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Living Fossils A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders The Evolutionary Truth About Living Fossils Appearances to the contrary, no species is exempt from selection, even when changes are difficult to detect in the fossil record. Alexander J. Werth and William A. Shear o gaze upon a horseshoe crab to have gone extinct in the Late Cre- is to glimpse a prehistoric Or- taceous until the discovery of living dovician sea of nearly half a Latimeria in 1938. Likewise, the dawn billion years ago. So moved redwood tree, Metasequoia, was known Twas Charles Darwin by the appear- only from fossils (10 to 100 million ance of these and similarly ancient- years old) before it was found alive in looking creatures—lungfish, lampreys, a remote Chinese valley in 1943; today lampshells, and lycopods—that he it is a common ornamental that grows coined the term living fossil to describe readily in temperate regions. them. In his landmark 1859 treatise On Living fossil is a perfectly appropri- the Origin of Species, he wrote that such ate and evocative term for such extant apparently primitive species are “rem- forms even today. It conveys the jar- nants of a once preponderant order… ring surprise that such species conjure which, like fossils, connect to a certain by retaining anatomical structures out extent orders now widely separated in of deep geologic time. This curious the natural scale.” These “anomalous phenomenon raises evolutionary and forms,” he wrote, nonetheless “have ecological questions at once simple endured to the present day.” and profound, such as why these taxa Like modern biologists, Darwin was appear to have persisted unchanged Visuals Unlimited/naturepl.com struck by the unusually archaic form for so long, and how we can distin- of the three widely scattered lung- guish species over long stretches of evident, apparently retained from long- fishes (Protopterus, Neoceratodus, and Earth history without gene pools or lost eras. This morphological conserva- Lepidosiren of Africa, Australia, and other reproductive features essential to tism, readily seen in horseshoe crabs South America, respectively), whose our current comprehension of specia- and coelacanths, is made all the more large lungs; cartilaginous notochord; tion. No doubt living fossils offer, as remarkable by our knowledge of strik- and fleshy, lobed fins resemble those Darwin anticipated, an unprecedented ingly similar fossil forms from ancient of creatures known only from the fossil opportunity to study—with fascinat- geologic ages. Living fossils are repre- record. Similarly, the coelacanth (Lati- ing, attention-grabbing narratives— sentatives of otherwise extinct groups, meria), another lobe-finned fish, was major questions concerning extinction, often common in the fossil record. well known from fossils but thought competition, and rates of evolutionary Some were thought to be extinct. None- change, both morphological and ge- theless, a known fossil connection or Alexander J. Werth is Venable Professor of Biol- netic. They also reveal important and “twin” is not necessary for living fossil ogy and William A. Shear is Trinkle Professor often intractable difficulties of under- status. The eel species Protoanguilla pa- of Biology, both at Hampden-Sydney College in standing the concept of species over lau, discovered in 2010, is estimated to Virginia. Each earned a PhD in organismic and vast periods of time. have diverged from other eels around evolutionary biology from Harvard University, 200 million years ago and has been de- Shear in 1971 and Werth in 1992. Werth’s re- What Is a Living Fossil? scribed as a primitive living fossil with- search focuses on the functional morphology and The term living fossil itself, despite the out a known fossil record. biomechanics of feeding in baleen and toothed seductive appeal inherent in its apt de- Much of the surprise that greeted whales, and on the evolution of complexity. scriptiveness, poses challenges for the the coelacanth’s discovery in 1938 can Shear conducts research on the systematics of scientist and layperson alike. But even be ascribed to the fact that fossil coel- millipedes and other myriapods, and on the evo- lution of early terrestrial ecosystems. Shear has if this ambiguous term is not easily or acanthiforms were widely recognized published four previous feature articles in Amer- universally defined, it nonetheless re- by paleontologists, but no extant spe- ican Scientist. Address: Department of Biology, tains heuristic value. One of the chief cies were known. In truth there has Hampden-Sydney College, Hampden-Sydney, characteristics of living fossils is an an- never been any fossil find of the extant Virginia 23943. E-mail: [email protected]. cient or archaic form, at least externally Latimeria chalumnae or L. menadoensis photo courtesy of Jim Brace-Thompson 434 American Scientist, Volume 102 © 2014 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. The term living fossil has been used since Darwin coined it, referring to organisms that appear unchanged compared to fossil speci- mens, such as the gingko tree (Gingko bilo- ba). However, the term has been confusingly applied as the field of biology has advanced in understanding evolution and genetics. Is the concept useful? www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 November–December 435 with permission only. Contact [email protected]. photo courtesy of Jim Brace-Thompson Angelo Giampiccolo/naturepl.com An Ordovician fossil, Lunataspis aurora (left), looks nearly identical or so. Even if biologists could trace its evolution back ancestor by to today‘s Atlantic horseshoe crab (Limulus polyphemus, at right). ancestor, we could not define the point at which one species became Without genetic information from fossils, it is very difficult to know another. Nevertheless, we know that they could not interbreed and how much the organism has changed over the past 445 million years are therefore two different species. (West Indian Ocean and Indonesian species neither disprove nor provide forerunners of the diminutive living coelacanths), undercutting the claim any evidence counter to our under- fossil lycopod and horsetail plants un- that Latimeria is, strictly speaking, a standing of evolution, which remains derfoot today. living fossil, and thereby deflating as- the cornerstone of biological science. Conservative features alone are not sertions that such species demonstrate There are no “unevolved” species, no enough to paint a complete picture an absence of evolution. Still, living reanimated fossils that have literally of what constitutes a living fossil. Al- fossil taxa tend to have only fossil (not come back to life, and no living organ- though Escherichia coli and other bac- modern) counterparts. isms that are truly identical to extinct teria retain many traits of Earth’s first Unfortunately, creationists bent on species known in the fossil record. life forms (and as such are closer than denying the factual basis of evolu- What makes living fossils special, most species to the universal ancestor tion have increasingly misappropri- according to Harvard University evo- of all living things), no one would call ated the term living fossil. In the Atlas lutionary biologist Piotr Naskrecki, is them living fossils, not so much due of Creation, which pairs photographs simply that they superficially resemble to their simplicity as their ubiquity. If of living fossils with similar ancient their predecessors as members of an- living fossils were defined solely by fossils, Turkish author and Islamic cient genetic lines that have not been archaic form, one could claim we are creationist Harun Yahya erroneously extinguished, even as they “wither surrounded by them on land and at argues that “Darwinists are desperate through time and turn from roaring sea given the profusion of earthworms when confronted by these fossils, for rivers of species to a trickle before dis- and jellyfish. Scarcity is thus typically they prove that the evolution process appearing altogether.” Still, they may a key feature: Living fossils are usually has never existed.” Entry of “living live on in surprising ways. Naskrecki rare, with little taxonomic diversity. fossil” into Internet search engines points out that our fossil fuel economy They may stand out, like the ginkgo yields a preponderance of creationist is powered by once-dominant trees tree, as exceptional organisms that are websites, despite the fact that these of Carboniferous coal swamp forests, not closely related to any other living photo courtesy of Jim Brace-Thompson Doug Perrine/SeaPics.com The living coelacanth fish (Latimeria, shown at right) is strikingly same species. This fish was heralded as a fossil come to life upon its similar to and obviously closely related to extinct forms (at left, a De- discovery by South African naturalist Marjorie Courtenay-Latimer in vonian coelacanth). But appearances are deceiving: They are not the the Indian Ocean in 1938. 436 American Scientist, Volume 102 © 2014 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected].
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