Preliminary estimates of dinosaur size and speed at the Early Cretaceous Clayton Lake dinosaur tracksite, Union County, New Mexico

Abstract

The Early Cretaceous (late Albian) dinosaur tracksite at Clayton Lake in Union County encompasses approximately 533 dinosaur tracks, 182 of which are organized into 28 trackways. Most of the trackways were made by bipedal ornithopod dinosaurs, but two trackways were made by bipedal theropods, and one was made by a quadrupedal ankylosaur. To estimate dinosaur speeds from the trackways, we employed the methods of Alexander (1976, 1989) and Thulborn (1990) that rely on footprint lengths and strides as the raw data from which to estimate size and speed. On the Clayton tracks, we measured total length and width as highest edge to highest edge (rim to rim) of the maximum track shape. Though not ideal, this approach was dictated by the quality and nature of track preservation and provided a consistent methodology. For deeper tracks, these measurements are similar to the “negative vertical displacement” approach, but less than the “maximum zone of deformation.” This approach neither captures the true footprint dimensions nor the full extent of extramorphological variation but does represent the maximum lengths and widths on the bedding plane surface. Track length was measured from the anterior tip of digit III to the base of the “heel” margin, actual or inferred. In some cases, our measured track lengths exceed the true foot length of the trackmaker, and these tracks are unsuitable for size and speed estimates. However, some trackways have a “best” track(s) that exhibit(s) some combination of clear track outline, high angle footwalls, and “impressed” positives. We estimated size and speed for these. The results (using the method of Alexander, 1976) are that all of the ornithopod trackways show walking speeds of ~ 2-to 7 km /hour. The relatively fastest dinosaur speed at Clayton Lake may be