by Carl Strang
The Mesozoic Era continues to fascinate researchers as well as 8-year-olds. This is the time when the dinosaurs rose to their dominance of the terrestrial fauna, which they maintained through many millions of years, though evolution continued to shape and diversify them. The mammals and birds appeared, the latter as an offshoot of the dinosaurs, the former out of a different group. The flowering plants likewise made their entrance onto the stage of life. Today I share my notes from a grab-bag of studies published last year.
Timothy B. Rowe, Thomas E. Macrini and Zhe-Xi Luo. 2011. Fossil evidence on origin of the mammalian brain. Science 332: 955-957. Brain reconstructions from scans of two early Jurassic pre-mammal skulls from China, Morganucodon and Hadrocodium, added to other information, suggest that the mammal brain size increase that is so important in distinguishing that group happened in 2 stages involving portions of the brain associated with olfaction, with later elaboration to accommodate increased tactile receptors connected to the fur, and then increased motor areas to coordinate responses to such increased sensory input.
P. A. Selden, C. Shih, D. Ren. A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China. Biology Letters, 2011; DOI: 10.1098/rsbl.2011.0228 (Described in ScienceDaily article). The most remarkable point here (apart from the spider’s size: 5 inches across its legs) is the fact that this Jurassic fossil is from a genus of orb-weavers still alive today. They are mainly tropical, with one species, Nephila clavipes, reaching the Gulf Coast of the United States.
Ge Sun, David L. Dilcher, Hongshan Wang, Zhiduan Chen. A eudicot from the Early Cretaceous of China. Nature, 2011; 471 (7340): 625 DOI: 10.1038/nature09811 (As described in ScienceDaily article). This Liaoning fossil is of the top of a plant with leaves and flowers similar to buttercups. The authors place it in the Ranunculaceae (buttercup family). It is the earliest fossil, other than pollen, of a true dicot, at 125 million years ago. It is sufficiently advanced to point to an earlier origin of the family, and of eudicots generally, which might have appeared as early as the Late Jurassic.
Snively E, Theodor JM (2011) Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls. PLoS ONE 6(6): e21422. doi:10.1371/journal.pone.0021422 Their comparative study resurrects the possibility that head-butting was a feature of pachycephalosaur (bone-headed dinosaur) social behavior.
Lindsay E. Zanno and Peter J. Makovicky. Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution. Proceedings of the National Academy of Sciences, 2010; DOI: 10.1073/pnas.1011924108 Theropods include all the predatory dinosaurs. These researchers looked at the coelurosaurian theropods, a diverse group whose diets were known to be varied but uncertain for many species. Using clues such as fossilized gut contents, coprolites (fossilized excrement), tooth morphology, and presence or absence of grinding stones in the gut area, they found 21 skeletal features that are closely associated with an herbivorous diet. Having established that base, they reviewed the coelurosaurs and found that extreme carnivores (e.g., Velociraptor) are a minority in the group. Extreme carnivory may in fact have been derived from an earlier omnivorous or even herbivorous ancestry. One feature, the toothless beak (endpoint of an evolutionary process of progressive tooth loss, and evolving in more than one group independently), is associated with herbivory and is of special interest because the coelurosaurs include the birds. The toothless beak is associated with the gastric mill. In some cases (Therizinosaurians) there is no sign of a gastric mill, and a battery of grinding teeth is retained. Three groups of coelurosaurs, the tyrannosauroids, the Compsognathidae and Dromaeosauridae, are carnivorous, but most others in the group are found to be herbivorous. Of 90 species, 24 fall out as carnivorous and 44 as herbivorous (the remaining ones omnivorous or undetermined).
Fowler DW, Freedman EA, Scannella JB, Kambic RE (2011) The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds. PLoS ONE 6(12): e28964. doi:10.1371/journal.pone.0028964 They re-examined the functional morphology of the enlarged hind foot claw of Deinonychus and other “raptor” dinosaurs, and propose an alternative function: that instead of using it as a slashing tool in attacks on prey larger than themselves, the predators used it as a clasping tool to help immobilize prey of similar or smaller body size for killing and dismemberment with the teeth. The authors further suggest that the wing-like forearms could be flapped to increase the predator’s stability during this process, and that this might be relevant to the evolution of flight in birds.