A re-examination of lissamphibian origins: character construction and probabilistic inference of the largest early tetrapod dataset

Abstract

The phylogenetic relationships among early tetrapods (modern amphibians and their extinct relatives) remain ambiguous due to an absence of extant lineages and the resulting need to infer relationships solely using morphological data. One of the critical questions surrounding early tetrapods is the phylogenetic origin of modern amphibians, Lissamphibia (frogs, salamanders, and caecilians) and Amniota (reptiles, mammals, and birds), from this early stock of tetrapods. Three main hypotheses have been proposed to explain the origins of lissamphibians: the “temnospondyl hypothesis,” the “lepospondyl hypothesis,” and the “polyphyly hypothesis,” that different clades within lissamphibians originated from different groups, some from temnospondyls and some from lepospondyls. A phylogenetic dataset by Ruta and Coates (2007) is the largest morphological database produced to study determine lissamphibian origins, yet it has faced criticism for a combination of issues related to accuracy, including limitations in taxa sampled, algorithms employed, and errors in data input. Marjanović and Laurin (2019) sought to revise and improve this database, such as through thorough correction of data scoring errors, but with limited success. The present research builds on the work of Marjanović and Laurin (2019), particularly through revision of erroneous character constructions using the dataset construction framework proposed by Simões (2017). Analyses were completed on both the original dataset by Marjanović and Laurin (2019) and the updated dataset using maximum likelihood and Bayesian inference. The 277 original characters used by Marjanović and Laurin (2019) were reduced to 163 characters post-revision. Despite some discrepancies between analyses caused by differences in data and methods used, all results point to lepospondyl ancestry of Lissamphibia. All analyses place temnospondyls as a sister clade to lepospondyls, with one exception placing lepospondyls within the temnospondyl clade. Additionally, varying support for different possible outgroups to Lissamphibia are discussed, as well as differences in exact placement of groups often associated with crown lissamphibians, such as Aïstopoda and Eocaecilia.