As dramatic fossils go, it’s hard to beat the Mongolian fighting dinosaurs – a Velociraptor and a Protoceratops locked in mortal combat. The Protoceratops, an early horned dinosaur, has the raptor’s arm in its mouth, and the raptor appears to be kicking its prey in the neck. The two combatants were killed in this pose, around 75 million years ago. And according to a new study, they probably met and died sometime around dawn or dusk.
Most dinosaur reconstructions portray the animals walking about in bright sunlight but of course, we know that living animals are active at all times of the day. The diurnal ones prefer the daylight hours, while nocturnal species haunt the night. Crepuscular animals favour twilight hours, while cathemeral ones are active in short bursts throughout the day.
It’s easy enough to work out which group a living animal falls into, but the task becomes far more difficult if the animal in question is extinct. With the exception of tracks, burrows or other trace fossils, behaviour doesn’t fossilise easily. But Lars Schmitz and Ryosuke Motani have developed a clever way of working out when dinosaurs were active, using something we have in abundance – their skulls.
The eyes of all birds and many reptiles are reinforced by a bony disc called a scleral ring (which you can clearly see in my photo of Deinonychus above). The form of these rings closely follows their function. In nocturnal animals, the ring has a wide hole to let in as much light as possible. In diurnal species, the ring is thicker and has a narrower hole. That gives them sharper vision (think about how your focus gets better when you squint) without overloading the retina. Crepuscular or cathemeral species have rings that are somewhere in between, but they also tend to have larger-than-average eyes for their body size.
Schmitz and Motani developed a new model that takes the size of an animal’s scleral ring and eye socket, and works out when it would have been active. The model also accounts for the evolutionary relationships between different species. The duo tested it using data from 164 living animals, and found that it could accurately predict their daily habits. Next, they used it to analyse the skulls of 23 dinosaurs, as well as 10 other extinct reptiles, including eight of the flying pterosaurs.
They found that all the plant-eating dinosaurs in their sample, such as Diplodocus and Protoceratops were cathemeral or crepuscular, with only one truly diurnal species. Schmitz and Motani think that this pattern was probably driven by their size and diet. Even today, many large plant-eaters need to forage for significant chunks of the day to get enough food, while avoiding the hottest part of the day to avoid overheating. To keep cool while getting enough to eat, their lives bridge both night and day.
Most of the flyers in their sample were diurnal, including Archaeopteryx and three other extinct birds, as well as three pterosaurs (although four of the pterosaurs were nocturnal, including the famous Rhamphorhynchus). Meanwhile, the predatory dinosaurs in their sample, including Velociraptor and Microraptor, were mostly nocturnal, with a few cathemeral species. Again, modern animals share the same pattern. Most meat-eating mammals, for example, are nocturnal.
These new results fit with those of previous studies. For example, Martin Kundrát and Jiří Janáček speculated that the small predator Conchoraptor might have been a nocturnal hunter, based on brain structures that suggested excellent hearing. Meanwhile, based on the eye sockets of various meat-eating dinosaurs, Daniel Chure suggested that the small ones were nocturnal while the large ones hunted during daylight. Based on Schmitz and Motani’s study, Chure was at least half-right. Unfortunately, no one has yet been able to study the scleral rings of the large tyrannosaurus-sized hunters.
Schmitz and Motani say that their study goes against a commonly held assumption that dinosaurs generally walked around during the day, while the mammals scurried about under cover of darkness. Clearly, that’s not the case – the dinosaurs had the entire 24-hour cycle covered.
Reference: Schmitz & Motani. 2011. Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology. Science http://dx.doi.org/10.1126/science.1200043
More on dinosaurs:
- Dinosaurs grew fast, had teen pregnancies and died young
- Dramatic restructuring of dinosaur feathers revealed by two youngsters of same species
- Zombie hands to bird wings – the evolution of the dinosaur wrist
- Old wounds show that Triceratops used its horns for combat
- Dinosaur daddies took care of their young alone
April 14th, 2011 at 2:13 pm
Well, how cool is that? Sclerotic rings must have a better preservation rate than I thought! Very cool study (I’d be forever in your debt for a copy, por favor). I don’t know if you’ve read Nick Longrich’s Protoceratops paper in the IUP ceratopsian volume, but he suggested that the first horned face was a nocturnal burrower based partially on orbit size.
Given how recently (relatively) that book came out, I wonder if the authors here reference it at all?
April 14th, 2011 at 2:16 pm
Haven’t had a chance to read the study through, but I’m very glad they tested the model’s ability to predict extant behaviors before extrapolating the model to the prehistoric past.
April 14th, 2011 at 6:29 pm
So we must have what would be a third example of convergently-evolved sonar. Now we need to identify the structures they used to make noises and detect echoes. Most immediately, do any features of headgear correlate with nocturnality?
April 14th, 2011 at 6:31 pm
One thing else… I had been given to understand that sauropods needed to eat 18 hours per day. Does this method say when they did that?
April 14th, 2011 at 8:36 pm
Readers may be interested in first author Lars Schmitz’s description of their paper on his lab blog: http://wainwrightlab.wordpress.com/2011/04/13/nocturnal-dinosaurs/
for a behind-the-scenes look at the methods… Thanks for a great post!
April 14th, 2011 at 10:54 pm
I’ve always been interested in Dinosaurs, Pterosaurs, etc. and this study is just something that I’ve been waiting to hear about. It’s never occurred to me that there are nocturnal ‘saurs, so this article was really great to read about. Thanks.
April 15th, 2011 at 4:38 pm
Thanks for a great blog post, ED. Very good summary!
April 16th, 2011 at 5:26 am
@Nathan – Why the sonar assumption? Echolocating whales/dolphins aren’t exactly nocturnal, and there are plenty of nocturnal animals that don’t have sonar. Think cats, just for a start.
April 18th, 2011 at 2:40 pm
The modern animals that use sonar also often use it in place of vision, so we could anticipate that a nocturnal archosaur that uses sonar might, in fact, not show the same patterns of eye/scleral ring adaptions that differentiate diurnal and nocturnal animals that rely on vision. (ie, they might have “day” adapted eyes and still be nocturnal because they use sonar at night).
However, we already have a “third” example of convergently evolved sonar, in an archosaur, no less, though it is developed to a far less sophisticated degree than in bats and cetaceans. Cave swiftlets navigate with rudimentary sonar, too.
Which does bring up the interesting question of what their scleral rings are like.
I’ve wondered if the mammalian middle ear might be a preadaption that makes the future evolution of the really sophisticated sonar systems more likely (you have to be able to hear very well if you want to use sonar to navigate, after all), seeing as both examples convergently evolved in mammals, and to my knowledge there is no evidence in the fossil record of any other group possessing features that might suggest the use of sonar (Of course most of the most diagnostic ones aren’t likely to fossilize. Still, the deep water mesozoic marine reptiles, for example, tended to have very large eyes, which at least suggests that they were more likely to favor visual systems of navigation over auditory ones).
April 18th, 2011 at 7:46 pm
Ed: Cats don’t fly, in my limited experience. I suppose I could have mentioned that I was referring to nocturnal pterosaurs.
April 19th, 2011 at 12:01 am
Not aware of any evidence at all for echolocation among pterosaurs. As I said before, a lot of the pertinent structures may not fossilize well, but at least some should. Whales have particular skull structures used in their echolocation, for example. If something similar (and by similar, I mean functionally equivalent, not necessarily anatomically similar) was found in a pterosaur skull, structures that could conceivably be used in the generation and/or reception of sonar signals (larynx structures, ear structures, etc), that would be positive evidence.
And again, an echolocation-dependent pterosaur would very likely have small eyes (like bats and whales do), in which case their skeletons, if found without obvious evidence for their echolocation abilities, would probably be assumed not to be nocturnal using the criteria in the study discussed here.