Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies

Basal synapsids (‘pelycosaurs’) form the basalmost portion of the mammalian stem lineage and document the transition from primitive ‘reptile-like’ basal amniotes to derived, mammal-like therapsids. They dominated terrestrial ecosystems of the latest Carboniferous and Early Permian (∼300–271 million years ago), producing large-bodied terrestrial animals (3–6.5 metres long), high-fibre herbivores, and macropredators for the first time in vertebrate history, alongside an array of smaller-bodied forms. Despite numerous recent discoveries and reassessments of fossils collected over the past 250 years, and despite their importance for understanding the early diversification of terrestrial vertebrates, a comprehensive assessment of global relationships among basal synapsids has not been undertaken. A new phylogenetic dataset comprising 45 taxa (plus four outgroups and four therapsids) and 239 characters (147 cranial; 92 postcranial) reveals considerable uncertainty in the relationships of high...