Navahopus coyoteensis, Milàn & Loope & Bromley, 2008

Milàn, Jesper, Loope, David B. & Bromley, Richard G., 2008, Crouching theropod and Navahopus sauropodomorph tracks from the Early Jurassic Navajo Sandstone of USA, Acta Palaeontologica Polonica 53 (2), pp. 197-205 : 198-199

Navahopus coyoteensis isp. nov.

Derivation of name: After type locality.

Holotype: Trackway shown in Fig. 4 ; a cast will be prepared (permit pending) and stored in the collection of Museum of Northern Arizona.

Type locality: Coyotes Buttes locality at the border between Utah and Arizona, USA (36 ° 59'57.6''N, 112 ° 00'34.6''W) ( Figs. 4 , 5 ) .

Type horizon: Early Jurassic Navaho Sandstone ( Fig. 1 ).

Diagnosis.— Navahopus having pedal digit impression IV of equal or greater length than digit III. Pedal hypex III – IV markedly greater than hypeces I– II and II – III.

Description.—An approximately 2.5 m long segment of a trackway from a larger, quadrupedal track maker is found on a steeply−sloping surface representing the lee face of a dune ( Fig. 4 ). As in the case with the theropod resting trace, the present−day slope of the surface is around 25 °, compacted from an original 32 °. The tracks exhibit distinct heteropody. The pes impressions are around 0.2 m long, but the exact lengths of the original tracks are difficult to estimate because of the elongated drag traces from the claws being dragged forward through the substrate, elongating the apparent lengths of the digit impressions. The pes impressions are tetradactyl, and the impressions of digits IV and III are of approximately equal length, followed by the progressively shorter digits II and I. A deep hypex separates the two outer digits. The manus is tridactyl, consisting of two short, forward−oriented, clawed digit impressions and a prominent, inwardly directed impression of a large pollex claw. Like the pes, the manus impressions also bear evidence of the manual claws being dragged forward through the sand as the animal walked.

The trackway pattern changes along the exposed trackway segment. The first part of the trackway shows the orientation of the feet to be angled up the slope, while the direction of the trackway is at an angle to the slope ( Fig. 5 ). Midway through the exposed trackway segment, the trackway changes direction to directly upslope and the orientation of the foot axis corresponds to the axis of the trackway.

Figures

Fig. 4. Trackway of Navahopus coyoteensis isp. nov., as shown by Loope and Rowe (2003) and here reinterpreted as a sauropodomorph dinosaur walking up the lee slope of a dune. Note the elongate traces from the claws being dragged through the sediment. The present day slope is approximately 25°, due to compaction of the sediments. The original angle of slope was around 32° which is the residual angle of slope of dry sand after shearing (Allen 1984). Hammer is 30 cm long.

Fig. 5. Sketch of the trackway in figure 4. The sketch is redrawn from high−resolution digital photographs of the trackway. RM, right manus; LM, left manus; RP, right pes; LP, left pes. The solid arrow indicates the direction of progression and the broken−line arrow the orientation of the body during progression. Notice how the animal walked at an oblique angle upslope in the first half of the trackway, and then changed to progress head on, up the slope.

Fig. 1. Stratigraphic column of the Navajo Sandstone outcrop at Coyote Buttes. The studied tracks are all found at the top of the middle zone of bioturbation. The Coyote Buttes locality is located on Bureau of Land Management property (accessible by permit only) at the border between Utah and Arizona at 36°59'58''N, 112°00'35''W. Ichnofabric index 1–5 is zero to total bioturbation.