Eight Little Piggies

by Stephen Jay Gould

  Contingency is rich and fascinating; it embodies an exquisite tension between the power of individuals to modify history and the intelligible limits set by laws of nature. The details of individual and species’s lives are not mere frills, without power to shape the large-scale course of events, but particulars that can alter entire futures, profoundly and forever.

  Consider the primary example from American history. Northern victory was not inevitable in the Civil War, for the South was not fighting a war of conquest (unwinnable given their inferiority in manpower and economic wealth), but a struggle to induce war weariness and to compel the North to recognize their boundaries. The Confederacy had almost succeeded in 1863. Their armies were deep into Pennsylvania; draft riots were about to break out in New York City; Massachusetts was arming the first regiment of free black volunteers—not from an abstract sense of racial justice but from an urgent need for more bodies. In this context, the crucial Battle of Gettysburg occurred in early July. Robert E. Lee made a fateful error in thinking that his guns had knocked out the Union battery, and he sent his men into the nightmare of Pickett’s Charge. Suppose we could rerun history and give Lee another chance. This time, armed with better intelligence perhaps, he does not blunder and prevails. On this replay, the South might win the war, and all subsequent American history becomes radically different. The actual outcome at Gettysburg is no minor frill in an inevitable unrolling of events, but a potential setting point of all later patterns.

  Never apologize for an explanation that is “only” contingent and not ordained by invariant laws of nature, for contingent events have made our world and our lives. If you ever feel the slightest pull in that dubious direction, think of poor Heathcliff who would have been spared so much agony if only he had stayed a few more minutes to eavesdrop upon the conversation of Catherine and Nelly (yes, the book wouldn’t have been as good, but consider the poor man’s soul). Think of Bill Buckner who would never again let Mookie Wilson’s easy grounder go through his legs—if only he could have another chance. Think of the alternative descendants of Ichthyostega, with only four fingers on each hand. Think of arithmetic with base eight, the difficulty of playing triple fugues on the piano, and the conversion of this essay into an illegible Roman tombstone, for how could I separate words withoutathumbtopressthespacebaronthistypewriter.

  5 | Bent Out of Shape

  WE ALL DREAM about retirement projects that might recapture the lost pleasures of youth, or perfect what we had, perforce, abandoned when the practicalities of making a living and supporting a family intervened. Some day, in a rosy future after the millennium, I will take out my old stamp album or sit down at the piano and finally progress beyond the first of Bach’s two-part inventions and the Prelude in C Major from Book 1 of the Well-Tempered Clavier.

  Charles Darwin, my hero and role model, achieved this exquisite pleasure, so I may yet have hope for emulation. His last book, published a year before his death, treated the apparently arcane, but vitally important subject of earthworms and their role in forming the topsoil of England. This wonderful and disarming book unites Darwin’s end, in the calmness of old age, secure in the knowledge of accomplishment, with his more tumultuous youth, sparked by the fires of unrealized ambition. For Darwin wrote the précis of his worm book in 1838, just two years after the Beagle docked—a brilliant five-page article, presenting the entire argument that would fill a book more than forty years later. Darwin concluded:

  The explanation of these facts,…although it may appear trivial at first, I have not the least doubt is the correct one, namely, that the whole operation is due to the digestive process of the common earth-worm.

  Odd juxtapositions always intrigue me. I do not grant them deep meaning, and firmly believe that they represent nothing more than coincidence. Nonetheless, we do take notice, if only because we must find patterns to tell stories. Darwin published his paper in the fifth volume of the second series of the Transactions of the Geological Society of London in 1838. I was reading this paper a few months ago, and couldn’t help turning the last page to note the subsequent article, a four-page “Note on the dislocation of the tail at a certain point observed in the skeleton of many Ichthyosauri,” written by Richard Owen.

  Richard Owen, then a young man, became England’s greatest comparative anatomist and first director for the Natural History division of the British Museum when the collections finally escaped the shadows of the Elgin Marbles at Bloomsbury and won their own magnificent home in South Kensington (one of the world’s great Victorian buildings and an essential stop on any visit to London).

  Owen and Darwin had a long and problematical relationship. Darwin originally 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 did our understanding of their extensive convergence grow piece by piece. To be sure, the basic similarity with fishes had never been doubted. In fact, the first two published references, both in 1708, mistook ichthyosaur vertebrae for the backbone of a fish. Both the celebrated Swiss naturalist J. J. Scheuchzer, in his Querelae Piscium (Complaints of the Fishes) and the German J. J. Baier, in his work on fossils from the area of Nuremberg, presented figures of ichthyosaur vertebrae for a most interesting purpose: to maintain that fossils are true remains of creatures that once lived, and not some manifestation of a plastic force inherent in rocks and ordained to establish global order by eliciting parallel forms in the organic and inorganic realms (an idea that strikes us as absurd today, but that made lingering sense within a neo-Platonic ideology not yet fully dispersed by the causal theories of Newton and Descartes).

  Both Scheuchzer and Baier argued that these “fish” fossils recorded the devastation of Noah’s flood. Scheuchzer’s work is written as a humorous conversation among fossil fishes annoyed at humans who do not recognize their organic nature and affinity with living relatives. As for Baier, I recently had the pleasure of purchasing a copy of his rare work, without the slightest expectation that I would soon, or ever, have any practical or immediate use for this beautiful book. What a pleasure, then, to read his two page discussion of “ichthyospondyli” (fish vertebrae), with its conclusion that we must view them “pro piscibus vere petrificatis…pro universalis Diluvii reliquiis”—as truly petrified fish, remains of the universal flood.

  Better evidence, primarily from bones of the skull and paddles, revealed the reptilian nature of ichthyosaurs by the early nineteenth century, but strong convergence upon fishes remained the prevailing theme of most writing. Nonetheless, though skeletons revealed the streamlined body and fishlike paddles, two missing pieces conspired to prevent any full appreciation for the true (and awesome) extent of convergence—for the back and tail fins, as soft structures, had not been discovered. All the early reconstructions—by Buckland, Conybeare, de la Beche, Hawkins, and other worthies of early English geology—showed a slithering serpent without back or tail fins, not the reptilian embodiment of a swordfish. How, then, did the two key pieces fall into the piscine puzzle?

  Richard Owen’s note of 1838 stands as the chief document in this resolution. Thanks largely to keen insight and uncanny field work from Mary Anning, and to support from the demented and eccentric Thomas Hawkins (whose monographs of 1834 and 1840 must rank as the craziest documents ever written in paleontology), many good skeletons of ichthyosaurs were collected in England during the early nineteenth century. Owen had noticed an apparent peculiarity in one fine specimen—a sharp downward bend in the sequence of rear vertebrae at about two-thirds the distance from the back flippers to the end of the tail. Owen gave little thought to this tailbend, reasoning that it only represented an anomaly (probably a postmortem artifact) of a single specimen. But when skeleton after skeleton showed a tailbend in the same position, Owen realized that he had stumbled upon a phenomenon worthy of explanation. Owen wrote:

  Caricature of ichthyosaurs by Henry de la Beche, made in the early nineteenth century before the back and tail fins had been discovered. Courtesy of Department of Library Services, American Museum of Natural History.

  Having recently examined many saurian skeletons now in London, the greater part of which have been disencumbered of their earthy shroud by the chisel of Mr. Hawkins, a condition of the tail which, on a former occasion, in a single instance had arrested my attention, but without calling up any theory to account for it, now more forcibly engaged my thought, from observing that it was repeated, with scarcely any variation, in five instances [boy, did they love to write back then, as in Owen’s “disencumbered of their earthy shroud” for our modern “dug out of the rock”]. The condition to which I allude is an abrupt bend or dislocation of the tail…the terminal portion continuing, after the bend, almost as straight as the portion of the tail preceding it. In short, the appearance presented is precisely that of a stick which has been broken, and with the broken end still left attached, and depending [that is, hanging] at an open angle.

  Illustration of ichthyosaur tail bends taken from Richard Owen’s 1838 article. Courtesy of Department of Library Services, American Museum of Natural History.

  Owen then drew the right conclusion for the wrong reason and correctly inferred the existence of a tail fin. He argued that the constant position of the tailbend must record an attachment of some structure at this point. He rightly conjectured that this organ must be a tail fin, and he even predicted its vertical position (as in fishes) rather than a horizontal orientation (as in whales). But he wrongly assumed that the bend must represent a dislocation (probably after death) of an originally straight vertebral column—perhaps because the tail fin bloats with gas as the animal begins to decay, thereby fracturing the vertebral column at the front border of the fin. Owen then added other conjectures, and wrote:

  The appearance in the tail of the Ichthyosaurus…is too uniform and common to be due entirely to an accidental and extrinsic cause. I am therefore disposed to attribute it to an influence connected with some structure of the recent animal; and most probably to the presence of a terminal…caudal fin, which, either by its weight, or by the force of the waves beating upon its extended surface, or by the action of predatory animals of strength sufficient to tug at without tearing it off, might…give rise to a dislocation of the caudal vertebrae immediately proximal to its attachment.

  The puzzle finally achieved its solution in the 1890s when the perennial, but rarely granted, prayer of all paleontologists was answered by the powers that be. Ichthyosaurs with preserved soft parts were discovered in the Holzmaden deposits near Stuttgart. These sediments are so rich in organic oils and bitumen that they actually burn. (One fire raged beyond control from 1668 to 1674 and another from 1937 to 1939). Details of internal organs are not retained in these bitumen beds, but body outlines remain intact as black films upon the light gray rock. (Most of the fine specimens displayed at museums throughout the world come from the Holzmaden beds, and many readers are no doubt familiar with ichthyosaur body outlines preserved as blackened films on the rock under and behind the bones.)

  The Holzmaden ichthyosaurs finally proved the extent of external convergence upon the stereotypical form of a free-swimming fish. The dorsal fin, with no bony support at all, was revealed for the first time. And the caudal fin, correctly inferred by Owen from the tailbend, now stood out for all to see. The fin was vertical, as Owen had surmised, and composed of two nearly equal and symmetrical lobes. The vertebral bend did mark, again as Owen had conjectured, the anterior border of the fin—but as an item of normal anatomy, not a postmortem artifact or dislocation. The vertebral column bent naturally down to follow the lower border of the lower lobe of the tail right to the animal’s rear end. No other vertebrate displays this orientation. In fishes, the vertebral column either stops at the inception of the tail or extends, as in sharks, into the upper border of the upper lobe. No wonder that the ichthyosaur tailbend had provoked such confusion for more than fifty years.

  A tossil ichthyosaur with characteristic and excellent preservation from the Holzmaden deposits. Note the outlines of back and tail fins, and also the bending of the vertebral column into the lower lobe of the tail. Courtesy of Natural History.

  Nearly a century has passed since the Holzmaden discoveries revealed the true nature of the ichthyosaur tailbend by exposing its enclosure within the caudal fin. Yet the tailbend continues to provoke commentary and controversy for two main reasons as outlined by Chris McGowan of the Royal Ontario Museum, Toronto, and the world’s leading expert on ichthyosaurs
(my thanks to Dr. McGowan not only for his many illuminating articles, but especially for enduring a long phone call of inquiry during my research for this essay). First, and positively, the location, angle of downturn, and length of the vertebral column after the bend specify both the size and form of the caudal fin (only the Holzmaden ichthyosaurs preserve the fin itself as a carbonized film; all other specimens are bones alone, and the tailfin must be inferred from the vertebral column).

  Second, and representing yet another dubious triumph of expectation over observation (perhaps the most common of human foibles), many classic specimens have been reconstructed on the assumption that tailbends must be present. I raise no issue of fraud or delusion. In many specimens, the vertebrae (particularly the small items at the rear end) lie scattered over the rock surface. The wonderfully expert and professional Holzmaden preparators adopted the custom of removing these bones entirely from the matrix and then resetting them in the inferred position of the living animal—that is, with a tailbend. We have no doubt that several ichthyosaur species developed a pronounced tailbend, since perfect specimens with preserved body outlines clearly show the tail vertebrae extending into the lower lobe of the caudal fin. But perhaps other species (particularly the earliest forms) lacked a tailbend, and perhaps preparators have tended to exaggerate the amount of inclination in reconstructing their specimens.

  If the actual tailbends of most specimens on display are thus infected with doubt, how can we be confident about the existence and form of the caudal fin in most species? And, since this information is crucial to our understanding of swimming and maneuvering in ichthyosaurs, how can we hope to reconstruct the ecology of these fascinating animals? Obviously, we need a criterion of confirmation separate from the bend itself. Fortunately, McGowan has been able to establish such a criterion and to devise an ingenious way of putting it into practice.