Shocking Shark Teeth

Again I am going to pause on the genus Carcharhinus and talk briefly about another shark which is often misidentified.  This is the Tiger Shark (Galeocerdo).  I am always astonished when someone misidentifies a different species tooth as a tiger shark as tiger sharks have very distinctive teeth.  I am not going to go into detail on tiger shark teeth for now for various reasons.  In the tiger shark as there are 2 distinct lineages (the teeth are similar enough to identifying them both as Tiger Sharks).  Additionally there is a couple of other genera which can be confused with tiger shark teeth.  Physogaleus (which can be excused as it was formerly classified as Galeocerdo) and Squalicorax which went extinct at the end of the Cretaceous.  If it is a Cretaceous tooth, it is NOT a tiger shark. So these are tiger shark teeth, the uppers and lowers look just the same.  If it doesn’t look like below, it is not from a tiger shark.

I am going to take a detour from describing the Carcharhinus species and talk about Lemon Sharks (Negaprion) as we have had many postings misidentifying various species as Lemon Sharks.  There are 2 modern species of Lemon Sharks, the Lemon Shark (Negaprion brevirostris) which lives in the Atlantic Ocean and the Sickle Fin Lemon Shark (Negaprion acutidens) which lives in the Pacific and Indian Oceans.  Externally, the two species are told apart by the shape of their fins.  The dentitions however, are almost the same.  Upper teeth may be a little broader in the Sickle Fin but on an individual tooth basis, it would be very difficult to separate the two other than by location. The teeth are erect, long and have wide roots.  Upper teeth are broader than lowers and inclined or slightly inclined blades.  Rarely they will have extremely fine serrations on the blade.  There are irregular, bumpy serrations on the wide shoulders.  Lower teeth also have wide roots though not as wide as the uppers and lack the bumpy serrations.  Lower teeth are also long but not as wide as uppers.   The modern species can be found as fossils from the Late Miocene onward.   Earlier Miocene and Oligocene teeth are identified as Negaprion eurybathrodon, the main difference is that they are usually about 2/3 the size.  The Eocene species is Negaprion amekiensis, again smaller than N. eurybathrodon and the bumpy shoulder serrations are barely there.  Illustrated fossils show N. brevirostris followed by N. eurybathrodon and then N. amekiensis.

The Sandbar or Brown Shark (Carcharhinus plumbeus) is a near shore shark growing to a maximum of about 8 feet or 2.5 meters.  It is a popular shark in aquariums since it looks very sharky (somewhat like a bull shark) with a large dorsal fin, is very hardy and has a preference for eating fish (during a test, it rejected beef); thus it won’t be taking chunks out of the divers while they are cleaning the tank.  The US Navy attempted to train a bottlenose dolphin (Tursiops) to protect divers by using a live sandbar shark.  First they put a dummy shark in the tank with the dolphin and trained it to butt and push the dummy around.  Then they put in a dead shark and again the dolphin butted and pushed it around.  Then they added a live sandbar shark and again the dolphin bullied the shark around the tank.  Finally they removed the sandbar shark and replaced it with a live bull shark.  The shark ignored the dolphin.  The dolphin on the other hand began rapidly swimming around the tank, screaming.  They removed the shark and it took 2 days to calm the dolphin down.  The test wasn’t a complete success.  To a layman, the two sharks look very similar.  A dolphin can instantly tell the difference. The teeth are moderately sized for a Cacharhinid.  They are finely serrated and can be angular or curved.  The main distinguishing feature is that the teeth are very thin front to back (labial to lingual) compared to other Carcharhinids (best felt by rubbing the blade between your fingers).   Lower teeth have wide roots and lower lateral and posterior teeth roots thin out (top to bottom) towards the ends.  This feature can be easily confused with damaged teeth. Modern teeth illustrated are the white ones, colored ones are fossils. 

The next shark to be discussed is the Galapagos Shark, Carcharhinus galapagensis.  Like the Oceanic Whitetip, this is a medium sized shark and a rare fossil as the shark typically lives around islands and seldom near continents.  Its teeth appear much like a Dusky Shark in that they are curved and coarsely serrated.  The way to tell upper teeth apart from Dusky Shark teeth is that in a Dusky tooth, the tooth is usually approximately as long as it is wide.  In a Galapagos shark, the teeth are longer than they are wide (not a hard rule for posterior teeth).   Also ‘most’ Dusky’s are somewhat larger (again not a hard rule).  Lower teeth are pretty undistinguished and very hard to tell apart from a multitude of other Carcharhinids.   Again white teeth are modern, colored are fossils.

Side images of an upper Carcharodon hastalis tooth and a lower C. hastalis tooth showing the outward/inward curvature.  Note: this is not a solid rule, and can vary species by species or even within the same species particularly with pathologic (to be discussed later) or posterior teeth. 

The upper teeth in most sharks have a wider blade than the lowers.  The lower teeth are used to grab the prey, i.e., pin it with its spike like shape.  The upper teeth then come down and cut.  There are exceptions to this.  The 6 and 7 gill sharks have much wider teeth in the lower jaw.  Dogfish and their relatives (cookie cutter, etc.) can similar size teeth in both jaws and/or very small, narrow teeth in the upper jaw and wider teeth in the lower jaw.  Some sharks have similar size teeth in both jaws.  In many sharks, the upper teeth curve outward at the tips, while the lower teeth curve inward.  This allows the teeth which are already beyond razor sharp to slide past each other and cut like a pair of scissors.  Speaking of razor sharp, when the teeth are in the mouth, they have a protein coating over the enamel.  This can be as thin as 2 molecules at the cutting edge (a scalpel typically is 50-100 molecules thick at the edge).  When the tooth comes out of the mouth, this coating flakes off looking like thin onion paper.  Thus fossils are nowhere near as sharp as living teeth.  Rarely, this coating is fossilized but when it is, it presents a dirty appearance and is usually cleaned off by the collector.  When alive this coasting is clear allowing the shiny enamel to shine through. Attached is an image of a megalodon tooth with that coating partially preserved.  It is the dirty appearance on the right side of the tooth and covering most of the lingual (flat) side of the tooth.  A closeup shows that serrations were not actually the rounded, bumpy appearance we are used to but instead were triangular with thin cutting edges between the individual serrations the same as you would see on most steak knives.  

More Oceanic Whitetip: 

The next shark we will examine is the Oceanic Whitetip shark – Carcharhinus longimanus (not to be confused with the Reef Whitetip Shark or the Silvertip Shark).   Traditional sources state that this sharks grows to 15 feet in length.  While possible, a maximum length of about 9-10 feet is far more likely to be encountered.  Thus it is typically (and the teeth are) smaller than the Bull Shark (maximum length 13 feet) and the Dusky Shark (maximum length 14 feet).  An extremely aggressive shark which lives out in the open ocean usually far from land.   Because the open ocean is typically an empty desert, it treats anything it comes across (to include people) as potential food.   It seldom approaches land and then usually only when the water is deep or there is a nearby drop-off.   Once the most common large animal on earth (criminal shark finning has ended that), the fact that it usually stays away from land makes it a very uncommon fossil.  The upper teeth are very erect and triangular even in the distant lateral positions.   They are unusually thick labially to lingually (front to back) and coarsely serrated particularly towards the root.  The lower teeth are fairly large though not as large as from a bull or dusky shark.  The ends of the teeth are serrated while the base is usually not, often giving them a spear tip appearance. The images are from fossil teeth (colored specimens) and modern teeth (white).