courted Owen’s friendship and support. (Owen, at Darwin’s request, formally described for publication the fossil mammals that Darwin had collected on the Beagle . Darwin’s famous Toxodon , for example, was named, described, and illustrated by Owen.) But the relationship inevitably soured, in part because Owen’s vanity could not bear Darwin’s successes. Legend holds that Owen’s rejection of evolution prompted their final break, but such a falsehood only records our propensity for simplifying stories told in the heroic mode, thus making “bad guys” both nasty and stupid. Owen did reject natural selection, and with vigor, as an excessively materialistic theory depending too much on external environments and too little on laws of organic structure, but he embraced evolution as a guiding principle in natural history.
In any case, the juxtaposition of worm and ichthyosaur dates from 1838, an early period of their friendship. I couldn’t help noticing another link more interesting than mere spatial proximity. Darwin wrote, as quoted above, that his subject seemed trivial but really unleashed a cascade of implications leading to substantial importance. Owen then made the very same point, arguing that an apparently broken tail in an ichthyosaur might seem entirely devoid of interest, but that close study yielded generalities of more than passing concern. Since the conversion of detail to wide message, through links of tangential connection, forms the stock-in-trade of these essays, I could hardly avoid such a double invitation to discourse at greater length on the tail bend of ichthyosaurs.
Ichthyosaurs are a group of marine reptiles with bodies so fishlike in external form that they have become the standard textbook example of “convergence”—evolved similarity from two very different starting points as independent adaptive responses to a common environment and mode of life (wings of birds and bats, eyes of squids and fishes). Ichthyosaurs are not closely related to dinosaurs, though they arose at about the same time and became extinct before the great wipeout that ended the dinosaurs’ reign some 65 million years ago. (The god-awful spelling of their names, with its unpronounceable sequence chth , only records an orthographic convention in converting Greek letters to Roman. This four-consonant sequence represents two Greek letters, chi and theta , one transliterated ch , the other as th . Both belong to a five-letter Greek word for fish, and ichthyosaur means “fish lizard.” We meet the same orthographic problem in such words as ophthalmology. But never despair and remember that things could always be worse. What would you do if that four-letter sequence came right at the beginning of a word—as it does in a common barnacle with the most forbidding name of Chthamalus .)
In considering the convergence of ichthyosaur upon fish, we marvel most at the form and location of fins and paddles—the machinery of swimming and balancing. The fore and hind paddles are, perhaps, least remarkable, for ancestral structures are clearly present as front and back limbs of terrestrial forebears—and these can be modified, as whales and dolphins have done, to forms better suited for sculling than for walking. But the dorsal (back) and caudal (tail) fins are boggling in their precision of convergence with analogous structures in fishes. For the terrestrial ancestors of ichthyosaurs obviously possessed neither back nor tail fin, and ichthyosaurs therefore evolved these structures from scratch—yet they occupy the position, and maintain the form, that hydrodynamic engineers deem optimal for propulsion and balance.
The classic painting of an ichthyosaur by Charles R. Knight. Note the fish-like position of the fins. Courtesy of Department of Library Services, American Museum of Natural History .
Yet just as ichthyosaurs themselves developed these fishlike features in a graduated transition from terrestrial ancestors, so too
In any case, the juxtaposition of worm and ichthyosaur dates from 1838, an early period of their friendship. I couldn’t help noticing another link more interesting than mere spatial proximity. Darwin wrote, as quoted above, that his subject seemed trivial but really unleashed a cascade of implications leading to substantial importance. Owen then made the very same point, arguing that an apparently broken tail in an ichthyosaur might seem entirely devoid of interest, but that close study yielded generalities of more than passing concern. Since the conversion of detail to wide message, through links of tangential connection, forms the stock-in-trade of these essays, I could hardly avoid such a double invitation to discourse at greater length on the tail bend of ichthyosaurs.
Ichthyosaurs are a group of marine reptiles with bodies so fishlike in external form that they have become the standard textbook example of “convergence”—evolved similarity from two very different starting points as independent adaptive responses to a common environment and mode of life (wings of birds and bats, eyes of squids and fishes). Ichthyosaurs are not closely related to dinosaurs, though they arose at about the same time and became extinct before the great wipeout that ended the dinosaurs’ reign some 65 million years ago. (The god-awful spelling of their names, with its unpronounceable sequence chth , only records an orthographic convention in converting Greek letters to Roman. This four-consonant sequence represents two Greek letters, chi and theta , one transliterated ch , the other as th . Both belong to a five-letter Greek word for fish, and ichthyosaur means “fish lizard.” We meet the same orthographic problem in such words as ophthalmology. But never despair and remember that things could always be worse. What would you do if that four-letter sequence came right at the beginning of a word—as it does in a common barnacle with the most forbidding name of Chthamalus .)
In considering the convergence of ichthyosaur upon fish, we marvel most at the form and location of fins and paddles—the machinery of swimming and balancing. The fore and hind paddles are, perhaps, least remarkable, for ancestral structures are clearly present as front and back limbs of terrestrial forebears—and these can be modified, as whales and dolphins have done, to forms better suited for sculling than for walking. But the dorsal (back) and caudal (tail) fins are boggling in their precision of convergence with analogous structures in fishes. For the terrestrial ancestors of ichthyosaurs obviously possessed neither back nor tail fin, and ichthyosaurs therefore evolved these structures from scratch—yet they occupy the position, and maintain the form, that hydrodynamic engineers deem optimal for propulsion and balance.
The classic painting of an ichthyosaur by Charles R. Knight. Note the fish-like position of the fins. Courtesy of Department of Library Services, American Museum of Natural History .
Yet just as ichthyosaurs themselves developed these fishlike features in a graduated transition from terrestrial ancestors, so too
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