… except in Hollywood movies.
Let’s make this clear from the start, I don’t like the term “living fossils” at all. It’s as if we decided that certain species are second class organisms that should have gone extinct a long time ago. Unfortunately for me, the term crops up in the popular scientific press all the time. Especially the BBC seems to have a particular affinity for the concept. Even worse, scientific literature is polluted with this muddy term now and again1. Why would that be a problem? Because the term makes absolutely no evolutionary or scientific sense at all.
There are many reasons people refer to extant organisms as living fossils. They are all bullocks. Let’s go through the list.
1. Living fossils have been around for billions/millions of years and have changed little over time.
Even the morphologically most constant species will change and diverge over time. The horseshoe crab, the poster child of ‘living fossils’, can make this insightful.
It is true that the first recognizable horseshoe crabs make first appear in the fossil record around ~445 million years ago. But based on these fossilized shells alone, paleontologists can recognize dozens of different horseshoe crab species that lived in the past, in addition to the four different species of horseshoe crabs alive today. In fact, the extant Atlantic horseshoe crab doesn’t show up in the fossil record at all!
So the horseshoe crabs that live today did NOT exist millions of years ago. Furthermore, the only part of the horseshoe crab that fossilizes well is its shell, so we know almost nothing about the soft tissues of ancient horseshoe crabs. Any information about particular physiological organization of organs or genomic differences is lost forever. Maybe fossil horseshoe crabs maybe would be more recognizable as different species if we DID had access to that information. If anything, genetic drift will have made sure that the genetic composition of an ancestral horseshoe crab and an extant one are fully incompatible as millions of years passed. In other words, while it would make a cool experiment, trying to cross a ‘modern’ horseshoe crab with one of its ancestors will most likely fail.
Continuing with the completely arbitrary list of properties that define a living fossil (partly taken from this awful wikipedia page):
2. Living fossils… have all survived major extinction events
Newsflash: the ancestors of EVERY organism alive today have experienced ‘major extinction events’. That’s sort of why we’re here today.. This kind of reasoning can take completely ridiculous forms, as in the case of the Cypriot mouse2.
“All other endemic mammals of Mediterranean islands died out following the arrival of man, with the exception of two species of shrew. The new mouse of Cyprus is the only endemic rodent still alive, and as such can be considered as a living fossil,” said Dr. Cucchi.
The Cypriot mouse diverged approximately half a million years ago from its fellow mice and continued to evolve, adapted to the Cypriot island while genetically drifting away from its mainland nephews. So does the status of sole survivor of an extinction event make the Cypriot mouse a living fossil? Humans are the only surviving members of the genus Homo, but we don’t go about calling ourselves living fossils now do we?
3. A species which successfully radiates … has become too successful to be considered a “living fossil”
If you needed any reason to regard “living fossils” as complete bullcrap, this is it. “Successful radiation”?? How on earth can that be measured? For all we know, ‘living fossils’ may have ‘successfully’ radiated in the past, even if few species in that clade survive today. This definition also implies that species can lose their status of a living fossil if they divergein the future, making it a pretty worthless biological concept.
As an example. both Coelacanths and Elephantidae are genera that spanned dozens of species in the past. So subjectively, we could think of them both as ‘succesful’ groups of species. However, extinctions eliminated all but two species in both Elephantidae and Coelacantha. Yet, it are anoly the coelacanths who are viewed as ‘living fossils’ whereas elephants are apparently alive and kicking! If that’s not arbitrary, I don’t know what is.
The list of living fossils on wikipedia has no biological relevance at all. The troubles the wikipedians have in getting a clear definition of what constitutes a living fossil really says enough. To me it looks like a half baked attempt to provide some psuedoscientific justification for branding some species as ‘primitive’. Of course, the usual victims such as the highly derived platypuses and crocodiles feature on the list. Guess what: as Jonathan Eisen pointed out earlier, there are no ‘primitive’ species. Or ‘living fossils’ for that matter.
So please, spread the word, and whack people on the head with a fossilized trilobite if they ever use the word ‘living fossil’.
1.Amemiya, C., Powers, T., Prohaska, S., Grimwood, J., Schmutz, J., Dickson, M., Miyake, T., Schoenborn, M., Myers, R., Ruddle, F., & Stadler, P. (2010). Complete HOX cluster characterization of the coelacanth provides further evidence for slow evolution of its genome Proceedings of the National Academy of Sciences, 107 (8), 3622-3627 DOI: 10.1073/pnas.0914312107
2. T. CUCCHI , A. ORTH, J.-C. AUFFRAY , S. RENAUD , L. FABRE, J. CATALAN, E. HADJISTERKOTIS, F. BONHOMME & J.-D. VIGNE (2006). A new endemic species of the subgenus Mus (Rodentia, Mammalia) on the Island of Cyprus Zootaxa
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