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
No comments:
Post a Comment