Transylvanian Dinosaurs
Page 12
A more modern interpretation of fossil bones began to emerge with the rise of interest in comparative anatomy. During the seventeenth century, Robert Plot (1640–1696), curator of Oxford University’s Ashmolean Museum, was busy producing the first illustrated book of fossils from England.5 Among the plates was a figure of the lowermost part of a thighbone that looked very similar to that of humans, but was much greater in size (figure 5.1). Plot concluded that it “must have belonged to some greater animal than an Ox or Horse; and if so in all probability it must have been the Bone of some Elephant, brought hither during the Government of the Romans in Britain.”6 Another century passed, and Richard Brookes (dates unknown), also a natural historian from England, copied Plot’s figure into a compendium of natural history.7 In a caption to this illustration, Brookes now applied a name—Scrotum humanum. Although his designation was in apparent recognition of its general shape, Brookes was not persuaded that the fossil was an actual fossilized gigantic scrotum (although one French philosopher, Jean-Baptiste Robinet [1735–1820], apparently was convinced8). Instead, Brookes considered it, as Plot had before him, a portion of the thigh bone of a large animal. Although the original specimen is lost, we now know that this fragment was the lower end of a femur from the Middle Jurassic of Oxfordshire and that it must have belonged to some sort of large theropod dinosaur.9
In the seventeenth century, dinosaurs, and indeed deep time itself, had yet to be discovered; it took three people from England, the help of the great French comparative anatomist Georges Cuvier, and the first half of the nineteenth century to finally recognize them.10 William Buck-land (figure 5.2, left), Reader of Mineralogy, Reader of Geology, and then Canon of Christ Church College at Oxford University, sought to fuse geology and paleontology with the traditional Christian teachings of Noah’s flood and a divine Creator. A legendary eccentric, Buckland is also credited with the first scientific description of a dinosaur, a theropod that he named Megalosaurus, based on a fragment of a lower jaw with teeth and some postcranial bones from the Middle Jurassic Stonesfield Slate of Oxfordshire.11 Buckland’s contemporary, Gideon Mantell (figure 5.2, right), was trained as a physician, a career marked by considerable failure. It is Mantell’s other life pursuit, research on the paleontological riches of the English Weald, for which he is now remembered. As one of the world’s original dinosaur hunters, he energetically collected, swapped, and purchased teeth, isolated bones, and portions of a skeleton, from which he gave us the first-discovered herbivorous dinosaur, Iguanodon, from the Early Cretaceous of southeastern England.12
Figure 5.1. The first “named” dinosaur, Scrotum humanum, in actuality the lower end of a Megalosaurus femur. (Original plate from Brookes 1763)
Together, Buckland and Mantell recognized the unique nature of their material: the bones and teeth of reptiles whose great size eclipsed any reptile and most mammals known up to that time. Mantell, in particular, had the vision to impart animal form and habits to these fossil bones, as well as the intellectual courage to perceive that these were animals of the remote past, not relics of Noah’s flood. Deep time was about to be discovered.
Figure 5.2. William Buckland (1784–1856; left) and Gideon Algernon Mantell (1790–1852; right)
The actual recognition of Dinosauria, including Megalosaurus, Iguanodon, and an armored dinosaur known as Hylaeosaurus, was left in the hands of Sir Richard Owen.13 A comparative anatomist at the Royal College of Surgeons in London, Owen built on his personal insights and those of Mantell and Buckland and, with a flash of insight and no little political guile, hatched Dinosauria in April 1842.14 “The combination of such characters [in particular, a sacrum composed of more than two fused vertebrae] all manifested by creatures far surpassing in size the largest of existing reptiles, will, it is presumed, be deemed sufficient ground for establishing a distinct tribe or suborder of saurian reptiles, for which I would propose the name of Dinosauria.”15 This new group of reptiles called Dinosauria—the “fearfully great lizards”—was thought to consist of highly advanced terrestrial quadrupeds with the unmistakable character of enormous size.
From Owen’s day onward, the quintessential dinosaur, at least in the popular imagination, has been gigantic. These creatures were and still are the behemoths that pack the public into museum exhibits.16 With each thud of their feet, they have rumbled their way into books of all sorts, onto postage stamps and collecting cards, into children’s games (including the computer variety), and, of course, onto the big screen, from Gertie the Dinosaur (1912) through Jurassic Park III (2001).17
Dinosaur size records are set, only to be broken. Notable among the carnivores, the perennial favorite, Tyrannosaurus (or T. rex to its friends), a 12 m long predator from the Late Cretaceous of North America that may have tipped the scales at 7 metric tonnes, now has plenty of company among the great Mesozoic slayers.18 Carcharodontosaurus, a theropod from the middle Cretaceous of northern Africa, has been estimated to be as long as Tyrannosaurus, but it appears to have been slightly more massive,19 whereas Giganotosaurus from the Early Cretaceous of Argentina is thought by some to measure over 14 m and weigh up to 8 metric tonnes.20 Yet among the truly supergigantic, there’s nothing like the sauropods, which have long been known to be the largest of all terrestrial animals ever to have lived. For many years, the prize went to Diplodocus, reaching a length of 27 m,21 but this record has seen its challenges. In North America, there is Seismosaurus, an as-yet poorly known sauropod from the Late Jurassic of the southwestern United States, whose length has been variously estimated at upwards of 50 m.22 Not to be outdone, the Southern Hemisphere has offered up Argentinosaurus. Discovered in 1989, named in 1993, but with other aspects not yet fully published, this behemoth has also been estimated to be as much as 50 m long.23 These are clearly humbling animals.
The evolution of dinosaurian gigantism is but one example of the general trend of animals becoming larger over time. Other well-known examples include horses, ratites (ostriches and their kin), and ammonites (long-extinct relatives to today’s pearly nautilus, which grew to diameters of about 2 m). Such evolutionary trends in increasing body size—commonly referred to as Cope’s Rule, after E. D. Cope’s recognition that evolution often results in phylogenetic size increases24—are thought to be due to an extension of growth tendencies already present in ancestral ontogenies. These extensions are known as peramorphs (“beyond shapes”).25 Peramorphosis implies that descendants reach new end stages (for example, a larger body size) by passing along this course of ancestral ontogeny, but then, instead of standing still, advance still further.
TRANSYLVANIA: BUCKING THE GIGANTISM TREND
The production of peramorphs through an extension of growth has its opposite manifestation in the production of paedomorphs (“baby shapes”) through arrested growth.26 Said another way, paedomorphosis is the retention of ancestral juvenile characters into the adulthood of its descendants. Referred to by Ken McNamara (paleontologist and evolutionary biologist at the Western Australian Museum in Perth) as the Peter Pan syndrome, two modern organisms stand out as the most celebrated examples of arrested growth.27 The first is the axolotl (figure 5.3), otherwise known as the Mexican salamander Ambystoma mexicanum, which retains its larval characteristics into adulthood and thereby is able to reproduce while remaining in the form of an aquatic larva.28 The other modern epitome of paedomorphosis, as we shall see a little later, is ourselves—Homo sapiens. Furthermore, we encounter paedomorphs among dogs, horses, mice (including one named Mickey), finches, ratites, fish, insects, snails, and plants. They are even known from the fossil record, including the dwarfed dinosaurs from Transylvania.29
In chapter 2, we let Franz Baron Nopcsa introduce, for the first time, the notion of small dinosaurs from Transylvania in the context of his hypothesis that in the Cretaceous this region was an island. In his first synthesis of this subject, Nopcsa surveyed the Transylvanian fauna,30 emphasizing in particular the disparity in body size of its members: “While the turtles, crocodilians,
and similar animals of the Late Cretaceous reached their normal size, the dinosaurs almost always remain below their normal size.”31 He observed that most of the Transylvanian dinosaurs hardly reached 4 m in length, and the largest (what was to become Magyarosaurus dacus) was a puny 6 m long, compared with a more representative 15–20 m for other sauropods. After this 1915 paper, Nopcsa’s fascination with body size shifted to dinosaurian giants, but in both cases he equated the evolution of both dwarfism and gigantism to changes in the endocrine system, such as in the size and function of the pituitary gland.32
Nopcsa viewed Transylvanian dwarfing as the body-size consequences of island evolution. Thus these dinosaurs joined the ranks of other insular organisms, both living and extinct, that have evolved into smaller and larger versions of their continental relatives: dwarfed crocodilians, giant moas, downsized island deer, enormous Galápagos turtles, dwarfed donkeys, huge Komodo lizards, small Bali tigers, pygmy elephants, and dwarfed hippos.33 After 1914, Nopcsa’s efforts to define “small” in dinosaurs ceased, and he simply concluded that the Transylvanian dinosaurs were “less than the usual colossal forms” that tended to be found elsewhere. Whereas such a statement is certainly true, it belies a more complex set of questions about the acquisition of small body size. Yes, these dinosaurs were smaller, but were they, and they alone, miniaturized from a larger ancestor?
Figure 5.3. The axolotl (Ambystoma mexicanum), a living salamander that reproduces as an aquatic form; in this way, it is a modern exemplar of paedomorphosis. (After Lamar 1997)
Heterochrony: Timing (and Rate) Is Everything
Why is it that all babies are born looking like Winston Churchill, and vice versa? To put it another way, why do the faces of both young and old humans alike tend to look more like the faces of short-faced chimpanzee and gorilla youngsters than these latter two juveniles do to their respective long-faced parents (figure 5.4)?34 These similarities and differences in proportions have been attributed to changes in different aspects of facial growth and, when given an evolutionary context, are described in terms of heterochrony. We have already seen heterochrony in action through the gigantic (peramorphic) tendencies of dinosaurs and the paedomorphosis of the axolotl. In the case of humans, gorillas, and chimps, it is the retardation of growth in the jaw region that gives us that youthful look. So to the degree that the pattern of growth in chimps and gorillas is considered the primitive condition for the immediate clade of primates that includes these two groups and us, then we (or rather our faces) represent the paedomorphic retention of juvenile features into adulthood.
In their book of the same name, Michael McKinney and Ken Mc-Namara define heterochrony (meaning “different time”) as the “change in timing or rate of developmental events, relative to the same events in the ancestor.”35 In other words, heterochrony seeks to link development with evolution. In Darwin’s world, where external forces held sway, there was little attempt to link ontogeny with phylogeny through his view of natural selection. Instead, it was principally the German scientific community, notably the nineteenth-century school of Naturphilosophie, that attempted to link developmental sequences with similar patterns found in evolutionary history.36 Ernst Haeckel (1834–1919), in his attempt to reconcile Darwin’s evolutionary mantra of “descent with modification” with emerging issues in developmental biology, coined his own slogan, “ontogeny recapitulates phylogeny,” now known as Haeckel’s Biogenic Law.37
The most fundamental reconsiderations of how development interconnects with phylogeny to produce heterochrony have been through Stephen J. Gould’s now-classic book, Ontogeny and Phylogeny,38 and a subsequent paper cowritten by him and an impressive cadre of evolutionary and developmental biologists: Pere Alberch, George Oster, and David Wake.39 Both publications stress how the features of organisms change with size during both ontogeny and phylogeny, and the paper sets forth the schema of heterochronic terminology to express these changes. The concept of heterochrony has seen an incredible growth in evolutionary biology in recent years.40 It is now used to explain, with great success, the differences in skull forms of domesticated dogs, the evolution of fossil sea urchins, the outrageously large antlers in the so-called Irish elk (actually a giant deer), and sexual dimorphism in the forked fungus beetle.41 Each of these examples of heterochrony, and many more, describes how ontogenetic changes form the basis for phylogenetic transformations.
Figure 5.4. Neoteny and the human face. The left column (a) of three skulls represents the ontogenetic changes in the facial skeleton of a chimpanzee, while the right column (b) of two skulls represents a newborn and an adult human. The superimposed grids indicate how much change occurs in the skulls during ontogeny. Note the similarity of the faces of a newborn chimpanzee and a newborn human, the similarity of newborn and adult humans, and the dissimilarity between an adult human and an adult chimpanzee. (After Starck and Kummer 1962)
So what is this thing we call heterochrony? We began talking about the subject by introducing paedomorphs and peramorphs, the two ontogenetic patterns that are linked between ancestors and descendants. A paedomorph is an ontogenetically less well-developed descendant than its ancestor. That is, reduced or arrested development will produce a paedomorph. In contrast, if its development goes beyond that of its ancestor, a descendant is considered to be a peramorph. Peramorphs are produced by increased development. A sequence of successive paedomorphs is known as a paedomorphocline. Likewise, a continued trend of peramorphs is called a peramorphocline. Each of these states—paedomorphosis and peramorphosis—is the product of three growth processes: (1) the timing of trait formation, (2) the cessation of its development, and (3) the rate at which it is acquired (figure 5.5).
A change in the time that the growth of an organ or structure starts development, relative to others, results in either predisplacement (if the change is earlier) or postdisplacement (a later change). Predisplacement is seen in the nasal horns of the large, early Tertiary mammals known as titanotheres, in which earlier development results in peramorphosis.42 In contrast, postdisplacement involves the delayed onset of growth as, for example, is seen in the timing of the cellular differentiation of fins and limbs in vertebrates.43 In this way, later starters produce the paedomorphic condition.
When the development of a structure ceases also has an effect on the final body form of an organism. Earlier cessation/offset in a descendant, rather than in its ancestor, produces what is known as progenesis. Progenesis is thought to have altered the ancestral developmental program of arthropods and reduced the number of segments and limbs in insects, the paedomorphic descendants of their common ancestor with the multisegmented, multilegged centipedes and millipedes.44 Hypermorphosis produced by delayed offset—the extension of growth beyond that of an ancestor—has peramorphically produced large dogs such as Irish wolfhounds, Great Danes, and St. Bernards.45
Changes in the rate of growth can produce either acceleration, when growth rate is increased, or neoteny, when growth rate is retarded. Neoteny—that is, less shape change and a decrease in complexity—is thought to be behind why adult human faces look much like those of youngsters: the rate of growth of the face is decelerated, compared with that seen in ancestral ontogenies, to produce the paedomorphic human facial profile.46 Acceleration, by contrast, can be seen in the growth of the bones in the digits of the hand in bats, acting to produce the peramorphic scaffolding of their special kind of airfoil for flight.47
Figure 5.5. Heterochrony, its two manifestations (peramorphosis and paedomorphosis), and the six processes that produce them. (After McKinney and McNamara 1991)
As we have briefly introduced the concepts here, when descendants go beyond the size and shape of their ancestors, we have a pattern known as peramorphosis. And when descendants fail to achieve the size and shape of their ancestors, due to arrested growth, we have paedomorphosis. Although peramorphs and paedomorphs can be more-or-less easily recognized by allometric comparisons of size and shape, the processes that produce them (e.g., ne
oteny, progenesis, postdisplacement, etc.) are more difficult to discern, because they require a measure of actual ontogenetic timing. For living organisms, there is no barrier to establishing when various life-history events—the timing of protein expression, cell differentiation and interaction, hormone secretion, sexual maturity, and so on—take place (although this information is rarely available). The question is, can we do this with fossils?
We are fortunate to be living in a time when the insides of fossils are as important—and are nearly as easily seen—as their outsides are. Bone in thin-section is beginning to reveal how dinosaurs lived, reproduced, metabolized, and grew in ways for which, a generation ago, we had hardly a clue. For our purposes here, bone in thin-section appears to be able to allow us to calibrate anatomical changes in real time. This burgeoning research is called skeletochronology.48
How does it work? First, we know that bone growth can be episodic, and this ebb and flow is laid down in the bone itself, much like tree rings.49 These lines, found in both extant and extinct vertebrates, reflect disruptions in growth: periods of rapid growth followed by slowdowns or cessation. More simply put, growth lines—what scientists call lines of arrested growth (LAGs)—appear as thin, circumferential, avascular regions when bone is seen in thin-section. LAGs are thought to be annual, representing yearly histories of seasonality and ecological stress.
Provided that they are annual, LAG counts give an estimate of the longevity of a particular individual. When these are combined from individuals of different sizes within a single species—say, Tyrannosaurus rex—such longevity estimates, coupled with size data or mass estimates, can be used to make the classically sigmoidal age-versus-size curves. From such studies, we know that at least some of the extinct dinosaurs (e.g., ornithopods, theropods, and sauropods) grew quite rapidly, perhaps as much as is seen in modern birds.50 Yet how did these rates vary phylo-genetically among close relatives? By reconstructing and comparing the growth curves of closely related forms, it is possible to bring skeleto-chronologies to bear on real-time heterochronic problems. This has indeed been done for T. rex and other tyrannosaurids, Janenschia and other sauropods, the prosauropod Plateosaurus, the ceratopsian Psittacosaurus, and, perhaps most relevant here, a new species of dwarfed sauropod from the Late Jurassic of Germany.51