Binocular Vision in Pterosaurs

Birds
Binocular vision in certain birds is well known (Fig. 1). You can read more about it online here. Most birds have eyes on the sides of their heads. Apparently most have some degree of binocular vision. However, owls are different, with eyes rotated to the front for the skull. This occurs largely by the increased width of the back of the skull. The beak was always narrow, so narrowing of the beak was never an issue. Eagles and hawks have more binocular vision than most birds, other than owls.

Binocular bird vision.

Figure 1. An old image, perhaps from the 50-year-old Time/Life series, illustrating bird vision in a sparrow, a woodcock and an owl. The overlapping dark orange areas represent binocular vision. Most birds are like the sparrow, but many have some degree of binocular vision. 

Pterosaurs
Binocular vision in pterosaurs is a largely untouched subject. Most pterosaurs did not have much binocular vision. With eyes on the sides of its head, Pteranodon (Fig. 2) is such an example. It could view its entire world, but had to tilt its head one way or the other to see straight ahead. Many birds are similar.

The skull of Pteranodon in dorsal view without the possibility of binocular vision.

Figure 2. The skull of Pteranodon in dorsal view without the possibility of binocular vision.

Anurognathid Pterosaurs
Basal anurognathids, like Dendrorhynchoides (Fig. 3) also had eyes on the sides of their head, but note the slightly greater width at the back of the skull. Anurognathids evolved from a sister to Dimorphodon, Preondactylus and several other dimorphodontoids, including the IVPP embryo, all with small eyes at the back of their skull and little to no binocular vision. Perhaps binocular vision has been ignored by pterosaur workers largely because no others have bothered to accurately reconstruct the skulls of the only pterosaurs with substantial binocular vision, the owl-like anurognathids.

Dorsal and lateral views of three anurognathid pterosaurs.

Figure 3. Dorsal and lateral views of three anurognathid pterosaurs. From left to right, Dendrorhynchoides, Batrachognathus and Jeholopterus, all crushed dorsoventrally, due to the skull’s greater width.

Batrachognathus
The frontals were expanded laterally behind the very large eyeballs of Batrachognathus. The large upper temporal openings were likely filled with large muscles, larger than those of other anurognathids. The rostrum was shorter than in Dendrorhynchoides. Of all the known pterosaurs, Batrachognathus was the most owl-like. The scerlotic ring filled the orbit, which probably restricted the movements of the eye within the orbit. The skull had to be rotated to change the view. The large eyes also may indicate a nocturnal lifestyle.

Jeholopterus
The other derived anurognathid is Jeholopterus, the vampire pterosaur (Fig. 3). It had smaller eyes and a smaller pair of frontals, but they were angled like those of Batrachognathus. Considering the large size of the orbit and the small size of the eyeball itself (as revealed by the small sclerotic ring) one can imagine that such an eyeball had a greater ability to rotate within the skull, looking both sideways and somewhat forwards. Once latched onto its prey, there was no longer any need to look forwards, only to watch out for fellow vampires coming in from above to feast alongside it.

The other Jeholopterus(?)
The CAGS specimen had little to no binocular vision despite its closer relations with the above two anurognathids.  A relatively broad set of nasals and relatively small upper temporal fenestra provided excellent vision to each side, but not much up front.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again. Pterosaur workers, don’t be afraid to attempt your own reconstructions of anurognathid skulls. The bones are all there. I’ve provided guides to several specimens, but make your own observations. Don’t blindly follow the autapomorphic monstrosity of Bennett (2007). Test it with your own tracings and share your results.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
There are no published studies of binocular vision in pterosaurs. If you know of some, let me know.

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