A new stem saurian reptile from the late Permian of South Africa and insights into saurian evolution

Abstract The evolutionary radiation of diapsid reptiles that includes all extant and most extinct reptiles is well-represented in the Mesozoic and Cenozoic fossil records, however, the earliest stages recorded in the Paleozoic Era are limited to comparatively few taxa. Consequently, the origins of Sauria, the crown-group of Diapsida, remains poorly understood and the phylogenetic positions of the few known taxa along the saurian stem are controversial. Here, we describe Akkedops bremneri sp. et gen. nov., a new early late Permian stem saurian from the Karoo of South Africa based on two skulls and show that the famous aggregation of "juvenile Youngina” SAM-PK-K7710 is also referrable to it, thereby making this one of the best-known stem saurians. The skull has a short rostrum, open lower temporal bar, large contribution of the postfrontal to the upper temporal fenestra, slender stapes, sliver-like supratemporal with a distinct lateral flange suturing to the postorbital, and lacks both postparietal and tabular bones. The saddle-shaped quadrate is rather saurian-like in being posteriorly emarginated with a tympanic crest and unique medial flange. The post cranial skeleton of Akkedops bremneri is lizard-like and notably shows a hook-shaped fifth metatarsal and thyroid fenestra. Phylogenetic analysis recovers Akkedops bremneri as sister to Sauria, which is especially surprising considering its rather small size and slender, lizard-like morphology prior to the split between the apparently similar lepidosauromorphs and many of the comparatively robust archosauromorph saurians. Our analysis also indicates that Youngina capensis falls outside the clade of Akkedops bremenri + Sauria and does not appear to form a clade with other “younginiform” reptiles. The available evidence indicates a surprising level of complexity related to the evolution of stem saurians and the origin of Sauria that occurred in the shadow of other Paleozoic amniotes.