Dinosaur tracks at the Nemegt locality: Paleobiological and paleoenvironmental implications

Accepted Manuscript Dinosaur tracks at the Nemegt locality: Paleobiological and paleoenvironmental implications

Judai Nakajima, Yoshitsugu Kobayashi, Tsogtbaatar Chinzorig, Tomonori Tanaka, Ryuji Takasaki, Khishigjav Tsogtbaatar, Philip J. Currie, Anthony R. Fiorillo PII: DOI: Reference:

S0031-0182(17)30677-6 doi:10.1016/j.palaeo.2017.10.026 PALAEO 8493

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

20 June 2017 23 October 2017 27 October 2017

Please cite this article as: Judai Nakajima, Yoshitsugu Kobayashi, Tsogtbaatar Chinzorig, Tomonori Tanaka, Ryuji Takasaki, Khishigjav Tsogtbaatar, Philip J. Currie, Anthony R. Fiorillo , Dinosaur tracks at the Nemegt locality: Paleobiological and paleoenvironmental implications. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Palaeo(2017), doi:10.1016/j.palaeo.2017.10.026

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Dinosaur tracks at the Nemegt locality: paleobiological and paleoenvironmental implications

Judai Nakajimaa, Yoshitsugu Kobayashib*, Tsogtbaatar Chinzoriga,c, Tomonori Tanakaa,

Department of Natural History and Planetary Sciences, Hokkaido University, Hokkaido,

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Ryuji Takasakia, Khishigjav Tsogtbaatarc, Philip J. Curried, and Anthony R. Fiorilloe.

Japan

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Hokkaido University Museum, Hokkaido University, Hokkaido, Japan.

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Institute of Paleontology and Geology, Academy of Sciences of Mongolia, Ulaanbaatar,

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Mongolia d

Department of Biological Sciences, University of Alberta, Alberta, Canada

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Perot Museum of Nature and Science, Texas, USA

*Corresponding author,

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[email protected]

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1. Introduction The Nemegt locality, located in the southern Gobi of Mongolia (Fig. 1A), has been known as one of the richest and the most diverse dinosaur localities in Mongolia

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for 70 years (the Upper Nemegt Beds sensu Gradziński et al., 1969; Jerzykiewicz and Russell, 1991; Eberth, 2017, this volume; Fanti et al., 2017, this volume). However, the

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majority of dinosaur studies of the Nemegt locality are focused on skeletal elements

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(Rozhdestvenskiy, 1952; Maleev, 1955; Kurochkin and Barsbold, 2000) despite the fact that several studies have reported dinosaur track sites from the Nemegt Formation at

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Bugiin Tsav, Hermiin Tsav, and Guriliin Tsav (e.g. Ishigaki et al., 2009; Eberth, 2017,

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this volume; Lee et al., 2017, this volume). Gradziński (1970) mentioned common occurrences of deformational structures in the Nemegt Formation (the Upper Nemegt

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Beds sensu Gradziński, 1970) at the Nemegt locality, which were probably dinosaur

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ichnites (Currie et al., 2003). Currie et al. (2003) identified numerous dinosaur footprints for the first time and described two theropod, two sauropod, and four hadrosaur footprints from three horizons. Since then, however, no comprehensive work

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has been published on Nemegt dinosaur footprints.

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Dinosaur footprints at the Nemegt locality were re-examined during the Nemegt Educational Expedition 2016 (NEE 2016). As Currie et al. (2003) reported, numerous dinosaur footprints were confirmed throughout the Baruungoyot and Nemegt formations. Instead of going over all of the footprints available, this study focused on a single footprint-bearing horizon in order to reveal dinosaur assemblages and behaviors within a short time interval. This study aims to report on newly discovered dinosaur footprints at the bottom of the upper Nemegt tongue of the Nemegt Formation at the Nemegt

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locality, to reconstruct the ichno-taxonomic composition of the area based on the footprints, and to compare it with the taxonomic composition of skeletal elements. Furthermore, population structure and size distributions of the Nemegt hadrosaurs are inferred based on the footprints, and are compared with comparable datasets described

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from elsewhere in the northern hemisphere (Vila et al., 2013; Fiorillo et al., 2014) .

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2. Geological setting

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At the Nemegt locality, the Baruungoyot (upper Campanian) and Nemegt (upper Campanian to lower Maastrichtian) formations are widely exposed in three

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major canyons, which are called Central, Northern, and Western Sayrs (Gradziński et al.,

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1977; Jerzykiewicz and Russell, 1991; Fanti et al., 2017, this volume). The early work by Gradziński and Jerzykiewicz (1974) reported that the Baruungoyot Formation

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consisted of inter-dune deposits with the intermittent lacustrine and fluvial deposits

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interpreted as a semi-arid environment with significant rainfall. This interpretation is supported by a later study along the Central Sayr (Fig. 1B) (Eberth et al., 2009) that shows the formation is composed of a stacked succession of tabular redbeds, consisting

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of alluvial, lacustrine, and eolian deposits. Fanti et al. (2012) examined the exposures of

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the formation in the Northern Sayr (Fig. 1B) and found arid or semi-arid eolian deposits -- characterized by abundant extraformational clasts, manganese oxides, and well-developed caliches -- with no indication of alluvial or lacustrine deposits. Eberth (2017) in this volume refined the sedimentological descriptions and interpretations of Eberth et al. (2009) at the Nemegt locality and suggested that the upper Baruungoyot Formation (paleoenvironemental Zone 3), the lower Nemege Formation (Zone 5), and the transitional zone of these formations (Zone 4) are exposed at the locality. The upper

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Baruungoyot Formation is posed of deposits at the distal margins of alluvial fans (fluvial, lacustrine, paludal, and eolian deposits), which indicate arid to semi-arid and seasonally dry paleoenvironments (Eberth, 2017, this volume). The Nemegt Formation conformably overlies the Baruungoyot Formation and is composed of a stacked

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succession of light grey alluvial deposits (channel, crevasse splay, and floodplain),

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indicating dominant fluvial environments (Gradziński et al., 1977; Eberth et al., 2009;

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Fanti et al., 2012; Eberth, 2017, this volume). This formation has been interpreted by all authors as a more humid environment than the Baruungoyot Formation. The lower

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Nemegt Formation is mainly composed of small channel and sheet flood deposits, which indicate seasonally wet-dry paleoenvironment (Eberth, 2017, this volume).

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Gradziński and Jerzykiewicz (1974) suggested that the transition between the Baruungoyot and Nemegt formations is gradual. Subsequent study based on four

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stratigraphic sections in the Central Sayr (Figs 1B, 2) further revealed that these

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formations are interfingered with two tongues at the boundary (Eberth et al., 2009). This interfingering interval (“gradual sedimentary passage series” sensu Gradziński, 1970) is

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at least 23 m thick and produces abundant vertebrate fossils, such as birds, crocodiles, and dinosaurs (Fanti et al., 2017, this volume). The Nemegt tongues are pinched out

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southeastward, and the upper tongue is thicker than the lower tongue (Fig. 2). The number of tongues may vary laterally because only one tongue is recognized in the Northern Sayr (Fanti et al., 2012; Fanti et al., 2017, this volume). The interfingering interval of the two formations indicates coexistence of eolian and predominantly fluvial environments at the formational transition, as a result of a gradual spreading of a drainage system and increasing wetness in the area (Gradziński and Jerzykiewicz, 1974; Eberth et al., 2009; Eberth, 2017, this volume).

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Currie et al. (2003) mentioned that the footprints are recovered from at least three horizons within the Nemegt Formation at the Nemegt locality. Based on stratigraphic sections by Eberth et al. (2009), eight horizons within the interfingering interval are known to yield dinosaur footprints. Most of the footprint-bearing horizons

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are located at the bottom of medium to coarse sandstone layers (including rounded

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pebbles, load casts, and rip-up clasts) that overly silty to muddy layers. Four horizons

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are within the Baruungoyot Formation below the lower Nemegt tongue, and one horizon is located at the base of the upper Nemegt tongue. The rest of the horizons are located

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within the main body of the Nemegt Formation.

A geological section (NEE 2016 section; Fig. 3A) containing the

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footprint-bearing horizon at the base of the upper Nemegt tongue was measured near the south end of the Central Sayr at the Nemegt locality (Fig. 1B). This footprint-bearing

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horizon is one of the richest in dinosaur footprints at the locality, and all dinosaur

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footprints used in this study are from this horizon. The NEE 2016 section is composed of more than 3 m of Baruungoyot facies and approximately 2 m of Nemegt facies

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unconformably covered with alluvium. In the section, the Baruungoyot facies are mainly composed of poorly-sorted, sub-angular, red sandstone. The lower part (2.4 m

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thick) of the Baruungoyot facies shows extraformational clasts, syntopic caliche clasts, parallel laminae, and soft sedimentary deformations. Invertebrate trace fossils are abundant in multiple horizons of fine- and medium-grained sandstones of the Baruungoyot facies, but not in the footprint-bearing horizon (Fig. 3). The trace fossils in the fine-grained sandstone are dominated by linear sand pipes that are each approximately 1.0 cm in diameter, which invaginate perpendicular to the bedding plane. These are similar to Type III traces in the Djadokhta Formation in Mongolia, suggesting

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these are potentially traces of arthropods such as sand wasps, wolf spiders, and tiger beetles (Ahlbrandt et al., 1978; Fastovsky et al., 1997). The upper part of the Baruungoyot facies (approximately 0.6 m thick) is composed of fine sandstone and brown-red silt to mudstone with numerous organic inclusions and syntopic caliche

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clasts. This Baruungoyot facies corresponds to “Big Red” in the Avimimus Quarry

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section (Eberth et al., 2009) based on the lithofacies (e.g. feeding traces, shale/clay

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laminae, and soft sediment deformations). This suggests that the footprint-bearing horizon of the NEE 2016 section is stratigraphically at the base of the upper Nemegt

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tongue (Fig. 2).

The contact with the Nemegt facies is erosive and is composed of medium to

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very coarse sand, rounded pebbles, and reworked-caliche conglomerates. Soft sediment deformations are common in the lower part of the Nemegt facies. All of the dinosaur

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footprints, which are the subject of this study, are from this horizon (Figs. 2, 3;

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Footprint horizon). The upper part of the Nemegt facies consists of a stacked succession of grey colored, coarse to medium sandstones with inclined bedding and trough

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cross-bedding, which appear as the fining-upward succession (approximately 1.5 m thick). These sediments are interpreted as the channel deposits in a meandering river

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system. The fine sandstones with parallel laminae overlie the channel deposits, and are interpreted as point bar deposits. The footprint-bearing horizon is composed of CaCO3 sandy sheets containing reworked-caliche conglomerates and well-rounded pebbles. The horizon occurs approximately 8 m below the Avimimus sp. bonebed (Fig. 3; Eberth et al., 2009; Funston et al., 2016; Funston et al., 2017, this volume). Most of the natural casts of footprints are massive and lack bioturbation (Fig. 4), indicating that the dinosaur tracks were buried rapidly by coarser sediments. The footprints are filled with

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sediments that contain abundant reworked-caliche conglomerate. It is assumed that these footprints are remarkably well-preserved due to post-depositional CaCO3 cementation (Eberth et al., 2009; Eberth, 2017, this volume).

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3. Materials and methods Thirty-eight newly discovered dinosaur footprints (FS-58 – FS 95; Table 1)

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from a single horizon, extending 3.25 km along the Central Sayr of the Nemegt locality,

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are utilized in this study. The footprints were named following the FS numeration system utilized for the footprints in the Nemegt basin (Currie et al., 2003; Stettner et al.,

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2017, this volume) GPS coordinates of all of the dinosaur footprint sites known at the

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Nemegt locality are recorded in the field in degrees, minutes and seconds (Table 2) and plotted on satellite imagery from Google Maps (Fig. 1B). Taxonomic identifications of

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the trackmakers were conducted based on comparisons with previously known dinosaur

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footprints from the literature. Linear measurements were obtained from all of the 38 footprints, following Currie et al. (2003) (Fig. 5). Photogrammetric models were constructed using Agisoft PhotoScan Professional 1.2.6 for FS-68 and FS-85 using 66

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and 50 images, respectively. Photographs used for 3D image reconstructions are taken

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by a Nikon COOLPIX P310 camera (resolution 4608 x 3456 and pixel size 0.078125) and a Nikon D3100 camera (resolution 4608 x 3072 and pixel size 0.084635). Focal Length varies in each photograph. Contour maps were created using Kasimir 3D ver.8.9.8. Ichno-taxonomic and taxonomic compositions of footprints and skeletal elements of the Baruungoyot and Nemegt formations are based on the results of NEE 2016. 4. Result

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4.1 Taxonomic identification Most of the newly found footprints are infillings (natural casts). Twenty of the 38 dinosaur footprints discovered are identified as hadrosaur (11), sauropod (4), and

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theropod (5) footprints. The trackmakers of eighteen footprints could not be identified

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due to poor preservation or absence of unambiguous taxonomic characters.

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4.2 Description

Footprints (n = 38) ranged from 13 cm to 109 cm in length with an average of

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56.8 cm. All except two of the footprints (FS-59 and FS-62) are longer than 30 cm. Currie et al. (2003) did not report on any footprints smaller than 30 cm in length. The

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two small footprints (FS-59 and FS-62) found for this study are poorly preserved and only partially exposed; therefore, it is not clear whether these are footprints of a small

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sized dinosaur or manus tracks of a medium to large sized dinosaur. Footprint sizes are distributed bimodally (Fig. 6) based on measurements (Table 1).

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4.2.1 Hadrosaur Footprints

Dinosauria Owen, 1841 Ornithopoda Marsh, 1881

Ichnofamily Iguanodontipodidae Vialov, 1988 Hadrosauropodus Lockley et al., 2003

4.2.1.1 Locality. Nemegt locality, Omnogov Province, Mongolia. 4.2.1.2 Description. The Nemegt hadrosaur footprints are ranged from 33 cm to 72 cm in length, with an average of 51.3 cm. The footprint lengths are bimodally distributed, suggesting the presence of two groups with different sizes: six belong to medium sized

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(30 cm to 59 cm) and five belong to large (more than 60 cm) individuals. The best preserved specimens of large (FS-82) and medium (FS-91) footprints (Figs 7A, B) are described in this paper. FS-82 is 60 cm long and 67 cm wide, and has three digits with hoof-like claw

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impressions (Fig. 7A). Digit III is the most robust toe. It has a bluntly U-shaped tip,

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whereas digits II and IV have broad but tapering claw impressions. Each digit has digital phalangeal pad traces but lacks skin impressions. The posterior part of the

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footprint is well-preserved and has an asymmetrical bilobed “heel” pad (Fig. 7A). The

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lateral lobe of the bilobed heel of a hadrosaur footprint from the left side (MPD 100F/11) described by Currie et al. (2003) extends more posteriorly than the medial

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lobe. This suggests that FS-82 is a right footprint. Width of this heel is much wider than the width of the proximal part of the digit III impression. Digit IV is longer than digit II

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(Table 1). The divarication between digits II and IV is 74°, which is larger than the

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criterion proposed by Schulp et al. (2008), but similar to the angles of Nemegt hadrosaur footprints described by Currie et al. (2003).

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FS-91 (Fig. 7B) is approximately equal in length (36 cm) and width (34 cm), and is similar in size to the smallest hadrosaur footprint described by Currie et al.

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(2003). Although the distinction between the left and right sides of this footprint is not clear because of poor preservation of its heel, the divarication between digits II and IV is 62° (Table 1). This is approximately 10° less than the divarications of FS-82 and MPD 100F/11 (Currie et al., 2003). Digit III is robust and has the widest basal width among the three digits. The lengths of digits II and III are significantly different (20 and 31 cm) in contrast with the large footprint (FS-82). The hoof-like claw impression of digit III is wider and more blunt than those of the other digits, which are narrower and

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more pointed, which is similar to the large hadrosaur footprints (FS-82 and MPD 100F/11). The footprint is deeper at the anterior end compared to the posterior end (Table 1). 4.2.1.3 Remarks. Schulp et al. (2008) listed the following criteria for identifying

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ornithopod footprints: sub-equal digits in length and width, digits with bluntly rounded

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tips, third digit with a width/length ratio of 0.5, lack of pronounced digital curvature,

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absence of a hallux impression, a divarication of 65° between digits II and IV, and a smooth and convex posterior margin. Fiorillo et al. (2014) identified a footprint as

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hadrosaur if it was wider than long, tridactyl with bluntly terminated toes, and had a wide, bilobed heel. The bilobed heel is a key feature for its further taxonomic

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assessment. This feature is known only in Hadropodus, represented by three ichnospecies from Canada, Korea, Mexico, and USA (Lockley, 1987; Lockley et al.,

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2003; Lim et al., 2012; Diaz-Martinez et al., 2015), suggesting that the Nemegt

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hadrosaur footprints should be assigned to Hadropodus. Morphological differences in pes among these three ichnospecies are width of heel and digit III and position of

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proximal pads of digits II and IV (Diaz-Martinez et al., 2015). Proximal and heel pad of the Nemegt hadrosaur footprints are not clear but the heel is wider than the proximal

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part of digit III, suggesting its affinity with Hadropodus langstoni (Lockley et al., 2003). Regardless of its ichnotaxonomic designation, the dominant (and possibly only) hadrosaur known from the Nemegt Formation on the basis of skeletal material is Saurolophus angustirostris, which would have been the generator of most or all of the Hadropodus ichnites. 4.2.2 Sauropod Footprints

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Sauropoda Marsh, 1878 Ichnofamily indet. 4.2.2.1 Locality. Nemegt locality, Omnogov Province, Mongolia. 4.2.2.2 Description. FS-80 is the best preserved of the four sauropod footprints and has

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four claw and one digit impressions (I, II, III, IV, and V) (Figs 7C to F). It is an imprint

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of a right foot with a well-defined outline. It is longer than wide and has a subtriangular

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outline with the approximate length/width ratio of 1.2. This footprint is 49 cm wide and is smaller than previously reported sauropod footprints at the Nemegt locality (Currie et

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al., 2003, this volume; Stettner et al., 2017). The claw impressions of digits I and II are well-preserved and have pointed tips. The tips of digits III and IV are indistinct because

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of erosion. The claw impressions of digits I-IV are anterolaterally curved, as seen in the sauropod footprints reported by Currie et al. (2003), and is even larger than the footprint

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reported by Stettner et al. (this volume). The impression of digit V is a bulbous

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protrusion that is directed laterally; these are key characters of sauropod pedal footprints (Castanera et al., 2016). Oval interphalangeal pads are preserved behind the claw

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impressions of digits II and III (Fig. 7C) and are 19.2 cm and 16.2 cm in diameter, respectively. An interphalangeal pad behind digit I , reported by Currie et al. (2003), is

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absent. No skin or scale impressions are preserved. The “heel” is triangular and rounded in shape.

FS-81 preserves both manual and pedal tracks (Figs 7E, F) although the digital impressions are missing in both. The manual track is 35.4 cm long and 32.2 cm wide. It has a U-like kidney-shape with two posteriorly positioned protrusions, which is a general feature of manual tracks of sauropods (Vecchia, 1999; Castanera et al., 2016). The pedal footprint, on the other hand, is a huge and cylindrical track and is 100 cm

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long and 76 cm wide. This sauropod pedal footprint is approximately twice as long as the sauropod footprints previously reported by Currie et al. (2003), and is even larger than the footprint reported by Stettner et al. (2017, this volume). The margin of the footprint is nearly vertical, and several slide marks perpendicular to the bedding plane

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are present in both the manual and pedal tracks.

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FS-64 and FS-69 are also identified as sauropod footprints, but these are poorly

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preserved. Both footprints have vertical margins with several slide marks as seen in FS-81.

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4.2.2.3 Remarks. Sauropod pes tracks are similar in general shape throughout Mesozic (Castanera et al., 2016). Despite of their conservativeness in morphology, there are a

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few key features for their taxonomic assessment: laterally oriented pes digits and a lateral notch behind digit V, which are present in sauropod tracks in this study. Pes

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digits in basal sauropods are anteriorly orientated, but become laterally oriented near the

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origin of neosauropods. A lateral notch behind digit V is common in sauropod tracks such as Brontopodus birdi from the Glen Rose Formation (late Aptian to early Albian)

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in Texas, associated with titanosauriform body fossils (e.g., Paluxysaurus from the Glen Rose Formation) (Farlow et al., 1989; Rose, 2004; Castanera et al., 2016), suggesting

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that the Nemegt sauropod footprints were probably produced by this group. Sauropod pes tracks from the Maastrichtian Fumanya Formation in Spain are similar in shape (anterolaterally directed digits) to the Nemegt footprints but lack this lateral notch (Vila et al., 2008). Regardless of their ichnotaxonomic designation, there are a maximum of two sauropod taxa known from the Nemegt Formation (Currie et al., this volume), and both are titanosauriforms.

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4.2.3 Theropod Footprints Theropoda Marsh, 1881 Coelurosauria Gauthier, 1986 Tyrannosauridae Osborn, 1905

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Tyrannosauripodidae indet.

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Ichnofamily Tyrannosauripodidae McCrea et al. 2014

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4.2.3.1 Locality. Nemegt locality, Omnogov Province, Mongolia.

4.2.3.2 Description. Five theropod footprints are tridactyl, although the impressions of

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either digits II or IV are missing in each. Three of them (FS-60, FS-72, and FS-89) are larger than the theropod footprint that was previously reported from the Nemegt locality

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( MPD 100F/12; Currie et al., 2003) (Table 1). FS-88 is approximately half the length of MPD 100F/12 (Currie et al., 2003). FS-94 is an incomplete footprint.

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FS-60 preserves two digital impressions (Fig. 7G). The length of digit III (70

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cm) is longer than that of the other digit. The basal width of digit III is 19.8 cm, which is wider than that of digit II (or IV). The free part of digit III is 19.5 cm long, but its tip

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is missing. No claw impression or pedal interphalangeal pad is present due to its poor preservation. The divarication between digits III and II (or IV) is approximately 25°,

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which is slightly larger than MPD 100F/12 (20°) (Currie et al., 2003). Its heel is rounded.

FS-72 is a large, 70 cm long theropod footprint (Figs 7H, I). It has slender toes with sharp claw impressions. The basal width of digit III is 25.3 cm, which is wider than that of the other preserved digit (II or IV). The divarication between digits III and II (or IV) is 31°, which is larger than that of FS-60. Its surface is severely damaged and no interdigital pads or skin impressions are preserved.

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FS-88 is a small footprint with slender toes. The basal width of digit III is 13.6 cm, which is wider than that of the other digit (II or IV) that is 8.9 cm wide. Although the divarication (40°) between digits III and the other digit (II or IV) is wider than that of any of the other theropod tracks (FS-65, FS-72, and MPD 100F/12), the basal width

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of digit III is remarkably slender. In digit III, the claw impression is not preserved due

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to poor preservation, but the tip of the other digit tapers to a point.

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4.2.3.3 Remarks. McCrea et al. (2014) coined and diagnosed the ichnofamily Tyrannosauripodidae, which includes Tyrannosauripus pillmorei from the Raton

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Formation (Maastrichtian) of New Mexico, USA (Lockley and Hunt, 1994) and Bellatoripes fredlundi from the Wapiti Formation (Campanian-Maastrichtian) in British

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Colombia, Canada. Theropod footprints described in this study are not well-preserved but match with the diagnoses of the ichnofamily such as large functionally tridactyl,

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mesaxonic tracks with distal metatarsal pad impressions, footprint lengths greater than

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widths, and robust thick digits. The direction of a hallux and present/absence of digital pad impressions are the main differences between Bellatoripes fredlundi and

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Tyrannosauripus pillmorei (McCrea et al., 2014), but neither character is clear in any of the specimens in this study and the ones described by Currie et al. (2003). At this point,

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additional specimens are needed for further ichnotaxonomic identification. Regardless of the parataxonomic designation, only two tyrannosaurids (Alioramus, Tarbosaurus) are known from the Nemegt Formation, and at least the larger theropod ichnites would have been generated by them. 4.2.4 Indeterminate footprints Eighteen additional dinosaur footprints were studied. These are all tridactylous,

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which suggests that ankylosaurids, therizionsaurids and sauropods can be excluded as the potential trackmakers. However, they could not be identified as either hadrosaurs or theropods because of poor preservation, or because they had mixtures of characters of hadrosaur and theropod footprints (e.g., FS-68 and FS-85).

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FS-68 is longer than wide (Table 1), has a slender digit III with a sharp claw

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impression (Figs 8A, B), and an acute divarication between digits II and IV (51°) as in

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theropods (Thulborn, 1990; Currie et al., 2003). On the other hand, the outer digit is blunt and robust, unlike the slender toes of theropods (Thulborn, 1990). Because FS-68

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has several morphological conflicts, it is difficult to determine the trackmaker based on morphological information. Similarly, FS-85 has sharp claw impressions at the ends of

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slender digits II and IV, has fleshy interphalangeal pads (Figs 8C, D), and is remarkably longer than wide (Table. 1). At the same time, digit III has a robust, U-shaped tip (Fig.

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8D) and has a wide divarication between digits II and IV (Table 1) as in hadrosaur

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footprints. 5. Discussion

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5.1 Ichno-taxonomic compositions

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The ichno-taxonomic composition at the base of the upper Nemegt tongue is

composed of three major clades of dinosaurs (hadrosaurs, sauropods, and theropods). It is similar to the taxonomic composition of the Nemegt Formation although no bird footprint has been recorded yet (Figs 9A, B) and is different from that of the Baruungoyot Formation in the absence of ankylosaurs/birds and the presence of hadrosaurs (Fig. 9C). The abundant occurrence of hadrosaur footprints from the horizon and the distribution of skeletal elements from the Nemegt Formation together suggest a

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preference of hadrosaurs for wet environments (Butler and Barrett, 2008; Lyson and Longrich, 2011). A histogram of hadrosaur footprint lengths shows two peaks at the intervals of 30-39 cm and 60-69 cm, suggesting that two different ontogenetic stages are

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represented at this horizon (Fig. 6). The peak of the large footprints represents

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hadrosaurs with body lengths of 8.4–9.8 m (foot length × 4 × 3.5; Alexander, 1976;

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Lockley et al., 1994). The largest hadrosaur footprint is 72 cm and its estimated body length is 10 m. The body length of Saurolophus angustirostris, the most common and

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possibly only hadrosaur taxon from the Nemegt Formation, is estimated to reach 11 m (MPC-D 100/706). The other hadrosaurid taxon from the formation, Barsboldia

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sicinskii, is likely to be even larger because the estimated length of its tibia (140 cm; Prieto-Marquez, 2008; Prieto-Márquez, 2011) is longer than that of S. angustirostris

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(100 cm, MPC-D 100/706). However, it is also possible that the only know specimen of

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B. sicinskii is in fact a relatively large specimen of S. angustirostris based on allometric projections of the neural spine height of the latter. Therefore, the estimated body lengths

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of the trackmakers indicate that the trackmaker may have been S. angustirostris as suggested by Currie et al. (2003). The absence of really large footprints suggests that

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hadrosaurs of the size of the holotype of B. sicinskii were rare. The smallest, well-preserved sauropod footprint recorded in this study matches titanosaur footprints in terms of its morphology (subtriangular outline with length/width ratio of 1.2, anterolaterally angled pedal unguals I – IV, laterally angled bulbous digit V, (Castanera et al., 2016) and resembles the Opisthocoelicaudia skarzynskii. footprint reported in Currie et al. (2003). The sizes of the other three sauropod footprints are between 95 and 109 cm and are approximately twice as large as the minimum pedal

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width of the type specimen of O. skarzynskii

(46cm, Currie et al., 2003). Because the

type specimen is supposed to be a fully-adult individual (Borsuk-Bialynicka, 1977), the large footprints recorded in this study may not represent O. skarzynskii. Although there is another sauropod taxon known from the Nemegt Formation (Nemegtosaurus

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mongoliensis), several papers (Currie et al., 2003, this volume; Wilson, 2005; Currie et

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al., 2017) suggest the possibility that they might be a single taxon because there is no

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overlap of elements between N. mongoliensis (only skull preserved) and O. skarzynskii (no skull). Therefore, the three large sauropod footprints recorded in this study suggest

larger type of sauropod in the formation.

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either that O. skarzynskii grew much larger than the holotype, or that there is another,

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The smallest theropod footprint recorded in this study (30 cm long) is estimated to have had a body length of 5.9 m (foot length1.14 × 3.06 × 4; Thulborn, 1990; Weems,

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2006). Based on the equation for large theropods (foot length0.85 × 8.60 × 2 + 3.5;

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Thulborn, 1990; Weems, 2006), the three tridactyl theropod footprints longer than 70 cm are estimated have had body lengths longer than 9.8 m. The equation for

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tyrannosaurids suggests the hip height is larger than 3.1 m (29.8 × foot length 0.711; McCrea et al., 2014). Although the Nemegt Formation is rich in theropod dinosaurs of

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various sizes, troodontids and dromaesaurids are unlikely to be the trackmakers of these trydactylous footprints because their feet were functionally didactylous (Kim et al., 2008; Li et al., 2008). Similarly, Therizinosaurus cheloniformis can be excluded because therizinosaurs have four-toed feet (Perle, 1981) and would have made four-toed footprints (Fiorillo and Adams, 2012). Oviraptorosaurs known in the Nemegt Formation (Avimimus portentosus., Conchoraptor gracilis, Elmisaurus rarus, Nemegtomaia barsboldi, Nomingia gobiensis, and Rinchenia mongoliensis) are also unlikely to be the

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trackmakers of the recorded footprints because their body lengths (less than 2.0 m; Paul, 2010) are less than half of the estimated body lengths of the trackmakers. Therefore, medium sized theropods such as Alioramus altai, Alioramus remotus (5 - 6 m; Kurzanov, 1976), and Gallimimus bullatus (6 m; Paul, 2010) or an immature large

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theropods such as Tarbosaurus bataar could all be the trackmaker of the 30 cm long

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theropod footprint. The other four large theropod footprints are likely to belong to either

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Deinocheirus mirificus or T. bataar because these two taxa are the only known theropods in the Nemegt Formation that make tridactyl footprints that exceed 9.8 m in

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body length. However, D. mirificus had broad, hoof-like unguals and is unlikely to have

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made the tracks that have sharply tapering digital impressions. 5.2 Comparison between ichno-taxonomic and taxonomic compositions

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Recent discoveries of dinosaur tracksites point out that dinosaur footprints are

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more consistently abundant than dinosaur skeletal elements (Lockley and Hunt, 1994; Lockley, 1997). However, the occurrence of abundant dinosaur footprints in most sites

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that are rich in skeletal elements is still rare (Currie et al., 2003; Ishigaki et al., 2009). Rich occurrences of dinosaur footprints and skeletal elements in the Nemegt Formation

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allow comparison between ichno-taxonomic and taxonomic compositions of the dinosaur assemblage living in Nemegt times. Ichno-taxonomic composition suggests that hadrosaur footprints are the most abundant and that two other groups have smaller ratios that are similar to each other (approximately 20% each). The taxonomic composition of the Nemegt Formation based on bony elements shows that the taxa are the same but are represented by different proportions (Figs 9A, B). While the ichnofaunas are dominated by hadrosaur footprints

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(57.2%), hadrosaur bones compose only 9.8%. Similarly, whereas sauropod footprints constitute 19.0% of the ichnofauna, sauropod bones comprise only 2.6% of the collected bones. Theropods make up 87% of the bones collected, but only 23.8% of the recorded footprints. Furthermore, there is a conflict between ichno-taxonomic and taxonomic

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compositions of large and small sized theropods. While 80% of theropod footprints are

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large, skeletal elements of large theropods comprise only 11.9 % of all of the collected

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bones. Instead, small theropods -- such as dromaeosaurs, ornithomimosaurs, oviraptorosaurs, and several unidentified small taxa -- are far more commonly recovered

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bones in comparison with large theropods.

Currie et al. (2003) suggested that small footprints are absent at the Nemegt

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locality because water covering over the soft sediments prevented small dinosaurs from leaving their footprints. Even if small animals left some footprints, it is known that there

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is bias against preservation of smaller footprints (Lockley, 1997). Furthermore, the

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vertical exposure of the footprint horizon also makes it more difficult for small footprints to be exposed and discovered in comparison with large footprints. These

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problems regarding preservation of small footprints might explain the scarcity of small theropod footprints compared to theropod skeletal elements.

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On the other hand, the ichno-taxonomic structure based on large footprints is likely to have fewer preservational issues. The large proportion of hadrosaur and sauropod footprints agree with the herbivore-dominant trend present in other dinosaur tracksites (e.g. Lockley, 1986; Lockley and Hunt, 1995; Paik et al., 2001; Xing et al., 2015). Therefore, although disproportionately high numbers of Tarbosaurus partial and complete skeletons have been collected from the Nemegt Formation (Lee et al., 2014;

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Currie, 2016), the ichno-taxonomic structure suggests the possibility that the area was dominated by herbivorous dinosaurs, as would be expected in a normal ecosystem. 5.3 Population structure of Nemegt hadrosaurs

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Hadrosaur footprints are common in the Upper Cretaceous deposits of other

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continents, and herding behavior appears to have been common for hadrosaurs (Currie, 1983; Lockley et al., 1983) based on evidence from bonebeds and footprints Fiorillo et

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al. (2014). Previously, Currie et al. (2003) noted the richness of hadrosaur footprints at

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the Nemegt locality by reporting ten hadrosaur footprints. The richness of hadrosaur footprints was confirmed in this study based on high footprint density and at least five

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hadrosaur footprints were located in an area of 2.8 m (Fig. 1B, Fig. 10). Because the richness of hadrosaur footprints was confirmed from a single

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horizon, the population structure of the Nemegt hadrosaurs can be interpreted based on

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size. The population structure of the Nemegt hadrosaur is compared with that of the Maastrichtian Cantwell Formation in the Denali National Park and Preserve, Alaska

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(Fiorillo et al., 2014) to test if the difference in paleolatitudes (at least 65° N and higher in Denali National Park and Preserve and 40.8° N in the Nemegt locality, Central Sayr;

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Tomsich et al. (2010) and the Paleobiology Database, www.paleobiodb.org) affects hadrosaur population structure. In addition, it is also compared with the hadrosaur population structure in the Tremp Formation, which is reconstructed from multiple localities of the formation in the southern Pyrenees, Spain (Llompart, 1979; Barco et al., 2001; Llompart, 2006; Vila et al., 2013), in order to test the influence of insular environments on hadrosaur composition.

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Unfortunately, accessibility (the horizon located in the middle of a vertical cliff) and preservation of footprints allowed only eleven hadrosaur footprints to be recorded from the single horizon in this study. Among the eleven hadrosaur footprints, a cluster analysis on footprint lengths and widths was undertaken. The analysis grouped the

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eleven footprints into two clusters, whose average lengths and widths are statistically

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different from each other (p < 0.001; t Test). The group of smaller footprints is 33 cm to

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46 cm long and the group of larger footprints is 50 cm to 72 cm long. These ranges overlap stages 3 and 4 hadrosaur footprints from the Cantwell Formation (Fiorillo et al.,

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2014) (Fig. 11A), although the maximum size of the Nemegt hadrosaur footprints slightly exceeds that of the Cantwell hadrosaur footprints. The size of these Alaskan

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hadrosaur tracks is corroborated by a new study of hadrosaur tracks from the correlative Prince Creek Formation of Alaska, a rock unit deposited even farther north than the

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lower Cantwell Formation (Flaig et al., 2017). This new study includes larger tracks

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than those from the lower Cantwell Formation, and these larger tracks are attributed to overstepping of footprints by hadrosaurs (i.e., one footprint overlain by a second track).

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However, given that these tracks from the Prince Creek Formation are preserved only in cross-section, some of the smaller tracks of Flaig and others’ Cluster C (2017) may in

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fact represent single tracks. If so, some of the tracks attributed to Flaig and others’ Cluster C (2017) are of the same size as the large tracks found in the Nemegt Formation. The presence of two groups of different size classes is likely to represent two different ontogenetic stages, suggesting multigenerational population structure of the Nemegt hadrosaurs. These two groups seem to correspond to stages 3 and 4 of the Cantwell hadrosaur, and form 84% of the population. The dominance of adult sizes and

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presence of multigenerations are similar to the Cantwell Formation (Fig. 11B). The Nemegt hadrosaur, on the other hand, lacks equivalents of stages 1 and 2 of the Cantwell hadrosaur, although the small sample size and preservational bias may explain their absence.

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The hadrosaur footprints of the Tremp Formation show a different population

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structure from the Nemegt and Cantwell hadrosaurs. A cluster analysis on lengths and

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widths of the footprints of the Tremp Formation resulted in four clusters, whose average lengths and widths are statistically different from each other (p < 0.05; Tukey-Kramer

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HSD). The result demonstrates that 78% of the hadrosaur footprints of the Tremp Formation are comprised by stage 1 and 2 footprints, suggesting a dominance of

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immature hadrosaur individuals in the Tremp Formation in contrast with the adult-dominated Nemegt and Cantwell hadrosaur populations. Presence or absence of

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large predators may be a major factor influencing ontogenetic herd structure because

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younger animals have higher incidence of being killed (e.g. Palmeira et al., 2008) and the higher proportion of adults in a herd is effective for defensive behavior (Tortato et

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al., 2015). The adult dominant population structure in the Nemegt Formation hadrosaur might be a result of the co-existence of large sized predators such as Tarbosaurus

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bataar. However, it may also have been influenced by the relevant timing of the life cycles of the animals compared with the times of the year when the tracks were produced. The lengths of adult hadrosaur footprints (stages 3 and 4) of the Nemegt ranged between 50 and 72cm, which is similar to the Cantwell hadrosaur (19 to 64 cm) but larger than the Tremp hadrosaur (32 to 51.5 cm). The hadrosaur footprints in the mid-latitude Nemegt Formation indicate that population structure was similar to that in

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the high-latitude Cantwell Formation rather than to the mid-latitude Tremp Formation. The maximum body sizes of the Nemegt and Cantwell hadrosaurs also resemble each other. Fiorillo and Tykoski (2014) argued that the smaller body size of an Alaskan tyrannosaurine theropod Nanuqsaurus hoglundi is a result of seasonal light variation

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and accompanying reduction of resource availability. However, similar maximum body

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sizes of the Nemegt and Cantwell hadrosaurs suggest that the effect of resource

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limitations on body size is limited to carnivores, but not on hadrosaurs. Instead, there might have been enough primary production at the Cantwell hadrosaur habitat area to be

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as adequate as at the Nemegt area. Vila et al. (2013) demonstrated that the small size of the hadrosaur footprints from the Tremp Formation represents dwarfism, caused by an

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insular environment (e.g. insufficient foraging areas, resource limitation, absence of gigantic predators; Benton et al., 2010). The lower boundary of stage 3 is similar to

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other hadrosaurs, but the maximum size of adult Tremp hadrosaurs is less than those of

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the Nemegt and Cantwell hadrosaurs, suggesting that the Nemegt Formation represented a more open environment and that the hadrosaurs had better access to food

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resources. 6. Conclusions

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Thirty-eight newly discovered dinosaur footprints from a single footprint-bearing horizon within the Nemegt Formation consist of three types of dinosaur footprints (hadrosaurs, sauropods, and theropods). The majority of the identified footprints are attributed to hadrosaurs. This trend contrasts with the domination of Tarbosaurus and other theropod skeletal elements collected at the Nemegt locality. Footprint records indicate that the Nemegt locality was dominated by herbivorous dinosaurs, including sauropods. Three large sauropod footprints indicate

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that either sauropod taxa known from skeletons were not fully mature, or that larger sauropod taxa remain to be found as skeletons at the Nemegt locality. Although the scarcity of small footprints is likely to be the result of preservational biases, the ichno-taxonomic composition based on large footprints

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suggests a herbivore dominant Nemegt fauna. The size distribution of the hadrosaur

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footprints infers a multigenerational, adult dominant population structure for the

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Nemegt hadrosaurs. The population structure and size range of the Nemegt hadrosaurs are similar to those of the high-latitude Cantwell hadrosaurs, but contrasts with those of

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the mid-latitude Tremp hadrosaurs. It suggests that the Nemegt area may been a more open habitat characterized by higher plant productivity than the insular Tremp

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environment.

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Acknowledgements

This study was a senior thesis of JN, done under the supervision of YK. We thank K. Kubo and R. Suzuki for their help in prospecting for footprints. We also thank to the

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staff of the Institute of Geology and Paleontology of the Mongolian Academy of

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Sciences, and members of NEE 2016 for managing the expedition and helping us out in all stages of the field work. The first author is grateful to C. Luigi, E. Koppelhus, F. Fanti, G. Funston, P. Bell, and Y.N. Lee for their valuable suggestions on this project. Finally, the authors are grateful to the reviewers and the Editor in Chief, Professor T. Algeo for their help to improve this paper and manage the special volume. This work is sponsored in part by Tokyo Institute of Technology Research Fund and the Clark Memorial Foundation Grant of Hokkaido University.

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Prieto-Marquez, A., 2008. Phylogeny and historical biogeography of hadrosaurid dinosaurs. The Florida State University.

Rose, P.J., 2004. A new titanosauriform sauropod (Dinosauria: Saurischia) from the

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Methodist University.

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Early Cretaceous of central Texas and its phylogenetic relationships. Southern

Rozhdestvenskiy, A., 1952. A new representative of the duck-billed dinosaurs from the Upper Cretaceous deposits of Mongolia. Doklay Akademii Naukk SSSR 86,

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Schulp, A.S., Al-Wosabi, M., Stevens, N.J., 2008. First dinosaur tracks from the

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Arabian Peninsula. PLoS One 3, e2243. Stettner, B., Person, W.S., Currie, P.J., 2017. A giant sauropod footprint from the Nemegt Formation (Upper Cretaceous) of Mongolia. Palaeogeography, Palaeoclimatology, Palaeoecology. Thulborn, T., 1990. Dinosaur Tracks. Chapman and Hall, London. Tomsich, C.S., McCarthy, P.J., Fowell, S.J., Sunderlin, D., 2010. Paleofloristic and paleoenvironmental information from a Late Cretaceous (Maastrichtian) flora of the lower Cantwell Formation near Sable Mountain, Denali National Park, Alaska. Palaeogeography, Palaeoclimatology, Palaeoecology 295, 389-408.

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Tortato, F.R., Layme, V.M.G., Crawshaw, P.G., Izzo, T.J., 2015. The impact of herd composition and foraging area on livestock predation by big cats in the Pantanal of Brazil. Animal Conservation 18, 539-547. Vecchia, F.M.D., 1999. A sauropod footprint in a limestone block from the Lower Cretaceous of northeastern Italy. Ichnos 6, 269-275.

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Vialov, O., 1988. On the classification of dinosaurian traces. Ezhegodnik Vsesoyuznogo Paleontologicheskogo Obshchestva 31, 322-325.

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Vila, B., Oms, O., Fondevilla, V., Gaete, R., Galobart, A., Riera, V., Canudo, J.I., 2013. The latest succession of dinosaur tracksites in Europe: Hadrosaur ichnology,

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track production and palaeoenvironments. PLoS One 8, e72579. Vila, B., Oms, O., Marmi, J., Galobart, A., 2008. Tracking Fumanya footprints

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(Maastrichtian, Pyrenees): historical and ichnological overview. Oryctos 8, 115-130.

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Weems, R.E., 2006. Locomotor speeds and patterns of running behavior in non-maniraptoriform theropod dinosaurs. The Triassic-Jurassic Terrestrial Transition. New Mexico Mus. Nat. Hist. Sci. Bull 37, 379-389.

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Wilson, J.A., 2005. Redescription of the mongolian sauropod Nemegtosaurus

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mongoliensis nowinski (dinosauria: Saurischia) and comments on late cretaceous sauropod diversity. Journal of Systematic Palaeontology 3, 283-318. Xing, L., Lockley, M.G., Marty, D., Zhang, J., Wang, Y., Klein, H., McCrea, R.T.,

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Ornithopod-Dominated Tracksite from the Lower Cretaceous Jiaguan Formation (Barremian-Albian) of Qijiang, South-Central China: New Discoveries, Ichnotaxonomy, Preservation and Palaeoecology. PLoS One 10, e0141059.

Figure captions Fig. 1. Locality maps. (A) The geographic position of the Nemegt locality. (B) Detail of the Nemegt locality (Map data: ©Google). The red stars indicate the footprints from a single horizon that are described and analysed in the present study. The orange asterisks

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indicate other known footprint sites in the Nemegt. The solid squares (Avimimus quarry, Camp, Recon A, and Recon B) indicate the localities of the geological sections by Eberth et al. (2009). The open square indicates the locality of the measured section (NEE 2016) of the present study. The open circle indicates the locality with dense

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dinosaur footprints.

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Fig. 2. Simplified correlation diagram of 5 measured sections (Camp, Recon A, Recon

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B, NEE 2016, and Avimimus Quarry) at the Nemegt locality (modified from Eberth et

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al. 2009).

Fig. 3. Correlation of two stratigraphic columnar sections. (A) NEE 2016. (B) Avimimus

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Quarry (modified from Eberth et al. 2009).

Fig. 4. (A) The dinosaur footprint FS-85 in lateral view from the footprint horizon of

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NEE 2016 section. (B) Diagrammatic representation of footprint seen in (A), showing the cross section of the deposits filling the track.

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Fig. 5. Histogram of the footprint lengths used in this study. Fig. 6. Dinosaur footprints from the footprint horizon at the Nemegt locality. (A)

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ventral view of large right hadrosaur footprint (FS-82), (B) ventral view of medium hadrosaur footprint (FS-91), (C) ventral view of right sauropod footprint (FS-80) showing five claw and digital impressions, (D) diagrammatic representation of FS-80, (E) lateral views of manual and pedal ichnites (FS-81), (F) diagrammatic representation of FS-81, and (G) ventral view of a theropod footprint (FS-65). Fig. 7. 3D reconstructed models with contours of dinosaur pedal ichnites from the

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footprint horizon (see Fig. 3) at the Nemegt locality. (A) Indeterminate footprint (FS-68), (B) 3D model of FS-68, (C) indeterminate footprint (FS-85), and (D) 3D model of FS-85. All of the footprints are ventral views. The contour interval of (B) is 1

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mm and that of (D) is 1.5 mm. Fig. 8. Taxonomic compositions determined from (A) the footprints at the base of the

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upper Nemegt tongue, (B) the skeletal elements collected from the Nemegt Formation,

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and (C) the skeletal elements collected from the Barun Goyot Formation.

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Fig. 9. Dense occurrence of dinosaur footprints on a single horizon. Each footprint is located at the tip of a grey arrow. Also note the displaced footprint blocks on the slope

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below the ledge. Scale = 1 m.

Fig. 10. (A) Bivariate plot of hadrosaur footprint length-width from three different

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formations. Different shaped marks indicate different stages indicated by the cluster analyses, (B) Relative frequencies of each growth stage. Notice the similarities between the Nemegt ichnological assemblage and the Cantwell ichnological assemblage,

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trajectories.

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suggesting that hadrosaurs across a wide range of latitudes had similar growth

Table 1. Linear measurements of the footprints used in this study. m stands for the manual track and p stands for the pedal track at FS-81. Table 2. List of the GPS coordinates of the footprint sites at the Nemegt locality.

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Table 1 ID FS-58 FS-59 FS-60 FS-61 FS-62 FS-63 FS-64 FS-65 FS-66 FS-67 FS-68 FS-69 FS-70 FS-71 FS-72 FS-73 FS-74 FS-75 FS-76 FS-77 FS-78 FS-79 FS-80 FS-81m

Length Width 36 13 70 50 19 95 33 33 41 44 109 63 70 68 82 46 40 50 35

32 18 55 56 13 80 32 20 80 38 45 20 68 65 57 36 44 36 32

II-IV div 80 105 65 120 48 66 51 62 85 95 84 82 -

Le digit L 32 11 15 27 60 60 31 64 78 45 24 -

Digit III W 14 6 15 25 2 12 12 9 20 22 15 12 10 -

D E

T P E

C C

A

Le digit W 8 4 3 9 13 8 7 18 22 9 16 7 -

R digit L 34 11 57 42 16 26 57 67 74 44

R digit W 11 11 9 2 8 9 14 17 14

-

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Depth ant 5 4 47 1 4 23 16 23 13 25 39 28 1 25

Depth pos 7 6 34 1 11 25 10 14 9 31

T P

I R

C S U

N A

M

Le-III div 39 58 60 20 41 66 32 25 48 61 49 52 -

Taxa Hadrosaur Indeterminate Theropod Hadrosaur Indeterminate Indeterminate Sauropod Hadrosaur Indeterminate Indeterminate Indeterminate Sauropod Indeterminate Indeterminate Theropod Indeterminate Hadrosaur Indeterminate Indeterminate Indeterminate Hadrosaur Hadrosaur Sauropod Sauropod

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FS-81p FS-82 FS-83 FS-84 FS-85 FS-86 FS-87 FS-88 FS-89 FS-90 FS-91 FS-92 FS-93 FS-94 FS-95

100 60 49 80 68 60 30 79 57 36 64 72 43

76 67 59 66 58 56 74 28 64 69 34 64 62 52 36

104 72 117 62 85 76 -

53 42 61 52 44 23 49 31 58 -

10 11 14 16 18 7 14 7 16 9 -

D E

T P E

A

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18 16 16 20 20 6 15 14 9 17 24 13 -

53 68 49 63 40 20 53 54 28

66 56 32 37 80 40 56 28 55 27 -

19 30 11 13 16 3 8 10 5 19 14 21 12

T P

I R

C S U

N A

M

11 15 16 9 19 8 9 15 10 9

22 11 18 2 28 3 14 10 17 22 -

Sauropod Hadrosaur Indeterminate Indeterminate Indeterminate Indeterminate Hadrosaur Theropod Theropod Indeterminate Hadrosaur Hadrosaur Hadrosaur Theropod Indeterminate

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Table 2

43°29'10.1"N 101°03'58.8"E FS-68

FS-03

43°28'55.3"N 101°04'16.5"E FS-69

FS-04

43°30'41.8"N 101°04'01.7"E FS-70

FS-05

43°29'06.6"N 101°04'09.9"E FS-71

FS-06

43°29'05.5"N 101°04'11.7"E FS-72

FS-10

43°29'14.6"N 101°03'54.0"E FS-73

FS-13

43°29'38.0"N 101°03'14.1"E FS-74

FS-14

43°29'39.6"N 101°03'09.9"E FS-75

FS-15

43°29'40.4"N 101°03'07.9"E FS-76

FS-16

43°29'48.1"N 101°02'58.9"E FS-77

FS-17

43°29'48.7"N 101°02'56.5"E FS-78

FS-18

43°29'51.0"N 101°02'53.9"E FS-79

FS-19

43°29'49.2"N 101°02'55.1"E FS-80

FS-20

43°29'01.4"N 101°04'20.2"E FS-81

FS-21

43°30'10.5"N 101°02'57.3"E FS-82

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43°29'54.5"N 101°02'37.8"E

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FS-22

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FS-02

FS-84

43°29'11.3"N 101°04'05.5"E FS-85

FS-24

43°30'03.2"N 101°03'09.5"E FS-86

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FS-23

FS-25

GPS coordinate 43°29'08.0"N 101°04'09.1"E 43°29'41.8"N 101°03'28.4"E 43°29'31.1"N 101°03'40.9"E 43°29'31.1"N 101°03'40.9"E 43°29'31.1"N 101°03'43.9"E 43°29'31.1"N 101°03'43.9"E 43°29'36.7"N 101°03'10.1"E 43°29'37.7"N 101°03'14.3"E 43°30'04.9"N 101°03'11.5"E 43°29'50.2"N 101°03'05.9"E 43°29'10.0"N 101°04'07.3"E 43°29'41.0"N 101°03'06.6"E 43°29'11.2"N 101°04'05.6"E 43°29'05.3"N 101°04'01.6"E 43°29'14.7"N 101°03'54.1"E 43°29'02.5"N 101°04'16.5"E 43°29'04.1"N 101°03'39.1"E 43°29'04.1"N 101°03'39.2"E 43°29'12.8"N 101°04'06.4"E 43°29'42.0"N 101°03'34.6"E 43°30'04.7"N 101°03'09.7"E 43°30'06.1"N 101°03'10.1"E 43°30'03.7"N 101°03'13.8"E 43°30'03.4"N 101°03'09.8"E 43°30'04.1"N

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ID

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GPS coordinate

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ID

43°29'59.5"N 101°03'07.7"E FS-87

FS-26

43°29'34.7"N 101°03'19.7"E FS-88

FS-27

43°29'43.7"N 101°03'01.9"E FS-89

FS-30

43°28'54.7"N 101°04'15.0"E FS-90

FS-38

43°30'10.7"N 101°03'28.4"E FS-91

FS-39 FS-40

43°29'57.4"N 101°03'52.9"E FS-92 43°29'58.5"N 101°03'40.6"E FS-93

FS-54

43°29'52.6"N 101°03'55.4"E FS-95

FS-58

43°28'57.2"N 101°04'23.4"E FS-96

FS-59

43°28'55.5"N 101°04'18.3"E FS-97

FS-60

43°28'54.4"N 101°04'14.9"E

FS-98

FS-61

43°28'54.8"N 101°04'15.5"E FS-99

FS-62

43°28'59.5"N 101°04'21.7"E FS-100

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FS-102 FS-103 FS-104

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FS-67

43°28'56.5"N 101°04'19.6"E

FS-101

43°28'56.5"N 101°04'19.6"E

FS-105

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FS-66

43°28'55.0"N 101°04'15.4"E

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FS-65

43°28'55.0"N 101°04'15.4"E

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FS-64

43°28'59.6"N 101°04'21.1"E

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FS-63

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43°29'46.0"N 101°03'58.1"E FS-94

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FS-53

101°03'03.1"E 43°30'10.4"N 101°02'58.7"E 43°30'03.5"N 101°03'06.8"E 43°28'55.3"N 101°04'17.2"E 43°29'43.6"N 101°03'02.1"E 43°29'57.2"N 101°04'29.8"E 43°30'08.6"N 101°03'04.3"E 43°30'24.5"N 101°04'07.3"E 43°30'24.5"N 101°04'07.3"E 43°30'24.5"N 101°04'07.3"E 43°30'24.5"N 101°04'08.8"E 43°30'23.5"N 101°04'09.4"E 43°30'16.6"N 101°04'07.0"E

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Abstract The Nemegt locality is one of the most famous dinosaur localities in Mongolia ever since the site was discovered in 1946. It yields abundant dinosaur skeletons; however, little attention had been given to dinosaur footprints at the locality. The only

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Nemegt dinosaur footprint study focused on descriptions of the footprints, gave only a

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few taxonomic implications, and provided no comparison with other dinosaur tracksites.

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This study reports newly recorded dinosaur footprints (hadrosaurs, sauropods, and theropods) at the Nemegt locality during the Nemegt Educational Expedition of 2016. A

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single footprint-bearing horizon that extends several kilometres was examined within the Nemegt Formation to determine the ichno-taxonomic assemblage of the Nemegt

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dinosaurs. A significant difference was identified between taxonomic compositions based on skeletal remains and ichno-taxonomic compositions based on footprints.

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Although the vast majority of the skeletal elements collected in the area belong to

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theropods, the footprints suggest that the Nemegt locality was dominated by herbivorous dinosaurs. This suggests that the previously inferred Tarbosaurus dominant

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taxonomic composition at the Nemegt locality is a result of a preservational bias. The size distribution of the newly studied footprints suggest that the Nemegt hadrosaurs had

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an adult-dominant and multigenerational population structure. Comparisons with dinosaur tracksites at the Cantwell (Alaska, USA) and Tremp (Spain) formations show that the population structure and body sizes of the Nemegt hadrosaurs were similar to those of the high-latitude Cantwell hadrosaurs. It suggests that the Nemegt area was more open and had higher plant productivity than the Tremp area.

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Highlights

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Dinosaur footprints on a single horizon within the Nemegt Formation were examined Footprint-based and bone-based taxonomic compositions are compared numerically

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Skeletal elements and footprints suffer different preservational biases

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Nemegt hadrosaur herd structure and body size resemble to the Alaskan hadrosaurs

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Nemegt area may have had been more open and food rich compared to

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southwestern Europe