title: Nocturnality in synapsids predates the origin of mammals by over 100 million years.
abstract: Nocturnality is widespread among extant mammals and often considered the ancestral behavioural pattern for all mammals. However, mammals are nested within a larger clade, Synapsida, and non-mammalian synapsids comprise a rich phylogenetic, morphological and ecological diversity. Even though non-mammalian synapsids potentially could elucidate the early evolution of diel activity patterns and enrich the understanding of synapsid palaeobiology, data on their diel activity are currently unavailable. Using scleral ring and orbit dimensions, we demonstrate that nocturnal activity was not an innovation unique to mammals but a character that appeared much earlier in synapsid history, possibly several times independently. The 24 Carboniferous to Jurassic non-mammalian synapsid species in our sample featured eye morphologies consistent with all major diel activity patterns, with examples of nocturnality as old as the Late Carboniferous (ca 300 Ma). Carnivores such as Sphenacodon ferox and Dimetrodon milleri, but also the herbivorous cynodont Tritylodon longaevus were likely nocturnal, whereas most of the anomodont herbivores are reconstructed as diurnal. Recognizing the complexity of diel activity patterns in non-mammalian synapsids is an important step towards a more nuanced picture of the evolutionary history of behaviour in the synapsid clade.
authors.affiliation: Integrative Research Center, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, IL 60605, USA.
authors.forename: K D
authors.initials: KD
authors.lastname: Angielczyk
authors.affiliation: W. M. Keck Science Department, Claremont McKenna, Pitzer, and Scripps Colleges, 925 North Mills Avenue, Claremont, CA 91711, USA lschmitz@kecksci.claremont.edu.
authors.forename: L
authors.initials: L
authors.lastname: Schmitz
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002940:Circadian Rhythm; D005580:Fossils / Q000033:anatomy & histology*; D008322:Mammals / Q000033:anatomy & histology* / Q000502:physiology*; D009915:Orbit / Q000033:anatomy & histology*
keywords: Synapsida; ancestral state reconstruction; diel activity pattern; phylogenetic flexible discriminant analysis; scleral ossicles