Theropod trackways associated with a Gallimimus foot skeleton from the Nemegt Formation, Mongolia

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Highlights

  • The first report of theropod trackways from the Nemegt Formation in Bügiin Tsav, Mongolia

  • Taphonomic interpretation of a Gallimimus foot skeleton associated with theropod tracks

  • An interpretation of disarticulation and distortion of the foot skeleton is proposed.

Abstract

A theropod tracksite was discovered in the Nemegt Formation (Maastrichtian) at Bügiin Tsav, Mongolia by the Korea-Mongolia International Dinosaur Project in 2009. A total of 67 tracks (14 trackways [one didactylous, 13 tridactylous] and 12 isolated tracks) belonging to four ichnomorphotypes were mapped on a single horizon. This indicates at least four different theropod trackmakers lived in the same area at the same time. This tracksite consists of laminated gray mudstone-yellowish brown siltstone couplets interbedded with eolian yellowish brown sandstone deposited on a distal floodplain. Abundant footprints with V-shaped profiles (cross-section) within in the vertical section indicate that dinosaurs repeatedly walked across in this area. Before the discovery of the tracks, the site was illegally excavated by fossil poachers, a widespread problem in the Gobi Desert. During excavation of the track horizon, a clenched, inclined Gallimimus foot skeleton was found in the mudstone, extended down 20 cm below the track-bearing sandstone layer. The occurrence of tracks closely associated with body fossils is unusual and taphonomically intriguing. It is possible that the foot skeleton represents an animal that died in its tracks. However, the depth of the foot in mud is probably too shallow for the animal to have been mired. Sedimentological and taphonomic evidence also suggests that the pes of Gallimimus may have passed straight through the track-bearing sandstone layer. The inclined right pes indicates that the body lay on its left side on the substrate. During decomposition in the mud, all digits were flexed but the distal phalanges were stuck and anchored in the stiff lower mud. Consequently, as more proximal phalanges were able to accommodate flexing, they were pulled away and dislocated from the anchored distal phalanges. Subsequent trampling by dinosaurs in the track-bearing sandstone would have further distorted the underlying foot.

Introduction

In August 2009, an abandoned quarry marked by excavation pits, plaster, felt, garbage (vodka bottles and broken car parts) and empty glue bottles, along with scattered ornithomimid bone fragments was found along a low hillside in the Nemegt Formation at Bügiin Tsav (43°51.801′N, 99°58.327′E) (Fig. 1A). Up to four pits are separated less than 10 m from each other, and it seems that the illegal excavations removed several skeletons of Gallimimus (Fig. 1B). Like most poached quarries, there were many broken bones scattered around the floor of the quarry. The remains represented ornithomimid vertebrae, gastralia, limb bones (including metatarsals from both sides of the body), and gastroliths. Many of the bones were encased in hard nodules in the mudstone. Subsequently the pits were filled in with wind-blown sand, leaving shallow depressions. Reopening one of the depressions revealed a dinosaur track-bearing sandstone horizon and an articulated Gallimimus foot in a mud layer immediately underlying the sandstone (Fig. 1C). The foot was obliquely positioned in the mud and metatarsals were broken away in mid shaft at the level of the track-bearing horizon, implying the body, if originally present, had been removed by previous collecting activity. About 40 m2 was excavated for mapping the trackways (Fig. 1D, E). This footprint locality is known as FS045, but the site is also referred to as Gall035 because of the presence of articulated bones of Gallimimus.

Although Mongolia has a long history of collecting and abundant vertebrate skeletons from Cretaceous beds, it was long believed that vertebrate trace fossils are relatively rare. Since the first description of footprints (Currie et al., 2003) from the Nemegt site (sensu Gradzínski et al., 1968), many dinosaur footprints have been recognized from other localities. Except for theropod trackways from the Djadokhta Formation in Abdrant Nuru (Ishigaki, 2010), however, few are fully described (Eberth, 2017, this volume; Nakajima et al., 2017, this volume; Stettner et al., 2017, this volume). For example, more than 1400 footprints were found in Bügiin Tsav (Ishigaki et al., 2009), but most of them remain unstudied.

Sixty-seven theropod tracks belonging to four ichnomorphotypes were found on the exposed surface. Thus, this site offers an opportunity to describe in detail for the first time a variety of theropod trackways from Bügiin Tsav, helping to define the contemporary theropod faunal composition. Additionally, the unusual posture of the Gallimimus foot provides taphonomic clues regarding the death and preservation of this animal. The distorted morphology of the foot is significant evidence of post-mortem processes. Because the close association of bones with footprints is very rare in the fossil record, this occurrence is intriguing and requires elucidation regarding the relationships of bones to tracks.

Section snippets

Bügiin Tsav

Bügiin Tsav is located at the northeastern rim of the Ingeni Khobur Basin, northwest of Altan Uul in the South Gobi Aimak, Mongolia (Suzuki and Watabe, 2000, Eberth, 2017, this volume). The Nemegt Formation is widely distributed in this region with extensive outcrops, many of which face to the southwest of a mesa (plateau) or as cliffs, hills, and monadnocks. The lower beds are variegated fluvial sandstone and thick mudstone that have produced a variety of dinosaurs (Barsbold, 1983, Kurochkin

Trackways

The 40 m2 excavated, track-bearing surface is horizontal and continuous on the sandstone but interrupted by mud concretions (Fig. 1D). One of them was opened in search of fossils, but none was found inside. Sixty-seven theropod tracks (55 tracks within 14 trackways plus 12 isolated tracks) were mapped (Supplementary Table S1). The major azimuths of all trackways are ENE and SSE (Fig. 1E).

The footprint depth is well correlated with footprint size (Supplementary Table S1). In order of decreasing

Skeleton

A right foot (MPC-D100F/17) was found lying obliquely inside the mudstone layer below the track-bearing surface (Fig. 3A, B, C, Supplementary Table S2). There is no evidence of tooth marks or anything to indicate predation during the Cretaceous, and the middle shafts of three metatarsals were clearly cut in modern times. Our experience in Mongolia suggests the other parts of the skeleton that would have accompanied such a well preserved foot were excavated illegally and removed before

Miring?

Miring has been proposed as the cause of death of some dinosaurs (Sander, 1992, Varricchio et al., 2008, Eberth et al., 2010). For a bipedal dinosaur to become helplessly mired would depend on viscosity and adhesiveness of mud, the weight and health condition of the animal, and the depth of penetration of the hindlimbs. The shallow footprints on the track layer indicate that the sandstone surface could be stiff to support the weight of walking dinosaurs yet plastic enough to deform (Nadon and

Conclusions

This tracksite indicates that at least four different kinds of theropods were living and traversing the same area at essentially, if not exactly, the same time. If it is true that ichnomorphotype 1 was made by Gallimimus, this track site is concordant with the dominant skeletal record of ornithomimids in the Nemegt Formation. An unusual association of dinosaur bones and footprints from Bügiin Tsav, was interpreted using taphonomic and sedimentologic data. There is probably no direct association

Acknowledgments

Thanks go to all members of Korea-Mongolia International Dinosaur Expedition (KID) in 2009. The KID expedition was supported by a grant to Y.-N. Lee from Hwaseong City, Gyeonggi Province, South Korea. We thank Dr. Matteo Belvedere and one anonymous reviewer for improving an earlier version of this manuscript with their comments, and Prof. T. Algeo (Editor in Chief). This research is supported by the Basic Research in Application and Development of Geological Samples and Geo-technology R&D Policy

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