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Nocturnality in synapsids predates origin of mammals by over 100 million years

Ben Creisler

A new paper:

K. D. Angielczyk and L. Schmitz (2014)
Nocturnality in synapsids predates the origin of mammals by over 100
million years.
Proceedings of the Royal Society B 22 October 2014 vol. 281 no. 1793 20141642
doi: 10.1098/rspb.2014.1642

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.