 A nocturnal lifestyle has been suggested for the primitive ceratopsian Protoceratops . However , comparisons between the scleral rings of Protoceratops and Psittacosaurus and modern birds and reptiles indicate that they may have been cathemeral , active throughout the day at short intervals . 
 A nocturnal lifestyle <a href='been_done' title='1'><mark>has</mark></a> been suggested for the primitive ceratopsian Protoceratops . However , comparisons between the scleral rings of Protoceratops and Psittacosaurus and modern birds and reptiles indicate that they may have been cathemeral , active throughout the day at short intervals . 
A nocturnal lifestyle done been suggested fa the primitive ceratopsian Protoceratops . However , comparisons between the scleral rings of Protoceratops and Psittacosaurus and modern birds and reptiles indicate that they may have been cathemeral , active throughout the day at short intervals .