NO. 3 GRAND CANYON FOSSIL FOOTPRINTS — GILMORE 3 1 



OCTOPODICHNUS DIDACTYLUS, new species 



Plate 10, fig. 2' 



Type. — Catalogue number 11,501, U. S. N. M. Consists of a slab 

 440 mm. long, having a trail traversing the entire length. A small 

 portion of the obverse slab is also present. 



Type locality. — Hermit Trail (500 feet to left of trail going down ) , 

 Hermit Basin, Grand Canyon National Park, Arizona. 



Geological occurrence. — Coconino sandstone (about 150 feet above 

 the base), Permian. 



Description. — The trail here described consists of two lines of im- 

 prints arranged in groups of four, the groups of the two sides alter- 

 nating. These groups are arranged in a row of three regularly spaced 

 tracks with the fourth offset inward and slightly behind the most 

 posterior imprint of the line of three. A line passed through the 

 three tracks has its axis everted 45° to the line of direction of move- 

 ment (see fig. 13), this inclination of course being reversed in the 

 groups of tracks on the opposite side. The direction of movement 

 is clearly indicated by the displaced sand caused by the impact of 

 the heel (see pi. 10). The tracks are subequal in size, the two anterior 

 imprints being bifurcated, with the outer toe or claw slightly longer 

 and more robust than the inner ; the two posterior imprints seem to 

 be unidactyle. The outer toe of the third imprint of each group, 

 enumerated from the front, especially of the right side, has a heavy 

 inward projecting heel. The toes have the same direction as the line 

 of tracks. The stride, if the movement may be so designated, is 

 106 mm. The greatest width of trackway is about 94 mm., space 

 between single imprints usually 21 mm., there being a slight variation ; 

 the fourth or offset impression is about 15 mm. inside the third. The 

 three tracks in line occupy a linear space of 58 to 63 mm. Single 

 tracks have a length of 13 mm., a width of 7 mm. 



Much uncertainty exists as to the nature of the animal that made 

 this trail. Some of the living crustaceans have didactyle extremities 

 and that is the chief reason for the suggestion about to be made that 

 the trail may be the tracks of a member of that group. While there 

 seems to be no living crustacean that would make such a trail, in 

 Permian times there may have been such an animal. The trackway 

 is distinct from all others found at this locality and in all of the 

 hundreds of square feet of sandstone surface examined only one 

 other such trail was discovered. A second poorly preserved specimen 

 (No. 7,846, U. S. N. M.) was collected in this same general locality 

 in 1924, but the preservation was such that its principal characteristics 

 were not recognized at that time. 

 3 



