Diverse Dinosaur-Dominated Ichnofaunas from the Potomac Group (Lower Cretaceous) Maryland

Abstract

Until recently fossil footprints were virtually unknown from the Cretaceous of the eastern United States. The discovery of about 300 footprints in iron-rich siliciclastic facies of the Patuxent Formation (Potomac Group) of Aptian age is undoubtedly one of the most significant Early Cretaceous track discoveries since the Paluxy track discoveries in Texas in the 1930s. The Patuxent tracks include theropod, sauropod, ankylosaur and ornithopod dinosaur footprints, pterosaur tracks, and miscellaneous mammal and other vertebrate ichnites that collectively suggest a diversity of about 14 morphotypes. This is about twice the previous maximum estimate for any known Early Cretaceous vertebrate ichnofauna. Among the more distinctive forms are excellent examples of hypsilophodontid tracks and a surprisingly large mammal footprint. A remarkable feature of the Patuxent track assemblage is the high proportion of small tracks indicative of hatchlings, independently verified by the discovery of a hatchling-sized dinosaur. Such evidence suggests the proximity of nest sites. The preservation of such small tracks is very rare in the Cretaceous track record, and indeed throughout most of the Mesozoic.

This unusual preservation not only provides us with a window into a diverse Early Cretaceous ecosystem, but it also suggests the potential of such facies to provide ichnological bonanzas. A remarkable feature of the assemblage is that it consists largely of reworked nodules and clasts that may have previously been reworked within the Patuxent Formation. Such unusual contexts of preservation should provide intriguing research opportunities for sedimentologists interested in the diagenesis and taphonomy of a unique track-bearing facies.

Keywords:

• Maryland
• Potomac Group
• Cretaceous
• dinosaur tracks
• pterosaur tracks

ACKNOWLEDGEMENTS

We acknowledge the following colleagues for their help: Sheila M. Stanford for the discovery of two tracks in the collection and for her encouragement; Mark Symborski for his help in geological matters, interpretation of tracks, and help in the field; John Menas for his discovery and contribution of two tracks to the collection; and David B. Weishampel, Peter Dodson, and Robert T. Bakker for their enthusiasm and encouragement. Partial costs of manuscript preparation and specimen curation were provided by the University of Colorado at Denver Dinosaur Trackers Research Group. We also thank John Foster (Museum of Western Colorado) and Spencer G. Lucas (New Mexico Museum of Natural History and Science) for their helpful reviews of this manuscript.

Notes

*Indicates length with metatarsal impression

*Denotes clear claw or digit impressions preserved.