When speaking of living fossils , it refers to organisms whose lineage originates in the remote past—tens and even hundreds of millions of years—and whose body scheme has changed little or not since then.
Classic examples of living fossils are the coelacanth, the nautilus, the tuatara, the Moluccan pan ; and among plants, the ginkgo . All of them are, without a doubt, living representatives of ancient lineages, who maintain an appearance very similar to that of their ancestors. And it is that, contrary to what is usually thought, evolution does not tend to generate changes unless environmental pressure induces them. Nature does not lightly discard a functioning life form , and if the environment in which an organism lives is stable, or the changes that occur do not generate any alteration of selective pressure, those successful forms tend to be preserved.
However, little change does not mean that nothing changes, and speciation events—that is, the emergence of new species from previous species—with subtle but defining changes, frequently occur .
For example, although the coelacanth lineage dates back more than 400 million years, modern species, belonging to the genus Latimeria, are only a couple of hundred thousand years old. The tuatara has more merit; its lineage dates back to the mid-Triassic, about 240 million years ago, but the fossil record shows organisms that could be associated with the species we find today, Sphenodon punctatus , from the Miocene, less than 20 million years ago.
Oldest is the species Ginkgo biloba , the only species of ginkgo currently extant. While the genus Ginkgo is estimated to be up to 170 million years old, the species we find today arose during the Eocene, about 50 million years ago.
However, among all these species that are living fossils in themselves, there is one that stands out above the rest. A species that has an invariable fossil record, and that has probably remained without significant changes for more than two million centuries.
Triops cancriformis , the oldest species
Triops are a group of branchiopod crustaceans , with a flattened, oval shell lining the front of the body. They have three eyes, they feed on algae, protozoa, insect larvae, worms, small fish and amphibian eggs, and even tadpoles.
They are animals very resistant to inclement weather ; their eggs can withstand dry soil and even sub-zero temperatures. When mud or mud forms, the eggs stick to the feet of animals —or to the soles of hikers’ boots— and as a result, they disperse. A few days after the heat arrives, and as long as they are hydrated, the eggs hatch. They grow rapidly, reaching 8 or 10 centimeters. When there are not enough males, females can reproduce by parthenogenesis. Their life expectancy is very short , they die just six weeks after hatching.
These traits have made the triops an easy animal to keep in aquariums , its dehydrated eggs are even sold as part of educational toys. It is, however, extremely important not to dump or abandon these animals or their eggs in the wild; These are potentially invasive species that can put aquatic ecosystems at risk.
Of the various species of the genus Triops , some are of more recent origin than others. But when analyzing the data from the fossil record, and using comparative anatomy, everything seems to indicate that the different species come from an evolutionary radiation event that took place 220 million years ago , during the Triassic. And the species T. cancriformis would be the oldest. If this hypothesis is correct, this would be the oldest of all living species, with an origin located 220 million years ago.
Not all studies agree
The implications of this statement are highly relevant in evolutionary terms. Some studies refer to a mitochondrial gene , the one that codes for cytochrome oxidase 1, or the COI gene . Although it is only due to mere genetic drift —changes suffered at random and without selective pressure—, the usual rate of evolution measured by this gene in invertebrates is around 1.2% every million years. However, an age of 220 million years for a species such as cancriformis would place this mutation rate at 0.16%.
For the team of researchers led by Bram Vanschoenwinkel, from the Catholic University of Leuven, in Belgium, this mutation rate is implausible , and in reality, the evolutionary radiation that gave rise to the different species of the genus Triops must have happened between 35 and 56 million years, in the Eocene, from a relict ancestor, this yes, with origin in the Triassic.
The scientific discussion is still in the air. However, there is no doubt that, whether this species has remained unchanged since the Triassic, or since the Eocene, Triops cancriformis is one of the oldest surviving species .
Kelber, K.-P. 1999. Triops cancriformis (Crustacea, Notostraca): A Remarkable Fossil from the Triassic Period of Central Europe. En N. Hauschke et al. (Eds.), Triassic – A Whole Different World: Vol. III.16 (pp. 383-394).
Suno-Uchi, N. et al. 1997. Morphological stasis and phylogenetic relationships in Tadpole shrimps, Triops (Crustacea: Notostraca). Biological Journal of the Linnean Society, 61(4), 439-457. DOI: 10.1111/j.1095-8312.1997.tb01801.x
Vanschoenwinkel, B. et al. 2012. Toward a Global Phylogeny of the “Living Fossil” Crustacean Order of the Notostraca. PLOS ONE, 7(4), e34998. DOI: 10.1371/journal.pone.0034998