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Research Article
First fossil frog and snake assemblage from southern Taiwan: a window into Pleistocene herpetofauna and palaeoenvironments in subtropical East Asia
expand article infoChien-Hsiang Lin‡, Si-Min Lin§, Chi-Wei Chien|, Te-En Lin, Haroon Nazir#, Ningthoujam Premjit Singh#¤
‡ Academia Sinica, Taipei, Taiwan
§ National Taiwan Normal University, Taipei, Taiwan
| CPC Corporation, Miaoli City, Taiwan
¶ Taiwan Biodiversity Research Institute, Nantou, Taiwan
# Wadia Institute of Himalayan Geology, Dehradun, India
¤ Academy of Scientific and Innovative Research, Ghaziabad, India

Corresponding author: Chien-Hsiang Lin ( chlin.otolith@gmail.com )

Corresponding author: Ningthoujam Premjit Singh ( ningthoujampremjit11@gmail.com )

Academic editor: Johannes Müller
© 2025 Chien-Hsiang Lin, Si-Min Lin, Chi-Wei Chien, Te-En Lin, Haroon Nazir, Ningthoujam Premjit Singh.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Lin C-H, Lin S-M, Chien C-W, Lin T-E, Nazir H, Singh NP (2025) First fossil frog and snake assemblage from southern Taiwan: a window into Pleistocene herpetofauna and palaeoenvironments in subtropical East Asia. Fossil Record 28(2): 293-308. https://doi.org/10.3897/fr.28.164568
ZooBank: urn:lsid:zoobank.org:pub:CD488CEE-E71A-4875-AE67-BECB421A0661
Open Access

Abstract

Herpetofauna, particularly amphibians and reptiles, exhibit high levels of endemism and distinct diversity pattern on Taiwan island. However, the biogeographic history of these groups remains obscure, in part due to the lack of a herpetofaunal fossil record. Here, we report the first fossil record of frogs and snakes from Taiwan, based on Middle Pleistocene vertebrate assemblage recovered from the Chochen–Tsailiao area in southern Taiwan. The collection includes a vertebra of a bufonid frog and multiple vertebrae of colubrid and viperid snakes. Despite their fragmentary preservation, several vertebrae are identifiable, including a toad (Duttaphrynus melanostictus), rat snakes (Ptyas mucosa and P. cf. mucosa), a wolf snake (Lycodon rufozonatum), and a pit viper (Protobothrops sp.). Additional vertebrae are attributed to Colubridae indet. and Alethinophidia indet. The preservation of these delicate skeletal elements in a tectonically active and humid subtropical setting highlights the exceptional nature of this discovery. Palaeoenvironmental reconstruction based on ecological preferences of extant analogues suggests that the region supported a humid fluvial and open woodland environment with abundant water bodies.

Key Words

biogeography, toad, colubriform snakes, diversity, ectothermic indicators, Middle Pleistocene, subtropical Asia

Introduction

Taiwan is located within a critical transitional zone between the Palearctic and Indomalayan biogeographic regions, making it a hotspot for faunal exchange, lineage diversification, and endemism (Ota 1998; Päckert et al. 2009; Yang et al. 2018; Dufresnes and Litvinchuk 2022). This biogeographic boundary is a complex suture zone further shaped by mountainous topography, latitudinal range, and varied climate in Taiwan. Many terrestrial lineages exhibit biogeographic structuring associated with these northern and southern influences (Ali and Vences 2019). Among its terrestrial vertebrates, herpetofauna exhibits notably high levels of endemism in Taiwan (Shang et al. 2009). Excluding marine species, the island supports 37 native amphibians and 85 reptiles (Shang et al. 2009), of which 5 salamanders, 14 frogs, 18 lizards, and 14 snakes are endemic to Taiwan (Lee et al. 2019). This diversity and endemism reflect complex evolutionary histories influenced by both ancient colonization events and geographic isolation (Kaito and Mamoru 2016).

However, the absence of herpetofaunal fossil records in Taiwan has left a substantial gap in our understanding of how these modern patterns originated and evolved. Without palaeontological evidence, it remains unclear whether Taiwan’s current herpetofaunal diversity reflects recent dispersal events or long-standing lineage persistence across glacial and interglacial cycles (Rage and Roček 2003; Bailon and Blain 2007).

In this paper, we present the first fossil herpetofaunal assemblage from Taiwan. These fossils, comprising an amphibian and snakes, have been discovered from the Middle Pleistocene deposits in the Chochen–Tsailiao area of southern Taiwan. These new findings significantly advance our understanding of the historical diversity and evolutionary lineages of herpetofauna in the region.

Geological setting

The fossil herpetofauna described herein were collected from the riverbed of Tsailiao River, in the Chochen (alternatively spelled as Tsochen or Zoujhen) area of Tainan, southwestern Taiwan (Fig. 1). This region is renowned for its rich Pleistocene terrestrial and marine assemblages, which have yielded a wide range of taxa. Findings from the Chochen–Tsailiao area include primates, proboscideans, suids, cervids, rhinoceroses, crocodiles, a bird, and fishes (Shikama 1937, 1972; Shikama et al. 1975a, 1975b, 1976; Otsuka and Lin 1984; Qi et al. 1999; Tao and Hu 2001; Shieh and Chang 2007; Chang et al. 2012; Kawamura et al. 2016; Lin et al. 2021; Tsai and Mayr 2021).

Figure 1. 

Geological and stratigraphic context of the Chochen–Tsailiao fossil locality in southern Taiwan. A. Generalized geological map of Taiwan. The Chochen–Tsailiao area is highlighted by a dashed rectangle (modified after Chen 2016; Lin et al. 2021); B. Stratigraphic column of the Chochen–Tsailiao area (modified after Chen 2016; Lin et al. 2023); C. Detailed map of the Chochen–Tsailiao area (modified after Qi et al. 1999). Fossils were collected from reworked deposits within the riverbed along the Tsailiao River (see text for details).

Despite being one of the most important localities yielding mammal and terrestrial fossils in Taiwan, most of the fossils are being transported and lack stratigraphic information (Hayasaka 1932). Most studies indicate the fossils originate from the nearby Middle Pleistocene Chiting Formation (Qi et al. 1999; Ho et al. 2005; Shieh and Chang 2007; Tsai and Mayr 2021; Fig. 1), which yield in situ mammal fossils (Kawamura et al. 2016). The Chiting Formation, reaching approximately 2000 m in thickness, predominantly consists of interbedded sandstones, siltstones, and mudstones. Stratigraphically, the formation is subdivided into three ascending members: Kangtzulin, Kuoling, and Takengwei (Ho et al. 2005). Fossils from the Chochen–Tsailiao area predominantly originate from the Kuoling and Takengwei members, dating from approximately 0.7 to 0.4 Ma (Middle Pleistocene), as indicated by nannobiostratigraphic analyses and cyclostratigraphic estimations (Chi and Huang 1982; Ho et al. 2005; Chen et al. 2011; Kawamura et al. 2016).

The Chiting Formation reflects a complex depositional environment characterized by marine transgression-regression cycles. Although originally deposited in marine conditions, abundant terrestrial vertebrate fossils, including mammals and reptiles, indicate significant fluvial and terrestrial influence, likely due to periodic terrestrial inputs and intense seasonal flooding events. Many fossils have been secondarily reworked, leading to mixed marine and terrestrial assemblages within the formation (Chen et al. 2011; Chen 2016; Lin et al. 2021). Additionally, some of the marine fossils may also have been reworked from the underlying Plio–Pleistocene Gutingkeng Formation, which is primarily exposed to the east (Lin et al. 2023; Hsu et al. 2024).

Ongoing collection and research have clarified the stratigraphic contexts of some fossils, although many specimens have been discovered without precise stratigraphic positions due to their retrieval from riverbeds after seasonal flooding (Otsuka and Lin 1984; Kawamura et al. 2016). Considering the uncertainty in precise stratigraphic provenance, we conservatively estimate the geological age of the described fossil herpetofauna as broadly corresponding to the established age range of the Chiting Formation, approximately 0.8–0.4 Ma (Chen 2016).

Methods

The fossil materials described in this study were collected from the riverbed of the Tsailiao River in Chochen, Tainan, southern Taiwan (Fig. 1). All specimens were recovered from surface deposits through simple sieving and manual sorting. Due to the nature of riverbed collection, the fossils lack precise stratigraphic context, although they are inferred to originate from the Middle Pleistocene Chiting Formation (see above, Geological setting). The collection was made by Mr. Liang-Chieh Wang over the past three decades and represents an important accumulation of vertebrate fossil material from this region.

Fossils included in this study consist of isolated amphibian and snake vertebrae. Taxonomic identification was based on morphological comparisons with extant and fossil materials, using relevant published literature (Ikeda 2007; Chen 2020; Ratnikov 2024) and a reference collection of extant vertebrae sourced from roadkills (Chyn et al. 2019) housed at the Biodiversity Research Museum, Academia Sinica, Taiwan (BRCAS) under the code ASIZAM for amphibians and ASIZRE for reptiles.

Specimen imaging was performed using a Nikon SMZ1270 stereomicroscope fitted with a digital camera. To enhance the depth of field, each image was compiled as a focus-stacked composite using Helicon Focus software. The resulting images were digitally optimized for clarity using Adobe Photoshop. All examined materials are housed in the Biodiversity Research Museum, Academia Sinica, Taiwan, under the registration code ASIZF. Measurements of the vertebrae were taken where preservation allowed, following the methodology of Bochaton et al. (2019), and are presented in Table 1. In addition, quantification of the vaulting (vaulting ratio)follows Georgalis et al. (2021). Detailed anatomical terminology used for vertebral descriptions is illustrated in Suppl. materials 1, 2.

Table 1.
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Measurements (in mm) of the vertebrae from the Middle Pleistocene Chiting Formation described in this study. Abbreviations: CL – centrum length; COH – condyle height; COW – condyle width; CTH – cotyle height; CTW – cotyle width; NAH – neural arch height; NAW – neural arch width; NH – neural spine height; NSL – length of the neural spine; PO-PO – width of the postzygapophyseal articular facets; PR-PO – distance between prezygapophyses and postzygapophyses of the same side; ZW –zygosphene width (after Ikeda 2007; McCartney et al. 2014; Venczel 2011; Bochaton et al. 2019).

Taxon Specimen (ASIZF) CTH CTW COH COW NAH PO-PO CL PR-PO ZW NAW NSL NH CL/NAW NAH/PO-PO NSL/NH PR-PO/PO-PO
Duttaphrynus melanostictus 101045 0.92 2.12 1.31 2.22 NA 4.86 3.66 4.63 NA NA NA NA NA NA NA 0.95
Lycodon rufozonatum 101048 3.00 3.31 2.94 3.03 2.15 9.50 6.37 8.1 4.42 5.85 3.62 2.39 1.08 0.23 1.51 0.85
Lycodon rufozonatum 101049 2.86 3.09 2.74 2.83 1.66 8.65 6.25 7.8 4.18 5.53 4.97 2.64 1.13 0.19 1.88 0.87
Ptyas mucosa 101050 2.76 2.88 2.62 2.68 1.84 7.60 6.46 8.9 3.75 4.95 3.21 2.30 1.31 0.24 1.40 1.07
Ptyas cf. mucosa 101051 3.40 4.17 3.48 4.14 2.21 9.90 7.87 9.6 5.50 6.44 8.96 2.82 1.22 0.22 3.18 0.95
Ptyas cf. mucosa 101052 3.66 3.74 3.35 3.46 2.48 10.00 8.17 10.2 5.11 6.30 7.52 4.60 1.30 0.25 1.63 0.96
Colubridae indet. 101046 2.05 2.37 1.96 2.14 1.20 5.22 5.45 8.30 3.00 3.42 4.71 2.92 1.60 0.23 1.58 1.41
Colubridae indet. 101047 2.01 2.39 2.23 2.20 NA NA 5.35 NA 3.10 3.78 2.73 1.26 1.41 NA 0.02 NA
Protobothrops sp. 101053 2.05 2.28 1.98 1.86 1.03 5.32 3.87 4.8 2.80 3.61 NA NA 1.08 0.19 NA 0.88

Systematic paleontology

Amphibia Linnaeus, 1758

Anura Fischer, 1813

Bufonidae Gray, 1825

Duttaphrynus Frost et al., 2006

Duttaphrynus melanostictus (Schneider, 1799)

Fig. 2A

Material.

Trunk vertebra (ASIZF 0101045).

Description.

The vertebra is large, wider than long, with a centrum that is procoelous and slightly dorsoventrally compressed. The neural canal is larger than both cotyle and condyle. The walls of the neural arch are robustly built, bearing a low but distinct carina neuralis. The prezygapophyses and postzygapophyses are dorsally elevated and extend laterally. A single transverse process is preserved and extends distally.

Remarks.

The substantial size of the vertebra (>4.5 mm in length), its proportions, and robust construction suggest that it belongs to the Bufonidae. Given its dimension and general morphology, the vertebra most likely represents a mid-trunk element, probably the 3rd or 4th presacral vertebra. Of the more than 30 native anuran species in Taiwan, only two bufonids, Bufo bankorensis (Barbour, 1908) and Duttaphrynus melanostictus, attain a comparable size. Trunk vertebrae of both species are morphologically similar; however, the centrum of B. bankorensis is more elevated in lateral view (Fig. 2C5), whereas that of D. melanostictus is flatter and more dorsoventrally compressed.

Figure 2. 

A. Duttaphrynus melanostictus (ASIZF 0101045) from the Middle Pleistocene Chiting Formation, southern Taiwan; B. Extant specimen of D. melanostictus (ASIZAM 000064), 4th trunk vertebra, 94.12 cm SVL (snout–vent length), 66 g; C. Extant specimen of Bufo bankorensis (ASIZAM 000062), 4th trunk vertebra, 10.7 cm SVL. A1–C1. Anterior view; A2–C2. Posterior view; A3–C3. Ventral view; A4–C4. Dorsal view; A5–C5. Lateral view. Scale bar: 5 mm.

Squamata Oppel, 1811

Serpentes Linnaeus, 1758

Caenophidia Hoffstetter, 1939

Colubriformes Günther, 1864 (sensu Zaher et al., 2009)

Remarks.

Most of our snake vertebrae describe here belong to the group of Colubriformes. Within Colubriformes, hypapophyses are typically present in mid- and posterior trunk vertebrae of several subgroups, among others, natricids, elapids, and viperids (Georgalis and Scheyer 2022). The absence of hypapophyses in mid- and posterior trunk vertebrae has long been considered characteristic of Colubridae (e.g., Boulenger 1896; Bogert 1940; Underwood 1967; Bourgeois 1968; Dowling and Duellman 1978) or of the paraphyletic group traditionally referred to as “Colubrinae” (Rage 1984; Szyndlar 1987, 1991a, 2012; Georgalis et al. 2018). However, recent studies have shown that more distantly related groups, such as dipsadids and psammophiids, may also lack hypapophyses in mid- and posterior trunk vertebrae (Georgalis et al. 2019, 2024; Zaher et al. 2019; Georgalis and Scheyer 2022; Georgalis and Szyndlar 2022). It is important to note that both of these non-colubrid groups are native to the New World and are absent from Taiwan.

Colubridae Oppel, 1811

Lycodon Fitzinger, 1826

Lycodon rufozonatum Cantor, 1842

Fig. 3A, B

Material.

Two trunk vertebrae (ASIZF 0101048–1049).

Description.

In anterior view, the cotyle is rounded in outline. Paracotylar foramina are present on either side of the cotyle. The prezygapophyses are well-developed, and the prezygapophyseal accessory processes are laterally extended. The zygosphene roof is thin and dorsally convex. The zygosphenal articular facets of the zygosphene are inclined dorsally. The neural canal is tunnel-like, with a width approximately equal to that of the cotyle.

In posterior view, the condyle is circular in shape and proportionally similar in height and width. The neural arch is extremely depressed to depressed (vaulting ratio = 0.19–0.23). The zygantral area is deep and contains paired endozygantral foramina.

In ventral view, the centrum is triangular and relatively short. Small paired subcentral foramina are visible flanking a well-developed, oblanceolate haemal keel that extends nearly to the condyle. Subcentral grooves are deep and long. Postzygapophyseal articular facets are oval. Synapophyses are clearly divided into diapophyses and parapophyses, with the absence of parapophyseal processes.

In dorsal view, the vertebrae are slightly wider than long. The anterior margin of the zygosphene is straight with two blunt lobes on the lateral sides. The posterior median notch is distinct and deep. Prezygapophyseal articular facets are oval.

In lateral view, both interzygapophyseal and subcentral ridges are straight. The neural spine is high, with the posterior end protruding above its base and the anterior end overhanging. Lateral foramina are present.

Remarks.

The two trunk vertebrae differ from those of Ptyas by lacking epizygapophyseal spines (Ikeda 2007; Nakamura et al. 2013). Additionally, several features, including the straight zygosphenal lip with two small blunt lobes on the lateral sides, exclude their allocation to Coronella, which possesses a crenate zygosphenal lip, and a wide haemal keel (Ivanov et al. 2018). The fossils can also be distinguished from Texasophis and Telescopusby their relatively large size and distinct subcentral ridges (Szyndlar 1987, 2005; Čerňanský et al. 2017).

The fossil vertebrae also closely resemble those of Elaphe and Lycodon, sharing characters including undeveloped parapophyseal process, well-developedprezygapophyseal accessory processes, and oval zygosphenal articular facets (Chen 2020). However, in Elaphe, the trunk vertebrae generally exhibit less prominent subcentral ridges and indistinctly developed subcentral grooves (Ratnikov 2004; Ivanov et al. 2018), which is not identical to the prominent subcentral ridges and deep subcentral grooves in the fossils, although Ratnikov (2022) noted that Elaphe trunk vertebrae show progressive morphological changes along the column, such as deepening of subcentral grooves, more distinct subcentral ridges, reduced haemal keel width, and increased neural spine height posteriorly.

Two extant species of Lycodon, Lycodon ruhstrati (Fischer, 1886) and Lycodon rufozonatus (Cantor, 1842), are found in Taiwan. The vertebrae of L. ruhstrati possess flat and very low neural spines, whereas in L. rufozonatus, these are substantially high as in our fossils (Fig. 3C, D).

Figure 3. 

A, B. Lycodon rufozonatum (ASIZF 0101048–1049) from the Middle Pleistocene Chiting Formation, southern Taiwan; C. Extant specimen of L. rufozonatum (ASIZRE 000007), 94th trunk vertebra, 74.2 cm TL (total length), 55.6 cm SVL (snout–vent length), 102 g. D. Extant specimen of Lycodon ruhstrati (ASIZRE 000005), 31st trunk vertebra, 71.8 cm TL, 53.2 cm SVL, 18 g. A1–D1. Anterior view; A2–D2. Posterior view; A3–D3. Ventral view; A4–D4. Dorsal view; A5–D5. Lateral view. Scale bar: 5 mm.

Ptyas Fitzinger, 1843

Ptyas mucosa (Linnaeus, 1758)

Fig. 4A

Material.

One trunk vertebra (ASIZF 0101050).

Description.

In anterior view, the cotyle appears rounded and flanked by paracotylar foramina. The neural canal is large and circular. The prezygapophyses are well-developed, with the ending portions of their accessory processes prominently directed forward and slightly ventrally inclined. The zygosphene roof is straight, and its articular facets are dorsally tilted.

In posterior view, the condyle is rounded and the neural arch is depressed (vaulting ratio = 0.24), with paired endozygantral foramina visible. In ventral view, the centrum is elongated and triangular in shape. Subcentral foramina are present, and both subcentral grooves and ridges are clearly defined. The haemal keel is well-developed and oblanceolate. Postzygapophyseal articular facets are oval. Epizygapophyseal spines are developed and directed posterolaterally. Diapophyses and parapophyses are clearly visible, though the parapophyseal processes are absent.

In dorsal view, the vertebra is slightly laterally compressed. The anterior margin of the zygosphene is straight with two small, blunt lateral lobes. The posterior median notch is markedly deep. Prezygapophyseal articular facets are oval, and the prezygapophyseal accessory processes are well-developed, pointed, and directed anterolaterally. In lateral view, the interzygapophyseal and subcentral ridges are straight. The neural spine is damaged. The lateral foramina are observed.

Remarks.

The fossil vertebra can confidently be attributed to colubrids or “colubrines”, further supported by several combined features: a well-developed haemalkeel (instead of a hypapophysis), the presence of paracotylar foramina, a pronounced division between the para- and diapophyses, and a moderately high neural spine. Moreover, its overall vertebral morphology closely matches that of Ptyas, which share characteristics such as epizygapophyseal spines, pointed prezygapophyseal accessory processes, and distinct subcentral grooves and ridges (see Ikeda 2007; Shi et al. 2023).

The strongly anteriorly directed prezygapophyseal accessory processes, however, are a distinctive feature seen only in the two extant Ptyas species from Taiwan, Ptyas major (Günther, 1858) and Ptyas mucosa (Linnaeus, 1758) (see Chen 2020). Among these, P. major is a medium-sized species, while P. mucosa notably larger, and the vertebrae of P. major are consistently smaller than those of Ptyas mucosa, as seen in our comparative specimens (Fig. 4B, C).

Figure 4. 

A. Ptyas mucosa (ASIZF 0101050) from the Middle Pleistocene Chiting Formation, southern Taiwan; B. Extant specimen of Ptyas mucosa (ASIZRE 000004), 55th trunk vertebra, 161.8 cm TL (total length), 120.0 cm SVL (snout–vent length), 584 g; C. Extant specimen of Ptyas major (ASIZRE 000006), 101st trunk vertebra, 80.5 cm TL, 51.3 cm SVL, 126 g. A1–C1. Anterior view; A2–C2. Posterior view; A3–C3. Ventral view; A4–C4. Dorsal view; A5–C5. Lateral view. Scale bar: 5 mm.

Ptyas cf. mucosa (Linnaeus, 1758)

Fig. 5

Material.

Two trunk vertebrae (ASIZF 0101051–1052).

Description.

In anterior view, the cotyle is rounded. In specimen ASIZF 0101052, paracotylar foramina are present, located on the lateral margins of the cotyle, whereas in specimen ASIZF 0101051, these foramina are not clearly visible. The prezygapophyses are well-developed, but the prezygapophyseal accessory processes are broken laterally. The zygosphene roof is dorsally convex and wider than the cotyle, with articular facets inclined dorsally. The neural canal is tunnel-like.

Figure 5. 

Ptyas cf. mucosa (ASIZF 0101051–1052) from the Middle Pleistocene Chiting Formation, southern Taiwan. A1, B1. Anterior view; A2, B2. Posterior view; A3, B3. Ventral view; A4, B4. Dorsal view; A5, B5. Lateral view. Scale bar: 5 mm.

In posterior view, the condyle is circular, with height similar to width. The neural arch is depressed (vaulting ratio = 0.22–0.25). The zygantral area is deep and the paired endozygantral foramina are present.

In ventral view, the centrum is distinctly longer than wide and triangular in outline. A well-developed, oblanceolate haemal keel is present instead of a hypapophysis, almost reaching the condyle. Subcentral foramina are not clearly visible. The subcentral grooves are deep and long. Postzygapophyseal articular facets are oval. The paradiapophyses comprise a laterallysalient diapophysis and an almost flat parapophysis.

In dorsal view, the anterior edge of the zygosphene is straight with two small pointed lobes on the lateral sides. The posterior median notch is distinctly deep. Prezygapophyseal articular facets are oval.

In lateral view, the height of the neural spine is indeterminate in specimen ASIZF 0101051 due to poor preservation, whereas it is very high in ASIZF 0101052. Both interzygapophyseal and subcentral ridges are distinct. Lateral foramina are present.

Remarks.

Although the prezygapophyseal accessory processes are only partially preserved, the remaining portions appear to be anteriorly directed. Combined with the large overall size of the vertebrae and the presence of distinct subcentral ridges with deep grooves, these features most closely match Ptyas mucosa, as described above. However, the poor preservation of both specimens—particularly the absence of epizygapophyseal spines, which may have been worn away—precludes a definitive assignment.

Colubridae indet.

Fig. 6A, B

Figure 6. 

A, B. Colubridae indet. (ASIZF 0101046–1047) from the Middle Pleistocene Chiting Formation, southern Taiwan. A1, B1. Anterior view; A2, B2. Posterior view; A3, B3. Ventral view; A4, B4. Dorsal view; A5, B5. Lateral view. Scale bar: 5 mm.

Material. Two caudal vertebrae (ASIZF 0101046–1047).

Description. In anterior view, the cotyle is rounded. Paracotylar foramina are present, located on the lateral margins of the cotyle. The prezygapophyseal accessory processes and pleurapophyses are developed and pointed in specimen ASIZF 0101046, but they are broken laterally in specimen ASIZF 0101047. The zygosphene roof is dorsally convex and about the same width as the cotyle, with articular facets inclined dorsally. The neural canal is tunnel-like.

In posterior view, the condyle is circular, with height similar to width. The neural arch is depressed (vaulting ratio = 0.23). The zygantral area is deep and the paired endozygantral foramina are present. In ventral view, the centrum is distinctly longer than wide. Well-developed haemapophyses are partially present in specimen ASIZF 0101046, almost reaching the condyle. Subcentral foramina are visible. Postzygapophyseal articular facets are oval in ASIZF 0101046.

In dorsal view, the posterior median notch is distinctly deep in specimen ASIZF 0101046. Prezygapophyseal articular facets are oval. In lateral view, the height of the neural spine is indeterminate in ASIZF 0101047 due to poor preservation, whereas it is high in ASIZF 0101046. Both interzygapophyseal and subcentral ridges, and lateral foramina are distinct in ASIZF 0101046.

Remarks. The caudal vertebrae closely resemble those of Elapheas described by Ratnikov (2022), particularly in terms of their elongated morphology from anterior to posterior, and well-developed haemapophyses and pleurapophyses. However, colubrid caudal vertebrae are often not suitable for genus-level determination.

Viperidae Oppel, 1811

Crotalinae Oppel, 1811

Protobothrops Hoge & Romano-Hoge, 1983

Protobothrops sp.

Fig. 7A

Material.

One vertebra (ASIZF 0101053).

Description.

In anterior view, the cotyle is large and circular. The prezygapophyses are dorsally inclined though their prezygapophyseal processes are not preserved. The zygosphene roof is slightly convex, with articular facets inclined dorsally. Paired, deeply set large paracotylar foramina are present on both sides of the cotyle. The parapophyseal processes are broken, but they extended beyond the cotyle.

In posterior view, the neural arch is extremely depressed (vaulting ratio = 0.19). The zygantrum is mediolaterally wide and deep. The condyle is rounded and relatively narrowerthan the neural canal.

In dorsal view, the anterior margin of the zygosphene is straight, with two pointed lateral lobes. Prezygapophyseal articular facets are oval. A deep posterodorsal notch exposes a large portion of the condyle. The neural spine extends longitudinally along the dorsal surface of the neural arch and terminates posterior to the posterior medial notch.

In ventral view, the centrum is triangular. Subcentral foramina are deep and restricted to the anterior part of the centrum. Parapophyses and diapophyses are distinctly observed. The parapophyseal processes project anterolaterally. Postzygapophyseal articular facetsare oval. Subcentral grooves and ridges are distinct.

In lateral view, the neural spine is broken ventrodorsally. The zygosphene articular facets are elliptical. The lateral foramina are deep and situated in the middle part of vertebra. The interzygapophyseal and subcentral ridges run straight anteroposteriorly. The hypophysis originates at the middle part of the centrum and extends beyond the level of the condyle.

Remarks.

The presence of well-developed hypapophysisacross the trunk column in Asian colubriformsindicates potential affinity with Elapidae, Viperidae, Homalopsidae, or Natricidae (originally considered as Natricinae; Szyndlar 1984, 1991b), as this character is considered apomorphic (Zaher 1999). Among these families, the referred specimen resembles more to the family Viperidae, particularly due to the following combination of features: presence of hypapophysis, depressed neural arch, dorsally inclined prezygapophyses and anterolaterally directed parapophyseal processes (Szyndlar and Rage 1999, 2002; Zaher 1999; Zaher et al. 2009, 2019).

Viperidae includes three monophyletic subfamilies: Viperinae (or “true vipers”), Azemiopinae, and Crotalinae (or “pitvipers”), with the latter two forming a sister clade to Viperinae (Wüster et al. 2008; Alencar et al. 2016). This vertebra can be confidently classified within Crotalinae due to its short and wide vertebra, thick hypapophysis that originates near the middle of the centrum, and large cotyle and condyle (Holman 2000), although these can also be found in some large viperines. Furthermore, the fossil vertebra most closely resembles Protobothrops or Trimeresurus rather than Gloydius, based on its undeveloped prezygapophyseal accessory processes, straight zygosphenal margin in the dorsal view; rounded shape cotyle and condyle, and straight subcentral ridge in the lateral view (Ikeda 2007). Among extant taxa, Trimeresurus stejnegeri—a small and common pitviper in Taiwan—is a possible comparison. However, the fossil vertebra is significantly larger than that of a large adult T. stejnegeri (Fig. 7C), suggesting that the specimen more likely represents a larger crotaline Protobothrops.

Figure 7. 

A. Protobothrops sp. (ASIZF 0101053) from the Middle Pleistocene Chiting Formation, southern Taiwan; B. Extant specimen of Protobothrops mucrosquamatus (ASIZRE 000002), 108th trunk vertebra, 91.0 cm TL (total length), 72.0 cm SVL (snout–vent length), 126 g; C. Extant specimen of Trimeresurus stejnegeri (ASIZRE 000003), 79th trunk vertebra, 63.1 cm TL, 52.2 cm SVL, 56 g. A1–C1. Anterior view; A2–C2. Posterior view; A3–C3. Ventral view; A4–C4. Dorsal view; A5–C5. Lateral view. Scale bar: 5 mm.

Within Protobothrops, the fossil is most comparable to P. mucrosquamatus, the only extant species of this genus occurring in Taiwan. However, the prezygapophyseal accessory processes in the fossil are more pointed and anteriorly directed, whereas in P. mucrosquamatus, the same processes are broader and more laterally expanded (Fig. 7B). Therefore, we conservatively retain the identification at the genus level.

Alethinophidia indet.

Fig. 8

Figure 8. 

Alethinophidia indet. (ASIZF 0101054–1059) from the Middle Pleistocene Chiting Formation, southern Taiwan. A1–F1. Anterior view; A2–F2. Posterior view; A3–F3. Ventral view; A4–F4. Dorsal view; A5–F5. Lateral view. Scale bar: 5 mm.

Materials. Six trunk vertebrae (ASIZF 0101054–1059).

Description. All the specimens referred here are fragmentary, but they share a few common characteristics such as the absence of hypapophyses, rounded cotyles and condyles, a large neural canal, a dorsally convex zygosphene, and distinct subcentral ridges and grooves. Prezygapophyses are preserved in a few specimens (e.g., Fig. 8A), displaying oval shape that extend laterally.

In specimens ASIZF 0101058 and 1059 (Fig. 8E, F), endozygantral foramina can be observed within the zygantrum. In ventral view, the haemal keel is well-developed, oblanceolate in shape, and subcentral foramina are observable in specimens ASIZF 0101055, 1056, and 1058 (Fig. 8B, C, E). The neural spine is damaged anteroposteriorly in all specimens. In lateral view, the interzygapophyseal and subcentral ridges are straight, and lateral foramina are present.

Remarks. The absence of hypapophysis indicates that these are mid- or posterior trunk vertebrae. However, due to the fragmentary nature of the specimens and the lack of genus-level diagnostic characters, more precise identification is not possible. However, based on the preserved features, all specimens can be attributed to Alethinophidia indet.

Discussion

Taxonomic composition and palaeoecology

The herpeto-assemblage recovered from the Middle Pleistocene deposits of southern Taiwan represents the first documented record of fossil amphibian and snakes from the region. These findings provide valuable opportunities to enhance our understanding of historical diversity and biogeographic patterns of Taiwanese herpetofauna. All the specimens discussed are fragmentary, making identification challenging, but a few can be confidently assigned to the generic or lower level. The amphibian is identified here as Duttaphrynus melanostictus, and snakes as Lycodon rufozonatum, Ptyas mucosa, Ptyas cf. mucosa, and Protobothrops sp. Additional isolated snake vertebrae are assigned to Colubridae indet and Alethinophidia indet. due to the lack of diagnostic features. Despite the relatively low taxonomic diversity, the significance of this assemblage lies in its status as the first fossil herpeto-assemblage from the subtropical East Asia.

The single Duttaphrynus melanostictus vertebra represents remarkable and noteworthy finding, especially given the low preservation potential of delicate skeletal elements in tectonically active and subtropical fluvial environments. In our collection, colubrid snakes are the most abundant group. Large, widespread species such as Ptyas mucosa occur in a range of habitats up to approximately 2,000 m in elevation across Taiwan (Shang et al. 2009), and their fossil presence is therefore not surprising. The only viperid vertebra recovered in this study is assigned to the genus Protobothrops, a medium- to large-sized snake distributed throughout Taiwan and adjacent parts of Southeast Asia; it is commonly known as the brown-spotted pit viper or Taiwanese habu (Shang et al. 2009).

The distribution of amphibians and squamates is strictly tied to environmental conditions such as temperature and precipitation, owing to their ectothermic physiology (Antúnez et al. 1988; Currie 1991; Rage and Roček 2003). Therefore, these vertebrates are commonly used as reliable indicators of past climatic conditions (e.g., Bohme 2003; Bailon and Blain 2007; Blain et al. 2013; Georgalis et al. 2019). The presence of fossil herpetofauna in the Pleistocene deposits of Taiwan is particularly valuable for palaeoenvironmental reconstructions, as many Pleistocene species belong to extant lineages with well-documented ecological preferences (Blain et al. 2008).

The co-occurrence of amphibian (D. melanostictus) and squamate fossils—including colubrids and a viperid—suggests the existence of fluvial and open terrestrial habitats. This general palaeoenvironmental reconstruction agrees well with previous findings of terrestrial vertebrate fossils and depositional environment (Chen et al. 2011; Chen 2016). The presence of D. melanostictus reflects freshwater or semi-aquatic environments nearby, while the occurrences of Lycodon and Ptyas point to moist, open woodlands or ecotones near water bodies (Kuntz 1963; Lue et al. 2002). The viperid Protobothrops further supports a mosaic landscape, as extant members of this genus occupy a wide range of environments, from dense forests to open habitats. Taken together, the composition of the herpetofaunal assemblage indicates a landscape with abundant water bodies that supported humid and warm climatic conditions, likely interspersed with riparian corridors and forested terrain (Fig. 9).

Figure 9. 

Reconstruction of Middle Pleistocene herpetofauna in southern Taiwan. The illustration highlights key taxa, including Duttaphrynus melanostictus, Lycodon rufozonatum, Ptyas mucosa, and Protobothrops. Note that the fossils were recovered from multiple localities within the formation and may not represent a strictly coeval assemblage; the reconstruction should therefore be viewed as a generalized palaeoenvironmental depiction. Artwork by Yun-Kae Kiang.

Biogeographical implications

Across the East Asian island arc, comparative genetic works have outlined a mixed biogeographic history, in which both repeated oversea dispersal and vicariance driven by sea-level changes have shaped modern reptile lineages. For example, in grass lizards (Takydromus), sea-level oscillations repeatedly separated and reconnected island groups, generating sequential isolation and divergence consistent with multiple colonization waves, leaving basal species on the most remote islands (Lin et al. 2002). By contrast, analysis of the Okinawa tree lizard (Japalura polygonata) supports a south-to-north stepping-stone expansion along the Kuroshio Current, providing clear evidence that oversea dispersal played a major role on these remote islands (Yang et al. 2019). At a broader scale, a synthesis across multiple endemic species traced most colonization events to the Early Pliocene (~5 Ma) and inferred contributions from multiple source regions.

Within this framework, Taiwan’s herpetofauna comprises elements of both the Indomalayan and Palearctic realms, reflecting its position at the biogeographic junction of these regions and at the tectonic boundary between the Eurasian and Philippine Sea plates (Ota 1998, 2000). Our Middle Pleistocene assemblage from the Chochen–Tsailiao area (ca. 0.8–0.4 Ma) documents widespread, low-elevation lineages (Duttaphrynus, Lycodon, Ptyas, and Protobothrops) that are still common today. Their fossil occurrence agrees with genetic hypotheses that emphasize the persistence of broadly distributed Oriental taxa through repeated sea-level oscillations. The absence of more northerly Palearctic or southerly tropical lineages in the assemblage may reflect the regional setting and sampling limits.

Taken together, comparative genetic syntheses and our new fossil evidence converge on a scenario of a multi-source, multi-pulse assembly of Taiwan’s herpetofauna, followed by lineage persistence and insular isolation through Pleistocene climatic cycles. Importantly, this assemblage provides the first direct fossil support for these genetic models and highlights the continuity of widespread Oriental lineages in Taiwan. Expanded surveys across additional regions in Taiwan and adjacent areas will be essential to further clarify the composition of Quaternary herpetofauna on the island, which occupies a biogeographic crossroads between continental Asia and the more distant Ryukyu Islands.

Acknowledgements

We thank Liang-Chieh Wang (Tainan City Zuojhen Fossil Park, Tainan, Taiwan) for providing the fossil specimens. We are grateful to Lai-En Lee (National Chiayi University), Hsi Yu (Biodiversity Research Center, Academia Sinica, Taipei, Taiwan, BRCAS), and Ching-Feng Lin (BRCAS) for providing extant comparative specimens, and Yun-Kae Kiang for the artwork. Hsin-Wei Liu (BRCAS), Chieh-Hsuan Lee (BRCAS), Siao-Man Wu (BRCAS), and Yu-Cheng Chen are thanked for preparing the figures and comparative specimens. We thank editor Johannes Müller and reviewers Georgios Georgalis and Hugues-Alenandre Blain for their constructive comments and reviews. This study was supported by the National Science and Technology Council, Taiwan (Grant No. 112-2116-M-001-017-MY3) and Academia Sinica, Taipei, Taiwan to C.-H.L.

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Supplementary materials

Supplementary material 1 

Basic anatomical views and terminology of a generalized toad vertebra

Chien-Hsiang Lin, Si-Min Lin, Chi-Wei Chien, Te-En Lin, Haroon Nazir, Ningthoujam Premjit Singh

Data type: tif

Explanation note: A. anterior view; B. posterior view; C. ventral view; D. dorsal view; E. lateral view. Abbreviations: c – centrum; cd – condyle; ct– cotyle; na– neural arch; nc– neural canal; ns – neural spine; po –postzygapophysis; pr–prezygapophysis; tp– transverse process.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
Download file (345.20 kb)
Supplementary material 2 

Basic anatomical views and terminology of a generalized snake vertebra (after Ikeda 2007)

Chien-Hsiang Lin, Si-Min Lin, Chi-Wei Chien, Te-En Lin, Haroon Nazir, Ningthoujam Premjit Singh

Data type: png

Explanation note: A. anterior view of trunk vertebra; B. posterior view of trunk vertebra; C. anterior view of caudal vertebra; D. ventral view of trunk vertebra; E. dorsal view of trunk vertebra; F.–G. lateral view of trunk vertebrae. Abbreviations: cd – condyle; ct– cotyle; d – diapophysis; ef–endozygntral foramen; es –epizygapophyseal spine; he –haemapophysis; hk–haemal keel; hyp–hypapophysis; lf– lateral foramen; na– neural arch; nc– neural canal; ns – neural spine; p –parapophysis; pf – paracotylar foramen; pl –pleurapophysis; pmn– posterior medial notch of the neural arch; po –postzygapophysis; pof–postzygapophyseal articular facet; pr–prezygapophysis; prf–prezygapophyseal articular facet; scf– subcentral foramen; scg– subcentral groove; scr– subcentral ridge; z –zygosphene.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
Download file (707.66 kb)
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