As we saw in chapter 5, Telmatosaurus, Magyarosaurus, and possibly Struthiosaurus were identified as being dwarfed, downsized from the primitive condition of their respective clades. In addition, Zalmoxes robustus (but not Z. shqiperorum) may have been dwarfed. However, the Transylvanian theropods are roughly the same size as elsewhere; the same applies to the region’s crocodilians, turtles, and mammals. We also noted the possibility of dwarfing in some of the Transylvanian pterosaurs, but certainly not in Hatzegopteryx, one of the largest among the large azhdarchid pterosaurs.
By integrating this cast of characters with their evolutionary relationships, and with their geographic and stratigraphic distributions, we can now begin to explore the biogeographic dynamics by which they achieved their dwarfed status. Sometime in the Early Cretaceous (or earlier), the ancestor of each of the Transylvanian lineages crossed the northern realm between North America or Asia and the mosaic of large and small landmasses that formed what is now Europe. We suspect that these migrants were no smaller than their closest stay-at-home relatives—since fasting endurance is proportional to body size,46 only large individuals would have been able to travel long distances without risking starvation, famine-induced disease, or susceptibility to predators.
Some of these migrants colonized the isolated, rapidly changing patches of terrestrial habitats in Europe. This region—isolated as it was from the grand coastal plains from which they came—was a place of serendipity, the product of the chance arrivals of various plants and animals. What were once more-or-less stable, evolutionarily well-honed ecosystems in North America and Asia appear to have been stitched together more by chance than by design as they developed in Europe. One hallmark of the kind of isolated habitats we envision here is the taxo-nomic and trophic imbalance of the European faunas.
Things now are starting to get really interesting, once each migrant had become established in these new habitats. Whereas those who traveled to Europe seem to be a random sample from their ancestral area (although they needed to be large enough in body size initially), selection appears to have favored downsizing. Very quickly, being large was hardly as advantageous as it had been before and during the journey to Europe. Many attempts have been made to account for dwarfing in isolated organisms. For example, large body size requires lots of resources—which were relatively plentiful in the now-forfeited coastal plains of Asia and North America, but perhaps critically rare in the new homeland. From this metabolic perspective, it would no longer pay to be big. Instead, it is reasonable to expect that the size reduction in these animals, living in restricted European habitats, was due to selection for small body size, based on resource limitations.47
From an ecosystem perspective, downsizing could also have been related to a release from predator pressure that comes from the chance jumbling up of colonizing predators and prey (chapter 5). The randomness of which predators arrived and evolved in Transylvania and elsewhere in Europe and which ones did not would have unpredictably altered the predator-prey relationships in these new regions. Selection for large body size as an antipredatory device, which may have existed in Asia and North America, would have been out of whack in Europe. It has been argued that with a loss of or decline in body-size selection, the distribution of body sizes in prey may evolve into an equilibrium that is not mediated by predator pressure—large prey get smaller and small prey become larger when they’re not using size to escape from their usual predators.48
Even though both of these ecological explanations of dwarfing are good enough by themselves, we argued in chapter 5 for the Roth model of downsizing: dwarfing was the product of selection based on life histories in an isolated setting.49 In this model, only a few individuals ever disperse to any isolated region. The resulting population bottleneck entails an immediate loss of genetic variation in the organisms colonizing the newly invaded environments. This small population is usually doomed to extinction through the consequences of genetic drift. However, in order to reduce the probability of extinction, a premium is placed on the invaders to increase their population size as rapidly as possible. Options on how this can be achieved are limited, but selection for early (precocial) sexual maturation—one aspect of what is called r selection—may be the colonizers’ best chance for success.50 By moving up the timing of its sexual maturity, an organism will be in a better position to increase its presence in the environment more quickly than its slower-developing ancestors and competitors. While this is applicable for whatever the quality of the colonized environment, it is even more effective in increasing survival rates in an ecologically unstable environment.51 In addition, precocial species are also better at exploiting patchy resources, and they disperse across their new habitats earlier than their more slowly developing competitors do.
Under this life-history approach, r selection, in the context of colonizing isolated regions, acts primarily on accelerating sexual maturity—a good colonizer is one that can quickly return to a healthy population size after the migrational bottleneck. Interpreted this way, dwarfing is not the direct consequence of selection, but instead is a tagalong, a predictable consequence of selection for those life-history parameters that relate to colonizing abilities.52
For our Transylvanian dinosaurs, early sexual maturity certainly accounts for the evolution of their small body sizes, especially in the context of their arrival in Europe. By means of their early maturation, they would have been better off than their larger ancestors during this time of population bottlenecking, simply by being able to recover better and increase more quickly in population size. These factors probably created a feedback loop with the other features that favor downsizing, such as limited space and resources and new predator-prey relationships. Taken together, the resulting morphology is an animal smaller than its ancestors.
How long it may have taken for this dwarfing to occur is hard to reconstruct from the fossil record. In other vertebrates, for instance in the dwarfed hippos of Crete, it is thought that dwarfing was achieved within a millennium, or even shorter periods of time.53 We don’t know if this time frame also applied to the Transylvanian taxa, due to our lack of information (concealed by their long ghost lineages). Nevertheless, we can reasonably conjecture that the isolation of these dwarfed populations, over whatever interval of time, not only reduced their genetic variability (by reducing the influx of expected variation, due to their migration), but also probably accelerated the divergence of the dwarfing populations from their mainland stocks.
AFTER THE DWARFS
Living as dwarfs in isolation may not have been all it’s cracked up to be, at least in terms of evolutionary legacy. For all but one of the Transylvanian dinosaurs, we are looking at the final pulse of their immediate clades. The likes of Zalmoxes and Struthiosaurus, in their isolated outpost within Europe, died out at the end of the Cretaceous without giving rise to other taxa. Magyarosaurus was part of only a small evolutionary radiation. Only in Telmatosaurus can we identify a link between dwarfed isolates and their subsequent migration and high levels of diversification. If we want to examine the great radiation of euhadrosaurian species in North America and Asia from non-hadrosaurid iguanodontians, we’ve got to go through Telmatosaurus and its European allies. In doing so, we hope to explain how this European occurrence of basal hadro-saurids is an important, but historically contingent factor in the rise of Euhadrosauria.
The construction of the hadrosaurid dentition is distinct from that of more primitive iguanodontians such as Tenontosaurus, Dryosaurus, Camptosaurus, and Iguanodon, so much so that it has been considered a hallmark for Hadrosauridae (chapters 2 and 5). Instead of having a dentition composed of a single large functional tooth and one replacement tooth per tooth position, hadrosaurids had hundreds of small functional and replacement teeth in each jaw, interlocked to form a complex dental battery that is ornate by anyone’s criterion (figure 2.21). Each tooth was composed of various kinds of dentine tissue and contained much less of the harder and more resistant enamel; yet, when w
orn down, this dentition provided a long, roughened surface to grind up tough plants, a dentition more effective at breaking down fibrous leaves than the teeth present in their ancestors. Teeth were continually replaced, such that this chewing surface was always present throughout the animal’s life. When the teeth in both the upper and lower jaws were worn, the opposing occlusal surfaces formed a complex arrangement of enamel and dentine, wonderfully designed to grate and rasp their way through the toughest plant material in ways not available to hadrosaurid precursors.
This dental battery certainly was a remarkable trophic invention, one that improved hadrosaurids’ chewing efficiency over the more primitive condition of mastication occurring among ornithopods. Surely it qualifies as a true adaptation—that is, an organismal feature whose origin and maintenance was the product of natural selection.54 Viewed in terms of the Cretaceous rise and diversification of flowering plants, a dental battery makes sense from such a Darwinian perspective: new kinds of plants provide new challenges (i.e., ecological problems and opportunities) for contemporary herbivores, which then select for those variants among iguanodontians that have an incipient dental battery (i.e., biotic solutions to these problems and opportunities). Using these arguments, it is always tempting to take an adaptationist perspective, rendering the rise of the hadrosaurids in terms of their coadaptation or coevolution with angiosperms.55
However attractive this inference is, we’re not particularly persuaded by this adaptationist explanation, whereby the evolution of the hadro-saurid dental battery was directly mediated by natural selection. We think it appropriate to look first at alternative explanations, using all the tools, perspectives, and information we now have at hand. In particular, that means an examination of dwarfing and migration to the isolated regions of Europe.
Dwarfing within the lineage of iguanodontians that gave us Telmato-saurus and the other basal hadrosaurids of Europe brought about the possibility of adding a few more teeth in the tooth row and another replacement tooth per position—the beginnings of an incipient dental battery. With this more complex dentition, made up of small teeth placed in small jaws, it is possible that these dwarfed hadrosaurids altered what they fed upon. As has been hypothesized for dwarfed, island-dwelling elephants, the Transylvanian dwarfs are likely to have experienced a decrease in their total metabolic requirements, coupled with a reliance on higher-quality food, assumptions based on their reduced body sizes and decreased population size.56 Whether the European ancestors of Telmatosaurus subsisted on a diet higher in nutrient-rich seeds and fruits than their larger North American and Asian relatives is unknown, but this may be anticipated, since dietary selectivity in many living animals is known to increase with smaller body size.57
Whatever the diet of these miniaturized hadrosaurids, their downsized stature—a consequence of early maturation—is likely to have promoted their success in the more restricted terrestrial habitats of Europe. Nonetheless, according to their phylogenetic history, the descendants of these animals migrated from Europe (most likely first to North America, but an Asian migration is also a contender here), going from a suite of environments that promoted small size back to a region of great coastal-plain openness. As far as we can tell, this out-of-Europe migration happened once, to produce what we now call euhadrosaurians (chapter 2).58 Once they were “back home,” their small size could not have been as advantageous as it had been in their restricted European habitats. These dwarf descendants would have again faced a great array of small and large predators (dromaeosaurids, tyrannosaurids) already present in North America and Asia. It’s possible that through the kinds of predator pressure their more distant ancestors experienced, they regrew to their previous larger sizes as a way to defend against predation. Whatever the case, these euhadrosaurians once more reached 10 m or more in length.
As far as we can tell, this size-increase phase equally affected just about all of their body proportions (as exemplified by Telmatosaurus). All, that is, but their dentition (chapter 4). Rather than scaling up to a bigger size (like the rest of the body), the teeth remained small, not much larger than those of hatchlings and youngsters. In other words, euhadro-saurians retained their dwarfed, juvenilelike dentitions into adulthood. In this way, hadrosaurids appear to have decoupled the increase in their overall body size from their dentition. Mediated by their European dwarfing phase, the teeth of these North American and Asian hadrosaurids, as they rebounded to a larger body size, paedomorphically remained small. Simply add more of these miniaturized teeth (i.e., baby teeth) along the length of their jaws and as replacement teeth, and voilá, the hallmark dental battery for which Euhadrosauria is famous had been created.
Having a dental battery must have counted for something among all hadrosaurids. It is retained in all of the nearly 50 species of euhadrosau-rians presently known worldwide, never deviating in its fundamental construction—small teeth, with three or more teeth per tooth position. Once formed, this dental battery became one of the most stable of all euhadrosaurian anatomical systems. From an engineering perspective, this dentition appears to have been a great invention, permitting the complex mastication of plant material. Once this structural complexity, and therefore better chewing efficiency, was achieved, natural selection would have acted to maintain the hadrosaurid dental battery. Only in this sense can the stability of the hadrosaurid dental battery be considered an adaptation, and we certainly wouldn’t be surprised if this was the case.59 However, the origin of dental batteries cannot be judged from the same perspective. This evolutionary innovation arose not as the primary object of selection, but instead as a serendipitous creation. Hadrosaurid dental batteries came about from the transformation of the dentition of the numerous, more basal iguanodontians to that of Telmatosaurus and other hadrosaurids, not for their properties related to chewing, but through the sequellae of paedomorphic dwarfing.
This notion of contingency in the evolution of hadrosaurid teeth may have its mirror in what we know about hadrosaurid reproductive biology. From work by Jack Horner of the Museum of the Rockies in Montana, we know that hadrosaurids such as Maiasaura and Hypacrosaurus— much larger forms than Telmatosaurus—laid large clutches of relatively small spherical eggs (between 16 and 22 eggs in a clutch, with the egg volume ranging from 900 to 4,100 cm3) and cared for the small (approximately 1% of adult body mass), immature hatchlings for some length of time until the juveniles left the nest (figure 7.9).60 Parental care of immature offspring is usually selected for in populations at or near the carrying capacity of the environment, what is known as the K life-history strategy. The logical opposite of r strategy that we previously discussed, K strategy is to be expected in animals of large adult body size inhabiting stable environments of the kind envisioned in western North America and Asia during the Late Cretaceous.61 K selection is also associated with small clutch size and parental care. In this way, the parents are able to invest a great deal of their energy promoting the survival of a few offspring. Humans, for example, are regarded as K strategists. However, a large clutch size is more consistent with an r strategy, in which the parents invest in abundant offspring, but do not care for them. Among vertebrates, sea turtles are an excellent example of this strategy. They lay plenty of eggs in the sand, but care for them not at all. After climbing out of their eggs, the hatchlings must fend for themselves.
Figure 7.9. A lambeosaurine hadrosaurid nest containing 18 eggs, from the Two Medicine Formation (Upper Cretaceous) of western Montana. Scale = 10 cm. (After Horner and Dobb 1997)
Hadrosaurids exhibit both r and K strategies in their reproductive biology. Is this a paradox, requiring an explanation in which natural selection is driving life-history traits in polar-opposite directions? Not necessarily, if we interpret these reproductive features along the lines of our schema of a European dispersal and dwarfing at the base of the hadrosaurid family tree. Recall that early maturation and its consequent reduction in body size make for good colonizers. In addition, selection on individuals in pop
ulations well below the carrying capacity of their environments favors the early and rapid production of large numbers of young. This aspect of r strategies makes good sense in such isolated terrestrial realms as Transylvania in the Cretaceous. Large clutch size is likely to have evolved in the context of colonizing the unpredictable, often severely fluctuating Neotethyan terrestrial habitats. The large clutch size of small hatchlings that we see in euhadrosaurians would then be primitively inherited from the isolated origin of the group, an evolutionary holdover from more stressful times. The K strategies (stable habitat, large body size, parental care) would then have evolved after hadrosaurids left Europe for North America and Asia. Now known as euhadrosaurians, these creatures evidently maintained their earlier r-selected reproductive strategies in much the same way as they ended up with their miniaturized dentitions.
In sum, unpredictability abounds in the details of what we’ve presented here. If we are correct about how the downsizing of the Transylva-nian dinosaurs was accomplished, then it would have been impossible to predict from their North American or Asian ancestors which taxa would make the journey and, once there, which ones would have been successful in the restricted environments of Europe. These colonizers then became dwarfed, not so much from a direct selection for body size, but instead as the consequence of selection for early sexual maturity.
Populations eking out a living in isolation, even with their odd genetic sampling and high levels of speciation, usually end up as evolutionary dead ends. This may have been true for the Transylvanian dinosaurs—Europe was their last stop. However, Telmatosaurus and its closest European relatives were the progenitors of the euhadrosaurians on the larger adjacent continental landmasses. Their reintroduction into Asia and North America (the postdwarf stage) has its own contingency overprint, with the same iffy-ness of dispersal and success or failure in these descendant regions. In this group, the effects of isolation on life histories indirectly produced evolutionary novelties, such as the hadrosaurid dental battery. These features were then bequeathed to descendant species that evolved into a diverse array of larger animals. In other words, the period of r selection experienced by the basal taxa of Hadrosauridae, which was related to their early sexual maturity in an uncrowded insular habitat, supplied a pool of dwarfed morphology that could be used as the raw material for the subsequent radiation of these herbivores. If true, then here we encounter one of the few examples of isolated populations successfully recolonizing, radiating, and dominating the biota in a broad continental setting.62
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