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The Debate Over Dinosaur Sight

Did Velociraptor hunt under the cover of darkness?

A reconstruction of Velociraptor, complete with a scleral ring in the eye, at the Wyoming Dinosaur Center in Thermopolis, WY. Photo by the author.

What’s scarier than a Velociraptor? A Velociraptor at night. That’s the hook I used last spring when a study published in Science used the fossilized bony rings that once supported dinosaur eyes to discern which species might have run around during the day and which stalked the night. (In truth, you wouldn’t have much to fear from Velociraptor at either time—the feathered dinosaur was about the size of a turkey and probably specialized in prey smaller than themselves.) Since the time that study was published, however, other researchers have raised questions about whether or not we can really use remnants of dinosaur eyes to study their behavior.

The idea behind the 2011 Science study by paleontologists Lars Schmitz and Ryosuke Motani was relatively simple. In dinosaurs, as in many other vertebrates except mammals and crocodylians, a ring of small bones helped support the pupil and iris of the eye. The structure is technically known as a scleral ring and acts not only as a proxy for eye size. A wider hole in the middle of the ring would indicate an ability to take in more light, and thus would be consistent with noctural habits, while a relatively smaller window would be more consistent with daytime behavior. Applied to dinosaurs, the study seemed to show that many predators hunted at night while large herbivores were most active during the mornings and evenings.

In a comment published in December last year, however, researchers Margaret Hall, Christopher Kirk, Jason Kamilar and Matthew Carrano pointed out that this correspondence may not be so simple. In addition to questioning the statistical analysis used by Schmitz and Motani, Hall and co-authors noted that there is a considerable degree of overlap in scleral ring anatomy between animals active at night and those active during the day. Among birds and lizards, for example, the scleral rings of some day-dwelling species are very much like those of nocturnal ones. The anatomy of the scleral rings may not be a clear-cut predictor of behavior.

That isn’t to say that the scleral rings can’t tell us some important things about the eyes of extinct animals. Hall and collaborators remarked that the inner diameter of the scleral ring corresponds to the diameter of the cornea—an essential measurement for figuring out how much light can enter the eye. The problem is that another measurement—axial length, or the distance from the front to the back of the eye—is essential for gauging the vision of dinosaurs, but no known specimen has the preserved soft tissue anatomy required to figure this out. Until other anatomical markers of eye shape and size are found, our inferences about whether dinosaurs were active during the night or day will be weak. “t is not yet possible to reconstruct the activity patterns of most fossil archosaurs with a high degree of confidence,” Hall and colleagues concluded.

Schmitz and Motani issued a rebuttal in the same issue of Science. In defense of their paper, Schmitz and Motani reject the criticisms as based on what they consider to be “unscreened data, untenable assumptions, and inappropriate methods” and affirm that their methodology properly categorized dinosaur behavior on the basis of what is known about modern animals. Regarding anatomical minutiae such as the axial length of the eye, Schmitz and Motani suggest that the outside border of the scleral ring is correlated with axial length and therefore can be used as a proxy to reconstruct an animal’s visual capabilities. Altogether, Schmitz and Motani affirm that “the inference of nocturnality in dinosaurs from scleral ring and orbit morphology is sound.”

A good deal of this disagreement deals with methods of statistical comparison and analysis that, I must admit, are over my head. Still, there remain important questions about the way skeletal anatomy relates to soft tissue anatomy. When dealing with animals that have been extinct for millions and millions of years, can we accurately reconstruct the shape and important features of their eyes? Some skeletal features definitely correspond to soft-tissue structures, but interpreting the capabilities of those reconstructed eyes is a more difficult task and the central point of contention. I have little doubt that there were dinosaurs that were active at night, in the heat of the day, and at dawn and dusk, but the trick lies in accurately figuring out which ones were which.

References:

Schmitz, L., & Motani, R. (2011). Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology Science, 332 (6030), 705-708 DOI: 10.1126/science.1200043

Hall, M., Kirk, E., Kamilar, J., & Carrano, M. (2011). Comment on “Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology” Science, 334 (6063), 1641-1641 DOI: 10.1126/science.1208442

Schmitz, L., & Motani, R. (2011). Response to Comment on “Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology” Science, 334 (6063), 1641-1641 DOI: 10.1126/science.1208489

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About Brian Switek
Brian Switek

Brian Switek is a freelance science writer specializing in evolution, paleontology, and natural history. He writes regularly for National Geographic's Phenomena blog as Laelaps.

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