Figure 5.11. Magyarosaurus as a dwarf. The solid silhouette represents a titanosaurian sauropod of ancestral body size; the open silhouette represents Magyarosaurus. Scale = 3 m
Figure 5.12. The relationship of humeral length and width (midshaft diameter) in titanosauroid sauropods. The interspecific regression of adults is indicated in the graph on the upper left. The regression of the composite growth series is indicated in the graph on the upper right. The combined adult and growth regressions—with Magyarosaurus superimposed on these lines—are indicated in the graph at the bottom.
What we report here comes from our analyses of the humerus, although roughly the same applies to the femur. When we plotted the data from the sample composed exclusively of adults, we got a reasonable linear relationship between humeral length and midshaft diameter (figure 5.12). Then, from our assembled growth series, we obtained another line with a similar slope to that of the adult sauropods, but which plotted slightly beneath the latter.68 This means that the humerus of a growing sauropod would be narrower than the same length of humerus for an adult. We then added data from Magyarosaurus specimens, and put a few other titanosaurs to the mix, to see how closely they resembled either the adult or the growth samples. Magyarosaurus plotted closest to the growth-series line, rather than to the adult line. Beyond mere inspection, the Magyarosaurus data are also statistically more similar to the growth series than to the adult sample.69 Therefore, we concluded that Magyarosaurus humeri appear to be more similar to those of subadults than to adults of other taxa.
Figure 5.13. A simplified cladogram of titanosauriform sauropods, showing two dwarfing events—one in Magyarosaurus and another in Rocasaurus—as indicated by the bars. (Modified after Curry Rogers and Forster 2001)
To judge whether these statistical insights had a phylogenetic context, we optimized body size at adulthood onto a cladogram of Magyarosaurus and its fellow titanosaurs.70 Figure 5.13 represents one such cladogram of these sauropods.71 Here we can see that small size evolved in the lineage leading to Magyarosaurus (as well as that leading to Rocasaurus). As did Telmatosaurus, Magyarosaurus joins Peter Pan’s merry band by retaining juvenile features into adulthood—that is, by paedomorphic dwarfing.
Was Zalmoxes a Dwarf?
At approximately 3–5 m long, Zalmoxes does not strike one as an extremely small iguanodontian dinosaur (figure 5.14). Nonetheless, the more relevant questions are: Is Zalmoxes bigger or smaller than its closest known relative, Rhabdodon from the Late Cretaceous of France and Spain? How do these two ornithopods compare with their next closest relatives, the iguanodontian ornithopods? And thereafter?
Zalmoxes certainly doesn’t appear to be especially different in its proportions from most other basal iguanodontians. The skull isn’t particularly shorter than that of Tenontosaurus, and the details of its construction don’t cry out for heterochronic interpretations. However, the postcranial skeleton appears to be more robust and heavy, and we would have been remiss if we didn’t pursue all heterochronic connections related to changing body size and skeletal proportions. Consequently, we needed to look at how the likes of Zalmoxes, Rhabdodon, Tenontosaurus, Orodromeus, and Hypsilophodon grew.
We approached growth and development in these ornithopods much as we had with Magyarosaurus, although we chose to examine only the femur, since it was much better represented in our sample of bones than the humerus.72 Measuring the length and midshaft width of these femora, we then analyzed the significance of this set of data, again using both statistics and phylogeny. However, unlike our analyses of Magyarosaurus, where we had to cobble together a single, generalized growth series for titanosaurs, virtually all of the ornithopods that we wanted to compare with Zalmoxes are known from adults, subadults, and even juveniles, so we had direct information on the growth patterns of each of these taxa. The other significant difference from our Magyarosaurus analyses was that we had a much better understanding of the phylogenetic history of ornithopods than we did for titanosaurs and other sauropods (chapter 2). This, in fact, provided us with a detailed, well-tested template for the comparisons we wanted to make.
Figure 5.14. Zalmoxes as a dwarf. The solid silhouette represents a basal iguanodontian ornithopod of ancestral body size; the open silhouette represents Zalmoxes robustus. Scale = 3 m
We began our work by rounding up as large a sample as possible of skeletal elements for both species of Zalmoxes. We selected the femur (as noted above), which, in the case of Z. robustus, turned out to represent samples from seven individuals with femora distributed from about 20 to 40 cm in length, and, for Z. shqiperorum, three individuals with femora ranging from 28 to 45 cm. Dimensions measured from several femoral landmarks were then used to produce growth curves for the Zalmoxes sample. The same data were collected for a selection of ornithopods that are thought to be closely related to Zalmoxes: Rhabdodon, Tenontosaurus, Orodromeus, and Hypsilophodon. These growth curves were then compared pairwise with each other.73
The overall pattern of growth for Zalmoxes—treated here in terms of both species (Z. robustus and Z. shqiperorum)—and for closely related ornithopods is quite similar (figure 5.15). Indeed, when we compared growth curves between any two of these ornithopods (say, between Orodromeus and Tenontosaurus), they were statistically indistinguishable from each other. The same applies to comparisons involving the two species of Zalmoxes, as well as comparisons with Rhabdodon—their growth curves are virtually identical. This overarching trend among these ornithopods indicates that the femur changed its proportions in the same fashion, regardless of whether it belonged to Hypsilophodon, Tenontosaurus, or Zalmoxes. The largest individuals, then, constitute the only difference among the samples. For the biggest Tenontosaurus or Rhabdodon, the femur grew to a length of nearly 60 cm. Right behind that comes Zalmoxes shqiperorum, with a femur length of just over 45 cm. Z. robustus, with a femur length of 37 cm, is smaller still, whereas Orodromeus and Hypsilophodon have the shortest femora (both between 15 and 20 cm long).
Figure 5.15. The relationship between femoral length and midshaft diameter in basal ornithopod dinosaurs. Included are growth series obtained from Orodromeus makelai, Hypsilophodon foxii, Tenontosaurus tilletti, Rhabdodon priscus, Zalmoxes shqiperorum, and Z. robustus. The line represents the regression of femoral length and midshaft diameter for the combined sample of all species.
As far as this simple scaling analysis goes, both species of Zalmoxes appear to grow their femora just like other ornithopods. One of them (Z. shqiperorum) does so to the degree exhibited by the existing sample of Tenontosaurus, but the other (Z. robustus) is known from femora that are greater in length than those of Orodromeus and Hypsilophodon, but smaller than those of Rhabdodon, Tenontosaurus, and Z. shqiperorum. Is this pattern one of successive peramorphosis in the clade leading to Tenontosaurus and the larger species of Zalmoxes, or of paedomorphosis leading to Orodromeus and Hypsilophodon? Or is it a mixture of both? Without full-grown adults from this taxon (see the discussion above on bone histology), we really can’t tell when Zalmoxes stopped growing. This ornithopod slowed down its growth rate and had an extended growth period, but it had not yet achieved its largest size. To see what this eventuality entailed in a phylogenetic context, we again optimized these size data onto the cladogram for basal euornithopods (figure 5.16). We can see that the more primitive trend within this part of the clade consists of a peramorphocline from basal euornithopods such as Orodromeus through more highly positioned taxa like Hypsilophodon, Tenontosaurus, Rhabdodon, and Zalmoxes shqiperorum. However, downsizing appears to occur in Zalmoxes robustus within the clade represented by the two last-mentioned taxa, all other thing being equal. If this is true, then we may have another Peter Pan paedomorph in Z. robustus.
Figure 5.16. A cladogram of basal ornithopods, showing the possible dwarfing event leading to Zalmoxes robustus, as indicated by the bar
Struthiosaurus, Theropods, and the Rest
Positive evidence exists for dwarfism in Telmatosaurus, Magyarosaurus,
and perhaps Zalmoxes robustus. What of the other members of the Transylvanian fauna? The ankylosaur Struthiosaurus and the unnamed pterosaur are quite a bit smaller than expected, compared with their phylogenetic compatriots, but we’re not yet certain whether they are represented by mature adult material or by subadult specimens. In contrast, the dromaeosaurid and troodontid theropods, although small, are no smaller than their close relatives in Asia and North America. Only Hatzegopteryx, poorly known though it may be, is large, but with an estimated wingspan of 12 m, it may turn out to be among the largest of all the azhdarchid clade.74 Turning to the Transylvanian crocodilians, turtles, and mammals, they are neither more diminutive nor more sizeable than what would be expected, based on their evolutionary cohorts. Thus this triumvirate of turtles, mammals, and crocodilians don’t seem to be playing the heterochrony game.
In the end, then, we have within the Transylvanian fauna a handful of cases of dwarfing through paedomorphosis, a roster of normal-sized animals, and an example of gigantism by way of peramorphosis. Maybe you’ve been expecting universal dwarfing among the animals found in Transylvania, but why should all of these forms have gotten small? We’re more than happy that paedomorphic downsizing can be identified in as many Transylvanian groups as it has—for here we have an important evolutionary signal. Nopcsa knew that such body sizes, compared with those elsewhere in the world, flew in the face of the basic tenets of dinosaur evolution, where large size reigns supreme, so he sought a causal basis for all of the changes in body size among dinosaurs.
THE BIOLOGY OF GETTING BIG AND SMALL
As Nopcsa saw it, the evolution of both large and small dinosaurs came about through some sort of neo-Lamarckian mechanism. For him, this inheritance of acquired characteristics was mediated by diseases or other physiological disturbances.75 For example, he looked to such endocrine diseases as acromegaly, in which hyperfunction of the pituitary gland leads to gigantism and a thickening of the bones in humans, to account for an increase in body size. His evidence for acromegaly was the enlarged pituitary fossa found in dinosaur braincases.
Other explanations of dinosaurian gigantism, even up to the 1950s, took a decidedly non-Darwinian perspective. Otto H. Schindewolf, one of the twentieth century’s great macroevolutionary paleontologists, devoted much of his long career to defending his theory of typostrophy. Schindewolf sought to understand evolutionary patterns as being internally regulated: groups of organisms, like individuals, go through phases similar to birth, childhood, adulthood, old age, and death as they evolve, and their extinction therefore is inevitable, predicated on this “evolutionary lifecycle.”76 In dealing with dinosaur extinction, he argued that these animals were driven to a sort of “racial senescence” that accompanied their growth to an exceedingly large size, as were other organisms, such as ammonites. How else to account for the “inevitable” poor design of dinosaurs? “It’s quite likely that these gigantic forms were no longer able to carry their enormous body weight around on land. They lived like amphibians, submerged in swamps and lagoons where water lightened the load and made locomotion easier.”77
The then-popular notion of dinosaurs as evolutionary failures because of their extinction at the end of the Cretaceous masked, for Schindewolf, just how these animals attained their gigantic size. He was forced to search for an ecological context, in which these animals were in their senescent phase. We now know that dinosaurs were not particularly relegated to the swamps, nor do taxa have well-defined “life stages,” but Schindewolf was right to seek evolutionary and ecological explanations for the changes in body size in dinosaurs. For example, their large size has been related to their physiology, in particular to the ways in which these animals regulate temperature: either through mass homeothermy (stable internal temperatures due to their large body’s low surface area / volume ratio) and endothermy (warm-bloodedness based on metabolic heat generation).78
Size increases through phylogeny not only accompany dinosaur evolution, but are also notoriously common elsewhere in the fossil record, so much so that they have been generalized under the rubric of Cope’s Rule, named after the prominent American neo-Lamarckian evolutionary biologist and vertebrate paleontologist Edward Drinker Cope (1833–1897).79 Ironically, its counterpart, evolutionary dwarfism, has received much less attention and is mostly centered on insular dwarfing, where changes in body size—downsizing in particular—are very common.80
Nopcsa viewed dwarfing as a consequence of body-size effects through island habitation; in other words, Transylvanian dinosaurs evolved to a smaller size as they became geographically isolated on an island in the Neotethyan Sea. There is ample indication that Transylvania, at the end of the Cretaceous, was a remote terrestrial region, a relatively small, isolated landmass revealed by the receding sea. This insular environment surely must have been large enough to accommodate viable populations of a variety of dinosaurs and other vertebrates, but apparently it was not so large as to support sauropods and ornithopods, at least in their normal, quite large body sizes.
We hope by now that it is obvious that the Transylvanian fauna includes a number of dinosaurs of small adult body size. The region in which these diminutive forms lived was an insular habitat, which leads us (in the same way as Nopcsa) to look for an explanation for our dwarfed Transylvanian dinosaurs by examining the relationship between body size and isolation.
The pattern of island dwarfing among mammals has been termed the Island Rule by Chicago evolutionary biologist Leigh Van Valen, who noted that it “seems to have fewer exceptions than any other ecotypic rule in animals.”81 The epitome of the Island Rule must assuredly be the dwarfing of insular elephants commonly present in the Pleistocene. These diminutive proboscideans are interesting—and relevant—for two reasons. First, among animals with living relatives, they most closely approximate dinosaurs in terms of body mass. However, even more importantly, their pattern of dwarfing is well enough understood, thanks to V. Louise Roth, for us to explore our own patterns of dwarfing in Transylvania. Roth, an evolutionary biologist at Duke University, conducts research on both large and small elephants, particularly in terms of their anatomy, feeding habits, growth, ecology, reproduction, fossil record, and evolution. Armed with this information, she developed a multi-faceted proboscidean model that relates interspecific competition, predation, and subsequent population densities, in order to explore what is causally behind the Island Rule.82
Roth’s model begins by taking up the events of island colonization. Getting to these islands is clearly a function of their remoteness from the adjacent mainland, and surviving such a migration depends on a combination of good luck and preparedness. In the case of nonhuman migrants, preparedness is a matter of body size. As it turns out, large individuals are more likely to be successful in surviving overseas dispersal and colonization than small individuals; hence, the founding population on an island may be of relatively large body size.83 This larger initial body size may be counterintuitive, but it provides a means for surmounting the problems of breaking through the barrier of isolation.
At this point, the largish elephants of Roth’s model have survived dispersal and are now island residents. Consider, though, that this new home base for the elephants must be much more confining than their previous range on the mainland. With this more restricted land area comes an ever-greater exploitation—and the eventual deterioration—of existing food supplies as the isolated populations reach high densities and the elephants suffer from increased overcrowding. Smaller body size is favored under these conditions and, according to Roth’s model, is most likely to arise when individuals reach reproductive maturity at earlier—and therefore smaller—stages. This connection between reproductive maturity and body size among dwarfed insular elephants again focuses our attention on the role of heterochrony and, in particular, on paedomorphosis. This shift of sexual maturity to younger and therefore smaller individuals is the most important connection with individual fitness.
So far, so good—the dwarfe
d elephant model provides some similarities with the Transylvanian dinosaurs. The paleogeography of the Tethyan realm during the Cretaceous indicates that terrestrial habitats were relatively well isolated from each other and often limited in area. Thus conditions in Transylvania are consistent with the geographic and ecological underpinnings of the elephant model.
Transylvanian Dinosaurs Page 14