Additionally, the ridges on the specimens from Wang Din So locality converge apically toward the carina on the labial side but do not extend to the carina on the lingual side; instead, they continue parallel to the crest. These features had also been previously described in Asian teleosauroids (Young, 1948; Lauprasert, 2006; Martin et al., 2019). However, relying only on differences in crown apex shape between the two localities makes it both difficult and challenging to determine whether they belong to Teleosauridae or Machimosaurini (Vignaud, 1997). As discussed above, given the similar characteristics of tooth shape in PRC-390 and PRC-391, it is likely that these teeth belong to the Teleosauroidea.

**5. Discussion and conclusion**

In Asia, teleosauroids have been discovered mainly in China and Thailand (Young, 1948; Li, 1993; Buffetaut et al., 1994b; Storrs and Efimov, 2000; Martin et al., 2019). The first Thai teleosauroid was reported in 1994 from southern Thailand, within the Sibumasu Terrane, based on the description of a lower jaw and femur (Buffetaut et al., 1994b). However, more detailed morphological descriptions and photographs are currently unavailable, and the whereabouts of the specimens are currently unknown. For nearly two decades, our understanding of teleosauroids in Thailand has been limited to the Phu Noi site in the central part of the Khorat Plateau within Indochina Terrane (Lauprasert, 2006; Martin et al., 2016, 2019; Johnson et al., 2020; Bhuttarach et al., 2023). Consequently, the teleosauroid specimens that were discovered at Ban Wang Din So, which is located west of the Khorat Plateau, offer additional evidence for the widespread distribution of teleosauroids throughout the Indochina terrane. Furthermore, specimens from Ban Mab Ching and Ban Khuan Kun in southern Thailand also demonstrate the presence of teleosauroids in the southern peninsula in the Sibumasu Terrane.

The new specimen of cf. *Indosinosuchus* sp. from Ban Wang Din So is the first known crocodylomorph taxon from the western region of the Indochina Terrane. Teleosauroids are typically found in marine sediments, except for Asian teleosauroids, such as *Peipehsuchus teleorhinus* and *Indosinosuchus* spp., which were unexpectedly discovered in continental deposits. This is confirmed by the discovery of a teleosauroid specimen, cf. *Indosinosuchus* sp., in non-marine rocks at Ban Wang Din So. This specimen exhibits the widest gap between the posterior nasal tips and the longest frontal process, suggesting that PRC-390 could be the largest *Indosinosuchus* ever discovered so far.
The new teleosauroid specimen (PRC-391) found in southern peninsular Thailand is different from *Indosinosuchus* of the Khorat Plateau because its postorbital bar has a different orientation, its dorsal surface exhibits stronger ornamentation, and its crown apex has a distinct shape. As a result, the teleosauroid specimens from the Sibumasu Terrane should be assigned to a newly identified category within the family Teleosauridae. Finding unique traits in these specimens points to the wide variety of teleosauroids that existed in Thailand and other parts of Southeast Asia.

Previous research revealed that Thai teleosaurids appeared solely in the Khorat Group, restricted to the Jurassic (Martin et al., 2019). PRC-391 and two isolated teeth (PRC-208 and PRC-209) were found in the Klong Min Formation, Thung Yai Group, in the southern part of Thailand. This suggested that teleosauroids lived in the Sibumasu Terrane from late Early Jurassic to early Middle Jurassic (Teerarungsigul, 1999; Meesook and Saengsrichan, 2011; Saengsrichan et al., 2011). This is consistent with the type specimen of the first Asian teleosauroid identified, *Peipehsuchus teleorhinus* Young, 1948 (fig. 6.4A), from the Lower Jurassic Ziliujing Formation (Li, 1993) and the upper part of Lower Jurassic Maanshan Formation of China (Lucas, 2001). Until the precise age of the Phu Kradung Formation from the Khorat Group is determined, it is postulated that the teleosaurids from southern peninsular Thailand are more primitive than *I. potamosiamensis* and *I. kalasinensis*.

In terms of paleoenvironments, cf. *Indosinosuchus* sp. from Ban Wang Din So, *Indosinosuchus* from Phu Noi, and teleosauroids from Ban Map Ching were all discovered in lacustrine or meandering river deposits. These freshwater occurrences resemble that of *Peipehsuchus* from the Lower Jurassic Ziliujing Formation of Sichuan, China (Li, 1993; Xing et al., 2017), suggesting a recurring association between early teleosauroids and freshwater ecosystems in Asia. Although the Thai specimens appear stratigraphically younger, their presence may indicate that some teleosauroids had adapted to freshwater conditions in the Sibumasu Terrane since at least the Middle Jurassic. To date, all known teleosauroid remains from Thailand have been recovered from fluvio-lacustrine contexts (Martin et al., 2016, 2019). While this pattern could suggest ecological preference, we recognize that the absence of marine records may reflect regional sampling or taphonomic biases. Although marine Jurassic deposits exist in the region, many are poorly fossiliferous or have yet to yield teleosauroid remains.

Nevertheless, the convergence of freshwater records from both Thailand and China raises the possibility of region-specific ecological plasticity or habitat preference among Asian teleosauroids — a behavioral pattern yet unrecognized in other parts of the world. Following the closure of the Paleo-Tethys in the Late Triassic, the region experienced renewed marine transgression during the late Early to early Middle Jurassic (Saengsrichan et al., 2011), leading to a complex landscape of shallow marine, lagoonal, and freshwater environments across southern Thailand. This paleogeographic context may have facilitated faunal dispersal across coastal and inland habitats. It is therefore plausible that teleosauroids in this region moved between marine and freshwater systems, exhibiting ecological flexibility possibly tied to environmental pressures or resource availability.

Given the focus of this study on paleobiogeography, these data warrant broader synthesis. The spatial and stratigraphic consistency of freshwater teleosauroid occurrences in Asia not only reflects localized adaptations but may also signal distinct
faunal exchange routes and ecological shifts following tectonic and sea-level changes in Jurassic. As such, the Thai fossil record offers a rare window into teleosauroid dispersal and habitat use in a region where fluvial Jurassic deposits are exceptionally scarce.

## Acknowledgments

The authors extend their sincere thanks to the Science Achievement Scholarship of Thailand (SAST) for their support throughout this study and research. We also express our gratitude to the Palaeontological Research and Education Centre, Mahasarakham University, for providing specimens, equipment, and accommodations for this research. This research project was financially supported by Mahasarakham University. We would like to express our sincere gratitude to the reviewers, Dr. Stéphane Hua and Dr. Paul Rummy, for their valuable comments and constructive suggestions, which have greatly improved the quality of this manuscript. We extend our gratitude to Paladej Srisuk, curator of Srisuk's House Museum; Nattakit Saengsuwan, a mineral resources technical officer (practitioner level); and Vipavee Keamsontie, a geologist from the Phunphin Geological Museum in Phun Phin District, Surat Thani Province, Thailand, for their invaluable contributions to the fossil excavation and preparation. We also sincerely thank Ongsa Budda for reconstructing the three-dimensional model and Kantanat Trakunweerayut for contributing to the geological map. Special thanks are due to radiological technician Sarayut Trinet and Pravina Kongrat from the Faculty of Medicine, Mahasarakham University. Last, we thank everyone who contributed to and supported this research.

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## Figure captions

Fig. 1. Simplified geological map of Thailand illustrating major tectonic units, Jurassic rock distribution, and study localities. Four key study sites are highlighted: Ban Wang Din So (Wang Thong District, Phitsanulok Province) in northern Thailand, Phu Noi (Ban Din Chi, Kham Muang District, Kalasin Province) in northeastern Thailand, and Ban Mab Ching (Thung Song District, Nakhon Si Thammarat Province) and Ban Khuan Kun (Sikhao District, Trang Province) in southern Thailand. Tectonic framework modified from Udchachon et al. (2024) and Morley and Chantraprasert (2025); Jurassic rock units modified from Department of Mineral Resources (2007) and Meesook and Saengsrichan (2011). Abbreviations: BRS: Bentong-Ruap Suture; CRS: Chinag Rai Suture; KF: Klaeng Fault; KMRF: Khlong Marui Fault; MPF: Mae Ping Fault; MYF: Mae Yuam Fault; NUS, Nan-Uttraradit Suture; RNF: Ranong Fault; SKS: Sra Kaew Suture; TPF: Three Pagoda Fault.
Fig. 2. Photograph of the frontal bone of cf. *Indosinosuchus* sp. from the Ban Wang Din So (PRC-390) in dorsal (left) and ventral (right) views in (A), and three-dimensional models in (B), highlighting the differences between the bones.

Fig. 3. Position of the width between the posterior nasal tips (WNT), modified from Martin et al. (2019).

Fig. 4. Photograph and interpretive drawing of Teleosauroidea indeterminate specimen (PRC-391) from the Ban Mab Ching. The frontal bone is shown in dorsal (A), and ventral view (B), and postorbital bone in dorsal (C), left lateral (D) and internal view (E).

Fig. 5. A comparison of the skull table of *Indosinosuchus*, including the holotype of *I. potamosiamensis* (PRC-11) in (A) and *I. kalasinensis* (PRC-239) in (B), with reconstruction of new teleosauroid materials from Ban Wang Din So (PRC-390) in (C) and Ban Mab Ching (PRC-391) in (D). Morphological features of PRC-390 are consistent with *Indosinosuchus*, as modified from Martin et al. (2019). In contrast, reconstruction of PRC-391 displays distinct differences from both *Indosinosuchus*.

Fig. 6. Photograph and drawing tooth morphology of Teleosauroidea indeterminate specimens from Ban Wang Din So, showing the largest teeth without enamel in PRC-386 (A), fine enamel ridges in PRC-387 (B), and the enamel ridges converge apically toward the mesial carinae in PRC-389 (C). And tooth of Teleosauroidea indeterminate specimens from Ban Khuan Kun exhibit well-preserved enamel ridges along the mesial carinae, as observed in PRC-208 (D) and PRC-209 (E).

Table 1. Measurements of the *Indosinosuchus* specimens used in the regression analysis of the width between the posterior nasal tips (WNT), skull length, and total body length, as modified from Martin et al. (2018) * and Young et al. (2016) **. All measurements are in cm.

Specimens WNT Skull length Total body length
<table><tr><td>PRC-10</td><td>1.72</td><td>44*</td><td>269.04**</td></tr><tr><td>PRC-11</td><td>2.56</td><td>48.4*</td><td>286.71**</td></tr><tr><td>PRC-239</td><td>2.42</td><td>57.5*</td><td>323.26**</td></tr><tr><td>PRC-390</td><td>5.36</td><td>80.51</td><td>415.69**</td></tr></table>
<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>
A

<figure><img src="image_8.png" /></figure>

B

<figure><img src="image_9.png" /></figure>

C

<figure><img src="image_10.png" /></figure>

D

<figure><img src="image_11.png" /></figure>

E

<figure><img src="image_12.png" /></figure>

or

<figure><img src="image_13.png" /></figure>

stf

or

<figure><img src="image_14.png" /></figure>

stf

cc

1 cm

<figure><img src="image_15.png" /></figure>

stf

itf

<figure><img src="image_16.png" /></figure>

pob

pof

<figure><img src="image_17.png" /></figure>

itf

pob
<figure><img src="image_18.png" /></figure>

A

5 cm

<figure><img src="image_19.png" /></figure>

B

5 cm

<figure><img src="image_20.png" /></figure>

C

5 cm

<figure><img src="image_21.png" /></figure>

D

5 cm

<figure><img src="image_22.png" /></figure>

frontal

<figure><img src="image_23.png" /></figure>

nasal

<figure><img src="image_24.png" /></figure>

postorbital

<figure><img src="image_25.png" /></figure>

prefrontal

<figure><img src="image_26.png" /></figure>
A

<figure><img src="image_27.png" /></figure>

B

<figure><img src="image_28.png" /></figure>

C

<figure><img src="image_29.png" /></figure>

D

<figure><img src="image_30.png" /></figure>

E

<figure><img src="image_31.png" /></figure>
**Declaration of interests**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☒ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Komsorn Lauprasert reports financial support, equipment, drugs, or supplies, and travel were provided by Mahasarakham University. Supanut Bhuttarachal reports equipment, drugs, or supplies, statistical analysis, travel, and writing assistance were provided by Science Achievement Scholarship of Thailand (SAST). Sita Manitkoon reports equipment, drugs, or supplies was provided by Palaeontological Research and Education Centre, Mahasarakham University. Komsorn Lauprasert reports a relationship with Mahasarakham University that includes: employment and funding grants. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.