Catching a Crab
Imagine yourself visiting the east coast of the United States in the early summer. You decide to take a romantic moonlight walk on the sandy shores of Delaware Bay. But wait … what are the primeval “giant bugs” coming out of the water by the thousands? Looking like creatures that time forgot, these “living fossils” mating and laying eggs on the shore are horseshoe crabs (or limulids). Horseshoe crabs have a long fossil record, but are represented today by only four species: one along the east coasts of North and South America, and three on the shores of the Indian and Pacific oceans. About four years ago, my colleagues and I found fossils that show these animals have been living in the same sort of shoreline environment for almost half a billion years…
The story of the fossil horseshoe crabs shows how scientific progress, though the result of knowledge, skill, and hard work, also depends on the following of hunches. In fact, it suggests to me that scientific instinct is a skill that can actually be trained and cultivated. This story also indicates that, if paleontologists are to continue to discover new and unknown fossils, we will have to change our approach to field research. Since we like to study superb fossils, and large numbers of them, paleontologists tend to quickly walk across potential sites to determine where the fossil-rich horizons are. They then home in on those horizons and focus their collecting there. This approach works beautifully for known groups, which are known because they are relatively common and because they can be found at fossiliferous sites. If you want to find the rare and unique things, though, you will need to look elsewhere. In other words, to find unusual fossils, you should be looking in the places where you think fossils are absent.
But enough of this philosophizing. Let’s get on with the story…
In about the year 2000, Ruth Bezys, who was then at the Manitoba Geological Survey, told me that a man working on their drill rig had found some interesting fossils in the Grand Rapids Uplands, while looking for rocks on which to do artwork. Ruth thought that the fossils were eurypterids (an extinct group commonly called “sea scorpions”). In the fall of 2001, I traveled to Grand Rapids with my associate Ed Dobrzanski. Grand Rapids is 400 km north of Winnipeg, and the site is some distance beyond that, in an area many kilometres from any habitation, so this is not the sort of trip undertaken at the spur of the moment.
The site was at a place where patches of exposed bedrock are spread over a large area, interspersed with loose rubble and vegetation. Although it now sits within a boreal wilderness characterized by long and bitter winters, the setting was very different when the rock was deposited. This dolostone formed as sediment during the Late Ordovician Period (about 445 million years ago), when this region was covered by broad tidal flats and lagoons under a hot tropical sun.
In two days we discovered one beautiful eurypterid specimen and a few lamp shells, but otherwise had nothing to show for the immense number of loose slabs that we had flipped and examined. The unusual fossils were present, but they were extremely rare. Since I was heavily involved in exhibit work at that time, I could not begin a new field project. When I had a spare moment I would think fondly of the promise this site held, telling myself that I would eventually get back there for some serious collecting.
After a long wait, in late 2004 I spent a week in the Grand Rapids Uplands with a group of Museum associates and volunteers. For the first few days we found very little. This was a frustrating time; we knew that the unusual fossils had to be there, but they were obviously rare and were probably “hiding” in a small area of the large exposure. One could almost imagine the eurypterids scuttling deeper into the crevices between the rocks as they sought to escape our prying eyes.
Finally, in the golden light of the evening before our last full day, volunteer Debbie Thompson discovered an almost complete eurypterid, giving us an idea of where we should be concentrating our efforts (this was par for the course for Debbie; she always finds more fossils than anyone else!). On that last day, we began to collect wonderful things: more eurypterids, but also jellyfish, cephalopods (relatives of squids), and other fossils that we couldn’t identify. It was clear that the site was going to yield many exciting creatures; it was also clear that it wasn’t going to give them up without a struggle.
In the early summer of 2005 I returned for a few days with Ed Dobrzanski and Sean Robson. Now that we knew where to collect, we were able to find fossils more steadily, but we still felt fortunate if a morning’s efforts resulted in one good specimen. Again, the last day of work produced remarkable results. Splitting apart a bed, I saw the fossil of a joint-legged animal with which I was not familiar. It looked like a tiny horseshoe crab, just a few centimetres in length (this is the specimen in the photo at the top of this entry). I knew, however, that the fossil record of horseshoe crabs started much later, so I thought it must be something else (I was not an expert on such animals). We packed it up with the rest of our samples and brought it back to the Museum.
Six weeks later, Ed and I were 600 km from Grand Rapids, working with Dave Rudkin and others near Churchill on the coast of Hudson Bay. We had spent several field seasons in the Churchill area since 1996, and we thought we had discovered many of its secrets, but we were in for another surprise. Among many other fossils occurring in this area, we had previously found an excellent eurypterid head, but it was within a boulder that had been transported from its original location. Dave and I had discussed where similar fossils might occur but had not tracked down the source bedrock. Looking for rocks with characteristics similar to those in the Grand Rapids Uplands, we quickly narrowed our search, focusing in on a quantity of likely-looking material. Washing slabs and turning them in the light, we began to find wonderful and strange fossils within the first hour. It is truly shocking is that this was less than 20 metres from a spot where we had parked the truck on many previous visits, but we had written off this rock as “unfossiliferous” and had paid it no further attention.
We found a lot of pieces of eurypterids, beautiful seaweeds, and intriguingly, arthropod heads that looked to be the same as the one from Grand Rapids. The next day, we hit the jackpot; Dave found a complete animal, and it was almost identical to the Grand Rapids one. In the evenings we carefully washed and examined the specimens. They certainly looked like horseshoe crabs, but Dave, with his greater knowledge of such groups, thought that this could not be the case. How could we, in the space of six weeks, have found specimens at two separate sites that would push the age of a known group back by 100 million years? They had to be something more common – but what?
Back at the Museum I photographed the fossils using many different kinds of lighting, so that all of their details could be assessed. The more we looked at them, the more we realized that they could not be fitted into any of the groups known to occur in the same geological period. They did share characteristics with younger fossil horseshoe crabs, though of course they were more primitive than any known so far. The preservation of some specimens is quite wonderful; they have unmineralized outer surfaces showing preservation of eyes and other features similar to those of the horseshoe crabs that are still with us. On the basis of our careful studies, we prepared a scientific paper, published by the journal Palaeontology in January 2008. We have named the fossils Lunataspis aurora, literally “crescent moon shield of the dawn.”
Why should this discovery be of interest? Of course, it gives us important new information about animals that are still with us. Horseshoe crabs are clearly among the oldest kinds of animals to share our planet. This discovery will also necessitate the development of new scientific theories, because these specimens are older than any member of a different group that was previously thought to be ancestral to horseshoe crabs! But it is the environmental story that intrigues me most: as we are discovering the ancient tropical shorelines of central and northern Manitoba, we are finding along those shores the members of a group that can still be seen on warm shorelines today. It is another piece of the wonderful puzzle of the evolution of life and environments through deep geological time.
(a shorter article containing some of this content was published previously in Features, © The Manitoba Museum)
Click here if you want to hear an interview I did about fieldwork at these sites.