We likewise argue that small size in Telmatosaurus, Magyarosaurus, and perhaps other Transylvanian forms appears to be favored under conditions of decreasing food supply, due to increased population densities in restricted habitats, just as it appears to be the case in the dwarfed, island-dwelling elephants. In this way, the dwarfing of Transylvanian dinosaurs would have arisen through paedomorphosis: individuals reaching reproductive maturity at earlier, smaller stages.
From a faunistic perspective, the dinosaurian herbivores are dwarfed relative to their close relatives elsewhere in the world, whereas the few predators known from Transylvania are not much different in body size from their closest relatives. In terms of the elephant model, these observations are not surprising. The lack of large individuals among the herbivores strongly suggests that immigration to this restricted area must have taken place considerably earlier than the latest Cretaceous. (The time interval for isolation and colonization of the Transylvanian region will be discussed further in chapters 6 and 7.)
As for body size among the Transylvanian theropods, as we have indicated, there were no behemoth, Tyrannosaurus-like predators living in this region in the Late Cretaceous. This, too, is not a surprise. The isolated habitats of the kind we envision for Transylvania were probably capable of supporting a lesser quantity and diversity of predators, mainly ones that had smaller home ranges; and, of these, those with smaller bodies fit the more restrictive Transylvanian ecological conditions better than large-bodied predators.
The downward trend in body size among large herbivores, which Roth’s model links with limited island resources, may also be driven by differences in predation between the mainland and an island. We know that large mammalian predators, though present on the mainland, were absent from islands inhabited by dwarfed elephants during the Pleistocene, and the same appears to have been true for most of the dinosaurian predators of the ancient Transylvanian region.84 Lower levels of predation on islands may act as a permissive factor, allowing large herbivores to evolve into smaller, perhaps more optimal sizes, since large size would not be selected as a deterrent to predators. In addition, this rationale for insular dwarfism may also help explain the evolution of gigantism among many small mammals and other animals on islands. Take, for example, the case of the modern giant rats on the Lesser Sunda Islands in the South Pacific.85 Such rat gigantism—a nearly half meter long rodent, without the tail that gives new meaning to The Princess Bride’s “rodent of unusual size”86—may be viewed as evolving closer to its optimal body size in the absence of the usual roster of predators from the mainland. Similar to the counter-case of island downsizing among large animals, very small size is no longer necessary as a defense against predators, producing, as a consequence, an increase from a smaller to a larger body size.87 This embellishment to the Island Rule provides an argument for there being an optimal intermediate body size that organisms would be free to evolve toward when predator pressures are significantly altered, as they would be on an island.88
Whether dwarfism in Magyarosaurus, Telmatosaurus, and maybe Zalmoxes was spurred by limited resources or by an alteration and perhaps a reduction in predation, these small dinosaurs beg for a hetero-chronic and ecological explanation. If Roth’s elephant model applies to the Transylvanian dwarfs, and we believe that it does, the adaptive significance of dwarfing is its immediate relationship with life-history strategies such as accelerated sexual maturation (i.e., paedomorphic heterochrony), rather than it being a development based on morphological advantages alone. As Gould put it, the “timing of reproduction is an adaptation in itself, not merely the consequence of evolving structure and function.”89 The morphological consequences of paedomorphosis—juvenile morphology—come along initially as biological baggage: a consequence of downsizing due to isolation. Thus, for example, it is not the retention of a juvenile dentition and the formation of a dental battery in Telmatosaurus that is the primary focus of selection. Instead, the overarching selection is for a small body—that is, paedomorphic dwarfism, based on the ecological particulars of the isolated region of Transylvania.
CHAPTER 6
Living Fossils and Their Ghosts
Being a Short Interlude on Coelacanths
and Transylvanian Ornithopods
For evolutionary biologists of all sorts, 23 December 1938 was a very important day. It was then that a trawler called the Nerine put into port at the town of East London, located about 850 km to the east of Cape Town, South Africa. The skipper, Captain Hendrick Goosen, made a living fishing the nearby coastal waters of the Indian Ocean. Having made friends with Marjorie Courtenay-Latimer, the curator of a small local museum, he would often have the dockman call Courtenay-Latimer to come look over the Nerine’s catch and to take any unusual specimens she wanted for her museum. On this particular day, the Nerine entered port after trawling off the mouth of the nearby Chalumna River. When the dockman called Courtenay-Latimer, she took a taxi to the ship, delivered her Christmas greetings to the captain and crew, and went through that day’s catch for anything unusual. There, beneath a pile of rays and sharks on the deck, was, as she put it, “the most beautiful fish I had ever seen, five feet long, and a pale mauve blue with iridescent silver markings.”1 Although she had no idea what the fish was, she did know that it had to go back to the East London Museum at once. After convincing the taxi driver to allow the reeking, 5 ft fish to accompany her, he drove her and the specimen back to the museum.
Back in her office, Courtenay-Latimer tried to identify the bizarre creature (figure 6.1) by combing through the few reference books in her library. A picture of a long-extinct fish bore the greatest resemblance, particularly in the structure of the head and the trilobed shape of the tail. She made a crude sketch of her discovery and sent the drawing and a short description to Professor J. L. B. Smith, a chemistry professor at Rhodes University in nearby Grahamstown, who also was locally well known as an amateur ichthyologist. Unfortunately, Smith was away for the Christmas holidays and the consensus back home was going against her increasingly odiferous fish—the director of the East London Museum dismissed it as a common rock cod. In an effort to preserve the fish by mounting it, much of the viscera had been discarded and then lost, and, to the great disappointment of all concerned, even the photographs taken of the preparation were spoiled.
When Smith finally visited the East London museum on 16 February, he immediately identified the fish as a coelacanth, a group of fish thought to have gone extinct toward the end of the Cretaceous, some 80 million years ago. Called the “most important zoological find of the century,” this discovery made Smith and Courtenay-Latimer overnight celebrities. In 1939, the fish was named Latimeria chalumnae (after Courtenay-Latimer and the river near where it had been collected) by Smith.
The saga of the coelacanth continues. On 21 December 1952, Captain Eric Hunt, a British sailor operating in the waters off the Comorean island of Anjouan, was approached by two islanders carrying a hefty bundle. One of them, Ahamadi Abdallah, had caught a heavy, grouperlike fish by hand line, while the other—a schoolteacher named Affane Mohamed—thought the fish might be the fabled coelacanth. Hunt had the fish salted on the spot, then had it injected with formalin to preserve its internal organs, and cabled J. L. B. Smith in South Africa.
After some delay, which was laced with confusion and frustration, Smith managed to reach the Comoros and, when he saw the dead fish, he is said to have wept. It was indeed a coelacanth, this time with its organs intact and captured most probably near the creature’s actual habitat.
Thereafter, quite a number of coelacanths have been caught for study, first by French researchers and, after the Comoros Islands became independent in the 1970s, by various international groups of scientists. In addition, Latimeria has been studied in its natural habitat by direct observations and by videos obtained by submersibles diving in the waters off the Comoros Islands.2
A most remarkable chapter in the history of the study of modern coelacanths began on 30 Ju
ly 1998. On this date, a population of these lobe-finned fish was discovered by American and Indonesian scientists about 10,000 km east of the Comoros Islands, off the coast of Sulawesi Island in Indonesia. When this coelacanth was first discovered, the only obvious difference between it and Latimeria chalumnae from the Comoros Islands was its color. The Comoros coelacanths are renowned for their steel blue color, whereas specimens from the Sulawesi population are brown. Described as a new species, Latimeria menadoensis, in 1999 by a host of Indonesians scientists and one Frenchman—L. Pouyaud, S. Wirjoatmodjo, I. Rachmatika, A. Tjakrawidjaja, R. Hadiaty, and W. Hadie—this discovery identifies living coelacanths as more widespread and abundant than previously assumed. Furthermore, it opens an incredible range of ecological, biogeographical, and evolutionary questions associated with these exceptional, albeit still very rare, living fossils.
Figure 6.1. The icon of living fossils, Latimeria chalumnae
Latimeria has dual significance in evolutionary biology. First, in the 1930s, extinct coelacanths were thought to be the direct ancestors of the tetrapods (land-living animals, which also include humans), and living coelacanths thus provided a wealth of new information on their non-skeletal anatomy, ecology, and—once living coelacanths had finally been observed in their natural habitat—their behavior. These rare glimpses into their biology have forced evolutionary biologists to reassess the closeness of the relationship between coelacanths and tetrapods, and we now know instead that lungfish hold such a position. Nevertheless, evolutionary biologists remain equally intrigued by Latimeria chalumnae through its eminent position as the icon of all living fossils.
What exactly is a living fossil, and why does Latimeria qualify? Along with horseshoe crabs, bowfin fish, and opossums, coelacanths are often cited as living fossils, but few scientists have agreed on the precise meaning of the term.3 Darwin introduced the phrase “living fossils” for forms that are the result of a long survival of lineages and remarkably slow evolutionary rates, both of which he attributed primarily to an absence of ecological competition.4 Since then, the concept of living fossils often took on an adaptational flavor. For example, Delamare-Deboutteville and Botosancanu envisioned living fossils as organisms that, by virtue of the narrowness of their adaptation, are restricted in their ability to change over time.5 In contrast, Simpson considered them to be characterized by broad adaptation.6 Adaptations aside, most assessments of living fossils agree that they share the following: they have survived for a relatively long time—measured in terms of geologic periods—and they exhibit a plethora of primitive characteristics that suggest that they have undergone little evolutionary change over this period of time.
The Transylvanian fauna were what formed the backbone of Nopcsa’s ideas in paleobiology and evolutionary theory, among them the connection between body size and habitat area (i.e., dwarfs and islands) and the evolutionary processes that mediated this relationship (endocrine disease and neo-Lamarckian inheritance). It also revealed to Nopcsa that his dinosaurs represented a depauperate assemblage of relicts of a much richer European fauna from earlier Cretaceous times. Although he never discussed this relictual nature of the Transylvanian dinosaur fauna using Darwin’s term, living fossil, in what follows, we are going to claim that—taken from the perspective of 70 million years ago—at least one member of the Transylvanian fauna can be regarded as a paleontological example of a living fossil. This is not an oxymoron—we really do mean living fossil, in the sense that it is separated by a long interval of time, and is little transformed, from its closest relatives or its potential ancestor. We will also show that other members of this region’s fauna changed at much more normal rates.
Evolutionary rates come in two varieties: taxonomic rates and rates of character change. In extinct organisms, taxonomic rates have generally been estimated from the first occurrence of a particular taxon, its temporal duration, and its diversity, and these rates are often expressed using survivorship curves.7 Rates of character change typically have been evaluated as changes in the size and shape of a feature (e.g., tooth-crown height) over a given time interval, although rates of change among character complexes have also been analyzed.8
Here we’re going to try something a little different: instead of measuring rates of character change from what has come directly from the fossil record, we’re going to calculate these changes on the basis of what we don’t have. In order to calculate the differences in rates of character change in some of our Transylvanian dinosaurs and to analyze their significance—in other words, to look for the presence of living fossils in the Late Cretaceous—we must also come to terms with their ghosts.
GHOSTS THAT TELL TIME
Our quest to understand the how and why of differences in evolutionary rates among the Transylvanian dinosaurs requires us to estimate their rates of morphological change, and to calculate these rates we need to know not only the time period, but also the historical pattern of common descent. As we have learned in chapter 2, the latter is what cladograms are all about: portraying the closeness of the relationships of different organisms. In so doing, cladograms also provide information about the relative sequence of evolutionary events—the mutual divergence of new lineages—that produced these organisms. As originally noted by Willi Hennig and later elaborated by Mark Norell, a vertebrate paleontologist and systematist at the American Museum of Natural History in New York City, it is axiomatic from evolutionary and cladistic theory that monophyletic sister taxa must have separated from each other at the same time through the same evolutionary splitting event.9 It is this historical continuity between paired sister taxa through their common ancestor at splitting events, when combined with temporal information from the stratigraphic record, that allows us to determine the minimal age of splitting and then use it as a “clock” to extend the minimal age of each group beyond the information that comes from stratigraphy alone.
Ghost Lineages and Their Durations
A ghost lineage is that part of an evolutionary tree for which there is no fossil record, but which must have existed because of the continuity through time between all of the ancestors and their descendant.10 In order to identify the existence of ghost lineages in a clade and thereby calculate their durations, we obviously need to know the clade’s phylogeny. That is, we must have as complete, as fully resolved, and as well-tested a cladogram as possible for the group in which we’re interested. All of the taxa in the cladogram should be monophyletic, without unresolved putative ancestors. When these aspects of a cladistic analysis are fulfilled, the resulting cladogram is the best we can expect from the most parsimonious treatment of the data. In addition, we must have the best data on the stratigraphic distribution of the various members of this clade. Anything less than all of what is currently available, on as precise a form as possible, will reap as much error in ghost lineage analyses as it would in an improperly constructed cladogram. If these aspects are kept in mind, ghost lineage analysis can meld information from phylogeny and stratigraphy to provide a measure of the quality of the fossil record.
To assess how well a set of first appearances in the fossil record corresponds to the prediction of specific phylogenetic hypotheses, we will use a method that has been called the Sum of Minimum Implied Gaps (SMIG).11 SMIG analysis pits the first occurrence of a taxon in the fossil record against its relationships, as determined by cladistic analysis, in the following way. Suppose dinosaurs X and Y are each other’s closest relatives (figure 6.2). As a result, they shared a common ancestor that is not shared by dinosaur Z or any other life forms. From different sources of information, we know that dinosaur X comes from rocks dated 100 million years ago and dinosaur Y comes from 125-million-year-old rocks. Since ancestors must come before descendants, the ancestor of X and Y has to be at least 125 million years old. We have thus discovered a ghost lineage leading to dinosaur X, amounting to 25 million years of not-yet-sampled history—its ghost lineage duration (GLD). The same follows for the ghost lineage of 5 million years leading
from the common ancestor of dinosaur Z and the common ancestor of dinosaurs X and Y GLDs are obtained by subtracting the earlier age (the earliest stratigraphic occurrence of the form that lacks a ghost lineage) of one taxon from the later age (the earliest stratigraphic occurrence of the form with the ghost lineage) of its sister taxon.
Short GLDs imply that there is not a great deal of missing history in the stratigraphically calibrated cladogram, while long GLDs indicate the opposite. The ghost lineages for the dinosaurs from Transylvania are of the 10-million-year magnitude, similar to those calculated for other dinosaur taxa. To provide a broader perspective, the ghost lineages for horses and humans are on the order of several million years.12 This difference between dinosaurian GLDs, on the one hand, and those of horses and humans, on the other, indicates not only that considerable time is missing in dinosaurian history in general, but also that the quality of the fossil record for at least some of the dinosaurs in the Mesozoic is significantly less than that for many of the mammals in the Cenozoic. Do these temporal lapses make it easier to speculate more and be less critical about the history of the Transylvanian fauna? We certainly hope not, and in chapter 7 we will take up this issue of the quality of the fossil record we might expect from a small, isolated region enveloped in the widespread tectonic dynamism that is the Late Cretaceous of eastern Europe.
Figure 6.2. Determining ghost lineages and their duration. The diagram in the upper left indicates the phylogenetic relationship of three dinosaur species (dinosaurs X, Y, and Z), while the diagram in the upper right provides the stratigraphic distribution of these same species. Together, the stratigraphic calibration of this dinosaur phylogeny is provided in the lower diagram. Ghost lineages are indicated by shaded rectangles in this lower diagram (see text for a further explanation).
Transylvanian Dinosaurs Page 15
