Dinosaurs under the Aurora

7. Cretaceous Dinosaur Pathways in the Paleo-Arctic and along the Western Interior Seaway

104 7 CRETACEOUS DINOSAUR PATHWAYS IN THE PALEO-ARCTIC AND ALONG THE WESTERN INTERIOR SEAWAY The record of Cretaceous age dinosaur trackways and their common association with ocean shorelines has been steadily accumulating since the 1940s.1 One of the most impressive and best-known records can be found in central Texas, an area that occupied the southern end of the great Western Interior Seaway. The Early Cretaceous (Albian) age Glen Rose Formation here contains numerous and widespread trackway complexes at several stratigraphic levels. Such widespread trackway complexes are now referred to as “megatracksite complexes.”2 The Glen Rose Formation includes fine examples of megatracksites that crop out over an area of some 38,000 square miles (100,000 square kilometers), including the famous Paluxy River site that was studied by the early dinosaur “tracker” Roland T. Bird, and is now a Texas state park.3 Sauropod and theropod tracks and trackways generally follow along Early Cretaceous marine shorelines. This is a pattern that is typical of Cretaceous-age tracksites.4 The recent documentation of abundant tracks and trackways in Upper Cretaceous rocks of the Kaskapau Formation in northeastern British Columbia is especially relevant to a discussion of paleoArctic dinosaurs and their association with ocean shoreline environments. This locality is on Quality Creek near Tumbler Ridge, British Columbia. It was located above the paleo-Arctic Circle at a high stand of global sea level. Three different coastal environments are recorded in a wedge of nonmarine sedimentary rocks that is part of a marine-dominated offshore formation. The area underwent a change from a strandplain with beach ridges and sandy coals to tidal channels then to a backshore freshwater lake and finally to a brackish lagoon with a complex of deltas. In two of these environments, abundant tracks and trackways are found in localized concentrations characterized as trampled or dinoturbated horizons (see figure 4.14). In contrast with North Slope Late Cretaceous dinosaurs, the Quality Creek dinosaurs have been directly associated with crocodilians, turtles, and oyster shells. It is interesting to note that a dinosaur-trampled oyster bank, or bioherm, was found in the brackish lagoon. This trampled bioherm is a first for British Columbia, as is the discovery of in situ dinosaur bones.5 If you look carefully enough at the bedding surfaces of Cretaceous rocks that include coal beds you are bound to find dinosaur and bird tracks. This statement has become a mantra for dinosaur “trackers” that scour the The Association of Dinosaur Trackways with Shorelines in Western North America Cretaceous Dinosaur Pathways 105 outcrops of Cretaceous terrestrial rocks throughout western North America. The environmental conditions responsible for the accumulation of peatrich sediments that are the first step in the formation of coal are also conducive to the preservation of dinosaur and bird tracks. The ideal conditions are water-saturated mud and fine silty sand surfaces that harbor abundant microbes such as cyanobacteria and green algae.6 If these conditions are accompanied by influxes of fine-grained, rapidly accumulating sediments, then the making of clear impressions and their subsequent burial and preservation are greatly aided. This combination of factors describes those encountered along most delta, lake, and ocean shorelines. These habitats also contain rich assortments of plants, such as grasses, scouring rushes, and ferns. During the Cretaceous, these plants were often joined by ginkgos, cycads, and a variety of needle-bearing gymnosperms—a plant smorgasbord that would attract herbivorous duckbills, ceratopsids, pachycephalosaurs, and ankylosaurs. It is now very clear, from several lines of evidence, including abundant trackways, that Cretaceous dinosaurs lived and flourished along paleo-Arctic lake, delta, and ocean shorelines. As noted in chapter 2, the first documentation of dinosaurs in the Arctic was the discovery of their footprints on the Island of Spitzbergen. Likewise, early evidence of dinosaurs in the Arctic of Alaska came from their footprints .7 However, the diversity and great abundance of this record along the Colville River on Alaska’s North Slope remained elusive despite over forty years of exploration by hundreds of field geologists and biologists that criss-crossed this area. This points up the value of having the “eye” for this type of fossil and the importance of field training that helps to acquire it. It is also critical to keep the mind open to paradigms other than those acquired in early schooling. Apparently, the rich record of dinosaur footprints and trackways was noticed, but the footprints were misidentified as abiogenic sedimentary structures instead of fossil tracks. Abundant and widespread...