How do you fish with a microscope?

Being a paleontologist means I spend warm spring and summer days exploring the outdoors for fossils eroding from crumbling cliffs, buttes, and river banks. In the fall and winter, I’m in my prep lab taking care of any number of tasks, including preparing fossils found during the previous year. Here in northeast Ohio, the local geology exposes wonderful slices of the Late Paleozoic Era. Our particular slice is Devonian which is between 358 and 360 million years old. The Devonian, also known as the “Age of Fishes”, represents a momentous time in the evolution of vertebrates; jawed fish exploded in diversity. Sharks had come into their own as dominant marine organisms. Jaws specialized, teeth sprouted, and by the end of the period vertebrates were exploring the banks of rivers and streams as fins lobed off into limbs.

When we go looking for fossils here, we are hunting for real sea monsters. The city of Cleveland and other ‘burbs in this region are built on a literal “Davy Jones’” museum of fossil wonders. Local rivers have carved into gray Cleveland bedrock that preserves the remnants of an ancient subtropical sea; geologists dubbed this the Kaskaskia Sea. The Kaskaskia flourished with life, and it was irrefutably ruled by a giant tank of a fish with hatchet-bladed jaws and a bone-splintering bite. Paleontologists named this trailer-sized swimmer Dunkleosteus (DUN-kul-AHS-tee-us [Dunkle’s bones]). A stunning diversity of other placoderm (plate-skinned) fish, small bony fish, and odd pocket-headed invertebrates proffered a kaleidoscope of morphological variety. Instead of a simplistic, “primitive” food web, marine niches were filled to the brim with filter-feeders, shell-crushers, and several trophic levels of predators including many species of early sharks.

Devonian sharks did not appear quite like the sharks we know today. Though the overall body shape would be hard to distinguish from their modern descendants, the major difference could be seen in the muzzle. Devonian shark snouts were blunt and rounded with a flat, terminal mouth. There was no pointed nose, and the jaws were fixed solidly at the front of the face. They really looked kind of cute, like a muppet, but that muppety mouth was packed with hundreds of needle-sharp, multi-cusped teeth.

 

One day last summer, Ashley and I were out with a museum crew on a fossil fishing expedition. We were tracing the banks of a creek looking for fossil sharks and placoderms and we were coming up with bupkis. Late in the afternoon Ashley’s rock hammer sang out on a small shale slab that split through perfectly, exposing a single delicate shark tooth. It was an amazingly lucky find, and our only find that day. Fossils do not easily reveal themselves in the Cleveland Shale, and many field days turn up fruitless despite our best efforts.

The most commonly preserved type of shark fossils are the teeth. Teeth are made from hard enamel, while the rest of a shark’s skeleton is composed of soft tissues wrapped around a cartilaginous skeleton. No bones. No extensive ossification. When sharks die, they rot completely away and nothing is left to fossilize but a pile of teeth. Being from the Late Devonian, Ashley’s tooth was epically ancient. It had grown and bitten things and fallen to the bottom of the sea to be buried in mud all well before there were such things as reptiles, before the formation of the supercontinent Pangea, and even before trees. But which species of ancient shark did the tooth belong to? The Cleveland Shale was host to several genera and species, each with unique dentition. I needed to go fishing with a microscope.

 

Back in the fossil prep lab, I placed the thin shale slab containing Ashley’s little shark tooth on a microscope stage and wheeled the focus knob until everything snapped into clarity. Although the Cleveland Shale appears dark gray in outcrop, under magnification it can take on ruddy hues where iron-rich minerals have oxidized. Flecks of chlorite and quartz sparkled under the intense illumination as I rotated the specimen around. The tooth looked to be perfectly preserved. There was one large central cusp, or spike, and flanking that were two shorter accessory cusps. I needed to see if there was a set of yet smaller cusps between those three primary spikes. The presence or absence of those, along with any ornamentation would be key in identifying the species.

 

 

Fossil preparation begins with a detailed examination of the specimen before touching it with a tool of any kind. I visually scrutinized the tooth for indications of fractures, loose pieces, matrix coverage, and anatomical landmarks to aid in identification. This tooth was smaller than a fingernail, so to reel in the data I was looking for, the matrix had to be removed.

From my tackle, I selected a slender dental pick that was as sharp as a fish hook. Gazing through the eyepieces, I carefully brought the tip of the tool into the field of view. At 60x, it appeared not as a needle tip but as a rounded steel cone. Lowering the tool to the shale, I gently began to work matrix away from the tooth’s perimeter. I proceeded horizontally across the surface of the slab in single, light pushes. Grain by grain, layer by layer, the shale yielded to the pick. Silica in the shale generally makes it very hard, but the surface along which the slab broke had been softened by weathering. It took only a few patient minutes to clear away much of the obscuring matrix. I marveled at the tooth for a moment. For the first time in over 1/3 of a billion years, it was free of the encasing stone. There were indeed two additional cuspules nestled between the primary and secondary cusps, and the waxy black enamel was unornamented. I had my answer. Without a doubt, this tooth belonged to Cladoselache.

The name Cladoselache (KLAD-oh-sell-ACK-ee) means “branched shark”, a name given because of the multiple points on the teeth. Cladoselache is the most common shark genus found in the Cleveland Shale. It was a small to medium-bodied shark that topped out around six feet (1.8 m) in length. We know this because the Cleveland Shale is a shark lagerstätte. Lagerstätte is German word meaning “storage place”. It is applied to special fossil deposits where there is either a huge abundance of material preserved (konzentrat = concentrated), or exceptional soft tissue preservation (konservat = conservation). Being a konservat lagerstätte, the Cleveland Shale is one of the few fossil deposits on earth that preserves entire shark bodies. Skin, muscle fibers, fins, scleral rings, 3-D skulls and brain cases, gill arches, and even stomach contents can be seen inside the dark gray outlines of shark bodies. It is a truly remarkable deposit, and the sharks found in the Cleveland Shale are among the oldest complete sharks currently known from the fossil record.

As amazing as the complete fossil sharks from the Cleveland Shale are, it is the elegantly simple anatomy of their teeth that coined the name for an entire genera. Ashley’s single tooth provides a wonderful example. We do find whole fish preserved in Cladoselache stomachs. These small bony fish were caught and swallowed tail first - clear evidence of high-speed pursuit. These fish are not cut up or torn apart, as Cladoselache teeth were spikes suited for ‘simple’ grab-and-gulp feeding. Prey were devoured whole. The “branched shark” was a living torpedo – a long, muscular cylinder with a broad tail propelling a mouth full of fish spears toward prey. One day, not so long ago, a persistent paleontologist made a lucky strike with her hammer on a special piece of shale, and found one of those little fish spears. And I went fishing with a microscope.

 

Photomicrographs in Figures 6 – 8 taken with Celestron 44206 Professional Stereo Zoom Microscope