The Hilda mega-bonebed is a complex of fourteen probable Centrosaurus apertus bonebeds discovered near the town of Hilda in Alberta, Canada. It was first described in the scientific literature by David Eberth, Donald Brinkman, and Vaia Barkas in 2010 after more than ten years of research. The Hilda mega-bonebed is significant because the behavior of the preserved dinosaurs themselves was the dominant cause of its existence, rather than the stratum's geological history like most bonebeds. It is also Canada's largest bonebed.
History of research
In 1959, Wann Langston, Jr. recorded evidence of a Centrosaurus bonebed near Hilda, Alberta. Later, between 1964 and 1966 Don Taylor of the Provincial Museum of Alberta (now called the Royal Alberta Museum) oversaw the collection of fossils from yet another bonebed in the same region. These prospective bonebeds attracted the attention of scientists working for the Royal Tyrell Museum in the 1990s because Hilda was an ideal location for maintaining physical continuity across the bonebeds' expanse. Physical continuity is important, because discontinuity in the rock strata can make it harder to tell if the bones were deposited at the same time or not. Eberth and the other researchers considered Hilda a prime site for a continuous bonebed for two reasons, one related to its history, one related to its present geography. First, during the Cretaceous the area was situated closer to the Western Interior Seaway where more sediment would have been deposited than at Dinosaur Provincial Park. Secondly, the modern Hilda area lacks the rough badland terrain that breaks up many probably equivalent deposits in Dinosaur Provincial Park. In 1996, the Royal Tyrell Museum finally performed a preliminary survey of the area studied by Langston and Taylor and entire new bonebeds were discovered. The next year, in 1997, research at Hilda began in earnest. In only two days, researchers discovered 14 separate bonebeds in one mudstone bed that extended for at least 7 km, with 3.7 km worth of visible outcrops. By the conclusion of the research program, the scientists mapped the bonebeds and excavated the bonebed cataloged as H97-04. They concluded that the Hilda bonebeds formed simultaneously when a herd consisting of thousands of Centrosaurus apertus drowned in a flood. The researchers further speculated that some of the 17 Centrosaurus bonebeds of Dinosaur Provincial Park likely formed simultaneously in a manner analogous to the formation of the Hilda bonebeds, which the researchers estimated to be spread over 2.3 km2. The researchers only collected fossils and taphonomic data from the bonebed H97-04, although all the others were also examined to check the quality and number of preserved bones. The research at Hilda was so complex that over ten years passed from the start of the project until David A. Eberth, Donald B. Brinkman, and Vaia Barkas published a formal description in the scientific literature.
Geographic location and extent The mega-bonebed is located 25 kilometers west of the town of Hilda, Alberta in a "steep-walled" valley cut into the landscape by glaciers during the Pleistocene epoch. Northward through this valley's interior runs the South Saskatchewan River. The individual bonebeds in the complex are recognizable as dense concentrations of ceratopsid bones. Research on the bonebeds of Dinosaur Provincial Park has discovered that ceratopsian bonebeds preserved in mudstone deposits tended to be circular to ovular in shape when viewed from above, so the similar Hilda bonebeds probably were as well. H97-04 is the largest of the fourteen bonebeds, at 150 m in width. If one assumes that H97-04's 150 m width is its longest linear dimension then it had an area of about 17,671 m2. Bonebeds other than H97-04 were less than 50 m wide. These individual component bonebeds tend to have areas of only a few hundred to a few thousand square feet and would have maximum areas of somewhat less than 2,000 square meters. The individual bonebeds at Hilda were generally smaller than those at Dinosaur Provincial Park. The researchers estimated the entire Hilda complex to be 2.3 square kilometers in area based on the length of its north-south and east-west axes. This is the size of about 280 football fields, and is one of the largest dinosaur bonebeds ever discovered. Bonebeds left by such large animals over wide areas are a rare discovery. However, completely excavating the bonebed would be too huge and complex an undertaking to be practical.
Lithology and stratigraphy
The Hilda mega-bonebed lies within the bottom 25 m of the Dinosaur Park Formation. Eberth, Brinkman and Barkas estimated that in the Hilda area, the Dinosaur Park Formation was almost 60 m thick between the over- and under-lying formations, but glacial activity and slumping had removed all but 43 m of this at the field site. When the researchers examined the stratigraphy of the Hilda site they found that the fourteen bonebeds occupied identical positions in the stratigraphic column. This continuity in the mudstone was what allowed them to infer that each bonebed was actually a component of a single large "mega-bonebed". At its thinnest point the mudstone bed hosting the centrosaur fossils is 25 cm thick and it measures 1 m at its thickest. Eberth, Brinkman and Barkas observed siltstone, claystone, and abundant organic matter in the composition of the "brown-grey" mudstone. Ironstone concretions can be found scattered throughout the bed, but were most abundant where fossils were also common. Fossils are most common in the lowest parts of the bed. There are signs of deformation in the sediment composing the mudstone. The stratum hosting the mega-bonebed has different sorts of contacts with other strata at different locations across its extent. The most distinct boundaries separate the Hilda bonebed mudstone from other mudstones above and below it. Less clear boundaries occur where the bonebed mudstone contacts the filled-in remains of an ancient body of flowing water. These contacts record where the channel cut down into the sediment now lithified as the fossil-bearing mudstone. The researchers couldn't find the bed's northern boundary, which they felt might extend north of the northernmost bonebed as part of a succession of stacked mudstone beds. An ancient channel cut all the way through the sediments now composing the mudstone, and its deposit forms the Hilda bonebed's southern boundary.
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