The Jinyuxi track site is a weathered siltstone surface located on the bed of a stream running through Yuanlin Village. Currently, there are at least three sauropod trackways on the original horizons, catalogued as JYX-S1–S3 and consisting of 8, 10, and 8 tracks, respectively (Table 1; Figs. 2 and 3). Six tracks in JYX-S3 are covered by river water. The tracks in situ were heavily weathered due to long-time exposure. There are also well-developed mud cracks, some shorter trackways, and isolated tracks on a slab that was newly stripped during the summer flood of 2020. Among these, the theropod trackway JYX-T1, which consists of four consecutive tracks, the theropod trackway JYX-T2, which consists of one continuous single step, isolated tracks JYX-TI1–TI4, and the sauropod trackway JYX-S4 which consists of five consecutive tracks, are identified.
The Jinyuxi track site is located at the southern edge of the Sichuan Basin (28°14′13.80″N, 105°49′35.92″E). Based on the regional geological survey report, the Cretaceous strata in the Jinyuxi area belong to the late Lower Cretaceous Jiaguan Formation (Fig. 4) (Sichuan Provincial Bureau of Geology Aviation Regional Geological Survey Team 1976; Xing et al. 2015b). The Jiaguan Formation comprises thick, brick-red, feldspathic, quartz-sandstone, and it shows conformable contact with the sandy conglomerate and mudstone of the overlying Upper Cretaceous Guankou Formation (Sichuan Provincial Bureau of Geology Aviation Regional Geological Survey Team 1976). The Lower Cretaceous Jiaguan Formation was deposited unconformably above the red mudstone of the Lower Cretaceous Tianmashan Formation and/or the Upper Jurassic Penglaizhen Formation (Gu and Liu 1997).
Numerous Early Cretaceous dinosaur tracks have been discovered in this area, and relevant geological information has been described in detail by the author team (Xing et al. 2015a, 2016a). Moreover, according to investigations by the Sichuan Provincial Bureau of Geology Aviation Regional Geological Survey Team (1976), the Jiaguan Formation consists of two members. The Lower Member of the Jiaguan Formation is 211–405 m thick and has a lithology of feldspathic quartz sandstone interbedded with multiple layers of mudstone, with a less than 10 m thick conglomerate layer at the bottom and a 2–10 m thick mudstone layer at the top. The Upper Member is 345–1000 m thick and is composed of feldspathic quartz sandstone interbedded with thin layers of lenticular mudstone and siltstone. The surface of the sandstone displays current ripples, and mud cracks are common in the siltstones. The sediments of the Jiaguan Formation are alluvial fan, river and desert deposits (Geng 2011). Chen (2009) argued that the Upper Member represents a meandering fluvial deposit interbedded with deposits from small braided rivers.
The Jinyuxi track site is associated with the feldspathic quartz sandstone of the Upper Member of the Jiaguan Formation.
Taenidium isp. (Fig. 5) mainly appears as unlined, cylindrical, straight to sinuous tubes with meniscate backfill (Ekdale et al. 2007). Taenidium tubes do not cross-cut one another. Arenicolites isp. (Fig. 5) co-occurs with Taenidium isp. and consists of two parallel U-shaped vertical tubes without spreiten (Hauck et al. 2009). The Arenicolites tubes are cylindrical and smoothly walled. These trace fossil assemblages occur alongside mud cracks, confirming a floodplain of braided river (Hu and Wu 1993; Chen et al. 2019).
Arenicolites and Taenidium also occur in direct association with the dinosaur tracks. Based on their cross-cutting relationships, the dinosaur tracks were formed first, in the wet mud substrate of the floodplain. Then Arenicolites and Taenidium were left by the invertebrate burrowing (both dwelling and feeding). During the dry season, the mud cracks were formed and cross-cut both the trackways and burrows. Subsequently, silty sediments covered this area, forming track casts. During the late weathering, argillaceous sediments and molds were denuded, and the silty casts were preserved and exposed.
Eubrontid tracks from China
More than 100 track sites in China have yielded various tridactyl theropod tracks. A considerable number of these specimens are simply classified as eubrontid tracks or Eubrontes-type tracks (e.g. Xing and Lockley 2016, Table 2) (see Fig. 6). In the case of small and poorly preserved samples, this situation is predictable and acceptable: i.e., precise and confident identifications may be difficult to make. Since the 1980s, ichnologists from China named some new ichnospecies of Eubrontes or new ichnogenera of Eubrontes-type tracks. Some of their validity has been discussed and verified (such as Lockley et al. 2013), and some have not been discussed in detail.
Eubrontes pareschequier (Xing et al. 2009b) Lockley et al. 2013
Changpeipus pareschequier ZLJ-ZQK1 and ZLJ-ZQK2 from the Lower Jurassic Lufeng Formation, Yunnan Province (Xing et al. 2009a) are represented by two tracks. Lockley et al. (2013) reassigned C. pareschequier to Eubrontes pareschequier. Xing et al. (2014b) designated the better preserved ZLJ-ZQK1 as the holotype and ZLJ-ZQK2 as the paratype (see Fig. 6h). However, the validity of E. pareschequier is questionable. The morphological characteristics of ZLJ-ZQK1 are similar to E. giganteus. The most distinct difference between these two ichnospecies is that ZLJ-ZQK1 has a comparatively low length/width ratio (1.3 vs. 1.7).
Eubrontes zigongensis (Gao 2007) Lockley et al. 2013
Gao (2007) preliminarily described one large theropod trackway from the Lower Jurassic Zhenzhuchong Formation of Weiyuan County, Sichuan Province, and named it Weiyuanpus zigongensis. Lockley et al. (2013) assigned W. zigongensis to Eubrontes zigongensis n. comb. Xing et al. (2014c) described E. zigongensis in detail and defined new diagnostic features (Fig. 6i). The most prominent diagnostic feature of E. zigongensis is the thin, anteromedially directed hallux trace.
Eubrontes platypus Hitchcock 1858 (Zhen et al. 1986)
Zhen et al. (1986) described a series of theropod tracks from Lower Jurassic Fengjiahe Formation of Jinning County, Yunnan Province, and classified it as E. platypus = E. giganteus. Lockley et al. (2013) retained this category. The author team of this article visited track sites and museum collections in 2015 and planned to publish detailed morphological features. According to the interpretive outline drawing provided by Zhen et al. (1986), the metatarsophalangeal pad of the Xiyang specimen was incomplete and had insufficient features to be assigned to a recognizable ichnospecies (Fig. 6j).
Eubrontes monax (Zhen et al. 1986) Lockley et al. 2013
Zhen et al. (1986) described Paracoelurosaurichnus monax from the Lower Jurassic Fengjiahe Formation of Jinning County, Yunnan Province. Lockley et al. (2013) assigned P. monax to Eubrontes monax n. comb., and provided a new interpretive outline drawing (Fig. 6k). According to this morphological evaluation, E. monax was registered three separated (non-connected) digit traces. It is possible that E. xiyangensis, E. monax and E. platypus are all extramorphological variants of Eubrontes-type tracks on a soft, wet and slippery substrate, such as Changpeipus carbonicus from the Middle Jurassic of the Turpan Basin, Xinjiang (Xing et al. 2014c) (Fig. 6m).
Eubrontes xiyangensis (Zhen et al. 1986) Lockley et al. 2013
Zhen et al. (1986) described Youngichnus xiyangensis from the Lower Jurassic Fengjiahe Formation of Jinning County, Yunnan Province. Lockley et al. (2013) assigned Y. xiyangensis to Eubrontes xiyangensis n. comb. and provided a new interpretive outline drawing (Fig. 6l). However, according to the new interpretive outline drawing, the metatarsophalangeal pad of E. xiyangensis was incomplete and had insufficient features to be classified at the ichnospecies level.
Changpeipus carbonicus Young 1960 (Xing et al. 2014c)
Changpeipus carbonicus from the Lower–Middle Jurassic of Jilin Province is a reasonably well-preserved theropod track and exhibits a digital pad formula that allies it to the Grallator–Eubrontes plexus (sensu Olsen 1980). Xing et al. (2014c) reviewed all the Changpeipus specimens of China, and proposed that Changpeipus is a monotypic ichnogenus, furthermore, Changpeipus and Eubrontes are similar (“sister”) ichnotaxa. The most prominent diagnostic feature of C. carbonicus is the inner hypex between digits II and III, which is situated distinctly posterior to the outer hypex between digits III and IV. Xing et al. 2009b, Xing et al. 2014c) also considered C. xuiana from the Middle Jurassic Yima Formation at the Yima opencast coal mine in Henan Province (Lü et al. 2007) a nomen dubium.
Eubrontes nianpanshanensis (Yang and Yang 1987) Lockley et al. 2013
Yang and Yang (1987) described one theropod trackway from the Middle Jurassic Xintiangou Formation of Sichuan Province, and named it Jinlijingpus nianpanshanensis. Lockley et al. (2013) re-assigned the ichnotaxon to Eubrontes nianpanshanensis n. comb. Xing et al. (2016c) described E. zigongensis in detail and provided new diagnostic features (Fig. 6n). The prominent diagnostic feature of E. nianpanshanensis is the low length/width ratio and wide divarication angle.
Lufengopus dongi Lü et al. 2006
Lü et al. (2006) described the first dinosaur footprint, an imperfectly preserved isolated large (37.8 cm in length) theropod track, from the Middle Jurassic Chuanjie Formation of the Lufeng Basin in Yunnan Province, and named it as Lufengopus dongi (Fig. 6o). Xing et al. (2014e) re-described this specimen, and referred it to Eubrontes isp.
Eubrontes (?) glenrosensis Shuler 1935; Farlow et al. 2012 (Li et al. 2010)
Langston (1974) had previously assigned Eubrontes (?) glenrosensis to the ichnogenus Irenesauripus. Li et al. (2010) assigned one theropod trackway from the Middle Jurassic Zhaogou Formation at the Hailiutu site in Inner Mongolia, China, and classified it as Eubrontes (?) glenrosensis. Xing et al. (2021) reviewed all the tracks from this site, and considered the length/width ratio and mesaxony of Eubrontes (?) glenrosensis Hailiutu specimen are more similar to Kayentapus, and then rejected the identification of the dubious ichnospecies Eubrontes (?) glenrosensis among any of the Hailiutu theropod tracks (Fig. 6p).
Lockleypus luanpingeris (Young 1979) Xing et al. 2018b
Changpeipus luanpingeris was discovered from the Lower Cretaceous of Luanping coal mine in Hebei Province by Young (1979). Xing et al. (2014c) considered it a nomen dubium. With further research reviewing the track specimens from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences, Xing et al. (2018b) considered C. luanpingeris as a new ichnogenus — Lockleypus, based on the free length of digit IV which is twice as long as the free length of digit II (Fig. 6q).
Chapus lockleyi Li et al. 2006
Li et al. (2006) described the large (42 cm in length) theropod track Chapus lockleyi (Fig. 6r) from the Lower Cretaceous Jingchuan Formation in Inner Mongolia, with relatively low anterior triangles (0.48) and low length of digit III/footprint length (0.60).
Asianopodus pulvinicalyx Matsukawa et al. 2005
Matsukawa et al. (2005) described Asianopodus pulvinicalyx from the Lower Cretaceous Kuwajima Formation of Japan, based on a somewhat gracile track of 29 cm long and 21 cm wide (length/width ratio = 1.38) from an incomplete trackway. The ichnotaxon is distinctive in possessing a distinct, bulbous “heel” impression (see Fig. 6s).
Asianopodus robustus Li et al. 2011
Li et al. (2011) described Asianopodus robustus as a larger, more “robust” and slightly wider ichnospecies (length, width and length/width ratio are respectively 33 cm, 23 cm and 1.43) than A. pulvinicalyx. A. robustus also has a distinct, well-developed, rounded “heel” pad (see Fig. 6t). The choice of the holotype was based on an illustration by Lockley et al. 2002, Fig. 4) for a track that is not a part of a trackway. Thus, trackway parameters are not known for this ichnospecies.
Asianopodus niui Li et al. 2020
Recently, Li et al. (2020) described a new ichnospecies of Asianopodus (A. niui) from the Upper Cretaceous Shenjinkou Formation in the Xinjiang Uygur Autonomous Region. The material they described was based on a large track with length of 56 cm and width of 42 cm (length/width ratio of 1.33) which more closely resemble Chapus than either of the previously described Asianopodus ichnospecies. However, the type material was poorly illustrated, and we consider it a nomen dubium. The senior authors have re-evaluated the status of this ichnotaxon elsewhere (Xing et al. 2021; Xing et al. in press), so it is not illustrated or further analyzed here.
Taxonomy of Jinyuxi site
Eubrontidae Lull 1904
Theropoda Marsh 1881
Eubrontidae Lull 1904
Eubrontes Hitchcock 1845, Olsen et al. 1998
Type ichnospecies E. giganteus Hitchcock 1836, Hitchcock 1845, Olsen et al. 1998
Etymology: The specific name honors Mr. Nobi Nobita (known as Noby in the English versions). He is the protagonist of the Doraemon series. Japanese animated films Doraemon: Nobita’s Dinosaur in 1980 and Doraemon: Nobita’s New Dinosaur in 2020 are unforgettable dinosaur films. In the film, Mr. Nobita’s dream is to have a dinosaur named after him.
Holotype: A complete natural cast of a pes track represented by four footprint trackways was catalogued as JYX-T1-L1 from the Jinyuxi track site (Table 1; Figs. 2 and 3). The plaster mold is stored in the Zigong Dinosaur Museum.
Type horizon and locality: Lower Cretaceous, Jiaguan Formation, Jinyuxi track site, Gulin County, Sichuan Province, China.
Diagnosis: A relatively large-sized tridactyl footprint with pes length/width ratio of 1.4, weak mesaxony of 0.37, and relatively small divarication angle (about 47°) between digit II and IV. Digit III is 65% of the total pes length. Digit II is 85% of the length of digit III. The metatarsophalangeal pads of digit II are fairly well developed, with a size almost as large as the phalangeal pad of digit IV. The metatarsophalangeal pads lie sub-symmetrically on either side of the midline of the track. This is a feature that differentiates the tracks from many less symmetrical theropod tracks including some Eubrontes, Asianopodus and Chapus. Step length is about 2.6× footprint length, and the mean pace angulation is high (about 160°). E. nobitai is different from the type ichnospecies E. giganteus by (1) wider digit II–IV divarication, (2) weaker mesaxony, and (3) lower length/with ratio; and, different from E. zigongensis, E. veillonensis and E. nianpashensis by (1) smaller digit II–IV divarication and (2) weaker mesaxony.
All tracks in the trackway are equally well preserved, on the scale of Belvedere and Farlow (2016), so each track is equally representative of the individual footprint morphology. The mean length and width of all four tracks (JYX-T1-L1–R2) are 31.4 cm and 22.5 cm, respectively, and the mean length/width ratio is 1.4. JYX-T1-L1 exemplifies the morphology, and was selected for replication as the holotype for the Zigong Dinosaur Museum. Digit III projects the farthest anteriorly, followed by digits IV and II. The mean ratio between the length of digit III and foot length is 0.65 with a range of 0.63–0.69. Two distinct metatarsophalangeal pad traces, a smaller one posterior to digit II and a larger one posterior to digit IV, can be seen. Both metatarsophalangeal pad traces are round and blunt and positioned on either side of the axis of digit III. The deep, concave digit impressions retain pad impressions that have a formula of x-3-3-4-x, including metatarsophalangeal pads II and IV. Each digit has a sharp claw trace, and digit II has the clearest and longest trace. The divarication angle between digits II and IV is 47°. The divarication angle between digits II and III (19°) is smaller than that between digits III and IV (28°). The other JYX-T1 tracks are consistent with JYX-T1-L1 in all general features; the divarication angles between digit II and digit IV of these tracks range from 43° to 50°. All the tracks are rotated ~ 7° outwards from the trackway midline. The outer trackway width of JYX-T1, measured from the outside margin of the pes, is 33–37 cm. The average pace angulation is 160°, and the footprint length to pace length ratio is 1:2.6.
The stride length relative to pes length of the trackway JYX-T1 allows for a speed (v) calculation using the formula of Alexander (1976): v = 0.25 g0.5 × SL1.67 × h− 1.17, where g is the gravitational acceleration in m/s, SL is stride length, and h is hip height, which is estimated to be 4.9 times the foot length using the ratio for large theropods proposed by Thulborn (1990). Then, we estimate a speed of ∼1.08 m/s or ∼3.89 km/h. The body length of the JYX-T1 track maker is approximately 4 m, further calculated using the average hip height to body length ratio of 1:2.63 (Xing et al. 2009c).
Eubrontes tracks were first discovered in the Connecticut River Valley of Massachusetts in the early nineteenth century, and were probably among the first non-avian dinosaur tracks discovered in North America (Hitchcock 1845; Lull 1904, 1953; Olsen et al. 1998). Hereafter, Eubrontes or Eubrontes-type tracks are commonly found in Lower Jurassic strata and have also been reported from the Jurassic series but less frequently from the Cretaceous. They are also known from Upper Triassic strata (Lucas et al. 2006; Lagnaoui et al. 2012; Xing et al. 2013a, b; Zouheir et al. 2018).
Several large-sized theropod tracks from the carbonate bedrock of Paluxy River in the Albian Glen Rose Formation (Lower Cretaceous) in Texas, were assigned to Eubrontes (?) glenrosensis (Shuler 1935; Farlow et al. 2012), but were transferred into the ichnospecies Irenesauripus glenrosensis by Langston (1974).
Eubrontes-type tracks are very common in the Jurassic and have also been reported from the Cretaceous of China, including some ichnogenera with similar morphology such as Chapus from the Chabu site of Inner Mongolia (Li et al. 2006) and Asianopodus from Junan site of Shandong (Xing et al. 2014a; Li et al. 2015). Compared with the classical Early Jurassic Eubrontes–Anchisauripus–Grallator assemblage (Olsen et al. 1998), Cretaceous Grallator–Eubrontes morphotypes from China have wider interdigital divarication (Lockley et al. 2013; Xing et al. 2016a), which can also be observed in Early Cretaceous theropod tracks from North America (Lockley et al. 1998).
The JYX-T1 track shows convergent traits with type Eubrontes tracks, such as the presence of a distinct metatarsophalangeal pad (MTP) trace posterior to digit II (MTP II). This characteristic is common in Eubrontes tracks, including type Eubrontes AC 151 (Olsen et al. 1998), and also distinguishes Eubrontes from Chongqingpus, a medium-large theropod track with a hallux impression (Xing et al. 2013a, b). This feature (MTP II) also distinguishes type Eubrontes and E. nobatai from most other Eubrontes, from common ichnogenus Kayentapus of the Early–Middle Jurassic (Lockley et al. 2011; Xing et al. 2020), and from such Cretaceous ichnogenera as Asianopodus (Fig. 6).
The JYX-T1 track has weak mesaxony (0.37) and low length/width ratio (1.4), similar to those of type Eubrontes (Olsen et al. 1998; Lockley 2009). The mesaxony and length/width ratio are respectively 0.58 and 1.7 for Eubrontes giganteus from the Hitchcock collection (Lockley 2009) (Fig. 6a), suggesting that Hitchcock Eubrontes tracks are slightly more elongate. However, other local specimens assigned to Eubrontes display wider interdigital divarications and lower length/width ratios (Ishigaki and Fujisaki 1989; and see Lockley et al. 2021 for additional data). Overall, the length/width ratio, mesaxony and divarication angle (47°) of the JYX-T1 track are relatively lower and smaller than these three parameters of Eubrontes reported from the Lower Jurassic of Utah, USA (Lockley et al. 1998, Lockley et al. 2021; Lockley 2000, Fig. 7). Eubrontes zigongensis from the Lower Jurassic Zhenzhuchong Formation of Sichuan Province, China (Fig. 6i) has stronger mesaxony (0.49) and wider divarication angle (55°) (Xing et al. 2014b). Eubrontes nianpanshanensis from the Middle Jurassic Xiashaximiao Formation of Sichuan Province (Fig. 6n) has much wider divarication angle (64°) (Yang and Yang 1987; Lockley et al. 2013; Xing et al. 2016c). Eubrontes HX-T3 tracks from the same Jiaguan Formation show similar mesaxony but wider divarication angles (53°–59°; Xing et al. 2016c) (Fig. 6v). Eubrontes BJA-T4, from the Lower Cretaceous Feitianshan Formation, has stronger mesaxony (Xing et al. 2016b) (Fig. 6w).
JYX-T1 also differs from all other Eubrontes tracks in two features. First, the metatarsophalangeal pad II of JYX-T1 is well developed, with a size similar to the phalangeal pad I of digit IV connected with the metatarsophalangeal pad IV; Second, JYX-T1 has a fairly long digit II, or shorter digit III. Excluding metatarsophalangeal pads and claw marks, the length of digit II is 85% as the length of digit III. The length ratio (85%) is obviously different from the other aforementioned Eubrontes, of which only E. nianpanshanensis has a ratio of over 80%.
The length of digit III was measured in relation to the overall pes length (Demathieu 1990; Lockley 2000; Lockley et al. 2021) to show that the digit III/pes length ratio of the Early Jurassic Eubrontes was ~ 0.70, compared with the ratio of ~ 0.60 for the Middle–Late Jurassic Megalosauripus (Lockley et al. 2021). A mean value of 0.64 was computed for the Early Jurassic Eubrontes from a large sample of well-preserved tracks from Utah, USA (Lockley et al. 2021). The digit III/pes length ratio has been calculated (Table 2) and illustrated (Fig. 6), mostly based on measurements obtained from individual tracks, for all the 23 morphotypes for the JYX-T1 series, including 18 morphotypes named at the ichnospecies level. The range of the digit III/pes length ratio varies from 0.57 to 0.71 with a mean value of about 0.65, indicating a slightly greater digitigrady than type Eubrontes but very close to the Utah sample. Meanwhile, the digit III/pes length ratio of the E. nobotai based on all tracks in the trackway is as similar as the case for Eubrontes Utah 2 (T3) (0.65 vs. 0.64). The relative length of digit III within the footprint is a potential measure of digitigrady. Arguably, the presence or absence of a digit II metatarsal phalangeal pad is also a measure of digitigrady and has been proved useful in ichnotaxonomic comparisons. For example, E. nobotai is closer to type Eubrontes giganteus than to other Eubrontes morphotypes in having a clear MTP II trace. For this reason, erecting the new ichnospecies under the unequivocal ichnogenus label Eubrontes is strengthened.
The Early Jurassic Eubrontes veillonensis tracks from Vendée, France (de Lapparent and Montenat 1967; Fig. 6g) are wider with stronger mesaxony. The Early Jurassic Eubrontes platypus (Hitchcock 1858) Xiyang specimen (Yang and Yang 1987) (Fig. 6j) and Eubrontes xiyangensis (Zhen et al. 1986; Lockley et al. 2013) (Fig. 6l) from Yunnan Province, China are poorly preserved, and then the original specimen needs more comparison. The Early Jurassic Eubrontes (?Changpeipus) pareschequier tracks (Xing et al. 2009b; Xing et al. 2014c) from Yunnan Province have stronger mesaxony and wider divarication angles. Eubrontes (?) glenrosensis (Irenesauripus glenrosensis, of Langston 1974) are larger than other Eubrontes, and have wider interdigital divarication angles.
Other theropod tracks
JYX-TI1 and JYX-TI2 (Figs. 2, 7) are small-sized tridactyl tracks located on one side of the same slab as the JYX-T1 trackway. The two footprints do not belong to the same trackway but are similar in morphology and length (12 cm and 11.3 cm). The tracks lack clear heel traces and thus have lower length/width ratios and wider interdigital divarication angles, the former ratios are 1.3 and 1.4 respectively and the latter angles are 47° and 50°. The phalangeal pads of JYX-TI1 and TI2 are unrecognizable, and the tracks may therefore be undertracks. Considering the length/width ratios and the relatively sharp claw marks, JYX-TI1 and TI2 show an affinity with theropod tracks.
JYX-TI4 (Fig. 3) is well-preserved. Its length and width are 19.7 cm and 16.1 cm, respectively, and the mean length/width ratio is 1.2. Digit III projects the farthest anteriorly, followed by a smaller metatarsophalangeal pad posterior to digit II and another larger metatarsophalangeal pad posterior to digit IV. The larger metatarsophalangeal pad is closer to the line of the axis of digit III. Digit pad impressions are unambiguous only in digit II and III; the formula (including metatarsophalangeal pads II and IV) is x-3-3-?-x. Each digit has a sharp claw trace, and that of digit II is the clearest and longest. In general, the digits have relatively wide divarication angles between digits II and IV (61°). The tracks display weak mesaxony (0.40). Based on the pattern of metatarsophalangeal pads, JYX-TI4 is similar to Early Cretaceous Eubrontid tracks commonly known in the Jiaguan Formation (Xing et al. 2016a).
JYX-TI3, a single track and JYX-T2, a pair of tracks likely representing a step (Fig. 7) are clearly peculiar in morphology due to extramorphological (preservational) factors. All three tracks are longitudinal ovate casts without discernable morphological features. The more distal one of the two consecutive tracks in JYX-T2 shows faint traces of a tridactyl morphology, consistent with the length/width ratio, step and apparently narrow trackway. JYX-TI3 is 12.4 cm in length, with a length/width ratio of 1.7. There is a short, round impression on the right side of the track, near the heel. This could conceivably be interpreted as the short digit II trace characteristic of deinonychosaurian tracks. However, although variably, even poorly preserved deinonychosaurian tracks are known from similar facies in the Cretaceous of China (Zhen et al. 1994; Li et al. 2007; Xing et al. 2009a), the sample size and quality of preservation in JYX-TI3 and JYX-T2 do not allow such tenuous speculation.
The mean length of the two tracks included in JYX-T2 is 12.6 cm, and the mean length/width ratio is 1.6. There are some shallow longitudinal indentations on the JYX-T2 tracks, which may be interdigital gaps or folds formed by deposition or weathering. Both tracks lie on a straight line, with a pace length of 61.4 cm, and therefore have a high affinity with theropod tracks.
Sauropod trackways discovered at the Jinyuxi site (Figs. 8, 9 and 10) are imperfectly preserved and variable in size. The mean length of the pes prints of JYX-S1–S4 is 37.4 cm, 31.2 cm, 49.0 cm, and 45.0 cm, respectively. The JYX-S1–S3 trackways contain tracks that are morphologically similar. The JYX-S2 and JYX-S3 trackways are both orientated from northwest to southeast. JYX-S1 is oriented from northeast to southwest. The slab bearing JYX-S4 (Fig. 10) has been stripped from the basement.
The newly exposed JYX-S4 is preserved best, consisting of a set of complete left tracks LP1-LM1, a set of complete right tracks RP2-RM2, and one incomplete right manus print RM1. The manus prints are oval, with obscure digit traces and metacarpophalangeal region. The pes prints are oval with long axes being outwardly rotated, and with anterior portion being wider than the posterior portion. The mean length of the pes and manus from the JYX-S4 trackway are 45.0 cm and 28.3 cm. The manus prints locate anteriorly to anteromedially to the pes prints. The mean length/width ratio of the manus and pes prints are 0.9 and 1.2 respectively. Traces of the claws on digits I–III are recognizable. Digit I is developed best with the deepest trace. There is an indentation in the middle of the trace, which may represent the gap between the phalangeal pad I and the claw. The boundary between traces of digits I and II, and the boundary between traces of digits II and III are both obscure. The preservation of digit IV is different and shows that the track LP1 has a distinct digit trace while the trace of the RP2 is not so clear, and the metatarsophalangeal region is smoothly curved.
JYX-S1–S3 trackways have been seriously weathered and the JYX-S1 is relatively best preserved. Both JYX-S1 and JYX-S2 have pes prints with anterior manus prints, while JYX-S3 only has pes prints. Though JYX-S1–S3 trackways are different in size, their morphology is almost identical. The mean pes length respectively for the JYX-S1–S3 trackways is 37.4 cm, 31.2 cm and 49.0 cm. In the JYX-S1 trackway, the manus and pes prints are oval; digits I and V are discernible and the metatarsophalangeal region is smoothly curved but the claw traces are unidentifiable. The long axes of the pes prints are outwardly rotated by approximately 32° on average, and the anterior portion of the pes prints is wider than the posterior portion. The manus prints are rotated outward by approximately 17° on average from the trackway axis. The average pace angulation of the manus traces is 78°, while the average pes pace angulation is 99°. The mean length/width ratios of the manus and pes prints of the JYX-S1 are 0.9 and 1.1, respectively. JYX-S2 and JYX-S3 are basically consistent with JYX-S1 in features and measurement data.
Pes and manus morphology and trackway configuration of the quadruped trackways at Jinyuxi track site are typical of sauropods (Lockley and Hunt 1995; Lockley 1999, 2001). Three Jinyuxi sauropod trackways JYX-S1–S3 show definite WAP/P’ML ratios of 1.2, 1.2, and 1.1 respectively. According to Marty et al. (2010), the WAP/P’ML ratio of 1.0 differentiates sauropod trackways between the narrow-gauge and the medium-gauge, and the WAP/P’ML ratio of 1.2 is selected as a divide between the medium-gauge and wide-gauge trackways. Therefore, the JYX-S1–S3 trackways are the medium- to wide-gauge.
The mean heteropody (ratio of manus to pes size) of JYX-S1 is 1:2.1 (2.5, 1.9 and 1.8, n = 3), the mean heteropody of JYX-S2 is 1:2.2 (2.1, 2.1 and 2.5, n = 3), and that of JYX-S4 is 1:1.7 (1.6 and 1.8, n = 2). They are all lower than that of Brontopodus birdi (1:3) and of Parabrontopodus (1:4 or 1:5) (Lockley et al. 1994). Most sauropod trackways in China are medium- and/or wide-gauge and are therefore referred to the ichnogenus Brontopodus (Lockley et al. 2002). The Jingyuxi sauropod trackways are consistent with the characteristics of Brontopodus type tracks like: (1) being medium- to wide-gauge; (2) having large outwardly directed pes tracks of which their length is longer than width; (3) having oval or U-shaped manus prints; (4) having a relatively low degree of heteropody (Farlow et al. 1989; Farlow 1992; Lockley et al. 1994; Santos et al. 2009; Xing et al. 2016a). Brontopodus trackmakers have long been considered to be Titanosauriformes (Wilson and Carrano 1999).
For sauropods, Thulborn (1990) estimated the hip height as: h = 5.9 × foot length, while González Riga (2011) estimated the hip height as: h = 4.586 × foot length. Further, the relative stride length (SL/h) is commonly used to determine whether an animal was walking (SL/h ≤ 2.0), trotting (2.0 < SL/h < 2.9), or running (SL/h ≥ 2.9) (Alexander 1976; Thulborn 1990). The SL/h ratios of the Jinyuxi sauropod trackways JYX-S1–S3 respectively range between 0.52–0.67, 0.54–0.7, and 0.51–0.66, accordingly suggesting that the sauropods were walking. Using the equation proposed by Alexander (1976) to estimate the speed from trackways, the mean locomotion speed of the trackmakers from JYX-S1–S3 is between 1.4 km/h and 2.12 km/h. This speed range is typical for sauropods, for which the estimated speed is always low.