Oh, the shark has pretty teeth, dear...

In the last post, I talked about the triple-layer enameloid that is one of the defining characteristics of a modern shark.  That enameloid has an outer single crystallite enameloid/shiny-layered enameloid (SCE/SLE) that resists the spread of cracks in the teeth; a middle parallel-bundled enameloid (PBE) that resists compressional (straight down) force; and an inner tangled-bundled enameloid (TBE) that resists rotational (twisting) forces.  These are tremendous properties for a shark that bites big chunks out of something else and needs to chew them up.  A Cretaceous predator, Squalicorax curvatus from the Western Interior Seaway of Kansas (about 90 million years ago), has teeth that everyone would recognize as shark's teeth.  Squalicorax has a labiolingually compressed (flat) shape, with a strong triangular cusp and a shoulder with serrated edges (like a steak knife).  These teeth are perfect for running up on something even bigger than the shark, slicing out a chunk of prey and chewing it up.

Squalicorax curvatus teeth coated with ammonium chloride to show detail

Squalicorax curvatus would be very recognizable to the casual observer as a shark.  It just LOOKS like a shark.  Mouth behind and below snout, large dorsal fin, fusiform body, mouth full of nasty teeth and big (up to about 10 feet long).  A few extremely well preserved specimens have been found, including a nearly complete skeleton (very rare for cartilage - even calcified cartilage) recently sold by PaleoSearch, Inc. in Hays, KS from the Smoky Hill Chalk of Kansas (about 85 million years ago).

Reconstruction of Squalicorax curvatus  by Dmitry Bogdanov

Exceptional Squalicorax skeleton - absolute once-in-a-lifetime find

Squalicorax skull detail

Squalicorax curvatus SLE and PBE - surface etch.  Size bars: (1,2,6)
1 micron, (3) 100 microns, (4) 50 microns, (5) 5 microns.

There is no argument in the literature that this particular shark is a selachimorph neoselachian (read SHARK).  The external anatomy of Squalicorax curvatus teeth was examined first using teeth that had not been exposed to 10% hydrochloric acid (HCl) (panel 1), treated with HCl for 30 seconds (panel 2) or 3 minutes (panel 3-6).  There is not much relief on the surface of the untreated teeth, but you can easily see randomly oriented enameloid crystallites on the surface of a tooth treated for 30 seconds with 10% HCl.  Longer treatments remove all of this SCE/SLE from the surface and expose the parallel bundled enameloid of the Squalicorax tooth.  Panel three shows the parallel bundles at the level of the serrations in the tooth.  As the parallel bundles approach the serration, they turn direction, so that they point towards the edge of the serration, instead of towards the apex of the tooth.  As you zoom in on the parallel bundles, it is possible to see that there are two populations of bundles; one that runs parallel to the long axis of the tooth, and one that runs perpendicular to the long axis.

Cross section of Squalicorax teeth.  Size bars: (1) 500 microns,
(2,3,6) 50 microns, (4) 20 microns, (5) 5 microns

It is possible to see all three enameloid layers simultaneously in sectioned teeth.  In panel 1, the lighter area is the enameloid and the darker area inside is the dentine of the tooth.  The tooth is embedded in plastic.  In the upper right and lower left of the picture, you can see the serrations (cutting edges) of the teeth.  In panel 2, we are looking at the interface between the enameloid upper right and the dentine (left).  The arrows show where the dentine and enameloid meet.  The TBE (T) is seen next to the dentine (left) in panel 3, and the PBE (P) is to the right.  Panels 4-5 show the interface between the TBE and dentine.  As the dentine is approached, the enameloid becomes more like an SCE/SLE.  The dentine has a structure that looks a lot like bone, with channels for odontoblasts that are surrounded by concentric rings of dentinal material.  Shark teeth have not changed a whole lot in appearance in the past 90 million years.  There are small changes, but those mostly reflect stress introduced onto the tooth what the shark eats.

Almost to the pay-off for the paper.....