This case is short but sweet.
The fossil record is what we scientists have always used to reconstruct past conditions in the ocean. Like any good investigator, we want to know who was there and what they were doing (and who was guilty, darn it!). Deep-sea sediments are hotbeds for microfossils, remnants of teeny animals like forams that leave behind their calcium or silica skeletons. And because the deep-sea is calm, cold and ancient, we can easily peel back layers of mud and glimpse back thousands of years. So we used to be like this:
But now we also have DNA evidence to build our case! A recent study by Lejzerowicz et al. (2013) found that those same deep-sea conditions that preserve microfossils are also pretty awesome at preserving “ancient DNA”. They could identify DNA fragments from some important eukaryotic taxa (forams and radiolarians) in abyssal plain sediments dated at 32,500 years old!!
And you’d think that would make our case even MORE open-and-shut. Buuuut….sometimes our relationship status with DNA is “complicated”. Here’s a side-by-side comparison of fossil and DNA results:So what’s the answer? Well we really don’t know, your honor. Some of the species we identify from ancient DNA are also found in the microfossil record (a very good sign). But the DNA data identifies a lot of animals that seem to be abundant, but just don’t show up in the microfossil counts (see all those big red bars on the right?). This might be because some animals bodies don’t preserve well in deep-sea mud, and only their DNA survives. Alternatively, the DNA evidence might be misleading – it’s almost impossible to deduce the original number of animals from such environmental DNA sequences, because one specimen could be represented by thousands of DNA sequences.
My closing argument: both microfossils and DNA are awesome, but neither gives us a complete picture of the ancient oceans.
Reference: Lejzerowicz F, Esling P, Majewski W, Szczucinski W, Decelle J, Obadia C, et al. (2013) Ancient DNA complements microfossil record in deep-sea subsurface sediments. Biology Letters, 9(4):20130283–3.