How can an angular bend be produced in a basically linear structure (like the vertebral column), built from a sequence of disks that must follow, one behind the other, without large spaces between? As the accompanying sketch shows, tailbends imply a change in the shape of the crucial vertebral disks at the bend itself—from their usual form (with upper and lower borders of equal width) to a wedge with a wider border on top and a narrower edge below. A succession of wedge-shaped disks will inevitably cause the tail to bend, and the greater the difference in width between upper and lower borders, the more pronounced the bend. In fact, by a simple construction akin to the problems we all worked in high-school plane geometry, the angle of the bend can be inferred from the number of wedge-shaped disks and their intensity of wedging.
But how can this wedging be assessed? The vertebrae of most skeletons are at least partially embedded in rock, and both ends are rarely exposed to reveal the extent of wedging (while museums rarely look kindly upon requests for sufficient mayhem upon their specimens to dig the vertebrae out of the enclosing matrix). McGowan solved this problem with a boost from modern medical technology—computed tomography as provided by a CT-scanner. These marvelous, donut-shaped x-ray devices can take a photographic slice right through a human body in any orientation (so long as the body fits into the donut-hole of the machine). Well, an ichthyosaur in its matrix is often about the same size as a human body. Why not take a CT-scan of vertebrae at the tailbend, thus producing a photographic image of the vertebral disks while still embedded in their matrix? (McGowan didn’t initiate the application of CT-scanning to paleontological material. Several successful attempts have been made in the past few years, including the resolution of cranial capacities and form of unerupted teeth in some important skulls of the human fossil record.)
Shape of vertebral disks in ichthyosaurs with tail bends. Note the necessary wedge shape at the bend itself. Ben Gamit.
McGowan used a CT-scanner to affirm that Leptopterygius tenuirostris, an early ichthyosaur with an uncertain tailbend currently subject to hot dispute, did grow a series of six wedge-shaped vertebral disks in the crucial region—not strongly wedged to be sure (none producing more than a five degree bend), but yielding in their ensemble a modest tailbend of some 25 degrees (see McGowan’s article, “The ichthyosaurian tailbend: A verification problem facilitated by computed tomography,” in the bibliography). Somehow, I feel a great sense of satisfaction in the affirmation of this continuity in human striving for knowledge through time—to think that a discussion beginning in two Latin treatises written in 1708, proceeding through the keen observations of England’s greatest anatomist in the 1830s, and on to the discovery of preserved body outlines in a famous German locality during the 1890s should be resolved, as we begin our last decade’s countdown towards the millennium, by the latest device of medical machinery!
Yet, however satisfying the particular resolution, this tale (and tail) would convey no message or meaning (to those outside the tiny coterie of ichthyosaurian aficionados) if the problem of the ichthyosaur tailbend did not illuminate something central in evolutionary theory. Ichthyosaurs are most celebrated for their convergence upon the external form of superior swimmers among fishes. Since English traditions in natural history place primary emphasis on the concept of adaptation, these similarities of fish and marine reptile have won the lion’s share of written attention—for we know how the threefold combination of flippers, backfin, and tailfin work in efficient hydrodynamic coordination, and we are awed that two independent lineages evolved such uncanny resemblance for apparently similar function. This awe even predates evolutionary theory, for an earlier attribution to God’s benevolent care inspired as much admiration as our current respect for the power of natural selection. William Buckland, Owen’s close colleague, had a special affection for ichthyosaurs. He also wrote the greatest paean of the 1830s to adaptation as proof of God’s benevolence. In Geology and Mineralogy Considered with Reference to Natural Theology, written in 1836, Buckland invoked the precise convergence of ichthyosaur and fish as a proof of God’s goodness. Buckland acknowledged that an ordinary reptile would be in severe trouble at sea, but ichthyosaurs have been granted by divine fiat (read “endowed by natural selection” for a modern version of the same argument):
…a union of compensative contrivances, so similar in their relations, so identical in their objects, and so perfect in the adaptation of each subordinate part, to the harmony and perfection of the whole; that we cannot but recognize throughout them all, the workings of one and the same eternal principle of Wisdom and Intelligence, presiding from first to last over the total fabric of the Creation.
Yet, in our complex world of natural history, almost any profuse enthusiasm also elicits its mitigating opposite. (Such a cautionary splash of cold water may then emerge as a primary theme with more enlightening implications in itself.) Yes, ichthyosaur convergences are remarkable; only a soulless curmudgeon could fail to be impressed by the fishlike form of this descendant from ordinary terrestrial reptiles. Only the most militant denigrator of Darwin and the entire English tradition could fail to utter the word adaptation with both confidence and admiration.
But another perspective demands equal attention—and Owen, the much misunderstood proponent of a continental tradition that viewed adaptation as superficial, and sought regularities of form underneath a garb of immediate design, discussed ichthyosaurs primarily in the light of this alternative. What are the limits to adaptation imposed by the disparate anatomical designs underlying a convergence (fishes and reptiles in this case)? To what extent must the ichthyosaur remain in the thrall of its past, quite unable to mimic the form of a fish exactly because the historical legacy of a reptilian body plan precludes a large set of favorable options? To what degree, in short, must an ichthyosaur remain an easily identified reptile in marine drag?
For a primary statement of this alternate theme (limits imposed by inherited design), we must look to the largely forgotten work of the great Belgian paleontologist Louis Dollo (1857–1931). Dollo gave his name to an evolutionary principle known as irreversibility (often called Dollo’s Law). In one of the cruel ironies often imposed by history, many fine thinkers win their posthumous recognition only by eponymous linkage with a principle so widely misunderstood that true views turn into their opposite. Many evolutionists interpret Dollo’s Law as an antiquated statement about inherent, directional drives in evolution—a last gasp of a mystical vitalism that the Darwinian juggernaut finally defeated. In fact, Dollo was a convinced mechanist, and a Darwinian in basic orientation (with some interesting wrinkles of disagreement).
To Dollo, irreversibility epitomized the nature of history under simple conditions of mathematical probability (Dollo had obtained an extensive education in mathematics and attributed his formulation of irreversibility to this training). Evolutionary transformations are so complex—involving hundreds of independent changes—that any complete reversal to a former state becomes impossible for the same reason that you will never flip 1,000 heads in a row with an honest coin. No mysticism, no vitalism, only the ordinary operation of probability in a complex world. A simple change (increase in size, mutation in a single gene) may be reversed, but the standard transformations that form the bread and butter of paleontology (origin of flight in birds, evolution of humans from apelike ancestors) cannot run backwards to recover an ancestral state exactly.
History is irrevocable. Once you adopt the ordinary body plan of a reptile, hundreds of options are forever closed, and future possibilities must unfold within the limits of inherited designs. Adaptive latitude is impressive, and natural selection (metaphorically speaking) is nothing if not ingenious. A terrestrial reptile may return to the sea and converge upon fishes in all important aspects of external form. But the similarity can only be, quite literally, skin deep and truly superficial. The convergence must be built with reptilian parts, and this historical signature of an evolutionary past cannot be erased. Dollo explicitly link
ed his principle of irreversibility with a concept that he called “indestructibility of the past.”
When we look again at the three great convergences of ichthyosaurs—the flippers, the dorsal fin, and the caudal fin—but this time from the alternate perspective of limits imposed by irrevocable starting points, we find that these features beautifully illustrate the three most important principles of irreversibility as a signature of history.
1. The flippers, or you must use parts available from ancestral contexts little suited to present environments. The flippers, by external form, are well adapted for swimming and balancing. But their internal bony structure reveals a terrestrial reptile under the marine adaptation. The front flipper begins with a stout humerus, followed by a shortened and flattened radius and ulna, side by side. The carpals and metacarpals (hand bones) and phalanges (finger bones) follow in a similar flattened modification. In an interesting change (still related to an irrevocable ancestral state), the phalanges are multiplied into long rows that mimic the rays of fish fins. Humans have three phalanges per finger (two for the thumb); ichthyosaurs can grow more than twenty per finger.
2. The dorsal fin, or you can’t get there from here. The dorsal fin of fishes generally contains a strengthening set of bony rays. Similar structures might well have benefited ichthyosaurs, but their terrestrial ancestors built no recruitable body parts along the back. Ichthyosaurs therefore evolved a boneless dorsal fin (that would have eluded us altogether if we had never discovered the Holzmaden specimens).
3. The caudal fin and its tailbend, or you must always build a converging structure with some distinctive difference, due to irrevocable ancestry, from the original model. The vertebral column of fishes, as noted above, either stops at the inception of the tail or extends into the upper lobe. Only in ichthyosaurs do the vertebrae bend down into the lower lobe of the tail. We do not know why ichthyosaurs developed this strikingly different and unique internal structure (I would need another essay to discuss the interesting structural and functional explanations that have been proposed), but all convergences evolve with distinctive differences based on a thousand quirks of disparate ancestries.
Louis Dollo has long been one of my private heros. I meant to cite his views on irreversibility as a centerpiece of this essay, but I didn’t know, until a chance discovery in the midst of my research, that he had written an entire paper on the caudal fin of ichthyosaurs—and at a most interesting time, in 1892 just after the discovery of fin outlines in the Holzmaden specimens. Dollo rejoiced that these beautifully preserved specimens had resolved “la curieuse dislocation de la colonne vertébrale, signalée depuis longtemps” (the curious dislocation of the vertebral column, recognized for so long). And he proposed an explanation rooted in uniqueness imposed by irrevocable history. I doubt that he was right in detail, but his conjecture is ingenious, and entirely in the spirit of an important and insufficiently appreciated principle of historical reconstruction. He argued that the tailbend arose because the two-lobed caudal fin of ichthyosaurs evolved from a skin-fold along the back (source of the dorsal fin as well), which extended itself in a posterior direction to form the upper lobe of the tailfin and then pushed the vertebral column down to form the lower lobe. Since several modern reptiles maintain such a skin fold along the back, but never along the belly, new fins could only evolve along the dorsal edge of the body, and the vertebral column could only be pushed down to form a two-lobed tailfin. But ancestral fishes maintained a fin-fold along both back and belly, and a two-lobed tailfin could evolve as a lower lobe pushed the vertebral column up.
Richard Owen, in contrast with his adaptationist colleague Buckland, appreciated the primacy of maintained reptilian design as the main lesson of ichthyosaur convergence. He wrote in his great monograph on British fossil reptiles (published between 1865 and 1881, and anticipating Dollo’s concerns):
The adaptive modification of the Ichthyopterygian skeleton, like those of the Cetacean [whale] relate to their medium of existence; [but] they are superinduced, in the one case upon a Reptilian, in the other upon a Mammalian type.
At about the same time, and in a more pointed commentary on the same theme of irrevocability in history, W. S. Gilbert (in Princess Ida) then penned a crisp epitome to remind his audiences of evolution’s major lesson:
Darwinian man though well behaved
At best is only a monkey shaved.
6 | An Earful of Jaw
THE MOST SUBLIME of all beauties often proceed from the softest or the smallest—the quadruple pianissimos of Schubert’s “Schöne Müllerin,” as sung by Fischer-Dieskau (and penetrating with brilliant clarity to the last row of the second balcony, where I once sat for the greatest performance I ever witnessed) or the tiny birds of brilliant plumage depicted in the marginalia of medieval manuscripts. But even the most refined and intellectual character may succumb without shame to the sheer din employed now and then by great composers to overwhelm the emotions by brute force rather than ethereal loveliness—Ravel’s orchestration of the “Great Gate of Kiev” at the end of Moussorgsky’s Pictures at an Exhibition, or the last scene of Wagner’s Die Meistersinger.
I once had the privilege of singing with the Boston Symphony at Tanglewood in the midst of numero uno among musical dins—the Tuba mirum of Berlioz’s Requiem. I had listened to the piece all my adult life; we had rehearsed (without orchestra) for weeks. I knew exactly what was coming as the dress rehearsal began. The four supplementary brass choirs enter one after the other, building and building to a climax finally joined by the timpani—eight pair, I think, although they seemed to extend forever in an endless row before the choral risers. And against this ultimate crescendo, the basses alone (including me) must sing the great invocation of the last judgment:
Tuba mirum spargens sonum
Per sepulchra regionum
Coget omnes ante thronum
(The wondrous sound of the trumpet goes forth to the tombs of all regions, calling all before the throne.)
So it should go, and so it went—but not for me. I had devolved into tears and spinal shivers—not in ecstasy at the beauty, but in awe at the volume. (Forewarned is forearmed; I was fine at the performance itself.) Great composers have every right to exploit the physiology of emotional response in this way, but only sparingly, for timing is the essence (and most of Berlioz’s Requiem is soft).
My memory of this extraordinary incident in my emotional ontogeny focuses upon a curious highlight of mixed modalities. The sound of the brass assaulted my ears, but the thunder of the timpani followed another, unexpected route. It entered the wooden risers under my feet and rose from there to suffuse my body; sound became feeling.
I am no disciple of Jung, and I do not believe in distant phyletic memory. Yet, in an odd and purely analogical sense, I had become a fish for a moment. We (and nearly all terrestrial vertebrates) hear airborne sound through our ears; fish feel the vibrations of waterborne sound through their lateral line organs. Fish, in other words, “hear” by feeling—as I had done through a set of wooden risers with a density closer to water than to air.
For an optimal combination of fascination with excellent documentation, no saga in the history of terrestrial vertebrates can match the evolution of hearing. Two major transitions, seemingly impossible but then elegantly explained, stand out at opposite ends. First, at the inception of terrestrial life: How can creatures switch from feeling vibrations through lateral lines running all over their bodies to hearing sounds through ears? How, in other words, can new organs arise without apparent antecedents? Second, at the last major innovation in vertebrate design: How can bones that articulate the upper and lower jaws of reptiles move into the mammalian ear to become the malleus and incus (hammer and anvil) in the chain of three bones that conduct sound from the eardrum (the tympanum in anatomical parlance, recalling my Berlioz story in the singular) to the inner ear? How, in other words, can organs switch place and function without destroying an animal’s integrity as a working creature
? How can we even imagine an intermediary form in such a series? You can’t eat with an unhinged jaw. Creationists have used this difference between reptiles and mammals to proclaim evolution impossible a priori—I mean, really, how can jawbones become ear bones? Get serious! Yet, we shall see, once again, that the domain of conventional thought can be much narrower than the capabilities of nature—although ideas should be able to extend and soar beyond reality.
The key to the riddle of both these transitions lies in the major theme of my Berlioz story—multiple modalities and dual uses. You can pat your head and rub your stomach, walk and chew gum at the same time (most of us, at least), feel and hear sound, chew and sense with the same bones.
Nature writing in the lyrical mode often exalts the apparent perfection and optimality of organic design. Yet, as I frequently argue in these essays, such a position plunges nature into a disabling paradox, historically speaking. If such perfection existed as a norm, you might revel and exult all the more, but for the tiny problem that nature wouldn’t be here (at least in the form of complex organisms) if such optimality usually graced the products of evolution.
I recently made my first trip to Japan to deliver a lecture at the opening of an annual series that will bring one American scholar to Japan and a Japanese counterpart over here to speak on a common topic. I was both pleased and intrigued by our assigned theme for this initial year (largely at Japanese request)—creativity. (Some Japanese apparently fear—although my superficial impressions included nothing to sustain such anxiety—that their scholars and industrialists excel at efficiency and alteration, but not at innovation.)
Eight Little Piggies Page 9