New Dinosaur Tracksite from the Upper Cretaceous Djadokhta Formation, Central Gobi Desert, Mongolia

Mainbayar, Bu;Ishigaki, S;Tsogtbaatar, Kh;Saneyoshi, M

2nd Asian Palaeontological Congress

東京大学

The Upper Cretaceous strata of Mongolia yield rich dinosaur fauna, but specimens of some clades, such as sauropods, are limited. Despite the long research history in the area, sauropod footprints had not been discovered until the first report from the Nemegt fossil site in 2001. A few previous studies have reported sauropod footprints from the Nemegt locality, Yagaan Khovil and Bugin Tsav, but the sauropod ichnofossils are scarce, and continuous trackways and manus prints had never been reported until 2015. Here we report the recent findings of sauropod footprints found by the Mongolian Institute of Paleontology - Okayama University of Science Joint Expedition (IPG-OUS JE) during 2015-2023. The findings include 31 trackways and abundant isolated tracks of sauropods from 9 fossil sites, all of which are represented by natural casts. These new discoveries significantly fill the gap in the sauropod fossil record of these strata.
1. Baynshiree Formation: 19 sauropod trackways are recorded at Shar Tsav. The most notable finds in this locality include (1) the trackway of medium-sized sauropod with an extremely wide gauge, associated with manus prints, (2) large sauropod trackways with digital prints, (3) very well-preserved pes and manus prints (both are isolated) of small sauropods. We also found a trackway consisting of four consecutive pes-prints in Khavirgiin Dzo in 2017. The best-preserved footprint of this trackway is 106 cm in length and 77 cm in width, which is the largest footprint in Mongolia and also one of the largest in the world. Additionally, three natural casts of pes and one natural cast of manus were recorded in Tsagaan Teg. All materials of this site are isolated.
2. Djaokhta Formation: Six trackways were recorded at Khongil, the Arts Bogd mountain area. One of the notable finds in this area is the discovery of four large sauropod trackways with manus prints associated with sliding marks and skin impressions. The length of the largest pes natural cast is 90cm. We also found two medium-sized pes-only trackways. Although the morphology of the pes prints is the same, they have different gauges. Abundant isolated but very well-preserved natural casts of pes and manus prints were also found at this site. At another locality called Abdrant Nuru, two medium-sized sauropod trackways with a wide gauge were recorded.
3. Nemegt Formation: Several trackways of medium-sized sauropods, one of which is a turning trackway, were recorded in Bugin Tsav. We also found an incomplete trackway of a large sauropod with manus and pes natural casts in Gurilin Tsav. This was the first sauropod trackway with well-preserved manus prints in Mongolia. Additionally, more than 500 isolated natural casts of pes and manus imprints of sauropod were discovered in Yagaan Khovil. Unlike the other sauropod ichnosites, there are abundant manus prints of various sizes and also sliding marks are clearly preserved.
4. Lower Cretaceous (?) bed: two small, a medium and a large-sized sauropod trackways were recorded at Bayan Undur in Bayankhongor, western Gobi Desert. The medium and large-sized trackways are associated with manus prints. The medium-sized one is a turning trackway.
The trackways reported here have large manus prints when preserved (40 to 50 % of pes print in the area) and have no digit I impression, which does not contradict the putative anatomical features on most of the sauropod taxon based on body fossils. The gauge of trackways often varies from narrow to wide, even among the trackways from the same horizon, suggesting that the difference in gauges does not reflect taxonomic differences but rather an intraspecific variation in gait. The estimated body size of the trackmaker from the giant tracks (112 cm in length) significantly exceeds the body size of the previously known sauropod body fossils in Mongolia, but we also discovered small sauropod tracks (40 cm in length). Most of the trackways suggest that the trackmakers were walking alone. The rich sauropod tracks throughout the Cretaceous strata revealed in this study suggest that the abundance of sauropods did not decline in the Late Cretaceous in Mongolia. In particular, the abundance of giant sauropod tracks, as well as recently described from the Djadokhta Formation, indicates that the Djadokhta fauna was not limited to small-sized animals. This evidence also implies the presence of a rich ecosystem capable of sustaining such giants in the Djadokhta age, which is often regarded as a dry environment with limited food resources.