Introduction

The non-avian theropod dinosaur clade Deinonychosauria includes the clades Dromaeosauridae and Troodontidae. Most deinonychosaurians were functionally didactyl, possessing highly modified second pedal digits (Makovicky and Norell 2004; Norell and Makovicky 2004; Longrich and Currie 2009) that were held off the ground during cursorial locomotion (Zhen et al. 1994; Li et al. 2007; Xing et al. 2009). Deinonychosaurians attract a great deal of attention because of their close phylogenetic relationship with birds (Currie 1997; Xu et al. 2009, 2010). The coincidental and fortuitous discoveries of numerous deinonychosaurian tracks (Ichnofamily Dromaeopodidae), beginning in 1994, promoted study of deinonychosaurian locomotion and behaviour and provided new evidence for the existence and distribution of deinonychosaurians around the Jurassic-Cretaceous boundary (Xing et al. 2009).

Fig. 1.

Photograph of Liujiaxia Dinosaur National Geopark sites I and II.

Dromaeopodid tracks were first discovered, and are particularly common, in China. The first dromaeopodid tracks, Velociraptorichnus sichuanensis, were from the ?Early Cretaceous of E'mei, Sichuan Province (Zhen et al. 1994). After a hiatus of more than a decade, a mass of new dromaeopodid track discoveries in China closely followed one another, including unnamed, Early Cretaceous dromaeosaurid tracks from Yanguoxia, Yongjing County, Gansu Province (Li et al. 2006), new specimens of Velociraptorichnus, the new ichnotaxon Dromaeopodus shandongensis from the Early Cretaceous of Junan, Shandong Province (Li et al. 2007), and Menglongipus sinensis from the Jurassic-Cretaceous boundary of Chicheng, Hebei Province (Xing et al. 2009). Reports outside China include unnamed ichnites from the earliest Cretaceous of Germany (Richter et al. 2007), unnamed ichnites of dubious quality from the Early Cretaceous of Utah (Lockley et al. 2004), Dromaeosauripus hamanensis and the recently-erected Dromaeosauripus jinjuensis from South Korea (Kim et al. 2008, 2012), Paravipus didactyloides from the Middle Jurassic of Niger (Mudroch et al. 2010), and unnamed tracks from possibly the Early Jurassic of Morocco (Ishigaki and Lockley 2010). Dromaeosaurid tracks have also been confirmed from the Late Cretaceous of Utah (Cowan et al. 2010).

Here we present a detailed description of dromaeopodid tracks from Yongjing County, China, which represent the largest-known accumulation and some of the best-preserved didactyl tracks anywhere in the world. This site was first reported by Li et al. (2006), who also noted the didactyl tracks and posited that theymay represent a new ichnotaxon. We provide evidence to support this claim and examine the ramifications for Dromaeosauripus trackmaker foot morphology.

Institutional abbreviations.—GSLTZP, Fossil Research and Development Center of the Third Geology and Mineral Resources Exploration Academy of Gansu Province, China; HDT, Huaxia Dinosaur Tracks Research and Development Center, Gansu, China; LDNG, Liujiaxia Dinosaur National Geopark, Gansu, China.

Historical and geological Background

In 2000, workers from the Research Center of Paleontology of the Bureau of Geology and Resource Exploration of Gansu Province discovered ten dinosaur track sites in the Hekou Group, Yanguoxia, Gansu Province (Du et al. 2001; Li et al. 2002). Zhang et al. (2006) reported a preliminary exploration of this and other, associated sites that had not yet been fully exposed in 2002, but their report did not include dromaeopodid tracks. Two subsequent Sino-Japanese and Sino-JapaneseKorean joint expeditions worked at the Yanguoxia dinosaur track sites in 2002 and 2004, which was followed by a preliminary report on two of the sites (Li et al. 2006) including the first report of dromaeopodid tracks at the locality. Beginning in 2004, the senior author was invited by the Fossil Research and Development Center, Third Geology and Mineral Resources Exploration Academy, Gansu Provincial Bureau of Geo-exploration and Mineral Development and the Geological Museum of Gansu to study dinosaur tracks in exposures of the Yanguoxia Formation (Hekou Group) in Liujiaxia Dinosaur National Geopark, Yanguoxia, Gansu Province (Fig. 1). During this study, new didactyl trackways were discovered and are described herein.

Liujiaxia Dinosaur National Geopark lies 54 km west of the city of Lanzhou, on the north shore of Taiji Lake (Yanguoxia Reservoir), Yongjing County, Linxia Hui Autonomous Prefecture, Gansu Province. The track locality sits at the southeastern edge of the Lanzhou-Minhe Basin. Specifically, the locality lies in the southeastern part of the Zhoujiatai low-uplift portion of the basin and occurs in the Yanguoxia Formation (Cai et al. 1999). Dinosaur footprints mainly occur in the ninth layer of fine sandstone-siltstone. The geological setting of the tracksite has been previously mentioned by researchers (Zhou et al. 2005; Li et al. 2006; Zhang et al. 2006).

Systematic ichnology

Fig. 2.

Deinonychosaurian track Dromaeosauripus yongjingensis ichnosp. nov. holotype GSLTZPS2-TE4L from the Early Cretaceous Yanguoxia track site, Gansu, China. A. Photograph. B. Computerized photogrammetry, with 0.2 mm contour lines; color banding reflects topography (bluegreen, highest; red-white, lowest). Based on the photogrammetric maps from Xing et al. (2009: fig. 6). C. Outline drawing.

Table 1.

Number of tracks in and directions of Dramaeosauripus yongjingensis trackways from Liujiaxia Dinosaur National Geopark sites I (S1) and II (S2).

Fig. 3.

Outline drawings of 19 of the best preserved Dromaeosauripus yongjingensis specimens (specimen numbers begin with GSLTZP-S2-) from Liujiaxia Dinosaur National Geopark site II, Early Cretaceous, including the holotype (TE4L).

Table 2.

Measurements (in cm) of the best-preserved Dromaeosauripus yongjingensis tracks from Liujiaxia Dinosaur National Geopark site II. Abbreviations: LDIII, length of digit III; LDIV, length of digit IV; ML, maximum length; MW, maximum width between the tips of digits III and IV; III–IV, angle between digits III and IV.

Fig. 4.

Turning trackway of Dromaeosauripus yongjingensis GSLTZPS2-TB at Liujiaxia Dinosaur National Geopark site II, Early Cretaceous. APhotograph. B. Outline drawing. C. Pace angulations.

Discussion

Turning trackway and walking velocities.—The six Dromaeosauripus yongjingensis trackways at LDNG S2 are comprised of at least two, and possibly three, turning trackways. Trackway GSLTZP-S2-TA turns 29° from E to SE. Trackway GSLTZP-S2-TB makes the most pronounced turn: 88° from E to NW. If GSLTZP-S2-TC-TF were made by the same track maker (see 6.2, below), then they collectively represent an individual making three turns: 41° from N to NW, then 30° from NW to N, and then 75° from N to NW.

Many theropod trackways appear to have extremely high pace angulations (nearing 180°) and stride lengths (Lockley 1991). As the velocity of a track maker decreases, pace angulations should decrease concurrently (Day et al. 2002). The first eight footprints of the GSLTZP-S2-TB trackway are the best preserved and the most continuous (Fig. 4). The pace angulations from GSLTZP-S2-TB1R-TB8L are 173°, 154°, 176°, 152°, 172°, and 143° (Fig. 4C). Using the equation of Alexander (1976), the walking velocity of the track maker decreases slightly along the trackway: 0.79 m/s, 0.73 m/s, 0.74 m/s, 0.75 m/s, 0.76 m/s, and 0.71 m/s. The slight decrease corresponds to the turn in the trackway, but is so slight that the track maker clearly made the turn without a marked drop in velocity.

Deinonychosaurians often have been inferred to be speedy predators. As yet, the only ichnological example for a speedy deinonychosaurian is a trackway of Dromaeosauripus from Korea that suggests a velocity of 4.86 m/s (Kim et al. 2008). All other known trackways indicate relatively slow, presumablywalking, locomotory modes. The velocities calculated for Dromaeosauripus yongjingensis tracks are slower than those calculated for Paravipus (1.67 m/s, 3.61 m/s; Mudroch et al. 2010) and Dromaeopodus (1.63 m/s; Li et al. 2007; Kim et al. 2008).

Behaviour.—Body fossil evidence of gregariousness in deinonychosaurians has been ambiguous. The occurrence of more shed Deinonychus teeth at one Tenontosaurus carcass than could be expected from a single individual has been used to infer gregarious, possibly pack-hunting behaviour for Deinonychus (Maxwell and Ostrom 1995; Ostrom 1990). However, Roach and Brinkman (2007) interpreted this evidence as indicating that Deinonychus had a Komodo dragonor crocodile-like feeding strategy in which individuals are usually solitary hunters but may be drawn in groups to carcasses, resulting in intraspecific agonism. In contrast, the famous “fighting dinosaurs”specimen (Carpenter 1998) suggests that a single, individual dromaeosaurid was capable of hunting on its own, without a pack.

Fig. 5.

Schematic map of the relative positions of six Dromaeosauripus yongjingensis trackways (GSLTZP-S2-TA to TF) at Liujiaxia Dinosaur National Geopark site II, Early Cretaceous.

Trackways, in contrast to body fossils, provide compelling evidence of deinonychosaurian group behaviour. Li et al. (2007) documented six parallel, closely spaced Dromaeopodus trackways that suggest at least occasional gregarious behaviour in the track-making animals. For Dromaeosauripus yongjingensis, the single trackway at LDNG site I is uninformative with respect to gregariousness. The six trackways at LDNG S2 (Fig. 5), however, provide a better sample to analyze gregariousness in deinonychosaurians (Table 1). Trackways GSLTZP-S2-TC, -TD, and -TE all may have been left by the same track maker: a portion of the track series has been obliterated by sauropod tracks made after the Dromaeosauripus yongjingensis tracks (Fig. 5), making interpretation ambiguous. Tracks in discontinuous trackway GSLTZP-S2-TF have the same walking orientation and were therefore most likely left by the same track maker. None of the six Dromaeosauripus yongjingensis trackways are parallel or closely spaced, suggesting that the track makers were, at least at the time the tracks were registered, solitary.

Dromaeosaurid or Troodontid?—Based on body fossils, members of the Deinonychosauria (Dromaeosauridae + Troodontidae) span one of the largest size ranges in Theropoda: the largest members, such as Utahraptor (Dromaeosauridae), approached 7min length (Kirkland et al. 1993), while the smallest, such as Anchiornis (Troodontidae), measured just 34 cm in length (Xu et al. 2009). The largest troodontid currently known is Saurornithoides, which approached 2.5–3 m in length (Norell et al. 2009). The size range of deinonychosaurians based on the track record is concordant with that of the body fossil record. Estimates of track-maker sizes for all currently known dromaeopodid ichnotaxa (Fig. 6, Table 3), including Dromaeosauripus yongjingensis, fall between 61– 300 cm, well within the size range established by body fossils of both dromaeosaurids and troodontids, although the largest tracks are at the uppermost end of the troodontid range. Furthermore, Lubbe et al. (2009, 2011) argued on osteological grounds that dromaeosaurids would produce tracks subequal lengths between digits III and IV whereas troodontids possess a relatively shorter digit IV. Based on this distinction, we contend that Dromaeosauripus yongjingensis (and probaby all Dromaeosauripus tracks) were likely made by a dromaeosaurid.

Table 3.

Comparison of the foot lengths and estimated track-maker body lengths of dromaeopodid ichnospecies. *calculated using the average hip height to body length ratio of 1:2.63 (Xing et al. 2009) and the formula: hip height = 3.983x + 69.453; where x is the length of digit III (Li et al. 2007). Note, only those taxa with length of digit III reported were calculated.

Fig. 6.

Schematic diagrams of dromaeopodid ichnotaxa to the same scale. A. Menglongipus (Xing et al. 2009). B. Shandong Velociraptorichnus (Li et al. 2007). C. Velociraptorichnus (Zhen et al. 1994; Xing et al. 2009). D. Dromaeosauripus yongjingensis (this study). E. Dromaeosauripus hamanensis (Kim et al. 2008). F. Dromaeopodus (Li et al. 2007). G. Paravipus (Mudroch et al. 2010). H. Morocco didactyl track (based on Ishigaki and Lockley 2010).