Middle Pleistocene fossil frog and snake assemblage from southern Taiwan identifies four genera amid limited resolution

A palaeontological study documents the first fossil amphibian and snake assemblage from Taiwan, based on isolated vertebrae recovered from Middle Pleistocene deposits in the Chochen–Tsailiao area of Tainan, southern Taiwan. The assemblage is taxonomically limited but includes representatives of four genera and provides direct evidence that several lineages now present on the island occurred there during the Middle Pleistocene.

The researchers identified one frog trunk vertebra as Duttaphrynus melanostictus and assigned snake vertebrae to Lycodon rufozonatum, Ptyas mucosa, Ptyas cf. mucosa and Protobothrops sp. Six additional fragmentary snake vertebrae were assigned to Alethinophidia indeterminate, while two caudal vertebrae were identified only as Colubridae indeterminate. Because the material consists largely of isolated and incomplete vertebrae, several assignments remain conservative.

Taiwan occupies a transitional zone between the Palearctic and Indomalayan biogeographic regions. Its mountainous topography, latitudinal position and varied climate have contributed to high levels of endemism among terrestrial vertebrates. Excluding marine species, the island supports 37 native amphibians and 85 reptiles, including 14 endemic frogs, 18 endemic lizards and 14 endemic snakes.

Before this assemblage, the absence of Taiwanese herpetofaunal fossils left it unclear whether modern diversity and endemism reflected recent dispersal, long-standing lineage persistence across glacial and interglacial cycles, or a combination of these processes. The new specimens offer direct evidence for the former presence of widely distributed lineages, but they do not by themselves establish the complete history of colonization or persistence.

The fossils were collected from reworked surface deposits in the bed of the Tsailiao River. Most fossils in the area lack precise stratigraphic positions because seasonal flooding transports and mixes them, although many are generally attributed to the nearby Chiting Formation. The formation is approximately 2,000 metres thick and consists mainly of interbedded sandstones, siltstones and mudstones deposited through marine transgression–regression cycles with substantial fluvial and terrestrial influence.

Fossils from the Kuoling and Takengwei members are dated to approximately 0.7–0.4 million years ago through nannobiostratigraphic analyses and cyclostratigraphic estimates. In view of the uncertain provenance of the herpetofaunal specimens, the researchers conservatively used the Chiting Formation’s broader age range of approximately 0.8–0.4 million years. The material was collected over three decades by Liang-Chieh Wang and recovered through simple sieving and manual sorting.

Taxonomic identification relied on morphological comparisons with published descriptions, fossil material and reference vertebrae from extant animals housed at the Biodiversity Research Museum, Academia Sinica, Taiwan. Specimens were photographed with a stereomicroscope and digital camera, and the images were focus-stacked and digitally optimised. Measurements were taken where preservation allowed, and the researchers also quantified vertebral vaulting. All examined fossils are housed at the museum under the registration code ASIZF.

The amphibian specimen was a trunk vertebra longer than 4.5 millimetres. Its flattened, dorsoventrally compressed centrum supported identification as Duttaphrynus melanostictus rather than Bufo bankorensis, whose centrum is more elevated in lateral view. Among the snakes, two trunk vertebrae were identified as Lycodon rufozonatum, including characters such as tall neural spines, deep subcentral grooves, distinct subcentral ridges and a depressed neural arch. One vertebra was confidently assigned to Ptyas mucosa, while two additional specimens were identified as Ptyas cf. mucosa; the latter assignment remained tentative because poor preservation, including the possible loss of epizygapophyseal spines, prevented a definitive determination.

One vertebra was classified as a crotaline viperid and compared most closely with Protobothrops or Trimeresurus rather than Gloydius. Its short, wide form, thick hypapophysis arising near the middle of the centrum, and large cotyle and condyle supported placement within Crotalinae. The fossil was larger than a comparative adult Trimeresurus stejnegeri vertebra and differed from the only living Protobothrops species in Taiwan in the form of its prezygapophyseal accessory processes, so the researchers retained the conservative genus-level identification of Protobothrops sp.

The co-occurrence of Duttaphrynus, colubrids and a viperid was interpreted as consistent with a warm, humid landscape containing water bodies, riparian corridors, moist open woodland and forested terrain. The frog was associated with nearby freshwater or semi-aquatic environments; Lycodon and Ptyas with moist open woodland or ecotones near water; and Protobothrops with a varied landscape mosaic. This reconstruction agrees with earlier evidence from terrestrial vertebrate fossils and the formation’s depositional setting, although the specimens may not represent a strictly coeval community.

The assemblage documents low-elevation lineages that remain present in Taiwan, including Duttaphrynus, Lycodon, Ptyas and Protobothrops. Its occurrence is consistent with genetic hypotheses concerning the persistence of broadly distributed Oriental taxa through East Asian sea-level oscillations, but it does not establish that each lineage persisted continuously on the island. The absence of more northerly Palearctic or southerly tropical lineages may reflect the assemblage’s regional setting and limited sampling. The authors state that expanded surveys across Taiwan and adjacent areas will be essential for clarifying the composition of Quaternary herpetofauna and testing the island’s multi-source, multi-pulse biogeographic history.

Subject of Research: Middle Pleistocene amphibian and snake fossils from southern Taiwan

Article Title: First fossil frog and snake assemblage from southern Taiwan: a window into Pleistocene herpetofauna and palaeoenvironments in subtropical East Asia

Article References: Lin, C.-H., Lin, S.-M., Chien, C.-W., Lin, T.-E., Nazir, H., & Singh, N. P. (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

Image Credits: AI Generated

DOI: 10.3897/fr.28.164568

Keywords: Taiwan palaeontology, Middle Pleistocene, fossil herpetofauna, frog fossils, snake fossils, Chiting Formation, Tainan, palaeoenvironment, vertebrate biogeography, First, fossil, frog