Living fossils... or not
All species are a mixture of primitive and derived attributes
Do living fossils really exist? The answer is clearly yes… and no. This oxymoron was coined by Charles Darwin and continues to fascinate scientists and laypersons alike. In the 1958 sci-fi movie Monster on the Campus, Arthur Franz, playing Dr. Donald Blake, takes possession of a coelacanth. Asked by a student if the fish is really a million years old, he explains that “It’s the species that’s old. No change in millions of years. See, the coelacanth is a living fossil, immune to the forces of evolution.” As Alexander Werth and William shear eloquently explain in an article published in American Scientist, this bit is fiction. The good doctor has drawn the same erroneous conclusion about living fossils as over-zealous Creationists. The species of coelacanth discovered since the 1930s are certainly primitive-looking and retain features seen in fossils; but they are not the same species as the fossils and fossils preserve only a fraction of the attributes of species. While conspicuous aspects of their morphology have been conserved, there are details of anatomy, internal organs, physiology, genes and behavior that we cannot compare.
One criterion of a living fossil is rarity. Burmagomphus chaukulensis, discovered in 2022, was not greeted as a living fossil in spite of having morphological structures shared with fossils more than 300 million years old. Perhaps this is because there are 5000 living species of Odonata (dragonflies and damselflies). Photo: Figure 9e in Joshi, S., Sawant, D., Ogale, H., and K. Kunte (2022) Burmagomphus chaukulensis, a new species of dragonfly (Odonata: Anisoptera: Gomphidae) from the Western Ghats, Maharashtra, India. Zootaxa 5133: 413-430.
Olivier Rieppel recently explained that every species is a mixture of primitive (or plesiomorphic) and derived (or apomorphic) features. When certain conserved attributes, including conspicuous morphological structures and body forms, appear little- or un-changed when compared to ancient fossils we are tempted to use the term “living fossil.” It is in the context of comparative studies and an understanding of phylogenetic relationships that the word has biological significance. So, I am of two minds with respect to the phrase “living fossil.” On one hand, in one sense, they do not really exist because every species is a mosaic of primitive and derived. In spite of superficial appearances, all species have evolved since the time when their ancestors were preserved in the fossil record even when suites of conspicuous characters have been retained in more or less the same form. On the other hand, I, like most people, find the urge to use the phrase for certain species discoveries irresistible.
Werth and Shear tapped into what draws us to the mystique of living fossils, brilliantly relating the concept to considerations as diverse as rates of evolution, forces of speciation, ecology, genetics, and paleontology. For a deeper, thought-provoking dive into living fossils and their implications, their article is a superb read.
I came away from their paper with several significant take-home lessons about living fossils, including the important observation that there is no single criterion by which we may judge a species to be a living fossil. There are, in practice, a number of factors that singly, or in combination, lead us to make that assertion. What are these factors associated with living fossil status?
First, living fossils may possess conservative features that have been retained or little-changed compared to ancestors in the fossil record. But, as already said, every species examined closely possesses a mixture of conserved and novel characteristics. This is a by-product of evolution. As Norman Platnick once put it, every heritable character of an ancestral species is faithfully passed on to its descendant species, either in its original form, or in some subsequently modified form. In this sense, species are all living fossils and, as was written in the book of Ecclesiastes, there is nothing new under the sun, only an accumulation of stepwise transformations through time. Even the most impressive and apparently unprecedented characters, seen in phylogenetic context, are modifications of ancestral characters and genes.
Second, living fossils may seem to have persisted unchanged over long periods of geologic time. Some of the most celebrated examples of living fossils have involved the discovery of one or more living species in a group previously thought to have gone extinct millions of years ago and we are astonished to suddenly realize that they are still around.
Third, living fossils bear a striking resemblance to ancient fossil species contributing to the impression that they have changed little. Again, a limited number of characters are preserved and observable in the fossil record, meaning that many subsequently evolved attributes are not visible to us. Werth and Shear summed it up masterfully saying that “the wonder of evolution is that although no organism is a complete living fossil, all are to some extent living fossils.”
Fourth, so-called living fossils may have limited or relict geographic distributions—but not always. Some, like horseshoe crabs, are both common and widespread.
And fifth, living fossils have low taxonomic diversity. When there are large numbers of living species in a group, we tend not to think of them as living fossils, even though they may exhibit conserved ancestral attributes. With 5000 species —about the same number as all living mammals— the discovery of a new species of dragonfly or damselfly does not make headlines as a living fossil, yet significant aspects of their body structure are little-changed in 300 million years.
Because there exists no single criterion by which we judge a species to be a living fossil, it is no wonder that the phrase is inconsistently defined and applied. Because all species are a mixture of primitive and derived attributes, the demarcation of living fossils is necessarily arbitrary. Even when questioned in respect to their biological significance, living fossils continue to awe, intrigue and inspire us. As Werth and Shear make clear, the idea of living fossils connects us to an amazing array of thought traditions, from genetics to ecology and evolution. And anything that reminds us of the wonder of life’s diversity and history, that prompts us to marvel and think, is a good thing. The muddled idea of living fossils may ultimately have its greatest importance in sparking our imaginations, but that is reason enough to continue to recognize and celebrate living fossils— and to keep the phrase in our vocabulary.
Spoiler alert: In Monster on the Campus, mild-mannered Professor Blake, after exposure to the blood of the coelacanth, turns into a menacing Neanderthal. As he explained to his students, man is the only creature that can decide whether to evolve or devolve; that “unless we learn to control the instincts we’ve inherited from our ape-like ancestors, the race is doomed.” As we witness thousands of species extinctions each year, including living fossils by whatever criteria, we recklessly focus on assuring that our own animal needs for resources are met while risking the loss of irreplaceable insights into the origins of life’s diverse species, including modern humans. Self-preservation is a powerful instinct, but we owe it to ourselves, science and future generations to do more to explore, inventory and document the diversity and history of life before it is too late. After all, if fossils can live, then they can die, too.
Readings
Cesane, Didier and Patrick Laurenti. 2013. Why coelacanths are not ‘living fossils.’ BioEssays 4 February.
Platnick, Norman I. 1979. Philosophy and the transformation of cladistics. Systematic Zoology 28: 537-546.
Rieppel, Olivier. 2023. “Living fossils” and the mosaic evolution of characters. Frontiers in Ecology and Evolution 11.
Werth, Alexander J. and William A. Shear. 2014. The evolutionary truth about living fossils. American Scientist, 102: 434-443.