Turtles as Hopeful Monsters: Origins and Evolution

Turtles as Hopeful Monsters: Origins and Evolution

Turtles as Hopeful Monsters: Origins and Evolution

Turtles as Hopeful Monsters: Origins and Evolution

Synopsis

Where do turtles hail from? Why and how did they acquire shells? These questions have spurred heated debate and intense research for more than two hundred years. Brilliantly weaving evidence from the latest paleontological discoveries with an accessible, incisive look at different theories of biological evolution and their proponents, Turtles as Hopeful Monsters tells the fascinating evolutionary story of the shelled reptiles. Paleontologist Olivier Rieppel traces the evolution of turtles from over 220 million years ago, examining closely the relationship of turtles to other reptiles and charting the development of the shell. Turtle issues fuel a debate between proponents of gradual evolutionary change and authors favoring change through bursts and leaps of macromutation. The first book-length popular history of its type, this indispensable resource is an engaging read for all those fascinated by this ubiquitous and uniquely shaped reptile.

Excerpt

Here they are, still with us, boxed up in a shell, seeming survivors of the distant geologic past: prehistoric creatures that both predated and outlived the dinosaurs. As a symbol in Hindu and Chinese mythology, the turtle supports the earth—but what supports the turtle? the philosophically intriguing answer is this: it’s turtles all the way down. Sluggish on land, seemingly stubborn in their behavior, turtles came to symbolize longevity, strength, and endurance in ancient China. the longevity of the turtle lineage is indeed remarkable, as is its evolutionary strength if measured by the numbers of species that have populated Earth for at least 220 million years, which is the approximate age of the oldest fossil turtle currently known. (For a discussion of fossils some 260 million years old, controversially interpreted as the oldest stem-turtle, see chap. 3.) in spite of their highly constrained body plan, turtles show a surprising potential for evolutionary diversification, with species that conquered a great variety of habitats: forests and grasslands, deserts and karst mountains, rivers, ponds, lakes, even the open sea, with some 331 living species currently recognized (Fritz and Havas, 2007; van Dijk et al., 2012).

As Adler has remarked, “Turtles are one of nature’s most immediately recognizable life forms” (2007:139), bizarre in their own way, which is what seems to attract people’s attention as well as scientists’ curiosity. Where do turtles come from, and how did their unique anatomy and bodily functions evolve? These are questions that have spurred intense research as well as heated debate for 200 years and more. Yet even in the wake of the development of revolutionizing techniques in modern molecular biology, the answer remains as elusive as ever. Progress has been made, but a lot still remains to be discovered.

The eminent twentieth-century evolutionary biologist Theodosius Dobzhansky once noted, “Nothing makes sense in biology except in the light of evolution” (1973:125). Philosophers of biology Kim Sterelny and Paul E. Griffiths paraphrased this famous line as, “Nothing in biology makes sense except in the context of its place in phylogeny, its context in the great tree of life” (1999:379). To ask where turtles come from is to raise the question of turtle ancestry. It certainly is uncontroversial that turtles are reptiles, but that does not tell us what group of reptiles gave rise to the turtle lineage in the distant part. What was the ancestor of turtles? That question was certainly a legitimate one to ask in the middle of the twentieth century and earlier, when the study of fossils was largely motivated by the search for ancestors of still-living descendants. the study of fossils retrieved from successive layers of rock was at that time compared . . .

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