Your Inner Fish: Chapter 4 Summary & Analysis

Though teeth might not be the most glamorous anatomical structure, looking at teeth is an integral part of learning about the lifestyle of an animal because teeth tell scientists what an animal most likely eats. For humans, we have a mix of blade-like teeth for cutting meat and flat teeth for grinding plant or meat material. Our upper and lower jaws also fit together precisely (a fit called occlusion) to break up food with maximal efficiency.

Teeth are one of the most common finds for paleontologists, because an easily-preserved hard mineral called hydroxyapatite makes up much of the outer layer of the tooth. Teeth are especially helpful for mammal fossils, as mammal species have distinctive teeth. While reptiles all have similar teeth that they replace many times over their lifetime, mammals have teeth that occlude and are only replaced once. During the time period from 225-million to 195-million years ago, paleontologists see a progression from dog-sized reptiles with fairly simple teeth to small animals with mammal-type teeth that fit together inside a smaller jaw.

Shubin began studying these early mammals at Harvard, under Farish A. Jenkins, Jr., who specializes in looking for the fossils of early mammals. With the help of expert fossil finders Bill Amaral and Chuck Schaff, Shubin learned how to spot the distinctive signs of bone in the ground. At first, this was incredibly difficult for Shubin. Schaff, a traditional “cowboy” type despite his New York upbringing, showed Shubin how to search for fossils without wasting effort.

Once Shubin is able to see the bone in the midst of desert rocks, he realizes there are fundamental rules to fossil hunting. Rule one: go to rocks that seem most likely to have fossils, based on geological search images or past experience of productive sites. Rule two: don’t follow in the footsteps of other fossil hunters, but search new terrain. Rule three: Only one person looking for fossils per area of the site. The more Shubin practiced looking for fossils, the easier it got to identify fossil from rock in all sorts of terrain and lighting conditions.

Jenkins’ site in the Arizona desert was full of tiny animals with bones no more than an inch or two long. Their teeth were even smaller, but Shubin was fascinated by the signs of occlusion in tiny mammals 190 million years old. It gave him a humbling perspective on the development of complex human anatomy.

Back in school after working with Jenkins all summer, Shubin decides to lead his own expedition. With limited funds, Shubin needs somewhere fairly accessible, yet with the right type and age of rocks. With the help of Paul Olsen, a professor at Columbia University, Shubin settles on a swath of 200-million-year-old rocks in Nova Scotia, Canada. Amaral and Schaff come along for a two-week dig.

Back in Boston, Amaral works as the fossil preparator for the rocks that Shubin’s team found in Nova Scotia. He uncovers a tiny reptile jaw from an animal called a trithledont that shows signs of wear on the cusps of the teeth – evidence of occlusion. Shubin is incredibly impressed with Amaral’s find and learns that some of the most important discoveries actually occur once paleontologists leave the field.

Shubin returns to Nova Scotia in the summer of 1985, hoping to find more trithledont fossils, but is disappointed to find that the dig site from the previous year is now weathered away from the tide. Still, Shubin and his team decide to investigate nearby rocks on the beach. One day, Shubin and Amaral get stuck on a spit of volcanic rock because the high tide blocks their path back to the base camp. Though volcanic rock was previously thought to be too hot to support fossil preservation, Amaral notices a whole area of small fossil fragments.

Shubin and Amaral dig out the volcanic rock site, finding that there are patches of sandstone that protected the fossils from the volcanic heat. They find several more trithledonts, which provide valuable clues to the progression from reptilian teeth to mammalian teeth. Though trithledonts do not have true occlusion, their upper and lower teeth scrape against each other like scissors, showing an intermediary step to the fit of the mammalian jaw. After trithledonts, the fossil record shows an explosion of new mammal species that have many different kinds of teeth specialized for different kinds of chewing to expand their possible food sources.

Teeth and Bones – the Hard Stuff. The most immediately special feature of teeth is how hard they are compared to other organs. The mineral that makes the outermost layer of teeth extra-hard, hydroxyapatite, is also found in lower concentrations in bones and the inner layers of the teeth. This mineral distinguishes human hardness from the hard exoskeletons of other animals. Shubin then turns to investigating where hydroxyapatite came from.

The most common fossils from the ancient oceans are conodonts, first discovered in the 1830s by Russian biologist Christian Pander. Conodonts are small, shelly organisms covered in spikes that have been found on every continent on Earth. At first, no one knew what conodonts actually were. Finally, a professor of paleontology at the University of Edinburgh found a slab of rock in university storage that showed a primitive jawless fish with the distinct impression of conodonts in its mouth. Conodonts are teeth.

Part of the struggle in identifying conodonts as teeth was that the teeth were the only hard part of these ancient jawless fishes’ bodies, and were therefore the only part of the jawless fish that was preserved as a fossil. These fish most likely developed teeth in order to break through the hard exoskeletons of potential prey. Once animals developed hydroxyapatite-rich teeth, the mechanism for creating hard structures out of hydroxyapatite then became a method of protection. Fish called ostracoderms actually have a disk-like shield of bone covering their head that is entirely made of the tissue that human teeth are made of.

Teeth, Glands, and Feathers. Teeth are also special due to their specific method of development. Teeth are made of two layers of tissue that fold together, with the outer layer becoming the enamel and the inner layer becoming the dentine and pulp of the tooth. This two-layer process is also used in the development of all body structures that form within skin, such as scales, fur, hair, or feathers. Shubin compares the process to a new assembly line process; once teeth were developed, animals reused the same system for creating teeth to create many different body parts. This underlying process links organs as different as feathers, teeth, or even mammary glands.

Shubin recaps what the book has argued so far, tracing how the same organ can be found in many different creatures. Chapter 1 focused on finding versions of human organs in ancient rocks. Chapter 2 compared the bones of fish and humans, while Chapter 3 looked at the genetic similarities in the development of those bones. Chapter 4 highlights teeth to once again show the deep similarities between different body parts and animals.