Dinosaur pupil shape

[tyrantqueen]

the slit pupils make it an easy choice for me.

Ya know, the eyes of crocodilians can actually be quite beautiful up close, but the way CollectA implemented the look in its older figures doesn't look good. It was a single black line and looked rather cartoony.

How likely is a vertical pupil on a herbivore? I daresay horizontal pupils would be more suitable, but I just don't know enough about the subject. I've noticed that John Conway has started giving some of his herbivores "goaty" pupils. I do know that vertical pupils are seen in carnivores.

[WarrenJB]

There's Mark Witton's centrosaurus here:

markwitton-com.blogspot.co.uk/2017/08/the-convention-of-shrink-wrapping.html



He elaborates his thoughts on the pupil shape of that painting and in general, on his Patreon feed, with a excerpt from his upcoming third book. The gist is that many extant herbivores have converged on horizontal slit pupils to limit the amount of bright sunlight passing through the pupil, as with the vertical pupils of some carnivores, but still allowing a wide field of view rather than binocular vision and related depth perception.

Centrosaurus meets several criteria from modern species for horizontal pupils - laterally facing eyes, a herbivorous diet and a (probable) need to forage in open setting where sunlight might be an issue - so their portrayal here is not unreasonable

[Lanthanotus]

For the starts, just in case it wasn't clear: The quote by me was ment as an easy choice against thisComfy figure.

There's a good chance we will never know about the shape of pupils in any dinosaurs, let alone in different species. However, observing how eyes developed in several kind of animals I dare say there's some educated guesses one can make. Vertical slit like pupils (which can widen to round in the dark) are a trait that offers way more benefits to actively hunting animals than it would to any herbivore for simple physical reasons. I know of no recent herbivore of any class with this trait. Several very different animals developed such a trait, such as crocs and cats.

Round pupils are al all around good choice it seems no matter if you hunt or graze, seeing how often this "model" is being used in such different animals as cows, birds, salamanders, fish (though, the several functions of the eye itself vary greatly in terms of color view, focussing etc.)

The horizontal "goat eye" pupil is a rather broad "slit" which offers a great overview of the surrounding area for animals that fullfill their role rather as hunted than hunter, though there's the exception of octopi. Their eye however seems to have developed independently of the vertebrae eye.

In my opinion round pupils would be the most likely model for any type of dinosaur. However, comapring to recent biology I can accept vertical pupils on hunting species and horizontal pupils on herbivores (haven't seen any figure yet with that feature, if not by a sloppy paint application).

[WarrenJB]

Round pupils are al all around good choice it seems no matter if you hunt or graze, seeing how often this "model" is being used in such different animals as cows...

Ahem.

www.relativelyinteresting.com/animal-eyes-macro-photos/

Number five.

[Faelrin]

If vertical pupils are probably better adapted for nocturnal predators, I have to wonder why owls do not have them, but still have round pupils (unless their ability of flight has something to do with it)? I wonder if that would apply for any non-avian theropod, like Velociraptor which might have hunted during twilight or night.

[Joey]

If vertical pupils are probably better adapted for nocturnal predators, I have to wonder why owls do not have them, but still have round pupils (unless their ability of flight has something to do with it)? I wonder if that would apply for any non-avian theropod, like Velociraptor which might have hunted during twilight or night.

Interesting thought about Owls' eyes. Maybe they didn't adapt vertical pupils because they already have a lot of other unique adaptations?

[Newt]

If vertical pupils are probably better adapted for nocturnal predators, I have to wonder why owls do not have them, but still have round pupils (unless their ability of flight has something to do with it)? I wonder if that would apply for any non-avian theropod, like Velociraptor which might have hunted during twilight or night.

Birds in general seem to be more "locked in" to a round pupil morphology than other tetrapod groups; as far as I know, skimmers (Rhynchops) are the only living birds with slit pupils. It is of course very hard to say if this dedication to the round pupil was true of any group of stem-birds as well.


Some other random thoughts on the subject:


I think too much is made of the "vertical slits for hunters vs. horizontal slits for grazers" dichotomy. It's a mammalian thing, and really a carnivoran vs. ungulate thing. The sample size is too small to make these generalities and just-so stories about adaptive advantage.  As a counterexample, consider the true toads of the family Bufonidae and the midwife toads and spadefoots of the families Pelobatidae and Scaphiopodidae, respectively. These anurans all have similar lifestyles and gross morphology, though the bufonids are not closely related to the other two groups. The bufonids have horizontal slit pupils, while the pelobatids and scaphiopodids have vertical slit pupils. The same is true of other groups of similar but not closely-related anurans; for example, centrolenid, hyperoliid, and rhacophorid treefrogs have horizontal pupils, while hylid treefrogs have vertical ones. That suggests to me that their different pupil morphologies are simply two approaches to the same goal, and are determined by the accidents of evolutionary history and the constraints of pre-existing genetics and morphology as much as differential selective pressure.


Translucent shades such as nictitating membranes, the eyelid-windows of some skinks, and extensive eyelashes or similar structures are viable alternatives or accessories to pupil modulation to achieve vision in both low- and high-light scenarios.

Some vertebrates go beyond the slit eye; some geckos, for example, can restrict the pupil to a series of tiny openings, almost like a line of micropupils.

Fish pupils are weird. Many are neither round nor slit-like, but rather oval, teardrop-shaped, or subtriangular.

[Lanthanotus]

Round pupils are al all around good choice it seems no matter if you hunt or graze, seeing how often this "model" is being used in such different animals as cows...

Ahem.

www.relativelyinteresting.com/animal-eyes-macro-photos/

Number five.

Hmm, I saw a lot of cows in my life, also from quite near, but it seems I didn't stare them in their eyes long enough to notice... always wondered tho where the actual pupil is in that big black eye.... now I know. Honest thanks to pointing it out.... thinking of it, I looked up several other images of eyes of ruminant animals (of which I was unsure of their eye morphology, like giraffes) and it seems to be a general trait in those animals.

[Joey]

Round pupils are al all around good choice it seems no matter if you hunt or graze, seeing how often this "model" is being used in such different animals as cows...

Ahem.

www.relativelyinteresting.com/animal-eyes-macro-photos/

Number five.

That is very interesting, I always thought that they just had round pupils...I guess you learn something new everyday.

[Newt]

Not just ruminants...horses also have slit pupils, though like cows the pupils are often hard to distinguish from the dark iris.

[WarrenJB]

Well to be fair, I wasn't sure myself, but the topic made me go fire up an image search!

Here's one more thing from Dr. Witton - a citation:

advances.sciencemag.org/content/1/7/e1500391.full

Nocturnal activity plays a part, but it seems that vertical pupils in ambush predators with forward-facing eyes are also strongly influenced by the height of the eye from the ground, due to greater peripheral blur caused by focusing on a ground-based object, from a position closer to the ground. The vertical pupil allows greater clarity of vertical contours, which in turn aids binocular focus between horizontally aligned eyes.

That hints at the reason (hunting) birds mostly have round pupils, with the one exception Newt mentioned. Their eyes are very high:

Nearly all birds have circular pupils (1). The relationship between height and pupil shape offers a potential explanation. A near and foreshortened ground plane is not a prominent part of birds' visual environment. The only birds known to have a slit pupil (and it is vertically elongated) are skimmers [Rynchopidae; (27)]. The primary foraging method for the black skimmer is to fly close to the water surface with its lower beak in the water, snapping shut when it contacts prey. The black skimmer is crepuscular or nocturnal. This niche is visually somewhat similar to the ones encountered by short terrestrial predators, and they tend to have vertical-slit pupils.

I think it's interesting that there's a steep drop-off in vertical pupil occurrence when quadrupedal shoulder height (~ eye height) rises above 42cm. Very specific! With a hip height of 50cm, that might just exclude the likes of Velociraptor, though I realise it's not so cut-and-dried as all that.

[Sim]

In my opinion round pupils would be the most likely model for any type of dinosaur. However, comapring to recent biology I can accept vertical pupils on hunting species and horizontal pupils on herbivores (haven't seen any figure yet with that feature, if not by a sloppy paint application).

The Bullyland Therizinosaurus appears to have horizontal pupils.

[UtahraptorFan]

I just randomly thought of one possibility: cervid (deer, elk, moose...) pupils where that's nearly, or actually, all you see when their eyes are open. Now that I think of it, that'd be really creepy on a theropod.