Bonebed 30 (BB30) in Dinosaur Provincial Park near Brooks, Alta., is about the size of a football field. It contains tens of thousands of fossil bones from the horned dinosaur, Centrosaurus apertus that were buried by a coastal flood 76 million years ago. Although there are hundreds of bonebeds in Dinosaur Provincial Park, only a small fraction of the bonebeds dominated by large (>5cm) fossil bones have been systematically excavated.
When we are excavating fossils, we aren’t just concerned with what animal(s) they are from and what part of the animal they represent; the locations and relationships among the bones are also very important. To map the site, a baseline is established to provide a frame of reference for the excavations in the bonebed. From this baseline, a one-metre by one-metre coordinated grid is established.
As each bone is exposed from the rock, we map its location on the map sheet for the map grid square where the bone is found, and record data about each bone including what animals the fossil came from (e.g., Centrosaurus, Gorgosaurus), what the fossil is (e.g., femur, parietal, tooth), which grid square it was found in (e.g., A2), the orientation, depth and plunge of the bone, and any other notes that can be observed in the field (tooth marks, missing fragments, abrasion, etc.).
The orientation of the bone is whether the bone is horizontal, vertical, sub-horizontal, or, less frequently, sub-vertical. The trend of the bone is measured for long bones like limbs bones or ribs and indicates the long axis of the bone relative to north (e.g., 140°-320° would represent a trend of roughly northwest-southeast).
If the majority of the long bones are deposited along the same general trend, this can indicate that the fossils were deposited under the influence of flowing water. Depth indicates how far the bone is below the baseline. If the larger, heavier bones were deposited lower than the smaller, lighter bones, this also suggests a fluvial (river channel) influence on the deposition of the bones. If the bone orientations and trends are widely variable with small and large bones all jumbled up together, this indicates a very different picture of the depositional history of the bonebed.
The baseline for this excavation in BB30 is 5.51 m long and runs west-east. On the map, the baseline is represented by the thick green line. The map area is divided into one meter squares, with the corners often marked with nails and flagging tape. The grid square is subdivided, using string, into 10 cm by 10 cm squares. Each grid square can be identified by a number and a letter—in the case of this map with letters A through D going from east to west along the baseline, and numbers 1 through 4 going from south to north away from the baseline. With a larger bonebed, more letters and numbers would be used. This map shows only a small portion of the bonebed. It was excavated during the summers of 2009 – 2012 by participants in the Guided Excavation Program, a public participation program operated jointly by Alberta Parks and the Royal Tyrrell Museum.
Find more information about the program here.
– David Lloyd, Research Assistant