A Comparative Analysis of Vertebral Morphology of Middle to Late Miocene Squamates from the Siwaliks of India: Paleoenvironmental Implications

The Siwalik strata of the Indian subcontinent are renowned for their rich vertebrate fossil assemblages, particularly mammals, spanning approximately 18 to 0.22 million years ago. However, squamate reptiles well known for their diverse morphology and ecological adaptations, remain underexplored in this region, reflecting a significant gap in our knowledge of Miocene herpetofauna. A comprehensive study of Miocene squamate vertebral morphology in India has yet to be undertaken. This research examined the vertebral morphology of squamate fossil remains from Siwalik localities, including the Middle Miocene Ramnagar (Jammu) and Late Miocene Haritalyangar (Himachal Pradesh), for taxonomic identification and ecological insights through comparisons with extant taxa. The collected specimens were analysed using a micro-CT and a Stereozoom microscope (10X). Identification and examination were conducted based on key morphological characteristics, including overall size and shape, cotyle and condyle dimensions, the presence or absence of zygosphene and zygantrum, the number and arrangement of ventral keels, the presence of foramina, the shape of the neural canal, and the occurrence of hypapophyses. The examined samples reveal the presence of various taxa, including Acrochordus, Anguimorpha, Colubridae, Constrictores, Python, and Varanus. Notably, the presence of fully aquatic piscivore Acrochordus from Late Miocene Haritalyangar (10–8.5 Ma), based on a single probably surface-collected specimen, appears to be artifactual and requires further investigation. However, the high diversity of thermophilic lizards and snakes dominated by Python and colubrids during the Miocene period indicates elevated mean annual temperatures. The coexistence of terrestrial and semi-aquatic squamates suggests a seasonally wet, sub-humid to semi-arid climate.