The Origin of Tetrapods : Reevaluating the Early Tetrapod Phylogeny

Abstract

Lissamphibians (modern salamanders, frogs, and caecilians) represent a living link between aquatic and terrestrial life, and clarifying their ancestry has implications not only for evolutionary biology but for conservation, climate resilience, and developmental biology. Yet our ability to trace this lineage with confidence is deeply shaped by data gaps, methodological inconsistencies, and analytical limitations. Fossil and ichnological records paint a conflicting timeline of tetrapod emergence, while disputes over the monophyletic versus polyphyletic origins of modern amphibians remain unresolved. Meanwhile, phylogenetic models continue to suffer from sampling biases, unresolved character dependencies, and incomplete or uneven datasets, each of which undermines the reliability of phylogenetic inference. Recent work has demonstrated that addressing these issues requires extensive revision and logical restructuring of existing morphological datasets. Building upon a recently revised early tetrapod morphological matrix, this study expands taxonomic sampling by scoring eight Devonian and Carboniferous taxa, in a curated 163-character dataset and analyzing them with maximum parsimony, filling an important chronological gap in taxon sampling. The updated matrix infers lissamphibians as the sister-group to lepospondyls, providing weak-to-moderate provisional support for the lepospondyl hypothesis for lissamphibians origin, consistent with Marjanović and Laurin (2019). All eight newly scored taxa were placed in the early-diverging stem tetrapod grade, though their phylogenetic positions did not strictly correspond to their stratigraphic ages. More broadly, these findings underscore the sensitivity of lissamphibian placement to dataset quality and taxon completeness and reinforce the need for continued methodological rigor in reconstructing early tetrapod evolutionary history.