The first crocodilian trckway from the Upper Cretaeous Bayanshiree Formation of Mongolia.

LEE, Y-N.; LEE, H-J.; KOBAYASHI, Y.; PAULINA CARABAJAL, A.; BARSBOLD, R.; FIORILLO, T.

Berlin

Congreso; Society of Vertebrate Paleontology annual meeting; 2014

Society of Vertebrate Paleontology

Crocodilian tracks are now known from the lower Upper Cretaceous Bayanshiree Formation (Cenomanian-Santonian) at the Shine Us Khudag locality in southeastern Mongolia. The tracks were discovered by a joint expedition from Korea, Japan, and Mongolia in 2013. Nine tracks are preserved as natural casts, forming a trackway with a quadrupdedal gait pattern without tail trails. All tracks are short and wide, and dominated by toe traces without plantar impressions. The average manus and pes width is 5.52 cm and 6.38 cm, respectively. The stride length, pace length, pace angulation, and gait-width of thepes (average 65.7 cm, 52.4 cm, 73.2°, 39.8 cm, respectively) are slightly larger than those of the manus (average 63.2 cm, 48.4cm, 64.4°, 30.6 cm, respectively). Pes tracks are characterized by four deep toe impressions and backward push marks behind them. Manus tracks have shallow toe impressions and well developed long, subparallel scratch marks behind. The number and length of these drag marks are not consistent among manus tracks. Pes tracks are anterior to the manus tracks in manus-pes sets, which is in the opposite order of crocodilian walking.The preferential association of the scratch marks with only the distal digit impressions, and an unusual and irregular trackway pattern support a subaqueous setting for this trackway, suggesting a semi-buoyant mode of propulsion or locomotion in fairly shallow water. The backward-directed push marks of pes tracks may have been made while pushing off the bottom with its hind feet to propel a crocodilian forward. Alternatively, long, subparallel scratch marks behind manus tracks may indicate that the animal´s front feet were dragging while their claws were touching the bottom. These crocodilian "swim tracks" (ichnogenus Hatcherichnus) are consistent with abundant neosuchian crocodylomorphs (Shamosuchusgradilifrons, S. major, S. ulgicus) found in the fluvio-lacustrine environments of the Bayanshiree Formation.