Living fossils are a tricky concept for evolutionary biology. In principle it seems simple: living organisms that closely resemble creatures seen in the fossil record going back millions of years. Usually they’re a single representative of a fossil record containing a multitude of diverse close relatives — survivors from a lost clade — and rare, or living in habitats that humans can’t easily access. As a consequence, living fossils’ shared ancestry with other living species goes back deep into the history of life, and they’re often treated as examples of “primitive” forms. The term has been applied to species including the coelacanth, dawn redwoods, ginkgo trees, and hoatzin.
However, as molecular ecologists well know, two organisms may look identical in terms of morphology, especially features like shells and skeletons that fossilize reliably, and still be quite different in ways less obvious to the human eye. Color or pheromones or behavior may change without leaving any fossil traces; and every living thing is locked in an ongoing coevolutionary race against parasites and pathogens, which necessitates rapid evolution at the molecular level. We can’t find evidence of these changes in fossils (well, except for relatively very recent ones), but we can see it in DNA sequence data from putative living fossils.
That’s the premise of a paper out ahead of print in Molecular Ecology, which reports the first substantial, genome-wide population genetic dataset for nautiluses. Nautiluses are cephalopod mollusks, most closely related to octopuses, with beautiful spiral shells. Though there are five recognized species in two genera, they are collectively considered living fossils, as the fossil record includes species from at least five other genera, and all of them are different enough from other cephalopods to rate placement in a separate suborder, the Nautiloidea. David Combosh, Sarah Lemer, and colleagues at Harvard, the University of Guam, and the University of Washington collected genome-wide SNP data from 140 samples of all five species using the ddRADseq protocol, and used it to take the first good look at nautilus diversity.