The Late Cretaceous Tucson Mountains Dinosaur

ASU Author/Contributor (non-ASU co-authors, if there are any, appear on document)
Andrew B. Heckert Ph.D., Professor (Creator)
Appalachian State University (ASU )
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Abstract: Historically, the “Tucson Mountains dinosaur” has been considered an Early Cretaceous iguanodont from a megabreccia block of the Amole Arkose in the Tucson Mountains caldera of southern Arizona. We demonstrate here that it is instead a large hadrosaur represented by an incomplete left hindlimb, including an incomplete ilium, proximal and distal femur, a distal tibia, a proximal metatarsal and unidentifiable bone elements. This specimen is diagnostically a hadrosaur because it is very large and has distal femoral condyles greatly expanded caudally and a very deep intercondylar groove on the distal femur. In the American West, hadrosaurs are restricted to strata of Late Cretaceous age, and large hadrosaurs typically indicate a Campanian or Maastrichtian age. The collecting locality of the hadrosaur lies ~550 m NNW of Gates Pass in ground exposing lenticular bodies of intracaldera megabreccia that interfinger complexly with Cat Mountain Tuff, the compound cooling unit of welded ash-flow tuff that forms the fill of the Tucson Mountains caldera. Megabreccia bodies were formed by landslides that slid into the caldera from its walls during eruption, and are blocks of extracaldera rocks encased in partially welded intracaldera tuff. The Cat Mountain Tuff has yielded multiple K-Ar (feldspar) ages of 68-72 Ma, and a single 40Ar/39Ar age (biotite) of 73.1 Ma. Approximately 8 km WNW of the dinosaur locality, the Tuff of Confidence Peak (~73 Ma), which was erupted from the Silver Bell caldera 30 km NW of the Tucson Mountains caldera, is interbedded with upper horizons of the Amole Arkose as exposed just outside the Tucson Mountains caldera. The stratigraphic relationship of the Tuff of Confidence Peak to the Amole Arkose is evidence that the latter includes strata at least as young as Campanian in age, even though older parts of the Amole Arkose are evidently correlative with Lower Cretaceous Bisbee Group. The sandstone matrix of the hadrosaur fossil thus is a block derived from an Upper Cretaceous horizon in the upper Amole Arkose.

Additional Information

Lucas, S.G., Lewis, C., Dickinson, W.R., and Heckert, A.B., (2005) The Late Cretaceous Tucson Mountains Dinosaur. New Mexico Museum of Natural History and Science Bulletin, v. 29, p. 111-113. Archived in NC DOCKS with permission of the editor. The version of record is available at:
Language: English
Date: 2005

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