First occurrence of brachyopid temnospondyls in Southeast Asia and review of the Mesozoic amphibians from Thailand

The non-marine Mesozoic sedimentary rocks of Thailand, which consist of the Indochina block and the Sibumasu block, have yielded several terrestrial and aquatic vertebrate fossils, but only few amphibian remains have been reported. Here, we present an overview on the Thai amphibian palaeo-diversity based on the literature, re-examination of published material, new findings, and unpublished material. Thai amphibian fossil remains are assigned to Stereospondyli (Cyclotosauridae, Plagiosauridae, and Brachyopoidea) and Anura and were discovered from four formations, ranging from the Upper Triassic to Lower Cretaceous of Thailand. The occurrence of Brachyopidae in Thailand, which are related to Chinese forms, supports the previous hypothesis of physical connections between the Indochina blocks and the Sibumasu block during the Mesozoic era.


Introduction
The amphibians of the Paleozoic and the Mesozoic can be dived into three major clades: the Seymouriamorpha, the Lepospondyli, and the Temnospondyli (Vitt and Caldwell, 2014). Basal members of temnospondyls were the most diverse and most successful group of amphibians in the Paleozoic, which lived during the Mesozoic Era (Damiani and Rubidge, 2003;Schoch, 2014). Temnospondyls are commonly retrieved in the tetrapod assemblages from Triassic continental deposits such as fluvial and lacustrine environments (Schoch and Milner, 2000;Dias-da-Silva and Dias, 2013).
We present here an update of the amphibian continental fossil record during the Mesozoic in Thailand. The Mesozoic rocks of Thailand consist of two sub-continent blocks. The western part, called "Shan-Thai block" or "Sibumasu block", includes the eastern part of Myanmar and northern, western, and southern parts of Thailand as well as the western peninsula of Malaysia (Fig. 1). The eastern part is the Indochina block, which includes northeastern and eastern parts of Thailand, southern parts of Laos and Cambodia, and the western part of Vietnam (Metcalfe, 1996;Buffetaut and Suteethorn, 1998;Racey, 2009). The Mesozoic rocks in the northeastern part of Thailand, which belong to Indochina, consist of a non-marine red bed sequence deposited in a continental environment (Racey et al., 1996;Racey, 2009). This block consists of seven formations considered Upper Triassic to Cretaceous in age based on invertebrate and vertebrate remains as well as palynomorphs (Meesook et al., 1995;Racey et al., 1996). However, fossils of Thai amphibians (i.e., Cyclotosaurus cf. posthumus, Plagiosauridae, Temnospondyli indet., and Anura) were discovered only from three formations of the Indochina block (Ingavat and Janvier, 1981;Suteethorn et al., 1988;Buffetaut et al., 1994a;Srisuk, 2002Srisuk, , 2005 which are as follows. 1. The Huai Hin Lat Formation is mainly formed by lacustrine bituminous limestone and shales (Buffetaut and Suteethorn, 1998). It is considered Upper Triassic based on palynomorphs, plant macro-remains, conchostracans, and vertebrate remains (Kobayashi, 1975;Racey et al., 1996;Buffetaut and Suteethorn, 1998;Department of Mineral Resources, 2014). The vertebrate assemblages from the Huai Hin Lat Formation consist of actinopterygian fishes (Martin, 1984), lungfish (Martin and Ingavat, 1982), temnospondyls (Ingavat and Jan vier, 1981;Suteethorn et al., 1988), and phytosaurs (Buffetaut and Ingavat, 1982).
In the Sibumasu block of the southern peninsula of Thailand, temnospondyls were discovered in the Klong Min Formation (Buffetaut et al., 1994b, c), which is formed by limestone and grey to brown clays, indicating a lacustrine depositional environment (Tong et al., 2002). The Klong Min Formation is considered Middle or Upper Jurassic in age, based on charophytes and vertebrate remains (Buffetaut et al., 1994b;Tong et al., 2002;Girard et al., 2020). The vertebrate assemblages of this formation consist of hybodont sharks, actinopterygian fishes, lungfish, turtles, and crocodiles (Buffetaut et al., 1994b, c;Tong et al., 2002).
Although thousands of vertebrate remains have been found, few amphibian remains have been reported. More recently, new specimens of amphibians (intercentra and a posterior part of a skull) were collected from new Mesozoic localities in Thailand, i.e., Huai Nam Aun locality, Phu Noi locality, and Khao Wong locality (Laojumpon et al., 2014;Chanthasit et al., 2019). In order to understand the diversity and biogeography of Thai Mesozoic amphibians, we describe and re-check the taxonomic status of all amphibian remains housed in the collections of the Sirindhorn Museum (SDM, Kalasin Province) and the Palaeontological Research and Education Centre of Mahasarakham University (PRC, Mahasarakham Province). The relationships between these amphibians and their Asian relatives are used in a palaeobiogeographical framework for a better reconstruction of the physical connections in Southeast Asia.

Locality and sediment
The specimen was discovered in 1981 at an outcrop near the Chulabhorn Dam, Khon San District, Chaiyaphum Province. The fossil assemblage layer consists of a grey to red conglomerate.
Description D.M.R. no. Ch.D 001 is a posterior part of a comparatively large skull (approximately 20 cm wide and 11 cm long), but the lateral part of the right side is missing. However, a pair of otic notches and a pineal foramen are preserved. The pattern of the ornamentation and the shallow interotic embayment of this skull allow an assignment to Cyclotosaurus (Fraas, 1913;Ingavat and Janvier, 1981;Damiani, 2001) and are similar to those of the holotype of Cyclotosaurus posthumus Fraas, 1913 from the Upper Middle Keuper, Bavaria (Germany).

Locality and sediment
Outcrop near the Chulabhorn Dam, Kon San District, Chaiyaphum Province. The outcrop is located in the upper part of the Huai Hin Lat Formation (Suteethorn et al., 1988). The specimen was embedded in a block of carbonaceous black shale rock.

Description
The specimen TF 1453, approximately 22 mm wide and 25 mm long, shows a peculiar ornamentation consisting of tubercles or pustules on its ventral surface. Dorsally, the surface shows some faint radiating grooves and ridges. Based on its ornamentation, TF 1453 was interpreted as a dermal bone fragment of a temnospondyl and was assigned to Plagiosauridae. A comparable ornamentation is observed in Middle to Late Triassic Plagiosauridae, e.g., Gerrothorax pulcherrimus and Plagioscutum ochevi from Europe (Shishkin, 1986;Warren and Snell, 1991;Suteethorn et al., 1988;Jenkins et al., 2008).

Formation/age
Klong Min Formation (Middle or Upper Jurassic) and Phu Kradung Formation (Upper Jurassic).

Locality and sediment
TF 3328 and TF 3329 were found in a road cut outcrop near Mab Ching village, Thung Song District, Nakhon Si Thammarat Province, in the southern peninsula of Thailand, which belongs to the Sibumasu block. This outcrop is formed by greyish clays and siltstones, with some freshwater limestone intercalations. It belongs to non-marine Middle or Upper Jurassic rocks based on charophyte algae, palynomorphs, and diatoms (Lei, 1993;Buffetaut et al., 1994c;Girard et al., 2020). TF 3144 was found in a road cut outcrop at kilometer 68, along Highway 12 from Chum Phae to Lom Sak, in Khon Kaen Province. This area consists of reddish-grey silts and fine-grained reddish sandstone deposited in a meandering river channel (Buffetaut et al., 1994a), which belongs to the Indochina block. All specimens are wedge-shaped intercentra. Both anterior and posterior surfaces are concave and show a marked circular notochordal canal, which is visible as a circular pit. The pleurocentra are reduced or absent, which is a typical feature of stereospondyls (Milner et al., 1994;Witzmann and Gassner 2008). The wedge-shaped morphology and the notochordal pit of TF 3328, TF 3329, and TF 3144 is consistent with the intercentrum morphology of the Brachyopoidea (Shishkin, 1991;Warren et al., 1997Warren et al., , 2011Warren and Dammiani, 1999), which are similar to the intercentra discovered from the Jurassic Indochina block, e.g., Gobiops desertus from the Gobi Desert of Mongolia (Shishkin, 1991).

Locality and sediment
The humeri (SHM-PT 529 and SHM-PT 530) were collected in a road cut outcrop near Phu Phan Thong village, Muang District, Nong Bua Lamphu Province. These fossils were embedded in a pale olive fine-grained siltstone (Srisuk, 2002).
The pelvic girdle (SHM-HY 231) was discovered in a road cut outcrop near the Huai Lao Yang reservoir road, Nong Bua Lamphu Province. The outcrop is formed by reddish-brown micaceous siltstones and lime-nodule conglomerates (Srisuk, 2005).

Description
The left (SHM-PT 529) and right humeri (SHM-PT 530) are represented by their distal parts. They are elongated with a slender shaft, slightly curved in lateral view, with rounded capitulum.
The partial pelvic girdle (SHM-HY 231) consists of parts of the ischium, pubis, and acetabulum, while the ilium shaft is not preserved. The acetabular fossa is nearly circular with a posteriorly widened acetabular rim. The base of the ilium shaft has a triangular cross section. These features are reminiscent of an anuran amphibian, but the taxonomic identification of these incomplete specimens is difficult.

Locality and sediment
These specimens were collected by a Thai-French paleontological team during an excursion in 2010 at Huai Nam Aun, an outcrop situated near the Nong Yakong village, Khon San District, Chaiyaphum Province. This locality contains various beds of limestone and mudstone, deposited in brackish water near a calcium carbonate source (Laojumpon et al., 2014). Other fossils such as Hybodus teeth, bony fish scales, and coprolites have also been found from this locality (Laojumpon et al., 2012(Laojumpon et al., , 2014.
consists of parallel or radial ridges without transverse ridges, while the polygonal sculpture, located close to the lateral edge of the bone, consists of short ridges connected and forming polygons in a honeycomb or hexagonal shape. The dorsal surface is smooth with a part of the ascending process located along the lateral edge. The polygonal ornamen-tation located close to the lateral edge and the presence of an ascending process indicate that CY-HN 364 is a left clavicle bone. The other small dermal fragments, i.e., CY-HN 365 (Fig. 5g-h), CY-HN-368 ( Fig. 5i-j), and CY-HN-377 ( Fig. 5k-l), are covered by radial and polygonal ornamentation in ventral view. These specimens, however, could be as- signed as either interclavicle or clavicle bones. Based on the shape of the intercentrum and ornamentation pattern, these specimens could refer to either the late Triassic Mastodonsauroidea (e.g. Cyclotosaurus) or Metoposauroidea (Chowdhury, 1965;Warren and Snell, 1991;Sulej and Majer, 2005;Sulej, 2007;Brusatte et al., 2015;Antczak and Bodzioch, 2018). Therefore, more evidence is required for a familylevel identification.

Locality and sediment
KS37-8 was collected at the Khao Wong locality in Khao Wong District, Kalasin Province. Sediments in this locality are grey to green siltstones.

Description
KS37-8 is a wedge-shaped intercentrum with the dorsal edge much shorter than the ventral edge. The parapophyses are close to the posterodorsal margin on the lateral surface. Both anterior and posterior surfaces are concave and show a marked circular notochordal canal, which is visible as a circular pit. The pleurocentra are reduced or absent. This intercentrum KS37-8 probably belongs to Brachyopoidea since it is the only Stereospondyli superfamily retrieved beyond the Triassic (Buffetaut et al., 1994b). Furthermore, the wedgeshaped morphology of KS37-8 resembles the intercentra of the Brachyopoidea (Shishkin, 1991;Warren et al., 1997;Warren and Dammiani, 1999;Warren et al., 2011), which are similar to the intercentra discovered from the Indochina block (TF 3144) and Sibumasu blocks (TF 3328 and TF 3329) of Thailand (Buffetaut et al., 1994a, c). However, the intercentrum is not sufficiently diagnostic for identification at the family level.

Description
The material consists of three temnospondyl specimens not found in association: the posterior part of the skull (KS34-1481, Fig. 6a-b) and two intercentra (KS34-1474, Fig. 3q-t and KS34-1489, Fig. 3u-x). The skull (KS34-1481) is short (approximately 14 cm width and 12 cm length), lacking otic notch and tabular horn in the supratemporal region. Ventrally, the interpterygoid vacuities are large, and the cultriform process of the parasphenoid is gracile and long. Posteriorly, the occipital condyles are rounded and projected well behind the posterior edge of the skull. This specimen is under study for a more detailed anatomical description. The intercentra (KS34-1474 and KS34-1489) are wedge-shaped with concave posterior surface and slightly convex anterior surface. The anterior surface shows a well-marked deep circular pit. The short skull, which lacks otic notch and tabular horn, and the position of occipital condyles are typical characters of Brachyopidae (Warren andMarsicano, 1998, 2000), similar to those of the Jurassic brachyopid temnospondyl skull from China such as Sinobrachyops placenticephalus (Dong, 1985).
Amphibian fossils from the Early Cretaceous of northeastern Thailand are described and related to anurans, although these specimens are too fragmentary to be more precisely identified. In the Cretaceous, Asian anurans have been discovered in Kazakhstan (Skutschas and Kolchanov, 2017), Mongolia (Gao and Chen, 2017), India (Prasad and Rage, 2004), Japan (Evans and Manabe, 1998), China (Wang et al., 2000;Yuan, 2004;Dong et al., 2013), and Myanmar (Xing et al., 2018). Thus, the discovery of this taxon suggests that during the Early Cretaceous, anurans were already present in Thailand.

Conclusions
The non-marine Mesozoic sedimentary rocks from Thailand were formed in the Indochina and Sibumasu blocks. These blocks have yielded at least four taxa of amphibians, including three taxa of temnospondyls (Cyclotosaurus cf. posthumus, Plagiosauridae, and Brachyopidae) and one taxon of anuran from four formations (Huai Hin Lat Formation, Khlong Min Formation, Phu Kradung Formation, and Sao Khua Formation) with ranges from the Late Triassic to Early Cretaceous. The Thai amphibian fossils show the most diverse Mesozoic amphibian record in Southeast Asia, and this agrees with the hypothesis of a large physical connection between the Indochina block and Laurussia during the Mesozoic era.
Data availability. All material included in this paper is deposited in the Palaeontological Research and Education Centre, Mahasarakham University and in the collection of the Department of Mineral Resources (Thailand).
Author contributions. TN and SM produced the images. TN and KL drafted the manuscript. All authors prepared the paper and contributed to the editing.
Competing interests. The authors declare that they have no conflict of interest.