Literature Review: Mesozoic Era

by Carl Strang

This week’s scientific literature focus is on recent studies of the Mesozoic Era, the “age of dinosaurs.”

Sun,Yadong, et al. 2012. Lethally Hot Temperatures During the Early Triassic Greenhouse. Science 338: 366-370. They used oxygen isotope ratios in conodont (small eel-like fishes) teeth to measure temperatures, and found that for 5 million years into the Triassic, temperatures were too high to permit terrestrial or marine vertebrate life in the tropics (50-60C = 122-140F on land, 40C = 104F ocean surface waters). Some ferns and shrubs could withstand this but their growth was limited, and there were no tropical forests. Without plants to sequester carbon dioxide, the runaway global warming resulted. Until this study it was thought that ocean temperatures could not exceed 30C.

Artist’s rendition of a prosauropod, subject of the following study, from an exhibit at the Field Museum of Natural History.

Artist’s rendition of a prosauropod, subject of the following study, from an exhibit at the Field Museum of Natural History.

Robert R. Reisz, David C. Evans, Eric M. Roberts, Hans-Dieter Sues, Adam M. Yates. Oldest known dinosaurian nesting site and reproductive biology of the Early Jurassic sauropodomorph Massospondylus. Proceedings of the National Academy of Sciences, 2012; DOI: 10.1073/pnas.1109385109 The grouped nests, eggs showing embryo details, and the presence of tracks of young twice the size of the embryos, points to a prolonged attachment to the nest sites. The eggs in the nests appear to have been carefully arranged by the adults. All of this points to prolonged parental care in a relatively early (prosauropod) dinosaur.

Balter, Michael. 2012. Flying dinos and baby birds offer new clues about how avians took wing. Science 338:591-592. This news review article discusses the origin of flight in dinosaurs and birds, focusing on Microraptor, a crow-sized Chinese dromaeosaurid. There has been some debate about this and other “4-winged” species which had wing-like rows of quill feathers on their legs as well as their forelimbs. An early idea was that the leg structures added gliding planes to those of the wings. Others argue that they would have served as rudders for quick turns, aided by stabilization from the animal’s long tail. Another idea with traction among many in this field is the “wing-assisted incline running” hypothesis, according to which the wings would have helped such animals quickly to climb trees to escape predators. Some recent research has focused on immature chukars, birds which have a number of features in common with the feathered dinosaurs but which are lost during development as they are transformed into the adult birds’ structures. The immature chukars cannot fly, but they are able to perform wing-assisted incline running. They lose much of this ability if their wing feathers are clipped.

Zelenitsky, Darla K., et al. 2012. Feathered non-avian dinosaurs from North America provide insight into wing origins. Science 338:510-514. The first fossil dinosaurs with feathers found in North America are ornithomimids from the Upper Cretaceous in which the young have only downy body coverings but adults add wings. These animals could not fly, so the quills were involved in adult activity such as courtship or brooding of young.

Achim G. Reisdorf, Michael Wuttke. Re-evaluating Moodie’s Opisthotonic-Posture Hypothesis in Fossil Vertebrates Part I: Reptiles—the taphonomy of the bipedal dinosaurs Compsognathus longipes and Juravenator starki from the Solnhofen Archipelago (Jurassic, Germany). Palaeobiodiversity and Palaeoenvironments, 2012; DOI: 10.1007/s12549-011-0068-y As described in a ScienceDaily article. Dinosaurs and relatives, including Archaeopteryx, often are found fossilized with the neck dramatically bent backward over the back. This had been thought to be a sort of death throes pattern, but Reisdorf and Wittke did experiments with chickens that revealed this to be a process of decomposition. A relatively decay-resistant ligament, the Ligamentum elasticum, is “pre-loaded” as a spring-like support along the top of the vertebral column in dinosaurs with long necks and tails. As decomposition proceeds in water (usually fossilized animals are preserved in aquatic sediments), the opposing muscles and connective tissues on the front of the neck break down first, so that the Ligamentum elasticum gradually pulls the neck back into the observed posture.

Gregory P. Wilson, Alistair R. Evans, Ian J. Corfe, Peter D. Smits, Mikael Fortelius, Jukka Jernvall. Adaptive radiation of multituberculate mammals before the extinction of dinosaurs. Nature, 2012; DOI: 10.1038/nature10880 From ScienceDaily article. They did a comparative study of multituberculates, an early mammal group that arose by 170mya (million years ago) and went extinct around 34mya. Their appearance as mouse-sized species preceded the diversification of angiosperms, but their own diversity increased after that happened, so that they ranged as large as beavers. There was a concurrent increase in tooth complexity, showing many facets facing in various directions especially in the molars, a change associated with a vegetarian diet (though many species had teeth more suggestive of an insect diet). Dinosaurs never matched this, even herbivorous ones either developing batteries of relatively flat-topped grinders, or swallowing plant material whole and using gastric mills. The small body size and dietary capabilities apparently contributed to multituberculate survival when the dinosaurs became extinct.

A model of a hadrosaur, Brookfield Zoo. These possibly hold the all-time record for tooth tissue complexity.

A model of a hadrosaur, Brookfield Zoo. These possibly hold the all-time record for tooth tissue complexity.

G. M. Erickson, Gregory M., et al. 2012. Complex dental structure and wear biomechanics in hadrosaurid dinosaurs. Science 338: 98-101. They examined the microstructure and mechanical properties of hadrosaur teeth, and found that these were more complex than molars of mammalian herbivores (with 6 rather than 4 tissue types), and were more effective as grinders of tough plant tissues. This helps explain why they became so successful, diverse and abundant in their time.

Gates TA, Prieto-Márquez A, Zanno LE (2012) Mountain Building Triggered Late Cretaceous North American Megaherbivore Dinosaur Radiation. PLoS ONE 7(8): e42135. doi:10.1371/journal.pone.0042135 They tie the unusual diversification of hadrosaurs and ceratopsians in western North America to the timing of its isolation by the inland seaway and the simultaneous development of 2 mountain ranges within it. This produced geographic isolation of populations over a large north-to-south gradient. They conclude that this was a special case, and it should not be assumed that such species diversity was characteristic of that time globally.

Stephen L. Brusatte, Richard J. Butler, Albert Prieto-Márquez, Mark A. Norell. Dinosaur morphological diversity and the end-Cretaceous extinction. Nature Communications, 2012; 3: 804 DOI: 10.1038/ncomms1815 As described in a ScienceDaily article. They reviewed morphological variation in the late Cretaceous in different parts of the world, working out of the premise that little variation may signal species headed toward extinction, while much variation may represent vigor and the potential for division into more species. Some groups in some parts of the world were found to represent each of these alternatives. Western North American dinosaurs after the incursion of the inland sea showed signs of deterioration. This may be tied to their more limited geographic area, the sea taking some land and western mountains further limiting the range. Large herbivores (hadrosaurs and ceratopsids) for the most part were declining, though carnivores and mid-sized herbivores (ankylosaurs, pachycephalosaurs) as well as sauropods were not. Asian hadrosaurs showed the opposite pattern, however.

Kerr, Richard A. 2012. Before the dinosaurs’ demise, a clambake extinction? Science 337:1280. News article describing a study of relatively thick strata from Antarctica that allow separation of effects of the Deccan Traps volcanic eruptions (comparable to the Siberian Traps associated with the end-Permian extinctions) from the impact that is generally regarded as ending the Cretaceous. The research group led by Thomas Tobin found results indicating that the Deccan Traps eruptions began 200,000 years before the impact, and this correlated with a warming of water in the bottom of the ocean and extinctions of clams and snails. Free-swimming ammonites were not affected by this, however.

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