A nestling-sized skeleton of Edmontosaurus (Ornithischia, Hadrosauridae) from the Hell Creek Formation of northeastern Montana, U.S.A., with an analysis of ontogenetic limb allometry

ABSTRACT

The Hell Creek Formation preserves one of the most intensely studied late Cretaceous terrestrial fossil units. Over 22 dinosaur genera are currently recognized from this unit, but the record of juvenile individuals is surprisingly limited. Here, we document a nestling hadrosaur that represents the first occurrence of an articulated nestling dinosaur skeleton from the latest Cretaceous (late Maastrichtian) of North America. The specimen (UCMP 128181) preserves a partial scapula, nearly complete rib cage, vertebral series from the shoulder to mid-tail, a large portion of the pelvic girdle, and both hind limbs through a combination of bone and/or natural impressions in the concretion. It is assignable to the genus Edmontosaurus based on the shape of the prepubic process, or blade, of the pubis. The specimen represents the earliest ontogenetic growth stage of Edmontosaurus cf. annectens and possesses a femur length of 148 mm. It greatly contributes as a new end member to a sample of associated Edmontosaurus skeletons that is well suited for allometrically testing the hypothesized ontogenetic gait shift in hadrosaurs from bipedal juveniles to quadrupedal adults using individual limb proportions. Although UCMP 128181 does not preserve forelimbs, regressions based on associated Edmontosaurus skeletons (N = 25) reveal overall isometry of the forelimb relative to the hind limb, and within each limb. These data indicate that Edmontosaurus nestlings were anatomically capable of fully quadrupedal locomotion and provide no compelling evidence to support an ontogenetic gait shift in hadrosaurids.

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Citation for this article: Wosik, M., M. B. Goodwin, and D. C. Evans. 2018. A nestling-sized skeleton of Edmontosaurus (Ornithischia, Hadrosauridae) from the Hell Creek Formation of northeastern Montana, U.S.A., with an analysis of ontogenetic limb allometry. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1398168.

ACKNOWLEDGMENTS

Access to specimens was facilitated by N. Carroll, M. Currie, P. Holroyd, M. Goodwin, C. Mehling, D. Pagnac, K. Seymour, and K. Shepherd. Our thanks to many across the globe who helped gather measurements and confirm locality information: D. Brinkman, N. Campione, R. Carr, N. Carroll, J. Eberle, J. Mallon, A. Mann, A. McGee, B. Roach, J. Scannella, V. Schneider, H. Seyler, T. Sonoda, and M. Whitney. Comments from R. Butler, A. Farke, D. McLennan, P. O'Connor, and an anonymous reviewer greatly improved the manuscript. We thank C. A. Masnaghetti (artist) and N. Turinetti from Saurian and Urvogel Games, LLC for working with us to design cover art associated with this article. We thank the Twitchell family for their long-term support of field work by the UCMP and crews on and around their ranch. D. Melton and G. Liggett of the Bureau of Land Management Office facilitated access to land under their management. We thank the M. Thomas family who supported Harley in the field during the collection of UCMP 128181. And most importantly, we thank the late H. Garbani, a long-term field associate and friend of the UCMP, who passed away in 2011. The nestling skeleton described in this paper is but one of a number of significant discoveries by Harley and his keen eyes and passion for hunting dinosaur bones over four decades working with W. Clemens and UCMP field crews.

Additional information

Funding

This work was supported by the University of California Museum of Paleontology Doris O. and Samuel P. Welles Research Fund to (to M.W.), a Natural Sciences and Engineering Research Council of Canada Discovery Grant RGPIN 355845 (to D.C.E.), a Royal Ontario Museum Department of Natural History Fieldwork Grant (to D.C.E.), N. Myrvold in support of the Hell Creek Project III (to M.B.G.), and the University of California Museum of Paleontology (to M.B.G.).