Transylvanian Dinosaurs
Page 17
Historical contingency—the unpredictability of life’s history from particular, often small-scale events—will take a prominent place as we build a foundation to answer these and other questions about the dinosaurs of Transylvania. Even one of the fundamentals of evolutionary processes—speciation—is fraught with its fair share of unpredictability. Speciation is the formation, through genetic divergence, of two or more descendant species from one ancestral species. This process is the motor for the diversification of life, and, when rendered in terms of cladogenesis (i.e., the generation of branches in a cladogram; see the discussion of cladistics in chapter 2), it forms the backbone for identifying the overall pattern of evolutionary history.4
Speciation is not just enhanced by, but, more importantly, it is dependent on a plethora of unpredictable conditions, particularly due to insularity. Contingency should be more common, and have greater effects, in situations of isolation; insularity should showcase historical contingency. What is crucial is reproductive isolation. When gene flow within a once-contiguous, interbreeding population is sufficiently interrupted, genetic divergence takes place between two or more portions of that original population. This divergence can occur in the same place as the ancestral population (sympatry), along the length of a geographically contiguous population (parapatry), or in geographic isolation from the ancestral population (allopatry).5 The evidence is extensive for allopatric speciation, which—despite some objections6—is generally regarded as the most influential theory of speciation. In addition to its prominence in evolutionary theory, we will be discussing it here in order to scrutinize the relationship of the Transylvanian fauna to geographic isolation.
The basis for allopatric speciation is the development of a geographic barrier separating an ancestral population into two subsequent descendant populations—think of a new mountain range or a river cutting through a formerly continuous population. If such barriers could split a species for a long enough period of time, gene flow between the two groups would cease and each would then evolve separately. Depending on the specifics, mutation, genetic drift, and/or natural selection would alter the genome of the two descendant populations over time. If these populations rejoined after they had diverged sufficiently, it is unlikely that they would successfully interbreed, either because mating would yield unviable or sterile offspring, or they might not recognize each other as members of the same species and hence would not interbreed. When the former applies, we speak of postmating or postzygotic isolating mechanisms, and, for the latter, premating or prezygotic isolating mechanisms. In either case, geographic isolation has led to reproductive isolation and thereby to speciation.
Preeminent among the cases of allopatric speciation are those involving peripherally isolated populations. Ernst Mayr, one of the twentieth century’s most influential evolutionary biologists, originally described the basis for speciation through peripheral isolation—sometimes called peripatric speciation—and argued that it was one of the most effective means and commonest way of producing two species where once there was one.7 Peripatric speciation involves the founding of a small second population—a few original individuals, or perhaps a single fertilized female in the most extreme case—through geographic isolation. The colonization of an island by a small population is the most cited example of what has been termed the founder effect.8 Should this group remain geographically separated from the main population for a sufficient period of time, they, too, will undergo the same divergence we’ve just discussed generally for allopatric speciation.
An especially important feature of peripatric speciation is that it would proceed very quickly. First, the founding population will carry only a small sample of the genetic reservoir of the ancestral population. Not only is there such random genetic loss, but a rare gene in the main population may also start being passed on with relatively high frequency in the founder population.9 These unpredictable sampling errors in gene frequencies—genetic drift—will be especially strong determinants of the subsequent genomes, so long as the population size remains small. Even though the chance of survival of this small peripheral population may be low,10 it is certainly not zero, and when the founder population increases to a less vulnerable size, natural selection will begin to take over, reflecting conditions that could be quite different from those of the more widespread population. In this way, the speciating population rapidly passes from one well-integrated and stable condition, through a highly unstable period, to another period of balanced integration.
There’s certainly a lot of dumb luck to peripatric speciation. Peripheral populations may be well adapted to their particular conditions, but, at the same time, their geographic separation from the main population is an unpredictable event. So are the details of genetic loss during separation, with the founders carrying a small, unrepresentative portion of the genome of the ancestral population. Chance also comes into play while the population remains small—gene frequencies are free to drift up and down randomly, without the controlling hand of natural selection. In this way, historical contingency is not just a possibility, but instead is a necessity in producing rapid speciation through isolation of the kind thought to exist in nature.
CONTINGENCY AND THE TRANSYLVANIAN DINOSAURS
If anywhere in the world should reveal the power of contingency, it would be somewhere like Transylvania in the Cretaceous, where historical possibilities and pathways can be altered by chance events in the small, insular populations living there. Because of the geographic isolation of the place, coupled with its considerable tectonic flux (chapter 3), we should find an unusual agglomeration of animals and ecological relationships in Transylvania when compared with those more typical of the nearby North American and Asian landmasses, where the coastal plains were much more extensive, luxuriant, and tectonically quiet. Just who arrived in the Transylvanian region, and from where, are both far from predictable. Equally serendipitous are differences in the structure of predator-prey relationships, patterns of growth and development, intra-specific life histories, social structures, and evolutionary dynamics. When might these colonizers have arrived? Who interacted with whom? What changes in their features might have occurred? These are some of the questions we’ll be pursuing in our attempt to assess contingency in the history of the dinosaurs from Transylvania. Whatever answers we can bring to these questions, they must certainly be couched in the one thing we learned about the Transylvanian fauna in this and the previous chapter—its susceptibility to the vagaries of unpredictable evolutionary events due to its isolation.
BACK TO THE PAST
In a shallow sea, a parcel of land becomes available for whatever terrestrial organisms come its way. Yet who’s to say which organisms? The issue of who successfully colonizes a new piece of ground is complicated by the size of the property; its proximity to other terrestrial regions; environmental differences in temperature, rainfall, and so forth; a species’ potential for transoceanic migration; which species populate neighboring regions; which species might already have arrived on the newly available land; and, above all, chance.
In this chapter, we will look at Transylvania as more than a place where dinosaurs once lived some 70 million years ago. With different eyes than we had at the beginning of this book, we now see more than a jumble of fossils, the remains of which have long been collected in the picturesque northern foothills of the Retezat Mountains of western Romania. First, by virtue of both the studies conducted by Franz Nopcsa and subsequent research efforts, we’ve gone well beyond merely recognizing various creatures from a collection of scattered bones found in western Romania: the duck-billed Telmatosaurus, the solidly built Zal-moxes, the long-necked Magyarosaurus, the armored Struthiosaurus, and the thus-far imperfectly understood predatory dinosaurs. Through paleobiological inference, these dinosaurs have become more than just entries in a Transylvanian faunal list. Second, we’ve used cladistics to put these Late Cretaceous denizens into their community of descent—siblings and cousins, as it were—wi
th dinosaurs elsewhere in the world. Third, we’ve see them in as much of their complex terrestrial habitat as data and conjecture allow us to reproduce. Fourth, the small size of these dinosaurs—initially noted by Nopcsa—is now given a heterochronic context. Fifth, we’ve established that Transylvania was a special haven that existed in relative isolation from other terrestrial habitats at the end of the Cretaceous.
This understanding of the Transylvanian dinosaurs as not only once living, but also evolving creatures has provided us with the basis for exploring themes introduced earlier in this book: the paleogeographic relationships of the Cretaceous landforms of Europe, colonization and faunal balance in isolated environments, and the nature of body-size changes and life-history consequences. We are now at the logical place to add historical biogeography to this soup. We have given reasons for expecting that historical pathways should reflect the influence of chance events within the broad mediation of the laws of nature, rather than merely straightforwardly following the ever-present operation of these laws. Such unpredictability gives organisms the opportunity to circumvent the status quo—and the risks of extinction—and thereby expand their own evolutionary opportunities and innovations. Consequently, we will go out of our way in what follows to emphasize the theme of historical contingency, not only when interpreting the biogeographic history of the Transylvanian dinosaurs, but also as we integrate this information with their paleoecology, changing body sizes, life-history strategies, and phylogeny. Finally, we’ll let our Transylvanian dinosaurs wander on into the realm of evolutionary theory, in particular the role of chance in the regulation of diversity—the stuff of what is known as the Red Queen hypothesis.
OF TRAVELING DINOSAURS AND MOVING CONTINENTS
Like organisms everywhere, the Transylvanian dinosaurs were the product of their individual histories. Obviously, one aspect of these histories is their arrival in the region of what is now western Romania. Nopcsa’s approach to where these dinosaurs (and the remainder of the Transylva-nian fauna) originated was to look solely within Europe. For example, he compared his Transylvanian hadrosaurid with Iguanodon from the rich faunas of the Early Cretaceous Wealden faunas of England, Belgium, and France,11 and why not—both Telmatosaurus and Iguanodon, although separated by 50 million years, were European members of Or-nithopoda. Likewise, Nopcsa directly compared the other members of the Transylvanian fauna with their European relatives from the Early Cretaceous as he attempted to understand how his peculiar dinosaurs arose. We agree that Nopcsa’s was a good and logical beginning, but why restrict ourselves to Europe? Why not expand the comparisons to all ornithopods throughout the world and evaluate their areas of origin using their phylogenetic relationships?
In order to build a global biogeographic history of the clades containing our Transylvanian dinosaurs, we will need to know not only about Iguanodon, Hypsilophodon, Hylaeosaurus, and Pelorosaurus from a Europe of earlier times, but a great many more dinosaurs from elsewhere. Now we’ve got to deal with the likes of Probactrosaurus, Gobisaurus, and Opisthocoelicaudia from Asia; Anabisetia, Gasparinisaura, and Saltasaurus from South America; Muttaburrasaurus, Leaellynasaura, Minmi, and Austrosaurus from Australia; Ouranosaurus, Valdosaurus, and Malawisaurus from Africa; and Tenontosaurus, Thescelosaurus, Pawpawsaurus, and Alamosaurus from North America. Bringing these additional taxa into our investigation, however desirable, should also give us pause, because of the uncertainties arising from the effects of patchy geographic coverage (now increased to worldwide scales) and additional aspects of stratigraphic incompleteness. What we don’t know is still more than we know, so any attempt to infer biogeographic patterns may be condemned to failure before we even begin. Nevertheless, we don’t intend to give up here without making an effort to reconstruct a history based on what we do know. In so doing, we turn again to the phylogenetic relationships of our Transylvanian dinosaurs, this time combining cladis-tic analysis with the global geographic occurrences of the groups within which they fall. This can be done by combining geographic data with phylogeny. If this approach sounds familiar, it should—we are again going to overlay a posteriori optimization of the geographic occurrences of the Transylvania taxa and their close relatives onto their phylogeny in order to reconstruct their biogeographic history (chapter 5). In this search for the source areas where particular dinosaurs evolved, we will rely not only on the paleogeographic reconstructions of this part of Europe through the Cretaceous (chapter 4), but also on any other landforms brought into consideration by the relationships of the taxa involved. This certainly means we will also have to take into account the paleogeo-graphic conditions and proximity of parts of Asia, South America, Africa, and Australia.
Before we begin looking at the biogeographic history of Transylvania from a worldwide perspective, a word of caution is necessary. Inferences about global biogeographic histories can only be as good as the fossil record will allow, and the biases inherent in this record should be admitted from the start. Ours has to do with the skewed geographic sampling of dinosaurs throughout the world. For example, we know that North America, Asia, and Europe each contain about 28% of the number of world-wide locations during the Early Cretaceous, whereas the remaining continents—South America, Africa, and Australia—each contribute an average of 5% (not surprisingly, Antarctica has thus far contributed nothing). In the Late Cretaceous, North America dominates at 31%, followed by Asia at 24%, Europe at 18%, and South America at 13%. Africa, Australia, and Antarctica together total a measly 6%.12 Even though our knowledge of dinosaur distribution around the world is always on the rise (as witnessed by the abundance of media accounts), it is also dominated by the big three—North America, Europe, and Asia. Consequently, any search for the area of origin for any of the Transylvanian dinosaurs and their immediate clade is biased in favor of the big three (which constitute the supercontinent of Laurasia) and away from the Gondwanan landmasses, simply because of the number of fossil locations available to us at this point, rather than being based on real biogeo-graphic history. For now, there’s nothing we can do about this problem.
With this cautionary aspect to inferring biogeographic history in mind, we turn first to Telmatosaurus transsylvanicus. Nopcsa originally thought this dinosaur lay at the base of Hadrosauridae, a position that has been confirmed over the ensuing years, both through the discovery of many new hadrosaurids and other iguanodontians, and by the application of cladistic methods to understand the general shape of the group’s family tree (chapter 2). We’ve already used this information, plus strati-graphic data, to identify hadrosaurid ghost lineages and their duration while examining rates of character change (chapter 6).13 What might this approach additionally tell us about the source area for the lineage leading to Telmatosaurus and its arrival in what is now Transylvania?
The closest relatives of Telmatosaurus—among them, euhadrosau-rians, Bactrosaurus, and Levnesovia (chapter 2)—probably originated in Asia at some time during the latest Early Cretaceous (roughly 103 million years ago).14 When all of these locations are plotted on a cladogram (figure 7.1), it is unclear where the source area of Hadrosauridae was. It could have been either Asia or North America; many present-day studies support an Asian origin for Hadrosauridae.15 Telmatosaurus and Tethys-hadros (or their ancestor) can therefore be interpreted as having dispersed on their own to Europe. Similarly, forms such as Jeyawati and the small clade of Eolambia caroljonesa and Protohadros byrdi,16 the former from the early Late Cretaceous and the latter two from the earliest Late Cretaceous, represent independent migrations to North America from Asia.
We’d like to conduct a little thought experiment concerning the area of origin for hadrosaurids. In doing so, we will push the biogeographic distribution data as far as they allow, hoping that what they indicate about the area of origin for individual clades will not be outweighed by information about them not yet known to us, because it is hidden in their ghost lineages. If that is the case, we can then only look to future discoveries that sometimes, despite the od
ds, produce better phylogenetic resolution for whatever questions are under consideration (here, biogeogra-phy). Expanding the data envelope involves our attempt to resolve the previously unresolved relationships of Telmatosaurus, Tethyshadros, the clade of Bactrosaurus and Levnesovia, and Euhadrosauria, and looking to Europe for other close relatives.
Figure 7.1. A simplified cladogram of higher ornithopods that also includes geographic information, indicating Asia (or possibly North America) as the area of origin for Hadrosauridae and the dispersal of Telmatosaurus to Europe, as shown by the bar
Tethyshadros insularis: Only a few years ago, the prospect of Europe being the source area of any dinosaur clade was poor to nonexistent. Nevertheless, in the vicinity of Villaggio del Pescatore (along the Gulf of Trieste in northeastern Italy, near its border with Slovenia), a few fragmentary dinosaur bones were discovered in the 1980s, found in the eroding walls in an old abandoned limestone pit.17 Excavation of the site began in 1992, and by the end of the century this quarry, whose limestones date to the late Santonian (approximately 84 million years ago), had yielded a nearly complete skeleton (figure 7.2a) and the remains of three other individuals, all pointing to a small (4 m long) iguanodontian. These wonderful specimens were studied in detail in 2009 by the Italian paleontologist Fabio Dalla Vecchia,18 who noted that this dinosaur has many features uniting it with Hadrosauridae,19 lacks critical characters that unite Euhadrosauria, and has several features of the skull, dentition, hand, and pelvis indicating that it is not Telmatosaurus.
Figure 7.2. (a) Tethyshadros insularis; (b) a maxilla and dentary referred to Pararhabdodon isonensis; and (c) a dentary of the Fontllonga hadrosaurid. Scale = 100 cm (a), 5 cm (b, c). ([a] after Dal Sasso 2003; [b] after Casanovas et al. 1999a; [c] after Casanovas et al. 1999b)