Museum scientists and staff made a fossil collecting expedition to North Dakota and Montana last summer to gather specimens for a new exhibit, The Last American Dinosaurs: Discovering a Lost World, which will highlight a complex ecosystem shared by dinosaurs and many other organisms just before the end of the Cretaceous. The exhibit is scheduled to open in November, 2014, and we've been processing the new fossils in FossiLab ever since they arrived at the Museum last August. The biggest job, by far, is a continuing search through bags full of loose rock for the fossils of small animals that lived alongside the dinosaurs.
In the lab, we soak the chunks of rock in warm water to break them apart (below, left) and pour the resulting mud onto a stack of sieves which lets fine sediment, but not the fossils, pass through. If the rock hasn't completely broken down at this point (below, right), we put the sieves into our sediment washing machine.
The sediment washer, shown on the left, is nicknamed "Duncan." It sits by the sink slowly dunking two stacks of sieves, over and over again, into buckets of water. We let it run until all the clumps have broken down and the sediment has washed away.
The photo below shows what was left in a set of sieves once all the fine sediment was gone -- mostly pebbles and pieces of charcoal, with a few fossils hidden among them. Each sieve in the set has a different mesh size (2mm, 1mm and 0.425 mm) because it is easier to find the very smallest fossils when they've been separated from the larger material.
The photo on the left shows pairs of fossils trapped in the different sieves in a set. The two fossils on the left, a broken piece of turtle shell (top) and a scale from a gar fish (bottom) were too large to pass through the 2mm mesh and stayed in the top sieve. The fossils in the middle, a fish vertebra (top) and a fish tooth (bottom) washed through the 2mm mesh but were caught in the 1mm mesh of the middle sieve. On the right, the tiny fragment of a fish jaw (top) and a small fish tooth (bottom) passed through the larger sieves, but were trapped by the fine mesh of the bottom sieve.
Below is the entire collection of teeth found in five bags of rock from one site after many hours of washing, sieving, and searching for fossil under the FossiLab microscopes. The teeth are from several types of fishes and crocodilians. We didn't find any teeth of small dinosaurs or mammals in these bags, but we expect to discover some as we search through rock collected at the other sites. Sometimes we have to screen large quantities of rock to find such fossils.
Photos by Abby Telfer.
Our sediment washer was designed and build by Ken Moffett at Macalester College.
Follow this link to read an earlier post about how we collected the fossil-rich matrix we are processing in FossiLab.