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RE: Horseshoe Canyon dinos

From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of KELL00BELL@aol.com
>     Thanks to Thomas Holtz for his answers.  I'm currently seeking more details and clarification: 
>      Is the fifth (top) unit of the Horseshoe Canyon formation early Late Maastrichtian in age?  
Actually, checking the Gradstein et al. Time Scale, the Lower Maa/Upper Maa boundary is placed between the clinolobatus and aff. nicoletti Ammonite biozones in North America.  This would actually be in the middle of the Paleomag 31r polarity chronozone, which (according to the Eberth et omnia paper cited previously) is within Unit 4 of the Horseshoe Canyon.  Incidentally, according to that paper the lower limit of the Horseshoe Canyon is within 32r, and hence deep inside the late Campanian.
Breaking down the Horseshoe Canyon into its subunits, Unit 1 and Unit 2 are Late Campanian, Unit 3 and the lower part of Unit 4 are Early Maastrichtian, and upper Unit 4 and Unit 5 are early Late Maastrichtian.
> Are centrosaurines,
> lambeosaurs and Saurolophus absent in this top unit?  Are any ornithiscians already absent in the underlying fourth unit? 
> (late-early or mid-Maastrichtian?) 
Preliminary work shows Edmontosaurus, Arrhinoceratops, Anchiceratops, Pachyrhinosaurus, and Edmontonia in Units 1 & 2, and Parksosaurus, Saurolophus, and Hypacrosaurus in Unit 4.  The theropods range throughout.

>      Dr. Eberth mentioned an Albertosaurus bonebed in the upper Horseshoe Canyon.  Are these remains in the top unit or
> the fourth one below it? 
The Barnum Brown Albertosaurus bonebed is in the boundary between Units 4 & 5.
> How well are tyrannosaurs represented in the top unit?
According to Eberth et al., "the vertebrate assemblage is poorly known" in Unit 5.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742      
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796