Data from: Nocturnality in Synapsids predates the origin of mammals by over 100 million years

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.