One of the great privileges of my job is that I get to participate on graduate student committees. Many times this allows me to get up close and personal with data sub-disciplines extend far from those of my particular expertise. You know that I have a fondness for fossils and the stories they tell. So participation on a graduate committee that allows me to get an up-close look at current research on fossils is especially gratifying.
Recently* I participated in a doctoral dissertation defense. These are typically two to four-hour affairs in which the committee is presented with an hour-long research seminar by the PhD candidate and then gets to explores every nook and cranny of a research project. Although doctoral defenses can be stressful to the candidate and the committee at times, more often than not they are a fun exploration and appreciation of the details and broader implications of several years of a doctoral candidates research.
During these intellectually simulating adventures I get to examine the data that lie behind the figures and tables, evaluate the methods used to collect that data and explore the validity of the conclusions drawn from that data. In this particular defense I was able to ask questions about dissertation chapters which reported the results of experiments with taphonomy—the study of how fossilization occurs—and details of measurements made of thousands of fossil clam shrimp—the focal organism at the heart of this candidates project.
What is a clam shrimp?
Clam shrimp are not clams but rather a small filter-feeding crustacean that produces a protective bivalve carapace that look superficially like a clam shell.
More than 100 species of clam shrimp have been identified. All of them are very small (3 to 15 millimeters in size), live in freshwater pools and have very short lifespans (a week up to two months). Most make a living in small temporary (ephemeral) pools. These could be depressions in a field, a rut on the side of the road, or shallow depressions on rock outcrops in a desert. Essentially anywhere water may collect for weeks to months every year or every few years you will probably find clam shrimp. Rock pools in deserts may completely dry up and the soil 120 degrees in the summer but add some water and a week later and they will be teaming with clam shrimp and other small organisms.
How do these animals suddenly appear in these pools?
Clam shrimp have a life cycle that is well adapted to these extreme environments. These crustaceans lay fertilized eggs at the bottom of the pool as it is drying out. The adults die in mass but the eggs can survive in a desiccated state for months, years or even decades. My colleague has collected bone-dry soil from rock outcrops around the world and stored that soil in Ziploc bags for more than 15 years. It is really amazing to see hundreds of clam shrimp suddenly appearing in an aquarium after a small cup of this soil added to a tank of water.
The eggs are just waiting for the next significant rainfall to fill the pools so they can hatch. Once they hatch they will molt several times and become adults within a week at which time they are able to produce a new round of eggs before the pond dries out once again.
Do we find fossils of clam shrimp in the geological column?
Yes and lots of them! We tend to get excited about dinosaur bones, fish skeletons or plant remains when we think of fossils but there are vast numbers of aquatic shelled organisms in the fossil record. There are some that are quite large like the scallop that I have presented in the past but most are very small like this clam shrimp and are found by the billions.
The fossil record of clam shrimp is extensive and deep. It stretches all the way back to the Devonian period (350 to 400 million years old). When fossils are found their carapaces are found in great abundance. However, they are not ubiquitous in the fossil record. Rather clam shrimp fossils are found almost exclusively in rock formations that have formed in a terrestrial condition rather than sedimentary deposits in the ocean.
The plants and other animals found fossilized with these clam shrimp reveal that the clam shrimp of the past lived in shallow freshwater pools rather than in the ocean. This is just the type of place they are found today.
During the dissertation defense I asked a number of questions about how these clam shrimp carapaces could have been preserved including whether these carapaces had been preserved in the place they were produced (eg. a small pool or wetland) or if there were indications that they had been transported over long distances before reaching their final destination in the fossil record. The student said that his observations of the fossils strongly suggest that most were preserved in the position where they had lived rather than being transported prior to preservation.
A Global Flood origin: An alternative explanation for clam shrimp fossils?
After the defense I asked our newly minted PhD if he could construct a scenario in which a vast global flood occurring not long ago could account for the fossils with which he was familiar. It probably won’t surprise you that he could not imagine any way to explain the origin of the fossils including the rocks they are preserved in within a recent global flood scenario.
There are several reasons why a global flood isn’t a viable explanation for these fossils.
1) Patchy distribution: While the fossils are abundant and found in most of the geological column, in general they are found in dense patches. How would a global flood pick up hundreds of billions of clam shrimp and then deposit them in dense clumps? How could they come to rest looking like they has just fallen gently to the bottom of a pool? The fossilized clam shrimp are distributed in the rock much like the scattered carapaces found in a small placid pools today. Chaotic massive floods would not deposit the same species of clam shrimp all together in one place. .
2) Fossil species are ordered: There are 300 named fossil species and those species are not found randomly in the fossil record. There are size and shape differences between clam shrimp species but size and shape do not follow a pattern vertically in the geological record. Creationist’ Flood Geology theory predicts that billions of shells would all be mixed up in the first stages of a flood and then get sorted out into layers. But it provides to explanation for how they could get sorted into species. Furthermore many species are only slightly different as one moves up the geological column. How could a single flood sort out slight shape differences between clam shrimp carapaces?
3) Unique to freshwater ecosystems: Most of the worlds sedimentary rock is formed in shallow oceans but clam shrimp are not found here even though they should easily be preserved in all layers of rock in a global flood. Creationist’ flood geology already has insurmountable problems with the evidence of rock formed in freshwater lake systems and here we have a specific example. Clam shrimp are found preserved with other organisms that are indicative of freshwater systems. How does a freshwater lake system get preserved in the middle of a 20,000 foot column of rock layers supposedly all laid down in a few months in the middle of a global flood?
Until creationists can propose an even remotely plausible scenario for the details in the fossil record observed by individuals like this student they should not be expected to be taken seriously. They claim that a global flood is a better explanation for the observed data. However, their claims about the fossil record don’t hold any weight with the majority of people that real experience with fossils. A viable hypothesis has to be able to accommodate new data that are being generated each and every day. An ancient Earth with a dynamically evolving land surface can easily accommodate all the characteristics observed in these clam shrimp fossils.
Some of the data from this project has already been published and can be found here:
Sexual discrimination at work: Spinicaudatan ‘Clam Shrimp’ (Crustacea: Branchiopoda) as a model organism for the study of sexual system evolution T.I. Astrop, L.E. Park, B. Brown, and S.C. Weeks. Article number: 15.2.20A Copyright Palaeontological Association, June 2012
*This is grammatically revised post from January of 2014