Discussion of Diet

In correspondence with me, C.A. Repenning, the person who first correctly identified the dinosaur bones originally found along the Colville River, speculated that "the hadrosaurs had fed on root mats of Equisetum in the shallow water of the Arctic Ocean and kept alive because the ocean was not frozen and would have been relatively warmer than the air" (Repenning, personal comm., 2004). As an intriguing attempt to understand the biology of Alaskan dinosaurs, it is in the spirit of this speculation that I will review possible specific food types for these animals.

Teaford and Oyen (1989) suggested that coarse wear on teeth indicated that more time was spent chewing. Hadrosaurs, however, have the most efficient tooth batteries of any dinosaurian group (Weishampel 1984; Horner et al. 2004). Further, many heavily worn teeth attributable to hadrosaurs are often found as components of Late Cretaceous microvertebrate assemblages. So despite the fine wear patterns observed on the teeth used in this study, hadrosaurs must have spent significant time chewing food items.

In her study of potential food items for the sauropod Brachiosaurus, Weaver (1983) estimated the range of caloric values for Jurassic plants based on their modern relatives. Horsetails and ferns had the lowest caloric values; ginkgos were intermediate, cycads the highest, with conifers ranging from high to intermediate in value. Ginkophytes and cycadophytes are present in the warmest part of the middle of the Cretaceous but have been squeezed out by the Campanian on the North Slope (Spicer 1987; Spicer 2003), and therefore cannot be considered further as a potential food source. Ferns, however, are present as part of the floral assemblages of the North Slope.

The author discounts longterm use of either ferns or horsetails by Alaskan hadrosaurs for three reasons. First is the low caloric value of either plant group as illustrated by Weaver (1983). Secondly, horsetails commonly include siliceous granules in their cuticle. Siliceous particles, because they are abrasive, would leave a much coarser wear pattern than that observed on the teeth here. Similarly, the last reason is that given the short stature of ferns and horsetails, such plants are almost assuredly coated to some degree with grit, which would also serve as an abrasive and leave a coarser wear pattern on teeth.

Given the abundance of conifer remains from the Cretaceous rocks of northern Alaska, conifers would seem to be the dominant food source for hadrosaurs on the North Slope (Spicer 1987, 2003). Conifers seem to have the presumed needed caloric value as shown by Weaver (1983), and they have been found in hadrosaurian coprolites to the south in Montana (Chin and Gill 1996). The similarity in wear patterns supports similarity in diet among hadrosaurs in these two general geographic regions, and similar wear among all populations of teeth examined suggests likewise.

The Cretaceous North Slope conifers are dominantly deciduous (Spicer 2003). Given the obliquity of light in the high latitudes during the Cretaceous (Spicer 1987; Fiorillo and Gangloff 2001; Fiorillo and Parrish 2004), the deciduous nature of the leafy part of the primary food source additional food sources should be considered.

Dead trees delignify and decay. Delignification with modern woods is brought on by wood-rot fungi (Tuor et al. 1995). Once delignification has occurred, wood, particularly conifer wood, has been recognized as a nutritious food source for domesticated ruminants (Orth and Orth 1977). Chin (2007) has recently described an intriguing specimen identified as a hadrosaur coprolite. The specimen contains small coniferous wood fragments. She attributes the appearance of partial decomposition to wood-rot fungi delignifying the wood prior to ingestion by this particular hadrosaur. Therefore, the author submits that it may be that during those parts of the annual cycle when deciduous plants lose their leaves, arctic hadrosaurs may have survived by consuming delignified wood, perhaps in combination with other adaptations (see next section).

Whereas delignified wood, in all likelihood, would be available all year, likely in limited quantities, given the deciduous nature of the vegetation, an additional food source during the Cretaceous spring of the ancient north may have been the buds and new wood. Anecdotally it has been observed along the Yukon River in central Alaska that black bears (Ursus americanus), after leaving hibernation, preferentially seek the new buds of trees because of the high nutritive value (O'Neill 2006).

To summarize, based on physical evidence found elsewhere such as coprolites, and nutritional inference from modern plants, a primary source of food for northern hadrosaurs was conifers. However, given the profound seasonality of light régimes, but not so much temperature seasonality during the Cretaceous, it seems likely other food resources were also utilized. These additional resources at this point can only be speculated upon. The only constraint provided by this study is that those foods qualitatively had a similar hardness. Further, the original prediction based on comparison to the dietary range of Ovis seems to be inconclusive for hadrosaurs as this study has pointed out a similarity in food hardness across the latitudinal range but an understanding of specific different food types remains ambiguous.