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 behavioral pattern for all mammals. However, mammals are nested within a larger clade, Synapsida, and nonmammalian synapsids comprise a rich phylogenetic, morphological, and ecological diversity. Even though nonmammalian synapsids could potentially elucidate the early evolution of diel activity patterns and enrich the understanding of synapsid paleobiology, 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 nonmammalian 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 nonmammalian synapsids is an important step towards a more nuanced picture of the evolutionary history of this behavior in the synapsid clade.

夜行性在现存哺乳动物中分布广泛，且常被视作所有哺乳动物的祖先行为模式。不过，哺乳动物隶属于一个更大的演化支——合弓纲（Synapsida），而非哺乳类合弓类拥有丰富的系统发育、形态学与生态学多样性。尽管非哺乳类合弓类或可阐明昼夜活动模式的早期演化历程，深化我们对合弓类古生物学的认知，但目前关于其昼夜活动的相关数据仍较为匮乏。
本研究借助巩膜环（scleral ring）与眼眶尺寸数据，证实夜行性并非哺乳动物独有的演化创新，而是在合弓类演化历史中出现更早的特征，甚至可能独立起源过数次。本研究样本中的24种石炭纪至侏罗纪非哺乳类合弓类物种，其眼部形态与所有主要昼夜活动类型均相符，其中晚石炭世（约300 Ma）的类群便已存在夜行性特征。
诸如惧狭盘龙（Sphenacodon ferox）、米勒异齿龙（Dimetrodon milleri）等食肉类群，以及植食性犬齿兽类长齿三瘤齿兽（Tritylodon longaevus），均大概率为夜行性动物；而多数异齿亚目植食类群则被复原为昼行性动物。
认识到非哺乳类合弓类昼夜活动模式的复杂性，是我们更细致地勾勒合弓纲演化支中该行为演化历史的重要一步。