ENVIRONMENTAL SETTING OF THE POLAR FAUNAS OF SOUTHEASTERN AUSTRALIA AND ADAPTIVE STRATEGIES OF THE DINOSAURS

概要

論文の詳細を見る
Paleomagnetic evidence palces southeastern Australia well inside the paleo-Antarctic Circle in the Early Cretaceous. Over the past two decades a small but moderately diverse dinosaur assemblage has been recovered from a variety of sediments laid down in a broad rift valley that was forming as Australia and Antarctica began separating at the beginning of the Cretaceous. The primary source of the sediments were volcanic vents that lay to the east on the Lord Howe Rise, east of Australia. A diverse flora of more than 150 species is associated with the dinosaur material. Analysis of this flora suggests a mean annual air temperature (MAAT) of +7℃-+10℃. Oxygen isotope investigations suggest a lower MAAT (-2℃ to +5℃). Because of the discrepancy in the temperature estimates, the plants indicating a MAAT about equal to that of modern day Denver or Chicago on the one hand and the oxygen isotopes suggesting conditions more similiar to present day Fairbanks, Alaska, finding a third, independent way of assessing the MAAT has been a primary goal of the southeastern Australian polar dinosaur project. A third method of estimating temperature was found in 1996 when cryoturbation structures were recognized at four localities in the Strzelecki Group to the southeast of Melbourne, suggesting the former presence of seasonally frozen ground. These cryoturbation structures suggest MAAT of -6℃-+3℃. The lower limit is set by the absence of sand-or ice-wedges, which form only at a MAAT<-4℃. In regions today with a similar MAAT, the coldest month of the year has an average temperature of -32.3℃ and -23.9℃. The first hint of a special adaptation of any of the dinosaurs from southeastern Australia to polar conditions was pointed out nearly a decade ago. Comparison with the few available endocasts of the dorsal surface of the brains of hypsilophodontids from lower paleolatitudes showed that the optic lobes of the hypsilophodontid Leaellynasaura amicagraphica were noticeably enlarged. This enlargement was interpreted as possibly allowing the animal to see under the low light conditions of a polar Winter because of enhancement of the lobes ability to process the visual signals received from the eye. This interpretation implied activity during the polar Winter. Histological analysis of femoral specimens of Leaellynasaura amicagraphica and two other hypsilophodontids from southeastern Australia shows that all taxa lack Lines of Arrested Growth, LAGs, in the compacta. LAGs, denser, darker layers of bone, form when the rate of growth is reduced such as when the animal is under food stress or hibernates. Animals which lack LAGs consequently are interpreted as not having undergone such phases. While LAGs occur in most dinosaurs examined, hypsilophodontids at both low and high paleolatitudes are a noticeable exception. The absence of LAGs in all hypsilophodontids coupled with the enlarged optic lobes in Leaellynasaura amicagraphica, but not in lower palaeolatitude hypsilophodontids, suggests that the family was preadapted for year around activity and then acquired greater visual acuity when they entered higher latitudes, if they, in fact, first evolved at lower paleolatitudes. The diversity of hypsilophodontids in the southeastern Australian dinosaur assemblage, where they constitute half the approximately dozen taxa known, would suggest that they lived in a habitat particularly suitable for them. Some, but certainly not all, polar dinosaur sites show this pronounced bias towards hypsilophodontids. The one other southeastern Australian dinosaur examined for LAGs was the ornithomimosaur Timimus hermani. This animal has strongly developed LAGs. Presumably, it may have survived the Winters by hibernating, unlike the hypsilophodontids which evidently did not.
国立科学博物館の論文

ENVIRONMENTAL SETTING OF THE POLAR FAUNAS OF SOUTHEASTERN AUSTRALIA AND ADAPTIVE STRATEGIES OF THE DINOSAURS

スポンサーリンク

概要

著者

関連論文

スポンサーリンク