Dinosaur size helps the understanding of evolutionary biodiversity and what researchers are describing as “adaptive radiation,” according to recent reports. The recent discovery of titanosaur bones in Argentina brings with it a fascination for the enormous, and in particular, helps illustrate the degree of diversity in dinosaur evolutionary history. The Argentine titanosaur, a sauropod, has attracted much attention as a result of its unparalleled size among fossils of dinosaurs to date. Outside of satisfying standards of gigantism, there are other fruitful implications of dinosaur size that are of concern to scientists. These implications pertain to the explanation of certain evolutionary trends in dinosaur morphology.
In a recent study published earlier this May, researchers assessed the rate at which certain dinosaurs evolved over a given geological time period. Adaptive radiation, as it is called, describes the process by which a group of animals undergo diversification. This diversification is contingent upon emerging ecological or environmental conditions. In particular, “rapid” rates of diversification are associated with changed conditions in the environment, the underlying logic being that changes in environmental conditions allow animals to develop different adaptations and means of surviving.
This basic description of adaptive radiation may come across as overly idealistic, because after all, not all animals are able to survive such ecological shifts. For that reason, and a host of other factors, extinction is important to (mal)adaptive radiation: the extinction of one group may allow for the thriving of another. This kind of a scenario can be described as a “niche-filling” model of adaptive radiation. Environmental changes welcome the possibility for biodiversity or niches, whether it is in lieu of another animal group’s capacity to survive, or simply as a result of more opportunities. Changes in dinosaur size or specialized behaviors are examples of species biodiversity.
The niche-filling model of adaptive radiation suggests that evolutionary diversity in a group occurs as a sizable “burst” and then dissipates over time as all the niches are filled. What makes the study in question significant is the argument that the niche-filling model helps explain the survival and continued biodiversity of avian dinosaurs at the expense of larger, more massive dinosaurs such as the recently discovered Argentine titanosaur. Avian dinosaurs were able to maintain evolutionary innovation more consistently, according to the study, thereby avoiding what researchers refer to as “niche saturation.” The proposed reason as to why the avian dinosaurs were able to avoid niche saturation: their comparatively small size. Accordingly, the study argued that smaller dinosaur size allowed for the highest possibility of continued adaptive radiation over the longest period of time, or put differently, their long-term evolvability.
What makes this study stand out is the attempt to include long-range time scales, or what the authors of the study refer to as “deep time,” within the niche-filling model. To date, the majority of research data maxes out at 50 million years ago as the time scale ceiling. By extending the time scale to include Mesozoic dinosaurs, the study was able to explain the 170 million years of persistent avian biodiversity in contrast to the adaptive radiation of extinct dinosaurs that were once avian contemporaries. The extension of the time scale allowed for the authors of the study to explore the possible underlying components of deep time evolvability in certain avian lineages.
By Courtney Anderson