Evolution, Anatomy, Biology and Classification of Non-Amniotic Tetrapoda: A Comprehensive Review with Emphasis on Lissamphibia

The transition from aquatic to terrestrial life represents one of the most significant evolutionary events in vertebrate history, fundamentally reshaping the trajectory of life on Earth. Non-amniotic tetrapods, comprising both extinct Palaeozoic forms and extant amphibians (Lissamphibia), provide crucial insights into this pivotal transformation that occurred approximately 390 million years ago during the Middle Devonian period. This comprehensive review examines the evolution, anatomy, biology, and classification of non-amniotic tetrapods, with particular emphasis on Lissamphibia as the sole surviving representatives of this ancient lineage. Through integration of paleontological, morphological, and molecular evidence, we explore thephylogenetic relationships within Tetrapoda, focusing on the contentious origins of modern amphibians and their evolutionary trajectory from early tetrapodomorph ancestors. The anatomical innovations that facilitated terrestrial colonisation, including respiratory adaptations, locomotor modifications, and sensory system evolution, are examined in detail. Contemporary Lissamphibia, encompassing Anura (frogs), Caudata (salamanders), and Gymnophiona (caecilians), exhibit remarkable morphological diversity whilst retaining fundamental characteristics that link them to their Palaeozoic predecessors. Recent phylogenetic analyses increasingly support a monophyletic origin of Lissamphibia within Temnospondyli, specifically among dissorophoid amphibians, challenging earlier hypotheses of polyphyletic origins. The evolutionary success of non-amniotic tetrapods, despite their physiological constraints and dependence on aquatic environments for reproduction, demonstrates the remarkable adaptability of vertebrate body plans and provides essential context for understanding the subsequent evolution of amniotic vertebrates.