What is a “Living Fossil”?
A number of extant species, that is a species that exists today rather than an extinct one, are referred to as “living fossils”. An example might be the remarkable Tuatara, that has just been introduced to the mainland of New Zealand. The Tuatara (Sphenodon punctatus) is the only surviving member of an otherwise extinct reptilian order the Rhynchocephalia. Fossils of Rhynchocephalians have been found in rock strata dating back to the Lower Triassic and from the fossil site locations, virtually every continent except North America, it is likely that this particular type of reptile was very common in the early Mesozoic. The Tuatara has been protected by the New Zealand Government for many years and recently, in an extension of the ongoing conservation programme a new mainland reserve for this little reptile was established.
An Example of a Living Fossil – The Tuatara
Picture Credit: New Zealand Conservation Trust
The picture shows an adult male Tuatara, the males have conspicuous triangular folds of skin that form a crest that runs down the back and tail of the males. The Maori word Tuatara means “peaks on the back”, a reference to the crests found along the backs of the males.
To read an article on the Tuatara re-introduction programme: Living Fossil to be Re-introduced to New Zealand Mainland
There are many other examples of living fossils, Charles Darwin, in his ground breaking work on natural selection “The Origin of Species” uses the phrase “living fossil”. In chapter 4 of our copy of the third edition (kept in the office), we first encounter the phrase “living fossil”. In this chapter, Darwin begins to outline his theory of natural selection, presenting evidence for the competition for organisms in the natural world. Darwin comments that in general, the larger the land mass or area within which an ecosystem exists the greater the competition between organisms and hence the greater the drive to create new species. He contrasts the paucity of species to be found on remote islands to those found on the continents and he states that those native organisms of the isolated country of Australia are easily out competed and threatened by European and Asian species once they are introduced. Darwin introduces the idea that there are circumstances in the natural world favourable to natural selection and circumstances which are not favourable to the process of natural selection.
In developing this point about circumstances favourable or unfavourable to natural selection; Darwin states that the world contains far more salt water than fresh water. Compared to the great oceans and seas of the world, freshwater lakes, ponds and rivers are very small. Consequently, he argues that the competition between organisms living in freshwater will be much less severe than the competition between organisms living in salt water. As a result new species of freshwater animals will take longer to develop when compared to salt water species. New forms take longer to develop and older forms persist for longer.
Darwin comments that in freshwater; seven genera of Ganoid fishes can be found, the likes of the Gar fish for example. These are the remnants of a much more common type of fish that dominated both freshwater and marine environments by the number of fossils found. It is also in freshwater, he notes, that the last of the Lepidosiren (lung-fishes) can be found. These remaining few species of once great families of fish are now widely separated in the natural scale and Darwin refers to these type of creatures as being:
“anomalous forms” that “may almost be called living fossils, they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition.”
If this is Darwin’s definition of a living fossil, then it is certainly good enough for us.