Brazil 2012 Fieldwork Diary Entry 6: Sharks Patrol These Waters
Fossil spine from a large xenacanth shark embedded in rocks of the Pedra de Fogo Formation. In life, the spine was located near the animal's head. Scale bar is in centimeters. Photo by Ken Angielczyk.
In my last entry, I noted that geological evidence suggests that some of the rocks that we're working in were formed near the shore of an inland sea, but that the environment became increasingly terrestrial as time passed, with some of the younger rocks representing sand dunes. Over the past two days, most of the fossils we discovered were preserved in the rocks that formed along the shoreline and are the remains of aquatic animals.
The most impressive of these animals are the large xenacanth sharks (an extinct group of sharks that were common in the Permian). Typically, very little of their skeletons are preserved because they were mostly made of cartilage, a material that decays before it can be fossilized. Most of what we find of them are large spines that were made of a mineralized tissue similar to bone, and which have a distinctive pattern of ornamentation. Last year we found a few of these spines, and the one in the picture above was found yesterday (April 15) by Christian Kammerer. We also found teeth of these sharks last year, but so far haven't discovered any this year.
A second denizen of the Pedra de Fogo are lungfish. Very few fossils of lungfish have been discovered previously in these rocks, but yesterday Jose (one of our drivers who is also a good fossil finder) discovered a very large lungfish tooth plate. Unlike us, lungfish only have four teeth in their mouths, and they have a very distinctive shape that makes them easy to recognize. We found additional lungfish toothplates today (April 16), although they were much smaller and may represent juvenile animals. Although rare today, lungfish have a long history in the fossil record, and are important in evolutionary studies because they are closely related to terrestrial vertebrates.
Lungfish toothplate from the Pedra de Fogo Formation. This is one of the first lungfish fossils to be discovered in these rocks, making it an important specimen. Scale bar is in centimeters. Photo by Ken Angielczyk.
Besides lungfish, we found members of the other main group of fish, the ray-finned fish. Several exciting specimens that we discovered yesterday that represent this group are jaws with teeth preserved. Today, in rocks that are slightly younger and probably formed in a more terrestrial environment, we found other specimens of ray-finned fish, but these were still articulated (i.e., the parts of the skeleton were still attached as in life). Martha Richter has seen similar specimens from that locality, and thinks they are early representatives of a group of ray-finned fish that were more common and diverse in the Mesozoic Era of Earth history (about 205 to 65 million years ago).
Jaw of a ray-finned fish showing teeth. Scale bar is in centimeters. Photo by Ken Angielczyk.
You may have noticed that I haven't mentioned any synapsid specimens yet. So far we haven't discovered any, but we have found fragmentary remains that seem to represent the archaic amphibians (or temnospondyls) that were common during the Permian Period. Many temnospondyl bones have a distinctive pattern of sculpturing on them that helps to differentiate them from similar-looking fish bones that we also find in these rocks. We only discovered a few of these bones so far, but they are important because they represent terrestrial vertebrates.
A probable skull bone from a temnospondyl amphibian. The sculpturing of the bone helps to differentiate it from similar fish bones found in the same rocks. Scale bar is in centimeters. Photo by Ken Angielczyk.
We'll carry on our search for the elusive synapsids tomorrow, but the fossils we collected so far are helping us paint a more complete picture of the animals that were living in the Parnaíba Basin during the Permian.