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Ostracoderm Classification Essay

Part 1 of 2: General Overview

Fossil shark image courtesy of Karl Stiger

Sharks may be on the top of the “Most Misunderstood Animal” list, but these creatures have been around a lot longer than humans, and their unique adaptations have led them to be the excellent sea predator that they are today.  These animals have an amazing sensory system that can detect electromagnetic waves put out by prey from long distances. Many sharks also have a fantastic sense of smell, leading them to injured fish or mammals. Through trial and error along their evolutionary journey, these creatures have developed amazing features, which can help us understand how they’ve survived as top predators in the ocean for so long.

Shark evolutionary tree. Sharks first began to diverge into their own type of animal during the Cambrian and were known as the Chondrichthyes. Image by Michael Benton 2005. Click image for larger view.

 

Sharks first began appearing in the fossil record as a distinguishable family around 450 million years ago. The first shark-like animals came about during the late Ordovician and had previously evolved from jawless fish known as ostracoderms. To begin distinguishing sharks from other ocean dwelling fish, we must back up in classification. All cartilaginous fish are classified under the class of Chondrichthyes and further subclassed into the Elasmobranchii. This Chondrichthyes class includes sharks, skates, rays and sawfish. An early class of sharks, called Hybodontids, existed until the late Cretaceous period and modern sharks directly evolved from this group of animals. Many Hybodonts can be recognized based upon their teeth alone, as they have distinctive shapes compared to both modern and prehistoric sharks. Hybodonts are unusual in their appearance in that they are also sexually dimorphic from one another. The males of these sharks had large dorsal and cephalic spines (spikes on the sides of the head). Females lacked prominent versions of these structures.

Fossil shark image courtesy of Karl Stiger.

Throughout their evolution, sharks have come to develop strange forms, some of which are extinct today, while others persist. Among these is the famed helicoprion, which is most famous for its stunning jaw formation. This buzzsaw-shaped jaw is the only part of this shark to have ever been preserved to the knowledge of paleontologists. Another oddity is the shark Stethacanthus, known for its large “spine brush” on its back that takes the place of the dorsal fin and the rough patch of skin on its forehead. This anvil shaped fin seems to be present only in the males of this genus. An extant species of shark that still retains its prehistoric appearance is the frilled shark. This  deep sea fish is one of only two sharks still surviving in the  family Chlamydoselachidae. These sharks and many like them, both extinct and extant, are the trials and errors of an evolutionary system progressing towards what will persevere the best in its environment.  

As many of us know, sharks do not actually have skeletons. Their “bones” are made almost entirely of cartilage. Cartilage is what makes up the noses and ears of humans and many other animals. This super flexible material, although helpful for the lifestyle of sharks, is incredibly hard to preserve. Cartilage is easily biodegradable, and is often destroyed soon after the death of an animal.Very few entire shark fossils have been found for this exact reason. The most common occurring fossils found of sharks are the teeth, which also happen to be one of the most common fossils in the world. There have been almost 3000 species of fossil sharks identified this way. The teeth are made of basically the same materials as ours, including an enamel-like substance called dentin that preserves very well. Interestingly enough, shark teeth are made of very similar materials to their scales (called denticles). These denticles can also be preserved given the correct circumstances, and have been found in mass quantities.

Sharks have evolved several unique characteristics during their evolution, one of which is the rapid replacement and regrowth of teeth. Considering this is the only part of the shark that is usually fossilized (excluding skin), we can identify many species (like the Hybodonts) based on tooth shape. Sharks continually grow and replace teeth in their lifetimes, and each tooth is connected to the jaw by ligaments. It has even been hypothesized that their teeth evolved from their scales, and as such, are simply modified scales. This would be helpful in explaining how they can easily develop new teeth. Much like our own skin, shark skin (and teeth) are constantly in a state of replacement so that it can stay fresh and get rid of any damage that may have occurred.

These microfossils of the skin are easy to find when you’re looking for them and also give a good indication of species, just like the teeth. Sharkskin is in essence a miniature version of their teeth. Their scales are tightly packed next to each other and look like chainmail. The very first scales found dated back 450 million years ago. These scales are widely debated by shark paleontologists, because they are similar, but not exact to what shark scales looked like in later evolution and now. The oldest undebatably “shark” scale fossils date back to 420 mya.

Shark scales; also known as denticles. Very similar to teeth on a microscopic level. Image by The American Museum.

It is also thought that sharks evolved from ancestors that were bony fish. Many prehistoric sharks have more durable skeletal forms (including the presence of ribs) that were selected against later in evolution and are not in most modern sharks. Similar to this is the unique structure of the vertebrae of these animals. The center of their vertebrae is very dense, and can be preserved occasionally. This is because the “backbone” of the shark calcifies for its entire life. In mammals, this usually signals degeneration of joints or bone surfaces. In sharks, it appears to be necessary for survival.

Sharks have some of the most amazing history in the story of evolution. They have evolved incredibly specialized systems by trial and error over the past 400 million years. The animal you see today is the result of this specialization for a perfect sea predator. Without all of these unique adaptations the shark would not have persisted for so long, as it still does.

**This article was based off of an essay written by Karl Stiger. Karl was born on March 23rd 1986 in New York City. As a child, growing up in Brooklyn, he frequently read and studied science books of varying disciplines including vertebrate paleontology, which led him to take on the basics of geology. And as a hobby, he collected rock samples near my home. As he got older, Karl wanted to go to college, so he returned to New York after spending a year in Baltimore, then took pre-college courses at the Jewish Guild for the Blind in 2009 and was soon accepted to a museum studies program at Queensborough Community College in the Fall of 2010. He majored in gallery and museum studies and participated in internships at various museums and galleries across the five boroughs, including one at the American Museum of Natural History, where Karl studied various cultural artifacts and fossil specimens.

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