Stereospondyli

Nonsrirach, Thanit, Manitkoon, Sita & Lauprasert, Komsorn, 2021, First occurrence of brachyopid temnospondyls in Southeast Asia and review of the Mesozoic amphibians from Thailand, Fossil Record 24 (1), pp. 33-47 : 38

Stereospondyli indet.

Reference material

CY-HN 378 ( Fig. 5a–d ), intercentrum ( Laojumpon et al., 2014); CY-HN 364 ( Fig. 5e–f ), left clavicle bone; and CY-HN 365 ( Fig. 5g –h ), CY-HN 368 ( Fig. 5i–j ), and CY-HN 377 ( Fig. 5k–l ), dermal bones.

Formation/age

Huai Hin Lat Formation (Carnian–Norian).

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, 2014).

Description

The material consists of an intercentrum and several fragments of dermal bones. A large intercentrum ( CY-HN 378, Fig. 5a–d ) is preserved over a length of approximately 65 mm and is 33 mm thick. CY-HN 378 is a disk-shaped intercentrum with reduced or absent pleurocentra ( Laojumpon et al., 2014). The anterior and posterior faces of CY-HN 378 are concave. Laterally, the parapophyses are located in less than half of the total length. These features share characters with Stereospondyli ( Milner et al., 1994; Witzmann and Gassner, 2008; Laojumpon et al., 2014). The shape of CY-HN 378 is comparable to those of the Metoposauroidea or Mastodonsauroidea intercentrum ( Moser and Schoch, 2007; Sulej, 2007; Fortuny et al., 2019; Marzola et al., 2017). The Plagiosauroidea possess cylindrical intercentra ( Warren and Snell, 1991; Konietzko-Meier et al., 2014). The Brachyopoidea commonly show a wedge-shaped intercentrum in lateral view ( Shishkin, 1991; Warren and Snell, 1991; Warren et al., 1997, 2011; Warren and Dammiani, 1999; Averianov et al., 2008).

The other undescribed specimens are dermal bone fragments. The largest fragment ( CY-HN 364, Fig. 5e–f ) shows two distinct types of heavy ornamentation in ventral view, i.e., radial and polygonal sculptures. The radial sculpture 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 ornamentation 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 assigned 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.

Figures

Figure 5. New Triassic amphibian remains from Thailand. An intercentrum of Stereospondyli indet. (CY-HN 378) in anterior (a), posterior (b), left (c), and right (d) views. Dermal bone fragments of Stereospondyli indet. of CY-HN 364 (e–f), CY-HN 365 (g–h), CY-HN- 368 (i–j), and CY-HN-377 (k–l) in ventral view and dorsal view. Abbreviations: ap, ascending process; pp, parapophyses; ps, polygonal sculpture; rs, radial sculpture.