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.....

Is you is or is you ain't my sharky?

Life was going swell, then work showed up and shot everything to hell.  My writing got in the way of my writing anyway.  So this is what I have been doing instead of writing blog entries.  One line of my research is examining the value of tooth enameloid characteristics in sharks as a way of determining whether they are a "shark ancestor/primitive shark" or a "modern shark".  "Sharks" are generally thought of as fish with a skeleton made out of cartilage (chondrichthyan), a mess of teeth in the mouth that are replaced over time, tooth like scales (denticles) in their skin, and a torpedo- shaped (fusiform) body.  As such, shark remains can be identified from the Devonian Period, over 400 million years ago.

1) Enameloid layers in shark teeth; 2) possible directions of
parallel bundles; 3) sectioning directions of teeth

Shark biologists spend a bit of time arguing about what characteristics make a chondrichthyan a modern shark or a shark ancestor.  The one telling characteristic of the teeth seems to be the structure of the tooth enameloid.  A shark's tooth has a core of dentine, surrounded by fluoroapatite crystals.  Primitive sharks such as the ctenacanths, symmoriids and hybodonts had a single-layered enameloid composed of randomly oriented single crystals of fluroapatite (single crystallite enameloid - SCE).  A modern shark (selachimorph) has a triple-layered enameloid, with an outer layer of SCE (called shiny layered enameloid - SLE) plus two underlying layers of bundled crystals.  The middle layer is composed of bundles arranged in parallel (parallel bundeled enameloid - PBE), while the inner layer next to the dentine is composed of interwoven bundles (tangled-bundled enameloid - TBE).

Looking at these enameloid crystals takes some doing, since they are held in place with smaller "cement" molecules.  Fortunately the enameloid crystals are more acid resistant than are the cement molecules, so 5 sec to 3 minutes exposure of fossil shark teeth to 10% hydrochloric acid is usually enough to see the enameloid, depending on what layer you want to look at, and how you have prepared the teeth.   With whole teeth, 15-30 seconds is enough to see the randomly oriented crystals of the SCE/SLE; while seeing the PBE may take 1-3 minutes.  You can see all of the layers at once if you cut the tooth open (or embed it in plastic and sand it down) and then treat with acid for about 5 seconds.

Gold coated Ptychodus tooth, about 1 cm across

Enameloid crystals are really small, so you have to use a very powerful microscope to see them.  A regular light microscope will usually allow for a 1000X magnification, but you need to magnify these teeth about 5000X to see the individual crystallites of the SCE/SLE well at all.  This requires the use of an electron microscope, which uses a beam of electrons in a vacuum to image a specimen instead of light.  Most biomaterials are natural insulators, which means that they will absorb electrons and not reflect or re-emit them.  To solve this problem, we coat the specimen with a one atom thick layer of gold.  These sharks are all pimped out.  The gold will allow us to see the shape of the surface they are deposited on by reflecting electrons (backscatter) or by absorbing an electron and emitting one in its place (secondary electrons).

Tooth embedded in plastic, ground sandpaper, and coated in gold.
Copper tape is used as a pointer

The Hybodont Tooth

One group of primitive sharks that are seen as being ancestral to the modern shark are the hybodonts.  The first record of hybodontiform sharks is seen in the Mississippian of the Carboniferous Period, about 340 million years old.  The hybodonts were very successful, surviving late into the Cretaceous Period.  This is an evolutionary life of about 270 million years.  Remember that dinosaurs went extinct only 67 million years ago.  The hybodonts had mouths that were not overhung by their noses (rostrum), large cranial scales, barbed spines in front of their dorsal fins, a variety of different denticle types, and SCE on their teeth.

Reconstruction of Onychoselache

One of the early hybodontiforms was Onychoselache.  They were very small, about 10 inches long with teeth that were about 1 mm in maximum dimension.  Their teeth had low, flat crowns for crushing hard shelled organisms.  They had barbed dorsal fin spines, oval hooked denticles on their pectoral fins, and c-shaped denticles along their lateral line (sensory region on flanks of fish).  I have found very similar remains from the 307 million year old Farley Limestone of the Kansas City Group.

Hybodontiform parts:  scale bars(1-5) 1 mm, (6-9) 0.5 mm,
(10-11) 0.2 mm

The remains at the right are from an indeterminate hybodontiform.  There is not adequate material to describe a fish, and while the remains are associated, they are separate bits and pieces.  This is just not enough material to describe an organism down to the species level.  Views 1-5 are various views of a tooth: 1-top (occlusal) view; 2-front (labial); 3-side (lateral); 4-broken side (medial); 5-back (lingual) views.  View 6 is a piece of dorsal fin spine with hooked denticles on the posterior (back) side.  Views 7-9 are denticles from the pectoral (front) fins and 10-11 are denticles from the lateral line.  I sectioned several pieces of these teeth and looked at the structure to make sure that they were really hybodontifom and had an SCE.

Sectioned hybodontiform teeth:  Scale bars: (1)  200 microns;
(2) 10 microns ; (3,4,6) 1 micron; (5) 50 microns

The tooth crown is composed of dentine, which extends in columns up into the enameloid.  In the electron micrographs at left, the dentine is dark and the enameloid is light colored.  In panel 1 you can see columns of dentine approaching the surface of the tooth.  The enameloid has several channels for conducting odontoblast (tooth-building cells) processes through the enameloid (panel 2).  In panels 3-4, randomly distributed enameloid crystals make up the outer layer of the tooth.  Dentinal tubules are seen in section in panel 5.  The junction (arrows) between the dentine (D) and enameloid (E) is seen in panel 6.  The SCE is clearly visible in this section as well.  A channel carrying an arm of an odontoblast cell can be seen between the left two arrows.

That is about 3 months worth of work to get everything done just right.  Next is the last two years of my research life.




Reconstruction by Kahless28