The conventional wisdom that dinosaurs were entirely diurnal, yielding the night to tiny contemporaneous mammals, appears to be incorrect. A new study by Lars Schmitz and Ryosuke Motani from the University of California, Davis suggests that many dinosaurs were active at night.
First, let’s define a few terms. Diurnal animals are active during the day, nocturnal animals during the night, and cathemeral animals all the time. By the way, although this study didn't address whether any animals were active at twilight (dawn or dusk), such a creature would be 'crepuscular'. Also, dinosaurs, birds and lizards all have a bony ‘scleral ring’ around their eye sockets that can be measured in both living creatures and fossils. The scleral rings of diurnal animals are small, cathemeral animals moderate, and nocturnal animals large.
The researchers measured the scleral rings of 164 living animals with known habits to see whether their predictions about those animals’ lifestyles matched up to reality. Upon finding that they could successfully use the scleral ring data to predict when those animals were active, they applied the technique to 33 fossils.
According to this new data, small carnivorous dinosaurs were active at night, whereas large herbivorous dinosaurs were active all the time. No doubt feeding those huge bodies required a cathemeral lifestyle. Flying reptiles like pterosaurs were mostly diurnal. These results mesh well with the habits of extant animals filling similar ecological niches today.
Close-up of the eye socket and ring of the dinosaur Protoceratops, active by day and night.
Credit: Ryosuke Motani and Lars SchmitzYou can see a gallery of some of the specimens under study here.