MARYLAND'S EARLY CRETACEOUS DINOSAUR DIVERSITY AS REVEALED BY A TRACK BONANZA

Until recently dinosaur footprints were virtually unknown from the Cretaceous of the eastern United States. Over the past decade, however, discovery of small dinosaurian trackways and isolated tracks, totaling approximately 300 footprints in iron-rich siliciclastic facies of the Patuxent Formation (Potomac Group) of Aptian age in Maryland, has undoubtedly become one of the most significant Early Cretaceous ichnological bonanzas since the Paluxy track discoveries in Texas in the 1930s.

The Maryland Patuxent dinosaur track discoveries reveal a far greater diversity of dinosaurs than the Texas Paluxy finds, and include ankylosaur, ornithopod, sauropod, and theropod footprints. There appear to be 14 dinosaur footprint morphotypes, and this does not include the discovered tracks of their pterosaur, mammal, and other vertebrate 'neighbors'. This diversity is at least twice the previous maximum estimate for any known Early Cretaceous vertebrate ichnofauna in the Eastern United States. Among the more distinctive forms are diagnostic examples of hypsilophodontid tracks and a surprisingly large mammal footprint.

A remarkable feature of the Patuxent track assemblage is the high proportion of small trackways, suggesting the presence of hatchlings. These tracks sometimes are accompanied by the tracks of a seeming adult of the same type or other tracks of seeming juvenile dinosaurs of the same type. Such evidence suggests proximity to dinosaur nest sites. The preservation (or at least the discovery) of such small tracks is very rare in the Cretaceous track record and indeed throughout most of the Mesozoic.

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