Friday, 3 October 2014

A Hadrosaurid Dinosaur trackway from the Denali National Park in Alaska.

The preserved tracks of ancient animals such as Dinosaurs can provide insights into their lifestyles and biology that could not be determined by examination of bones alone, although such data needs to be interpreted carefully. Such studies can potentially provide data on herding or other social structures among extinct animals, as well as data on the movement of individual creatures.

In a paper published in the journal Geology on 30 June 2014, Anthony Fiorillo of the Perot Museum of Nature and Science, Stephen Hasiotis of the Department of Geology at the University of Kansas and Yoshitsugu Kobayashi of the Hokkaido University Museum describe a newly discovered Dinosaur trackway in the Denali National Park in Alaska.

The trackway appears to show the movements of a large number of Hadrosaurid Dinosaurs. It is found in the lower part of the Late Cretaceous Cantwell Formation, which comprises fluvial (riverine) deposits preserved in a highly seasonal environment. The site is thought to be close to the same position that it occupied during the Late Cretaceous, i.e. it would have been within the Arctic Circle at the time when the tracks were laid down. Arctic ecosystems during the Cretaceous are thought to have been unlike anything found in the modern world, as the greenhouse climate of the period prevented the formation of permanent ice caps, but within the Arctic Circle the winter would still have been marked by several months of complete darkness.

Polar projection of tectonic plates during the Late Cretaceous, with inset map of Alaska. NA—North America. Dots in both maps indicate location of Denali National Park. Fiorillo et al. (2014).

The tracks appear to have been laid down over a short period of time (based upon analysis of plant and invertebrate fossils in the deposits, which are thought to have been highly seasonal) leading Fiorillo et al. to conclude they represent the passage of a single herd of animals. This herd apparently contained individuals with a wide range of sizes, from mature individuals, to very young juveniles. This is interesting as Hadrosaurid tracks in more southerly locations have previously shown only individuals of a single size, suggesting that the animals moved in packs determined by age. In contrast the Denali Herd appears to have contained animals of different ages, suggesting that younger animals were incorporated into a multi-generation herd, which in turn implies that extended parental care may have been occurring.

(A–C) Size ranges of tracks found at Denali National Park, Alaska, tracksite. (D) Adult Hadrosaurid track with skin impressions. Scale bar for (C1) is 5 cm.Fiorillo et al. (2014).

Fiorillo et al. also observe that while there appear to have been Hadrosaurs of all sizes in the pack, the size distribution is skewed towards larger, presumably adult, specimens. They suggest that this may be indicative of rapid growth in juvenile Hadrosaurs, with a lifecycle that includes a short growing period followed by an extended adult phase (something that has been suggested for Hadrosaurs previously).

Relative frequencies of each growth stage as function of body mass in the Denali Hadrosaur herd. This curve independently corroborates hypothesised growth curve for Hadrosaurid dinosaurs proposed by others. Fiorillo et al. (2014).

It has been suggested that Dinosaurs living within the Arctic Circle may have migrated seasonally to escape the dark winter season, in a similar way to the long distance migrations of modern Caribou. Based upon our understanding of the location of the Dinali site during the Cretaceous, and the sizes of the Dinosaurs within this pack (which affects the distance an animal can migrate by walking), Fiorillo et al. suggest that it would have been impossible for the Dinali herd to migrate seasonally from the site to a location outside the Arctic Circle, suggesting that they were able to overwinter in the Arctic.

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