In Alaska, dinosaurs lived where rainfall was optimal

Alaska during the Cretaceous Period was a different place from the state that exists today. Alaska was warmer and wetter, and supported an array of different plant species, as well as an abundance of large, herbivorous dinosaurs. During the late Cretaceous, large-bodied (>1000 kg) herbivorous dinosaurs in the Arctic were predominantly from the groups known as hadrosaurids (duck-billed dinosaurs) and ceratopsids (horned dinosaurs), and their fossilized remains have been found at numerous sites in the region.

In order to understand more about the relationship between the biota and the climate in the ancient Arctic between 100 and 66 million years ago, scientists from the USA and Japan have investigated the distribution of these megaherbivores in Alaska. The authors feel that this study might help them predict what the Arctic region might look like in the years ahead, if the climate continues to become warmer and wetter. 

Study co-author Paul McCarthy, a professor in the UAF Department of Geosciences, is a sedimentologist and a fossil soils specialist who has been studying the region’s ancient past for many years. 

“The reason we’ve been looking at Cretaceous environments up here is because Earth was in a greenhouse state at that point in time, and it offers the potential to provide analogs to what we might see, eventually, if global warming continues,” said Professor McCarthy.

“We can’t simulate the rates of change, which are likely to have been totally different in the Cretaceous. But we can simulate what an ice-free coast would look like and also see how rivers and floodplains would respond to spring snowmelt from the mountains if everything’s not frozen. And we can look at the distribution of plants and animals.”

Professor McCarthy led the analysis of the depositional environments and ancient soils of three rock formations: the fossil-rich Prince Creek Formation along the Colville River in northern Alaska, the Lower Cantwell Formation in the Central Alaska Range and the Chignik Formation on the Alaskan Peninsula.

The three formations are close enough to one another on the geologic time scale to allow for a climate comparison, according to the research paper. They all contain Late Cretaceous rocks that were deposited approximately 83 million to 66 million years ago.

In particular, the researchers estimated mean annual temperature (MAT) and mean annual precipitation (MAP) for each site using characteristics found in the fossilized soils. These geochemical and pollen proxies enabled them to get a better picture of climatic conditions that prevailed during this time period. Fossilized plants and animals and ancient footprints get most of the public attention, but fossil soil has equally important information to offer through its preserved features, mineral composition and chemical makeup.

“We can look at microscopic features preserved in the fossil soil samples and relate that to modern soil types to get an idea of where they formed,” said Professor McCarthy. “Are we looking at deserts? Are we looking at tropical rainforest or temperate forest? Or grasslands?” 

“Fossil soil also preserves pollen grains that can tell us something about the composition of the local vegetation. And it contains clay minerals, organic matter and the iron-carbonate mineral siderite, all of which can be used to determine precipitation and temperature using stable isotopic methods.”

The researchers estimated the relative abundances of dinosaurs at each site using the occurrences of fossilized tracks, as well as the frequency with which each type of dinosaur dominated in the fossil skeletal deposits. 

The findings of the study, published recently in the journal Geosciences, showed that dinosaur distribution in Alaska was correlated more strongly with the amount of annual precipitation than with the mean annual temperatures. Hadrosaurids, the family of duck-billed dinosaurs, became more dominant where climates were wetter and had a narrower annual temperature range. In contrast, ceratopsids, such as the well-known Triceratops, preferred a milder and drier climate.

At the three Alaskan sites studied, the ceratopsids, with their beaks and horns, were never dominant over the hadrosaurids in terms of population numbers.

The finding that precipitation has a greater influence on dinosaur distribution than temperature supported prior research from the Prince Creek Formation. That research, led by Celina A. Suarez of the University of Arkansas, suggested that ceratopsids preferred the drier, better-drained regions of the Late Cretaceous Arctic while the hadrosaurids preferred the wetter regions. Professor McCarthy was also involved in the study, which was focused on dinosaur teeth. 

Previous research has also found that variations in population sizes of large herbivores are related more to variations in precipitation than to variations in temperature. Precipitation has its effect on vegetation and the productivity of ecosystems, and these large-bodied megaherbivores would have depended on sufficient supplies of fresh food. 

The authors conclude that the formations studied “provide ample evidence for a flourishing high-latitude ecosystem during the Late Cretaceous greenhouse, an ecosystem where the local paleoclimate was a primary driver in structuring the relative abundances of large-bodied herbivores in local environments. The analysis suggests that MAP played a more direct role in determining the distribution of dinosaurs than did MAT, although seasonal temperature ranges between summer and winter may have also played a secondary role.”

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By Alison Bosman, Earth.com Staff Writer