300 Million Years of Fangs

[lpetrich]

I was let to consider this by checking out the cynodonts, a group of late-Permian mammal-like reptiles. That traditional description uses "reptiles" in the traditional sense of amniotes that are not birds or mammals, not in some more restrictive and cladistically-correct way.

Cynodonts ("dog teeth") were named after their fangs or canine teeth ("dog teeth"), and they are first known some 260 million years ago. Fangs, canine teeth longer than other teeth, are common among mammalian species, though many species have reduced or absent fangs, some species, like cetaceans, undifferentiated teeth, and some species, no teeth. Human canines are as long as other teeth, while chimp and gorilla and most other primate ones are longer than other teeth. Mammalian dentition has a lot of variety, though some features of it are widely shared.
 Mammal teeth
 Dentition
Mammalian teeth are often listed using a "dental formula": (upper jaw) / (lower jaw) where each jaw has for each side a count of its incisors (cutting), canines (piercing), premolars, and molars (grinding).

• The human formula is 2.1.2.2-3 / 2.1.2.2-3 with deciduous (initial) teeth 2.1.2 / 2.1.2 . The 2-3 on the end is for wisdom teeth only sometimes being present.
• Chimpanzee teeth are 2.1.2.3 / 2.1.2.3 - shared with other apes and Old World monkeys (collectively Catarrhini). New World monkeys have 2.1.3.3 / 2.1.3.3 or 2.1.3.2 / 2.1.3.2
• Dogs have 3.1.4.2 / 3.1.4.3 with deciduous teeth 3.1.3 / 3.1.3 and cats have 3.1.3.1 / 3.1.2.1 with deciduous teeth 3.1.3 / 3.1.2
• Bovines have 0.0.3.3 / 3.1.3.3 - they have a horny plate instead of upper front teeth.
• Horses have 3.0-1.3-4.3 / 3.0-1.3.3 with deciduous teeth 3.0.3 / 3.0.3
• House mice have 1.0.0.3 / 1.0.0.3 and squirrels have 1.0.2.3 / 1.0.1.3 - the incisors are continually growing
• Elephant tusks are enlarged upper second incisors, and elephants go through six sets of single molars (1 in each position, 4 total) over their lives.
• Armadillo: 0.0.7.1 / 0.0.7.1
• Cetacean teeth are undifferentiated. Baleen whales grow teeth as fetuses, though they never erupt and are later resorbed.
• Ancestral placental: 3.1.5.3 / 3.1.5.3 with 3.1.4.3 / 3.1.4.3 soon emerging from it
• Opossum: 5.1.3.4 / 4.1.3.4
• Kangaroo: 3.1.2.4 / 1.0.2.4
• Tasmanian devil: 4.1.2.4 / 3.1.2.4

Opossums are relatively primitive mammals, and they have prominent fangs.

An early mammal or mammaliform from around 200 million years ago (Late Triassic - Early Jurassic), Morganucodon, had dental formula 3-5.1.4-5.3-4 / 4-5.1.4-5.3-5 . Its had deciduous teeth and also no infancy teeth, suggesting that they were already secreting milk for their babies. The skull was about 2 - 3 cm (1 in) long and their body before the tail 10 cm (4 in) long. They likely had fur, thus looking much like a mouse -- a mouse with fangs and small incisors, like an opossum. Morganucodon was likely nocturnal, something inferred for ancestral mammals.

The Wikipedia article on them calls them "mammaliform" because their lower jaws are not quite at the single-bone mammalian state. They have two extra bones at the joint ends, bones that were larger in their mammal-like-reptile ancestors, and bones that became mammalian middle-ear bones. In fact, from early amniotes to mammals, one can watch the middle ("dentary") bone enlarge and the other bones get squeezed toward the joint ends and eventually detach or disappear.

 

[lpetrich]

We don't have direct evidence of whether or not Morganucodon laid eggs. The earliest-diverging present-day mammals, the monotremes, were egg-layers, something that suggests that Morganucodon also laid eggs. Such direct evidence is known for other species, like fossilized eggs or fossilized pregnancy. For instance, there are some fossils of pregnant ichthyosaurs, Mesozoic marine reptiles that converged on dolphins.

With Morganucodon ("Glamorgan tooth"), I had gotten up to 200 million years. I will now look further.

The late-Permian cynodonts ("dog tooth") had fully differentiated teeth, complete with fangs. They had other mammalian features, like a secondary palate (roof of the mouth), skull features that suggest enlarged jaw muscles, and the braincase bulging at the back of the head. They were the ancestors of mammaliforms and mammals.

 Evolution of mammals
Earliest warm-blooded species emerged some 250 million years ago - UPI.com
noting
Oxygen isotopes suggest elevated thermometabolism within multiple Permo-Triassic therapsid clades | eLife Lens
The evidence: oxygen-isotope ratios, more specifically O-18/O-16 ratios.

Not only cynodonts, but also dicynodonts were warm-blooded. Dicynodonts ("two dog teeth") were mid-Permian to late Triassic mammal-like reptiles that had enlarged upper canines -- tusks. Enlarged upper canines are familiar from sabertooth felines, and this feature has evolved several times. Elephant tusks are enlarged upper second incisors, however, and there are some fossil elephants that had corresponding enlarged lower teeth.

One dicynodont, Lystrosaurus, was very abundant just after the Permo-Triassic mass extinction, with as many as 95% of individuals in some fossil beds being in that genus.

In some carnivores' coprolites, fossil dung, one finds evidence of hair, so hair goes back to the late Permian. Meaning that cynodonts and/or dicynodonts likely had fur along with their warm-bloodedness.

 

[lpetrich]

Cynodonts and dicynodonts are two subgroups of therapsids, mammal-like reptiles that emerged in the Early Permian, around 275 million years ago.

I looked in several other therapsid groups, and I found lots of evidence of enlarged canines in them, as well as incisors and (pre)molars. So ancestral therapsids likely had fangs.

How the mouse outlived 'the giant': Groundbreaking research sheds light on ancestry of mammals and origin of hair -- ScienceDaily - a certain nerve departs from a channel in the skull less farther out in therapsids than in reptiles, suggesting a mammal-like flexible snout. Some other evidence is marks on the upper and lower jaw in species like Thrinaxodon that have been interpreted as evidence of blood vessels for whiskers. Meaning that these long stiff hairs go back to the late Permian.

I should mention that cynodonts had multiple fused bones in their lower jaws, much like multiple fused skull bones. But the largest of these bones was the dentary, the middle one, the one that became the only bone in mammalian lower jaws. In fact, this is a good example of a sequence of intermediates - the dentary becomes larger and larger and the other bones smaller and smaller, until the other bones either detach or disappear.

Jaws to ears in the ancestors of mammals has some nice diagrams of the transition. It uses an opossum as its mammalian example.


Therapsids are in the sphenacodonts, which are first known from the beginning of the Permian, about 299 million years ago. A notable non-therapsid of them was Dimetrodon ("two-measure teeth") from the early Permian. Its teeth were all conical, the ancestral shape for vertebrate teeth, but some were larger than others. It had prominent upper canines -- fangs.

Sphenacodonts are in eupelycosaurs, going back to the late Carboniferous, about 308 million years ago, and eupelycosaurs are in synapsids, going back to 312 million years ago. At this point, we see less tooth differentiation.

For instance, early-Permian synapsid Edaphosaurus had conical front teeth and peglike back teeth, but not much other differentiation. The late-Carboniferous one Archaeothyris had all-conical teeth that included some enlarged canines -- fangs.

So fangs go back to some of the earliest synapsids, one of the two survivors of early branching of the amniotes. The other is the sauropsids, which include the strict-sense reptiles, the dinosaurs, and the birds.

 

[lpetrich]

A notable feature of several early-Permian synapsids is their sailbacks. Their dorsal spines extend outward, and they likely had skin stretched between them. The main theories of that structure's function are for thermoregulation and as display organs.

Looking at the other main branch of amniotes, the sauropsids, teeth are usually much less differentiated, if at all. Lizard teeth are usually undifferentated, but some lizards have conical teeth in front and peglike teeth in back. Crocodilian teeth are somewhat differentiated in size, though they are all conical. Some venomous snakes have upper teeth that are enlarged into fangs. Hadrosaurs (duckbill dinosaurs) had lots of small molar-like teeth - Study into the Dentition of Hadrosaurs

I checked on fish, and most bony fish have undifferentiated teeth, often conical-shaped. Some fish have different-sized teeth, and some, like the sheepshead fish, have teeth that are shaped much like human incisors. Shark teeth are undifferentiated, as far as I can tell, and they are often flattened conical. Sharks have "denticles" on their skin that have very similar structure. There are two main theories of their relationship with teeth: outside-in (denticles -> teeth) and inside-out (teeth -> denticles).


Returning to the fang theme, spiders' fangs are on the ends of their frontmost limbs, their chelicerae. Their origin is very obviously separate from the origin of vertebrate fangs - once in early Synapsida and once in ancestral Viperidae (venomous snakes with fangs).

 

[lpetrich]

Back to mammals, their teeth have gone in a variety of different directions.

Among the carnivorans, the back teeth are modified for shearing -- carnassials.

Among the artiodactyls, pig tusks are fangs - long canines. Their upper ones can grow upward like their lower ones. Peccaries also have long canines, as to hippopotamuses. Hippos also have long incisors. Camels' canines are reduced to the lengths of the other teeth, much like human ones. Among the ruminants, the chevrotain or mouse deer has fangs. Most other ruminants lack canines.

Horses have an often-presented record of their evolution. Hyracotherium had fangs, but later equines had reduced canines that are often absent in present-day ones. Tapirs and rhinoceroses also have reduced or absent canines.

Rodents and rabbits have no canines. They have enlarged first incisors that continuously grow. Rabbits and some rodents also have continuously growing back teeth.

Present-day cetaceans have lookalike conical teeth, though early ones like Basilosaurus had some differentiation.

Elephants are weird. Their tusks are upper second incisors, they have no other front teeth, and they go through six sets of single back teeth over their lives:

1. 2 - 3 yrs
2. 4 - 6 yrs
3. 9 - 15 yrs
4. 18 - 28 yrs
5. 40 - 45 yrs
6. 60 - 70 yrs

The loss of the last set of molars means the end of their owner's life, because their owner can no longer chew their food very well. Elephants in this situation sometimes head to places with very chewy food, like in streams, but that doesn't help very much. Places like that may accumulate lots of bones of dead elephants, thus becoming "elephant graveyards".

Some extinct elephants had lower tusks in addition to upper ones, and some of them lost their upper tusks, thus having only lower ones.


Turning to marsupials, wombats have rodent-like dentition, Tasmanian devils have carnivoran-like dentition, fangs and all, and kangaroos have horse-like dentition, though with some unusual front teeth. Opossums have primitive mammalian dentition: incisors, canine fangs, and (pre)molars.

Present-day monotremes are toothless, at least as adults. Young platypuses have some baby-tooth molars that they later lose.