by Carl Strang
As I mentioned last week, the Mesozoic Era is a perennial hot topic in paleontological research. Lately, a hot topic within that hot topic has been the evolution of birds (plus the lead-up to the first birds in the feathered dinosaurs, some studies of which were included in last week’s listing). Here are notes from some studies published last year.
Mitchell, Jonathan H., and Peter J. Makovicky. 2014. Low ecological disparity in early Cretaceous birds. Proceedings of the Royal Society B, DOI: 10.1098/rspb.2014.0608 As described in a ScienceDaily article. They did an intensive study of a collection of fossil birds from China, early in the Cretaceous Period around 125 million years ago, when birds were a relatively new addition to the fauna. They concluded that the collection probably is a reasonably good approximation of what was there, and that it shows a remarkable lack of diversity. The size range and dietary breadth were limited, and large birds and water birds were missing. Most birds were sparrow to crow sized. There were some differences from today, as some species retained teeth or bony tails. Indications are that they lived in the forest and on the ground, and ate mostly insects and seeds. Though some of this limitation might have resulted from competition with established groups such as pterosaurs, the authors point to the lack of time for evolutionary diversification to occur as the main constraint.
Brusatte, Stephen L., Graeme T. Lloyd, Steve C. Wang, and Mark A. Norell. 2014. Gradual assembly of avian body plan culminated in rapid rates of evolution across the dinosaur-bird transition. Current Biology, DOI: 10.1016/j.cub.2014.08.034 As described in a ScienceDaily article. They looked at the evolutionary development of various structural components of birds, such as feathers, wishbone and wings. The elements of the bird body appeared separately over a very long period of time in the fossil record, with a slow convergence on the ultimate bird body plan in the line of theropod dinosaurs that led to them. Thus there is no stepwise appearance of the first bird. However, once the first birds had evolved, their diversification and continued evolution happened much more rapidly, demonstrating the advantages of that body plan.
Puttick, Mark N., Gavin H. Thomas, and Michael J. Benton. 2014. High rates of evolution preceded the origin of birds. Evolution, DOI: 10.1111/evo.12363 As described in a ScienceDaily article. They looked at the fossil record and used computer models to calculate rates of evolution of various traits. Two features essential to birds, small size and elongated forelimbs, began to appear 20 million years prior to Archaeopteryx, so that there were many species of small feathered dinosaurs (paraves) capable of flight well before the first birds appeared.
Meredith, R.W., G. Zhang, M. T. P. Gilbert, E. D. Jarvis, and M. S. Springer. 2014. Evidence for a single loss of mineralized teeth in the common avian ancestor. Science 346 (6215): 1254390 DOI: 10.1126/science.1254390 This portion of the whole-genome bird comparison study found that all modern birds point to a single common ancestor that lost the capability to grow teeth more than 100 million years ago, over a short span of time developing mutations inactivating 6 genes involved in enamel and dentin formation.
Lee, Michael S.Y., Andrea Cau, Darren Naish, and Gareth J. Dyke. 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds. Science 345:562-566. They did a detailed statistical study across the entire range (time and taxonomic) of theropod dinosaurs, and found a trend over the Mesozoic of reduction in body mass, culminating in the birds. This set the stage for other skeletal modifications that made birds possible.