A Tale of Taphonomy: Clam Shrimp Fossils and the Age of the Earth

One of the great privileges of my job is that I get to participate on committees of graduate students. This allows me to get up close and personal with data from sub-disciplines of biology that often extend far from those of my particular expertise.  You know by now that I carry a particular fondness for fossils and the stories they tell.  So when participation on a committee is combined with research on fossils, it is a really good day.  Recently I had one of those days as a participated in a doctoral dissertation defense.  These are typically 2-4 hour affairs in which we explore the every nook and cranny of a research project.  Although PhD defenses can be difficult 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 students work.   During these stimulating intellectual adventures I get to inspect  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 got to ask questions about his  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 organism at the center of this students study.

What is a clam shrimp?

Clam shrimp are not clams but rather a small filter-feeding crustacean that produces a protective bivalved carapace that look like a clam shell.

There are more than 100 species of clam shrimp alive today.  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 will collect for weeks to months every year or every few years you will probably find clam shrimp.  Rock pools in deserts may be dry as a bone and be 120 degrees in the summer but add some water and a week later they will be teaming with clam shrimp and other small organisms.

How do these animals suddenly appear in these pools?  These organisms 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 a cup of this soil added to a tank of water and then 4 days later find hundreds of little clam shrimp swimming around in the tank.  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 fossils of clam shrimp exist in the geological column? 

Yes and lots of them!  We tend to get caught up with dinosaur bones, fish skeletons or plant remains when we think of fossils but the vast majority of fossils are of aquatic shelled organisms.  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 that has formed in a terrestrial context rather than in the ocean.  The plants and other animals found fossilized with these clam shrimp tell us that the clam shrimp lived in a shallow freshwater pools rather than in the ocean. This is just the type of places 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.  He said that his observations of the fossils strongly suggest that most were preserved in the setting where they had lived rather than being transported prior to preservation.

Global flood: 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 wont’ surprise you that he could not imagine any way to explain the fossils and the context they are found within a recent global flood scenario.

There are several reasons why a global flood isn’t a viable alternative in light of these fossils?

1)  Patchy distribution:  while the fossils are abundant and found in a large portion of the geological column, in general they are found in dense patches.   How would a global flood pick up 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 fossils look much like the scattered carapaces found in a placid pool today.

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 how do they get sorted into species where some time layers of rock have both small and large species together? Further more why are there slightly different species (morphologies) as one moves up the geological column?

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 would be easily preserved in all layers of rock in a global flood.  Creationist flood geology already has huge problems with the evidence of rock formed in freshwater lake systems and here we see a specific example.   Clam shrimp are 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 come up with an even remotely plausible scenario for the details  in the fossil record observed by people like this student they should not 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 mean much to anyone that has experience with it.  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