More on Mesozoic Crayfish
Posted on February 20th, 2008 by blue collar scientistI recently did a little public outreach about astronomy, and because there were no clear skies to be had, we did a little lecture and question-answer session instead. In the last year or so, I’ve been making it a habit to start off the talk with a brief presentation on an unrelated science subject that is in the news. This time, I talked about Mesozoic crayfish and trace fossils from Australia. I hope to have the presentation file downloadable from here shortly; stay tuned.
Regular readers might remember that I covered this subject before - and now I have more of the story.
Dr. Anthony Martin, paleontologist at Emory University, is an acknowledged expert on trace fossils - that is, fossils that don’t preserve the body of an organism, but do preserve some indication of its anatomy and its behavior, such as footprints, burrows, and droppings. He’s also an expert about dinosaurs. He’s written important books about both subjects, as well as being heavily published in journals, mostly as the principal investigator.
He visited the Dinosaur Dreaming fossil site in Australia back in February, 2006. As he was walking around looking at the site on his first day, he discovered two large theropod dinosaur tracks. On the second day, he found a complex of crayfish burrows.
Crayfish burrows, 116 million years old, from Dinosaur Dreaming, Victoria, Australia. The scale card in this image is 10 centimeters long, or about four inches. Photo courtesy of Anthony Martin. Click to enlarge.
Paleontologists had been walking past the burrows for fourteen years, but either nobody noticed them, or nobody appreciated what they were. Martin saw them mainly because he was experienced and educated about what to look for - as they say, luck favors the prepared. At about 116 million years old, the burrows were an important find. Crayfish currently live on every continent except Antarctica and Africa. But many of the continents that crayfish are found on today are separated by large expanses of salt water, where crayfish can’t survive. Therefore, scientists thought that crayfish evolved and dispersed at a time when the continents they are found on were crammed together.
At the end of the Jurassic and beginning of the Cretaceous, Australia, Madagascar, South America, and India were all connected, but they had begun to move apart. If southern hemisphere crayfish had originated in Australia in the early Cretaceous, they would have had only a short time to expand to other continents. If this hypothesis were true, it suggests some specific predictions that could be made: Africa was already separated, so they wouldn’t be found there - and they aren’t. They might not have had enough time to make it to India, so they may not be found there either. It turns out that crayfish aren’t found in India, and neither are their fossils. So far, so good. And crayfish are found on Madagascar, South America, New Zealand, and Australia - all of which were connected, so this is consistent with the continental features of the time.
Another prediction can be made from this scenario: If southern hemisphere crayfish originated in Australia and expanded out from there, it would make sense if Australia to had more crayfish species today than its southern hemisphere neighbors. If it didn’t, that wouldn’t falsify the hypothesis, but if it did, it would lend it some support. And it turns out Australia does have more crayfish species - about 85% of all southern hemisphere crayfish species are found there.
There was a problem, though: although some crayfish fossils from around 150 million years ago are known from the northern hemisphere, none older than about 40 million years had been known from any of the southern continents. If southern hemisphere crayfish originated in Australia, the prediction of evolutionary theory and the theory of plate tectonics would be that crayfish body and trace fossils should be found from the early Cretaceous in Australia - fossils from sometime before 90 million years ago.
So with this discovery of 116 million year old crayfish burrows, these predictions are fulfilled. The burrows are of the right size and configuration for crayfish, and no other organism is known to produce just this morphology in their traces. In addition, the geology of the area supports a freshwater habitat favorable for crayfish. Everything was pointing to a significant find.
Having recognized the crayfish burrows, Martin asked the site director, Lesley Kool, the obvious question: “Do you have any crayfish body fossils from here?” It turns out they did not, but a crayfish body fossil had been dug up in Dinosaur Cove, another fossil site in Victoria, almost twenty years before. It had been sitting in the Museum of Victoria unstudied all that time.
Crayfish fossil, 106 million years old, from Dinosaur Cove, Victoria, Australia. Photo courtesy of Erich Fitzgerald.
One of the remarkable things about this fossil is how close it came to being destroyed by a rock saw. You can clearly see the slot sawed into the rock coming up from the bottom of the picture, slicing through the pincer, and heading toward the crayfish abdomen. There’s another saw cut that took out a chunk of rock that made up this crayfish’s upper back. It just goes to show that not all fossils - and not all important fossils, at that - are as clean and polished as the dinosaurs we see in museums. In addition to this fossil, fossilized claws from two other crayfish were found in the museum’s collections.
So, as a result of happening across the burrows, which led to asking about body fossils, Dr. Martin was able to describe the only crayfish fossils from the age of the dinosaurs in the southern hemisphere. Having written the paper, he saw it rejected twice, but got it published on the proverbially charmed third try.
In such haphazard ways is human knowledge advanced. In a lot of cases, paleontologists have already found interesting and important fossils - they just haven’t had an expert on that field around to recognize them or appreciate their significance. Gaps in our knowledge are caused not only by not having discovered important fossils (yet!), but also by not having studied the ones that have been discovered. This is how science works - it isn’t always in clean labs with white coats and microscopes, and it isn’t usually with perfect specimens. Sometimes it is considerably harder - and luckier - than that.
