Evidence for clinging arboreality in a Middle Jurassic stem lepidosaur

Lepidosaurs are an ecologically diverse and speciose group with more than 11 000 living species (squamates and the tuatara). Stem lepidosaurs are known from the Early Triassic onwards, but primarily from very incomplete specimens. Therefore, we have little information on their ecological diversity or the ecological context of deep evolutionary divergences of Lepidosauria. <i>Marmoretta oxoniensis</i>, from the Middle Jurassic of the UK, is one of the most completely known candidate stem lepidosaurs. Previous studies proposed that it may have been semi-aquatic, based primarily on its abundance in marginal marine rocks. We show here that <i>Marmoretta</i> was adapted for climbing, based on the post-cranial anatomy of a partial skeleton, visualized using micro-computed tomography (µCT)-in particular, the steep angles of thoracic zygapophyses, ungual phalanx morphology and elongate penultimate manual phalanges that curve distoventrally along their lengths. Linear discriminant analysis of the partial hand, using a training dataset of hand skeleton measurements and habitat use in extant squamates, returns strong evidence for clinging arboreality and <i>Marmoretta</i> clusters among scansorial/arboreal iguanians in manus shape space. Evidence of arboreality in <i>Marmoretta</i> provides the first information about habitat use in a probable stem lepidosaur and illuminates the vertical structure of ecological communities of the mid-Mesozoic.