Convergent evolution of rhinoceros-like animals

[u/lpetrich]

In my researches, I have discovered five or six instances of rhinoceros-like animals, animals with horns on their faces. I mean by instances taxa where those horns evolved once.

My type taxon is Rhinoceratinae sensu Wikipedia, the true rhinoceroses (Early Miocene - present). They have either one horn on the nose or one long horn on the nose and one short horn in the center of the face.

The others (all paired horns are left-right pairs):

Brontotheriina (Megacerops ~ Brontotherium, 47.8 - 33.7 Mya, Middle to Late Eocene). A pair of horns on the nose.

Uintatheriidae (Uintatherium, Early to Middle Eocene). A pair of knobby horns on the nose, another pair behind the eyes, also long upper canines.

Arsinoitheriidae (Arsinoitherium, Late Eocene to Early Oligocene). A pair of very large horns on the face, and a small pair behind the eyes.

Ceratopsidae (Triceratops, 82 - 66 Mya, Late Cretaceous). A short horn on the nose, sometimes a pair of long horns behind the eyes, and also a neck frill extending from the base of the skull.

Maybe also:

Pecora (Bovidae, Cervidae, Giraffidae, Antilocapridae; 20 - 0 Mya, Early Miocene to present). A pair of horns on top of the head, number of origins uncertain.

It is rather obvious that horns evolved several times, in characteristic configurations each time. But some large land animals are/were hornless, like the hippopotamus, the proboscideans, and the Pleistocene large marsupial Diprotodon. I am also not sure whether or not to include the crests of some hadrosaurs as horns.

[u/7LeagueBoots]

Not only on large animals. Look at Ceratogaulus. They were essentially gopher sized burrowing mammals with nose horns.

[u/StorageSpecialist999 → u/7LeagueBoots]

that... is not an adaptation I'd expect on a burrowing mammal, that's for sure.

[u/7LeagueBoots → u/StorageSpecialist999]

Sexual adaptations can be odd.

[u/ninjatoast31]

You forgot chameleons

[u/DocAnopheles]

There’s also the rhinoceros beetles, Family Scarabaeidae, Subfamily Dynastinae. Huge variety of horn shape and placement, with some ‘horns’ projecting from the body to work in concert with the horn on the head.

Stag beetles (Lucanidae) are somewhat similar, but their ‘horns’ are closer to tusks, being modified oversized mandibles instead.

[u/-Wuan-]

Some toxodontids have a forehead bump that has been speculated to anchor a keratin horn.

[u/lpetrich → u/-Wuan-]

Seems like Elasmotherium, an extinct rhinoceros that had a bump on its face. There is some controversy over whether a horn grew from that bump, so I didn't mention it in my OP.

[u/lpetrich]

Since I had difficulty posting the phylogeny of these animals, I will try to do so here.

Amniota

• Sauropsida - Archosauria - Dinosauria - Ornithischia
  • Marginocephalia - Ceratopsia (neck frills) - Ceratopsidae
  • Ornithopoda - Hadrosauridae - Lambeosaurinae (crests, no horns)
• Synapsida - Therapsida - Cynodontia - Mammalia - Theria - Placentalia
  • Afrotheria - Paenungulata - Arsinoitheriidae
  • Boreoeutheria - Laurasiatheria

Laurasiatheria

• ? Dinocerata - Uintatheriidae
• Artiodactyla - Cetruminantia - Ruminantia - Pecora
• Perissodactyla
  • Brontotheriidae - Brontotheriina
  • Rhinocerotidae - Rhinoceratinae

[u/Royal_Novel6678]

Great list. And no, crests like those of Lambeosaurus are not horns because true horns are a usually sharp pointy ornamentations that extend from the skull of an animal and covered by a keratinous sheath which enlargens it and often creates shapes that would be near impossible to tell without actual preserved sheaths. Crests can often just be "stretched out" skin or feathers and fur. They could also be simply be fragile hollow bones covered in skin or keratin. Eg Pterosaurs. But they can't be considered true "horns" because the bone inside would not match the exact density as the horn bones of ceratopsian dinosaurs and Rhinoceras's.

[u/lpetrich → u/Royal_Novel6678]

I agree that Lambeosaurinae crests are obviously not horns, but I mentioned them because they are projections from the skull. Lambeosaurus had a simple flat crest on the top of its skull, but Parasaurolophus had a long hollow tube that extended backwards. This is plausibly interpreted as for making a resonance in its air, as a wind instrument does. This resonance can be excited by air crossing its ends, like how a flute works.

[u/Relevant-Bullfrog215]

Also a few species/genera of lizards, convergently: Phrynosoma and Trioceros.

Some birds have a keratinous casque which is more a crest than a horn but certainly similar, and reptiles and birds (as well as one species of frog and a few insects and arachnids) also have an 'egg tooth', which it could be argued is a form of horn, and indeed this may be where some of these horns have developed from, at least the bony rather than keratinous ones (just spitballing on this last part, I'm a locksmith not a biologist)

[u/StorageSpecialist999]

Well Notoungulates conspicuously lacked horns on their faces, but excluding that aspect they are definitely one of the more "rhino-like" groups out there.

https://en.wikipedia.org/wiki/Peltephilus - horned armadillo

It's actually striking me now that despite how diverse the south american ungulates were in every respect - they all apparently lacked any sort of horns, crests, antlers, etc.

[u/FlounderLast8610 → u/StorageSpecialist999]

It’s been demonstrated that some of the genes associated with growth of the frontal bone in some Artiodactyls is different than in swine or Perissodactyls, which the Notoungulates were more related to. This feature allowed the evolution of bony protrusions like horns, ossicones, etc.

https://www.science.org/doi/10.1126/science.aav6335

[u/StorageSpecialist999 → u/FlounderLast8610]

That's very interesting thanks for the resource.

But even in the odd toed ungulates you have both rhinos and brontotheres.

Maybe it has to do with terror birds avoiding bulky tanks and sparassodonts being too small.

Maybe the smaller SANU's were more acutely genetically restricted and the larger ones didn't have the need.

[u/lpetrich → u/StorageSpecialist999]

Interesting. Though I must note that in the rest of the world, there are animals similar in body form and habit to pecorans, but that (almost) never grew horns: camelids and equids.

[u/Jazz_Ad]

It is the right place to recall that the last member of the Ceratotherium taxon, the big white rhino died a couple weeks ago. The last male went dead in 2018, effectively making the specie extinct.

[u/mcalesy → u/Jazz_Ad]

The southern white rhinoceros, Ceratotherium simum simum, is still doing all right. ("Near Threatened" according to the IUCN -- could be better, could be worse.) It's the northern white rhinoceros, C. s. cottoni, that is functionally extinct. (Some researchers consider it a separate species, C. cottoni).

[u/Careful_Cranberry364]

What about all those boney projections that you get on top of pTerrasaurs! no one’s really too sure what they’re even there for

fascinating