Embolomeri are a monophyletic group of large crocodile-like, semiaquatic predators, prevalent in the Carboniferous and early Permian (Cisuralian) (Panchen, 1970; Smithson, 2000; Carroll, 2009; Clack, 2012). The clade is generally considered to be a stem member of the Reptiliomorpha, taxa that are more closely related to amniotes than to lissamphibians (Ruta et al., 2003; Vallin and Laurin, 2004; Ruta and Coates, 2007; Clack and Klembara, 2009; Klembara et al., 2014; Marjanovic and Laurin, 2019). Its relationship with the Permian–Triassic Chroniosuchia is still uncertain, but it is generally accepted that Chroniosuchia are not a subclade of the Embolomeri (Carroll, 2009; Clack and Klembara, 2009; Klembara et al., 2010; Schoch et al., 2010; Clack, 2012; Arbez et al., 2018; Marjanovic and Laurin, 2019). The distribution and diversity of Embolomere are closely related to the paleogeography and climate changes of the late Paleozoic. The earliest members of the group started to appear in the Late Mississippian, exemplified by Proterogyrinus, which was collected from both Scotland and USA (Holmes, 1984; Clack and Smithson, 2020). The Pennsylvanian, especially the Bashkirian–Moscovian, was when the embolomeres were the most diversified, evidenced by the well-documented specimens of Anthracosaurus and Pholiderpeton (Eogyrinus) and the less well-known Calligenethlon, Leptophractus, Neopteroplax, Pteroplax, Carbonoherpeton, and Palaeoherpeton (Clack, 1987, 2012; Cope, 1873; Holmes and Carroll, 2010; Panchen, 1964, 1972, 1977). All these animals were distributed in the swamp forests in Euramerica, then positioned in the tropics near the Equator, where the weather was warm and humid, a favorable environment for the embolomeres (Clack, 2012). Coming into the Permian, embolomeres experienced a dramatic decline in diversity, with only two taxa reported from the early Permian. One is Acheria, represented by abundant material from Texas and Oklahoma of North America (Holmes, 1989), and the other is the much less-known Aversor, presumably an eogyrinid, from the Kungurian Inta Formation (∼ 273 Ma) of Russia (Gubin, 1985; Steyer, 2000). The decline of the embolomeres was probably related to the gradual aridification of Euramerica during the Permian (Roscher and Schneider, 2006; Roscher et al., 2011; Bernardi et al., 2017), which resulted in the decrease of suitable habitats.

Here we report a new embolomere taxon from the late Permian (Lopingian) of Yangquan, Shanxi Province, China. The fossil was collected from the red mudstone of the basal layer of the Sunjiagou Formation (Fig. 1). The new taxon expands the chronological and geographical ranges of embolomeres and bears important implications on the late Paleozoic paleogeography and climate (Abbreviations: YQZY, Yuanquan Land Resources).

Specimen. The specimen was collected from Cuifeng Mountain, Yangquan by a local (Bai Zhi-jun) during the 2010s and curated under YQZY. It was a partial palate and upper jaw from the red mudstone of the late Permian Sunjiagou Formation.

Phylogeny. We used the data matrix of Holmes and Carroll (2010), which was modified from Clack and Klembara (2009). We added three new characters based on the morphology of the pterygoid (see Appendix). The new matrix resulted in 10 taxa coded for 330 morphological characters. All coding followed Holmes and Carroll (2010) except for the newly added Seroherpeton. Caerorhachis was set as the outgroup. The parsimony analysis was performed in POY 5.1.1 (Wheeler et al., 2015) under the branch-and-bound algorithm.

Tetrapoda Jaekel, 1909

The holotype of Seroherpeton is a partial palate and upper jaw found from the late Permian (Lopingian) of North China. At first glance, its morphology is similar to tetrapodomorphs such as Eusthenopteron, especially with its large palatal tusks, the denticles on the palatal ramus of the pterygoid, and the oblique shaft on the pterygoid. However, the resemblance is largely due to the conservative nature of the palate throughout the evolution of tetrapodomorphs and early tetrapods (Kimmel et al., 2009). Large paired ectopterygoid tusks are common among lobe-finned fish (Jarvik, 1980; Clack, 2012). They were lost in the earliest tetrapods such as Ichthyostega and Acanthostega but were re-acquired in many later tetrapod clades such as temnospondyls (e.g., Eryops, Phonerpeton), baphetids (e.g., Megalocephalus, Loxomma), and embolomeres (e.g., Anthracosaurus) as independent derived conditions. Among embolomeres, the size of the tusk varies across taxa. With an estimated skull length of around 400 mm, the large tusk of the Seroherpeton is comparable to that of similar-sized Anthracosaurus, an assumed top predator of its time. Anthracosaurus has an ectopterygoid tusk of 45 mm long and a basal diameter of 15 mm (Panchen, 1977), slightly larger than the posterior tusk but smaller than the anterior tusk of the new taxon. Pholiderpeton of similar size, on the other hand, has much smaller tusks, with the largest one of only 29 mm long and a basal diameter of 7 mm (Clack, 1987). In both tetrapodomorphs (such as Eusthenopteron) and the new taxon, a row of teeth is very closely present lateral to the paired ectopterygoid tusks. However, in tetrapodomorphs, the teeth are on ectopterygoid, whereas in the new taxon, we interpret the teeth as maxillary teeth.

The presence of denticles on the pterygoid is another common character seen in various lobe-finned fish and tetrapods as a plesiomorphic condition. Among embolomeres, the degrees of denticle coverage vary. Anthracosaurus has a complete smooth pterygoid surface with no coverage of denticles (Panchen, 1977), whereas eogyrinids and proterogyrinids have extensive coverage of denticles over the ventral surface of the pterygoid (Panchen, 1972; Holmes, 1984). Seroherpeton has the denticles covering only part of the palatal ramus of the pterygoid, a presumably intermediate condition.

The presence of a prominent oblique shaft on the pterygoid of Seroherpeton recalls the similar condition in tetrapodomorph Eusthenopteron (Jarvik, 1980) and Medoevia (Lebedev, 1995). Jarvik (1980) interpreted the function of the shaft as providing a resting place for the ceratohyal and a possible water passage through the external gill in Eusthenopteron. None of the other known embolomeres has such a shaft. It is certainly a curious condition in the new taxon and may represent an autapomorphic condition for Seroherpeton among embolomeres.

Embolomeri is a monophyletic group characterized by synapomorphies such as the presence of the tabular horn, the absence of post temporal fossa, and the presence of a large Meckelian fenestra in the mandible (Smithson, 1985, 2000); its monophyly was supported in all recent phylogenies (Ruta et al., 2003; Ruta and Coates, 2007; Clack and Klembara, 2009; Arbez et al., 2018; Marjanovic and Laurin, 2019). Seroherpeton does not preserve the abovementioned synapomorphies but nevertheless has a derived pterygoid morphology that indicates its embolomere affinity. The muscle scars on the notch behind the pterygoid denticles and the medioventral border of the descending flange (Fig. 4) indicate extensive muscle attachment of M. pterygoideus (Holmes, 1989; Witzmann and Werneburg, 2017). Holmes (1989), in describing Archeria, considered the strongly developed descending flange and cartilago–torus transiliens as shared derived characters of embolomeres, and possibly their form and function are homologous with those of basal reptiliomorphs. A similar system was also known in Paleoherpeton (Panchen, 1964), Pholiderpeton (Clack, 1987), and a new possible embolomere from the Mississippian of Scotland (Clack and Smithson, 2020). This muscle attachment pattern is clearly missing in tetrapodomorphs (e.g., Eusthenopteron), in which the pterygoid lacks such an ornamented notch and has a smooth ventral edge of the quadrate ramus (Jarvik, 1980; Lebedev, 1995).

We ran a parsimony analysis to determine the phylogenetic position of Seroherpeton within embolomeres. It resulted in one most parsimonious tree with 199 steps (Fig. 5). The major difference between our tree and that of Holmes and Carroll (2010) is that Chroniosaurus lies more towards the crown than Eoherpeton in the new tree. Seroherpeton lies more towards the crown than Anthracosaurus + Calligenethlon, and forms the sister group of the clade consisting of Proterogyrinus, Pholiderpeton, and Archeria.

This result posts an interesting question of when and how the lineage leading to Seroherpeton occurred in the North China Block (NCB). Both the locality and age of the new taxon lie outside the previously known distribution of the clade. Embolomeri is predominantly a Euramerican group with the largest diversity in the Carboniferous. Sediments in NCB during the Carboniferous are mostly marine, and the block is distantly separated from Euramerica as an isolated island (Huang et al., 2018). It is therefore unlikely that embolomeres were distributed in NCB during the Pennsylvanian. Abundant fossils of Archeria in North America showed that although the clade as a whole were not as diversified in species number, it was still a major component in the Euramerican fauna during the early Permian. Although less studied, the appearance of Aversor, presumably an eogyrinid, in the Kungurian of Russia (Gubin, 1985) showed that the embolomeres had migrated to a slightly higher latitude by the beginning of the Guadalupian. The lineage leading to Seroherpeton may have existed as a “ghost lineage” for a long time and dispersed to NCB through Siberia/Tarim in the late Permian, depending on the availability of land bridges. Different studies have predicted different scenarios of land connection between NCB and other blocks in the late Permian, but Liu et al. (2020) showed that the connection must have existed since at least 256 Ma (evidenced by fossils found from the Shangshihezi Formation in NCB). NCB was warm and humid throughout the Permian (Tabor and Poulsen, 2008; Bernardi et al., 2017), providing embolomeres with favorable environments. In the meantime, with Euramerica gradually becoming more arid during the Permian (Roscher and Schneider, 2006; Tabor and Poulsen, 2008; Roscher et al., 2011; Bernardi et al., 2017), suitable habitats for embolomeres such as humid tropical forests became more sporadic in Euramerica (Fig. 5). As noted in Bernardi et al. (2017), the low-latitude faunas in the Lopingian tend to preserve the last-surviving members of clades better known from earlier ages (such as captorhinids and temnospondyls), serving as a “natural museum”. The occurrence of Seroherpeton in NCB is another example of it (Fig. 5). Nevertheless, the discovery of Seroherpeton showed that the Embolomeri as a clade have survived through the middle Permian extinction (Olson's Extinction) and, with the right environments, might occur elsewhere during the late Permian.