FOSSIL FOOTPRINTS FROM THE FORT UNION 

 (PALEOCENE) OF MONTANA 



By Charles W. Gilmore 

 Curator of Vertebrate Paleontology, United States National Museum 



INTRODUCTION 



In 1908 Mr. A. C. Siberling, while collecting mammalian fossils 

 for the United States National Museum in the Fort Union deposits 

 of south-central Montana, also discovered a series of fossil footprints 

 ^n this same formation. 



Apparently these were the first fossil tracks to be discovered in the 

 Paleocene, and as such they appear worthy of record. Recently a 

 series of fossil tracks found in deposits of equivalent age in the 

 Province of Alberta, Canada, have been described by Messrs. Ruther- 

 ford and Russell,^ but these tracks are thought to be mammalian in 

 origin, and although their details are somewhat obscurely preserved 

 their tridactjde nature effectually distinguishes them from the tracks 

 here considered. 



Originally the Montana tracks were preserved as one large slab, 

 but owing to the vicissitudes of transport this specimen was broken 

 into many pieces and the loss of connecting edges made it impossible 

 to reassemble them in their original relationships. The specimen 

 is now in three slabs, as shown in the accompanying plates. 



The tracks are impressed on the slightly undulating surface of 

 a fine grained sandstone that in some instances has preserved the 

 full details of the feet in addition to recording dragging tails, claw 

 scratches, and belly impressions. Due to the breakage of the original 

 slab, only short sections of trackways are now available. The best 

 one at hand, shown in Plate 1, is selected as the type. 



AMMOBATRACHUS MONTANENSIS. new species 

 Plates 1, 2, and 3 



Ti/pe. — Cat. No. 7635, U.S.N.M., parts of trackways on three 

 slabs that originally formed one large slab. Collected by A. C. 

 Silberling, 1908. 



Type locality. — Section 8, range 16, township 5, Bear Butte, Bear 

 Butte Pass, Sweetgrass County, Mont. 



Geological horizon. — " Silberling's Fort Union No. 3." Paleocene. 



1 Rutherford, R. L. and RusseU, L. S., Amer. Journ. Sci., vol. 15, 1928, p. 262. 



No. 2750.— Proceedings U. S. National Museum. Vol. 74 Art. 5 



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