My what big &#%!@*? teeth you have

You've heard them called tusks; you've heard them called fangs. You've seen these terms with quotation marks and without quotation marks. They all seem to refer to these &#%!@*? big teeth in temnospondyls (and other early tetrapod friends). "But wait," you think to yourself. Aren't tusks like the things that elephants or warthogs have? And aren't fangs things that snakes have? Did extinct amphibians have features like that??

I actually have no clue what you're talking about...
In temnospondyls, but also in many tetrapodomorphs, stem tetrapods, and other Paleozoic tetrapods (e.g., seymouriamorphs), there are often remarkably large teeth on the bones of the palate (roof of the mouth) and on the symphysis (front end of the jaw). In large temnospondyls, these teeth could be several inches long and could do some serious damage - I almost impaled my hand on one while doing summer fieldwork two years ago. Because of their size and perhaps as a means by which to differentiate different teeth in the temnospondyl mouth, these have often been referred to as "fangs" or "tusks." These are typically paired (although sometimes they are found as triplets), and because temnospondyls have alternating tooth replacement (every other socket is usually vacant), pairs of enlarged teeth usually consist of one tooth and one socket, although you can find specimens with double teeth or double sockets. The pair is thus sometimes called a "fang-and-pit," although this isn't too common anymore. 

The lectotype of Mastodonsaurus giganteus, a large isolated fang. Although the position of this tooth is unknown, it is clearly quite large (see scale bar)! Figure from Moser & Schoch (2007).

Palate of the Permian trematopid Acheloma dunni, modified from Polley & Reisz (2011). Blue shading indicates marginal teeth (premaxilla, maxilla), the same as your teeth, whereas red/orange shading indicates the palatal "fangs" (vomer, palatine, ectopterygoid). Tiny dots are denticles, or the smallest teeth that are found on the palate.

Where are they? 
These large teeth can be found on the major elements of the palate (vomer, palatine, ectopterygoid) as well as at the front of the mandible (jaw) - see the mocked-up figure to the left. Their presence / absence is pretty consistent (Warren & Davey, 1992 has some good summary tables), although many derived and/or paedomorphic temnospondyls tend to lose the pair of the ectopterygoid ('ect' on the left, the farthest back pair). There can be multiple pairs on the vomer ('v' here, the farthest forward pair), although one is typically larger than the other pair(s) except in small temnospondyls. The general configuration seen to the left is a pretty good example of a typical temnospondyl palatal dentition. Other permutations include large teeth adjacent to the marginal teeth but on the bones bearing the marginal teeth (premaxilla or maxilla), not the palatal bones. 

On the lower jaw, enlarged teeth are typically found at the front end, called the symphysis (see below slideshow for an example).

Why are these teeth so big?
Presumably, the teeth are for prey capture, not for something like intraspecific competition (like tusks in warthogs, for example). How exactly they were used is a bit uncertain because feeding mechanisms of temnospondyls remain largely speculative and were probably quite varied (but see a lot of the work by Josep Fortuny; Witzmann & Schoch, 2013 for some non-tooth-specific examples). The fact that these enlarged teeth occur in temnospondyls of all sizes and across ecologies (e.g., aquatic versus terrestrial) might suggest some degree of phylogenetic conservatism (i.e. they retained it from earlier tetrapods and it wasn't sufficiently detrimental to be selected against) since there are major differences between vacuuming up fish in a river versus crunching hard-shelled insects on land. Similar to a lot of other animals with imposing teeth (like saber-toothed mammals and the narwhal), the most extreme (and usually the most mainstream) ideas are probably not very accurate (i.e. they're not crushing bone). The bone that the palatal teeth sit on in temnospondyls is very thin and probably couldn't handle major compressional stresses associate with jamming those teeth onto a hard surface. A lot of work left to do to sort this area of temno paleobiology...

Tusks

Tusks take a lot of strange forms, but they are typically regarded as a mammalian feature because they usually are modified canine teeth (modified incisors in elephants), a type of tooth used only with respect to mammals (the term 'caniniform' being used for canine-like teeth in other animals). Mammals (but also a number of reptiles and various extinct tetrapods) have more differentiated teeth (e.g., variable shape, function, implantation), often reflecting different feeding behaviours and processes within the mouth - this is referred to as heterodont dentition. For example, in humans, we have incisors, canines, pre-molars, and molars. Each tooth type is adapted for a specific and distinct function - incisors have thinner edges and are better for cutting, whereas molars have a broad surface for grinding and chewing (masticating). By comparison, many other animals, particularly "lower tetrapods" like fish and amphibians (including temnospondyls), have relatively homogenous teeth that differ mostly in size - this is referred to as homodont dentition. Virtually all temnospondyls' teeth are some permutation of cones with a single sharp point (monocuspid), which in part reflects the fact that their diets were much more restrictive than animals with heterodont dentition (e.g., omnivores). As a result, we don't have any categories for their teeth. We only differentiate them by their position (e.g., palatal [roof of mouth] vs. coronoid [an element on the jaw]). There are general size patterns correlated with position -  palatal teeth are almost always distinctly smaller than marginal teeth, for example - but they're pretty vanilla compared to your average modern mammal. So as I noted above, calling the particular large teeth 'tusks' is one way to indicate which of the various teeth one is talking about in a temnospondyl. But are these actual tusks?

Let's see what the internet dictionaries say about tusks (definitions only listed for the noun form):

• Dictionary.com: (1) (in certain animals) a tooth developed to great length, usually one of a pair, as in the elephant, walrus, and wild boar, but singly in the narwhal; (2) a long, pointed, or protruding tooth; (3) a projection resembling the tusk of an animal.
• Cambridge English Dictionary: (1) either of the two long, pointed teeth of some animals such as elephants.
• Oxford English Dictionary: (1) a long pointed tooth, especially one which protrudes from the closed mouth, as in the elephant, walrus, or wild boar; (1.1) a long, tapering object or projection resembling a tusk.
• Merriam-Webster Dictionary: (1) an elongated greatly enlarged tooth (as of an elephant or walrus) that projects when the mouth is closed and serves especially for digging food or as a weapon, broadly : a long protruding tooth; (2) one of the small projections on a tusk tenon.
• Collins English Dictionary (British): (1) a pointed elongated usually paired tooth in the elephant, walrus, and certain other mammals that is often used for fighting; (2) the canine tooth of certain animals, esp horses; (3) a sharp pointed projection; (4) a tenon shaped with an additional oblique shoulder to make a stronger joint.
• Collins English Dictionary (American): (1) in elephants, wild boars, walruses, etc., a very long, large, pointed tooth, usually one of a pair, projecting outside the mouth and used for defense, digging up food, etc.; (2) any tooth or projection suggestive of a tusk.

By the way, if you have no idea what a "tusk tenon" is (I sure didn't), it's a woodworking term referring to a long process on a wood board that inserts into an opening on another wood board to form a joint. 

Hmmm....so overall, no additional clarity gained. It should be noted that defining tusks as paired teeth isn't very good because that means that the tusks of narwhals and unicorns are not tusks. Narwhals occasionally have paired tusks, but it's quite rare, and I at least have never seen a unicorn with paired tusks. Someone should comment if they have (photographic evidence required).
      A recurring theme in a lot of these definitions is that the tusks are not just enlarged teeth, but that they project out(side) of the mouth. Because temnospondyl "tusks" didn't serve the same suite of functions as mammalian tusks, perhaps this is a good place to draw the line. WRONG! There's always some oddball out there, and in this case, there are a few temnospondyls whose mandibular "tusks" poke through openings in the skull. Yes, you read that right. The teeth are large enough to go through either the nostrils (Microposaurus, see below) or through extra paired openings in the snouth (some trematosaurs, mastodonsaurids). 

Photographs of the Middle Triassic trematosaur Microposaurus averyi from Australia (figure from Warren, 2012).

Corresponding line drawings of the photographs to the left of Microposaurus averyi - pay particular attention to B. Modified from Warren (2012).

Probably the most helpful criterion is that tusks are constantly growing. They don't typically replace throughout life, although they can sometimes regrow if broken off provided the damage is limited to the crown (the exposed part) and not to the root (the embedded part). This is why tusks are typically more extensive (longer and sometimes more curled) in older mammals. As far as we know, temnospondyl "tusks" are replaced by just falling out and growing a new one, just like the rest of the teeth. 

Fangs

Like tusks, fangs are typically associated with particular animals, namely long and sharp teeth for prey capture in mammals, venom-delivering teeth in some snakes, and the things that look like teeth in spiders. In most cases, these are functions more readily associated with predation compared to tusks, which are primarily for almost anything else.

Back to the online dictionaries:

• Dictionary.com: (1) one of the long, sharp, hollow or grooved teeth of a venomous snake by which poison is injected; (2) a canine tooth; (3) a tooth resembling a dog's; (4) the root of a tooth; (5) one of the chelicerae of a spider; (6) a pointed, tapering part of a thing.
• Cambridge English Dictionary: (1) a long, sharp tooth that animals such as snakes and dogs have.
• Oxford English Dictionary: (1) a large sharp tooth, especially a canine tooth of a dog or wolf; (1.1) the tooth of a venomous snake, by which poison is injected; (1.2) the biting mouthpart of a spider.
• Merriam-Webster Dictionary: (1a) a long sharp tooth: such as one by which an animal's prey is seized and held or torn OR one of the long hollow or grooved and often erectile teeth of a venomous snake; (1b) one of the chelicerae of a spider at the tip of which a poison gland opens; (2) the root of a tooth or one of the processes or prongs into which a root divides; (3) a projecting tooth or prong
• Collins English Dictionary (British): (1) the long pointed hollow or grooved tooth of a venomous snake through which venom is injected; (2) any large pointed tooth, esp the canine or carnassial tooth of a carnivorous mammal; (3) the root of a tooth; (4) (usually plural, British informal) tooth.
• Collins English Dictionary (American): (1a) one of the long, pointed teeth with which meat-eating animals seize and tear their prey; canine tooth; (1b) one of the long, hollow or grooved teeth through which poisonous snakes inject their venom; (1c) the root of a tooth; (2) the pointed part of something.

I'm a particular fan of the definitions like "a pointed, tapering part of a thing." ​Most of these definitions are very specific to carnivorous mammals, venomous snakes, and spiders though. 

Does it really matter what you call them?
Does (did) the temnospondyl care? Unlikely. Does the average person? Definitely not. Linguistically speaking, there's no language police going around telling you what to use and what not to use - it would honestly be a lot easier to sort this all out if there was some arbitrary arbiter of terminology. From a practical standpoint, I do think it's important to be consistent in terminology when possible though, firstly because it implies homology (shared feature due to shared ancestry) to a certain degree (e.g., the femur is the same ossification in all tetrapods), and secondly because it makes it a hell of a lot easier for people unfamiliar with the system / structure to think they're comparing apples and oranges when it's really just different apples that are named as "apples" and "oranges." "Tusks" and "fangs" are pretty easy to sort out, but other names for specific parts of a certain element (e.g., semilunar curvature of the squamosal) can be harder to figure out when they're inconsistently applied and hard to discern from a label that could be pointing to some other structure or the element as a whole. Can't really do anything about the 18th-early 20th century when lots of fun and outdated terms were used (to say nothing of terms in languages other than English), but I do think it's something important to strive for.

So what do I prefer?
In my mind (and probably in a lot of other non-temno workers / non-scientists), fangs are primarily involved with prey capture, although that doesn't have to be primarily for inflicting serious physical injury (e.g., venom delivery in snakes). Seems about right with what we'd predict in temnospondyls. On the flip side, tusks are widespread in herbivorous, not carnivorous, mammals, and they are involved with a variety of functions that may be only tangentially related to feeding (e.g., digging) or totally unrelated (e.g., social signaling and competition) that often relate to the fact that they project outward compared to normal teeth. Therefore, I would say that 'fang' is more appropriate in terms of how we perceive the differences between these terms colloquially, since etymological origins and definitions are not going to help us out very much. To be honest, I've never paid much attention to which one I used in my publications, but after doing a quick check, that's apparently because I've only used 'tusk' once, and I put it in quotation marks, so my own notions of terminological consistency are apparently stuck in my psyche...

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Refs

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