Lab 6: Fossils - Chordata

 

 

Chordata

 

This group includes you and other vertebrates (birds, mammals, reptiles, amphibians, fishes), as well as primitive animals with worm-like bodies. Primitive chordates have several features that are common to the group, including a dorsal (toward the top of the body) nerve cord, which you know as the spinal cord. They also have a notochord, which is an elongate, stiff, fleshy structure running down the length of the body, giving structure to the body, enough so that muscles can fire down the body en echelon to form the familiar "wiggly" swimming motion of fish. The notochord is modified into the vertebral column in vertebrate chordates.

 

Lancelets

 

The living animal called the lancelet (lance = spear-like body) shows primitive features of Chordata:

 

Image Source: Wikipedia

 

The head is to the left, and tail down at lower right. Look along the top of the body for a long, linear tube-like structure. That's the notochord, above which is the spinal cord. Here's a sketch:

 

 

In the drawing, #2 is the notochord and #3 is the spinal cord.

 

Lancelets live in the ocean, burrowed in the sediment, but can swim. The notochord is a kind of stiffening structure, giving solidity to the axial frame of the body, upon which muscles down the body can work. Without the notochord, locomotion would be limited to the seemingly aimless wandering of organisms such as earth worms.

 

Pikaia

 

From the famous Cambrian Burgess Shale fauna comes a host of strange creatures, including a tiny one with a worm-like body. There is a notochord (or proto-notochord) and other features that indicate Pikaia represents a very early chordate animal:

 

Image Source: Wikipedia

 

Another feature of primitive chordates like the lancelet and Pikaia is a post-anal tail -- a tail that is positioned in the body posterior to the anal opening. Pikaia and the lancelet are quite small, only about as long as a finger or less.

 

Jawless Fishes

 

From an ancestor like Pikaia, jawless fishes evolved during the early Paleozoic. They had a kind of sucker-type mouth, but no bony jaw parts. Most of them were small, not getting longer than a meter or so. Armor is a special feature of these jawless fishes, present in plate-like bone embedded under the skin, and forming in a position over the head and shoulders, but not extensively around the body. Cephalaspis , of the early Paleozoic, is representative:

 

Image Source: Wikipedia

 

Jawed Fishes

 

Later, of course, we see the appearance of fishes with bony jaws. One early Paleozoic group of fishes, with no surviving descendants today, is Placodermi . Placoderms (Plac = bony plate; derm = skin) had bony jaws, but they were developed from different tissues and have different features from the jaws of the vertebrates we know today, and most other vertebrates with jaws. Dunkleosteus (Dunkle = a geological rock unit; osteus = bone) is representative of the group, and grew quite large, several meters long. Placoderms were among the major predators of the day, but were eventually replaced by sharks and other vertebrate fishes. Dunkleosteus is the big one in the back:

 

Image Source: Wikipedia

 

Sharks

 

Sharks go way back to the Paleozoic Era, finding the familiar torpedo-shaped body early on in their evolution, and sticking with it for the long haul of geological time. They had jaws with sharp teeth early on, but then, as now, most of their body was all cartilage (Grab your ear -- that stuff), which is not mineralized bone, so it doesn't fossilize. The teeth of sharks are made of hard bone -- enamel. Shark teeth are, thus, common fossils, but not so much for the rest of the body.

 

Image Source: Wikipedia

 

Shark teeth can be very common. Have you ever seen a shark feed on a large prey item? They take a big bite and violently twist and turn their body and head to take off a chunk, and in the process loose a few teeth often. Their teeth are constantly being replaced by new ones, though, so it is just the way they operate. As a result of this behavior, the sea floor receives a good supply of shark teeth over time. Sea bottom sediment accumulates slowly. Think about how many sharks would pass over an area of the sea floor over a period of hundreds, or even a few thousand years. That's a lot of shark teeth over time.

 

Other Vertebrates

 

We could keep this list going, but you generally know the groups from here. There are bony fishes of many types -- any fish that is not a shark or the others mentioned so far (which are mostly extinct), is a bony fish. By the Devonian there were land-dwelling vertebrates, the first tetrapods (tetra = four; pod = feet). And from there we have major groups including amphibians, reptiles, including birds, and mammals. Of course, tons and tons of vertebrate fossils have been collected, and fill up the shelves of many museums. We'll delve into the evolutionary history of these animal groups in the next lab.