Diverse dinosaur-, pterosaur-, and bird-track assemblages from the Hakou Formation, Lower Cretaceous of Gansu Province, northwest China

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1 Cretaceous Research 27 (2006) 44e55 Diverse dinosaur-, pterosaur-, and bird-track assemblages from the Hakou Formation, Lower Cretaceous of Gansu Province, northwest China Jianping Zhang a, Daqing Li b, Minglu Li a, Martin G. Lockley c, *, Z. Bai b a China University of Geosciences, Beijing , China b Center for Paleontologic Research, The Third Geological Exploration Institution, Lanzhou , China c Geology Department, University of Colorado at Denver, Denver, CO , USA Received 20 December 2003; accepted in revised form 20 October 2005 Available online 10 February 2006 Abstract Diverse and well-preserved assemblages of dinosaur (theropod, sauropod and ornithopod), pterosaur, and bird tracks from the Hekou Formation (Lower Cretaceous) in the Yellow River (Huang He) valley represent the first significant fossil footprint discoveries in Gansu Province, China. However, the sites are large, visually spectacular, and well-exposed thanks to labor-intensive hand excavation. The sites have the potential for development as educational and tourist destinations. These sites have become one of the National Geoparks in China. Dinosaur tracks include at least two theropod morphotypes that range in size from about 5 to >30 cm in length. Wide-gauge sauropod tracks (Brontopodus) range in size from 25 to 90 cm (pes length) and are the best-preserved examples known from China, with clear claw impressions. One trackway suggests an accelerating/running individual. Parallel ornithopod trackways indicate gregarious behavior. An enigmatic trackway may be a manus-only ornithopod trackway. A pterosaur trackway (cf. Pteraichnus), the first reported from China, consists of 24 consecutive footprints, and is the longest, well-preserved trackway on record. Bird tracks (cf. Aquatilavipes) are also very well preserved. The tracks occur at multiple stratigraphic levels in fluvio-lacustrine sequences of paleosol mudstones and sandstones with mud cracks and wave ripple marks. A minimum ichnodiversity of eight, the highest reported from the Cretaceous of China, is estimated. The saurischian component (theropods and sauropods) compares well with Inner Mongolia ichnofaunas from the Jing Chuan Formation. However, the co-occurrence of ornithopod and sauropod tracks is rare in Asia and globally, and compares with assemblages from South Korea at a similar Cretaceous paleolatitude (ca. 30 ). Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Pterosaur tracks; Bird tracks; Sauropod tracks; Ornithopod tracks; Gansu Province 1. Introduction The discovery of diverse assemblages of dinosaur-, pterosaur-, and bird tracks about 50 km west of Lanzhou City, the capital of Gansu Province, China (Fig. 1), represents the first * Corresponding author. Dinosaur Tracks Museum, University of Colorado at Denver, PO Box , Campus Box 172, Denver, CO , USA. address: martin.lockley@cudenver.edu (M.G. Lockley). significant discovery of Mesozoic vertebrate tracks in this province and one of the most significant of all Chinese fossil footprint sites (Li et al., 2002a,b). The site is referred to as the Yanquoxia site in Yongjing County and was first reported by Du et al. (2001). Preliminary illustrations of the site, which is visually spectacular, appear in Li et al. (2002b) and Lockley and Peterson (2002, p. 67). Small discovery exposures at three main sites in the Hekou Formation were hand excavated to expose more than 600 m 2 of bedding plane surface, revealing about 30 trackways /$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi: /j.cretres

2 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 45 Fig. 1. Locality map showing the location of the Yanguoxia site (modified after Li et al., 2002b). attributable to pterosaurs, birds, theropods, sauropods, ornithopods, and an unknown vertebrate. The local stratigraphic succession consists of a fluvio-lacustrine sequence of paleosol floodplain mudstones and siltstones with interbedded, locally ripple-marked sandstones known to yield fishes (e.g., Lycoptera and Sinamia) and conchostracans (Yanjiestheria). The main purpose of this paper, however, is to present preliminary maps of the three dinosaur track sites, to describe the main track types recorded, and to comment on the paleoichnological significance of the site in the broader context of East Asian vertebrate ichnology. Detailed morphometric studies of the trackways and their sedimentological and paleoecological context will be presented elsewhere. Although the site is at a location that is currently inaccessible by road, it can be reached by boat, along the Yellow River, and by foot, along the course of an infrequently used railway line. Owing to the visually spectacular nature of outcrops at the site (Fig. 2), it is being developed for tourism and public education after the establishment of a National Geopark. 2. Methods Natural exposures of the track-bearing surface were initially associated with small sandstone outcrops on a steep hillside. Because such outcrops cannot be accessed by road, they were excavated by hand. This method, though labor-intensive, has the benefit of not damaging the outcrop. This method has also facilitated the discovery of a number of isolated tracks from overburden, e.g., the bird tracks described by Li et al. (2002b). Each exposed surface was mapped using traditional compass and tape methods, generating the site maps presented herein. Each site was given an informal name that describes the main features of the site. The data obtained from mapping the main site were transferred to computer format to allow colorcoding and numbering of the different trackways. Selected tracks were also traced using acetate overlay. Tracings were then reduced to suitable size for reproduction. Fig. 2. Photograph of main site. Note the steep terrain and the larger exposed surface situated upslope behind standing people. A smaller, stratigraphically younger surface is exposed in the foreground. White flags in background are geologic/stratigraphic markers. Selected tracks were molded with latex and replicated in plaster of Paris. This permitted the distribution of duplicate replicas to the different institutions represented by participants in the project. Various original specimens were also collected from the overburden. Specimens in the collections of the Research Center of Paleontology, Gansu, are designated with the prefix RCPG. Replicas in the University of Colorado at Denver collections have the prefix CU. Tracings are also on file at the University of Colorado at Denver with the prefix T (e.g., T 662-T 690). 3. Description and mapping of main track-bearing outcrops 3.1. The main site The main site (Figs. 2, 3) consists of two large exposures of sandstone bedding plane that dip about 25e30 to the ENE (Fig. 2). Both were excavated by hand. The larger of the two surfaces, situated higher on the hillside but lower stratigraphically, consists of about 350 m 2 of exposure (Fig. 2) and was mapped (Fig. 3). The surface reveals at least seven theropod

3 46 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 and five wide-gauge sauropod trackways (cf. Brontopodus) and a single pterosaur trackway (cf. Pteraichnus). The pterosaur trackway, four of the theropod trackways, and four of the sauropod trackways are well preserved. The higher stratigraphic surface, situated lower on the hillside (Fig. 2), consists of about 100 m 2 of exposure revealing small sauropod and tridactyl tracks. This surface has not been studied in detail for the purposes of the present study The accelerating sauropod site Fig. 3. Preliminary map of main site with orientation corresponding to Fig. 2 (up-dip at top). Note the theropod (white arrows, T1-6), sauropod (black arrows, S1-5), and pterosaur (stippled arrows, P) trackway segments [in boxes] traced and measured for detailed analysis in Figs. 9, 11, and 14, respectively. Dotted circles indicate large, indistinct tracks. Track outlines simplified for clarity. Compare with Figs. 2, 9e11, 13 and 14. The accelerating sauropod track site also consists of a hand-excavated exposure of about 100 m 2 sandstone bedding surface that dips about 25e30 to the ENE. The outcrop reveals at least three theropod and three sauropod trackways (Figs. 4, 5) in addition to the trackway of an unknown vertebrate (Fig. 6). Two of the sauropod trackways represent normal walking animals that left wide-gauge trackways with both manus and pes footprints. The accelerating sauropod trackway (Fig. 5) consists of deep impressions of what appears to be the impressions of pes digits IeIII associated with 13 consecutive footprints (7 left and 6 right). The strides between consecutive right footprints, measured from the map (Fig. 5), vary in length as follows: 2.5, 3.2, 3.2, 2.9, and 4.0 m. Corresponding stride lengths on the left side are 2.8, 2.1, 4.0, 3.2, 2.9, and 3.5 m. Thus, the trend is not a regular lengthening of stride, but clearly increases in the direction of progression (Fig. 5). The preservation of pes claw traces without the impressions of the whole pes or associated manus does not seem possible if the animal was walking on the surface equivalent to the mapped bedding surface now visible. The most parsimonious explanation seems to be Fig. 4. Photograph of accelerating sauropod site. Compare with Fig. 5.

4 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 47 Fig. 6. A, trackway of an unknown vertebrate from Yanguoxia with trackways (BeC) of quadrupedal ornithopods showing manus impressions highlighted in black for comparison. B and C based on large and small trackways from the Dakota Group of Colorado and New Mexico (after Lockley and Hunt, 1995, figs and 5.22, respectively). See text for details. However, as noted below, this interpretation is conjectural, not least because no manus-only ornithopod trackways have previously been reported. Fig. 5. Map of accelerating sauropod site, with accelerating sauropod trackways highlighted in black. Compare with Fig. 4. Inset at lower left shows variation in length of left and right strides. that the track maker was walking on a higher or overlying stratigraphic layer, through which only the pes claws penetrated. A more speculative alternative interpretation is that the track maker was partially buoyed up by water. These explanations are interesting for two reasons, as discussed below. However, the former explanation, of a soft overlying layer, may have a bearing on our understanding of the trackway of the unknown vertebrate (Fig. 6). This trackway consists of five relatively deep, sub-rounded impressions between 6 and 8 cm in diameter (Fig. 5). They are separated by steps of about 85e95 cm, with a corresponding pace angulation of about 115. Such a configuration could conceivably be construed as a manus-only sauropod trackway. However, as indicated below, the smallest sauropod trackway found at this site has much larger manus impressions separated by shorter steps and lower pace angulation values. The only other well-known Mesozoic footprints that resemble these unknown ichnites are the manus impressions of quadrupedal ornithopods. Ornithopod trackways with manus and pes impressions are quite common in the Cretaceous of North America (Lockley and Hunt, 1995), and are also known from China (You and Azuma, 1995) and Europe (Lockley and Wright, 2001). As shown in Fig. 6, the pattern made by consecutive manus tracks, in trackways with pes impressions, is quite similar to the unknown ichnites from Yanguoxia The ornithopod herd site The ornithopod herd track site also consists of a handexcavated exposure of about 100 m 2 sandstone bedding surface (Figs. 7, 8). However, here the dip is somewhat steeper (about 35 to the east). The site reveals at least four parallel, well-preserved, ornithopod trackways associated with an unknown number of less-well-preserved, small sauropod trackways. 4. Description of trackway types The primary purpose of this report is to identify the main track types found at the Yanguoxia site. Research at the site is ongoing and is revealing additional tracks and data as studies progress. A full analysis of all morphometric parameters of exposed trackways is therefore premature and will be dealt with elsewhere Theropod tracks All well- or moderately well-preserved theropod tracks illustrated herein (Fig. 9) originate from the main site. They vary in size from 8 by 6 cm (length:width) to 32 by 26 cm (Fig. 9F and C, respectively). The latter (Fig. 9A) track forms part of a well-preserved trackway (T1; Fig. 3) consisting of 17 consecutive footprints of the largest individual represented on this surface. The average step length is

5 48 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 Fig. 8. Map of ornithopod herd site. Note trackways of small sauropods in background. Compare with Fig. 7. Fig. 7. Photograph of ornithopod herd site. Compare with Fig. 8. about 108 cm. Pace angulation is about 160 and the track axis (digit III) is rotated slightly outward. Digit divarication is about 40. Other tracks shown in Fig. 9 each represents a different trackway. Fig. 9B represents the second largest track maker and the second longest trackway (T2; Fig. 3), consisting of 12 footprints with foot length and width of 27 and 18, respectively. The average step length is about 87 cm, and the pes axis rotation is slightly outward. In comparison with the larger trackway the only obvious difference is size, and we infer that the two track makers may have represented the same species of theropod. Fig. 9D, E by comparison represent trackways of smaller animals that are not represented by continuous well-preserved trackways. Fig. 9D represents an animal with a relatively narrow foot (length 18:width 10 cm), in comparison with the other track makers, and may therefore represent a different species. Similarly the two smallest tracks (Fig. 9E, F) with foot length:width measurements of 16:12 and 8:6, respectively, clearly represent smaller individuals and may also represent different taxa. Thus, we infer that the theropod tracks indicate a diversity of forms. Precise diversity estimates are conjectural but based on the size and shape differences we suggest at least three morphotypes as follows: Morphotype 1, large, foot length >25 cm; wide digit divarication, outward rotation; Morphotype 2, intermediate, foot length (15e20 cm) narrow digit divarication; Morphotype 3, small, foot length less than 10 cm Sauropod tracks Sauropod trackways fall into at least three categories: large sauropod tracks with pes claw impressions from the main site, tracks dominated by pes claw impressions from the accelerating sauropod site, and smaller sauropod trackways without pes claw impressions from the ornithopod herd site. Trackways from the main site (Figs. 10, 11) are clearly wide-gauge with relatively large manus (Fig. 11) and thus can be assigned to the ichnogenus Brontopodus (Lockley et al., 1994a). Both the pes and manus show strong outward rotation. The trackways vary somewhat in size and include examples with and without overprinting of the manus by the pes. Two examples of trackway segments, without overprinting, are indicated on the map of the main site (Fig. 3). The larger of these trackways provided the measurements given in Table 1. This trackway has a mean outer trackway width of 182 cm (N ¼ 5; range 155e205). The inner trackway width, measured from the inside margin of the pes (cf. Fig. 11) is42 (N ¼ 4; range 40e46). A second wide-gauge sauropod trackway (Fig. 11) is somewhat smaller. Its precise dimensions were recorded by

6 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 49 quadripartite track morphology (i.e., three digital and one heel pad). Some heel pads appear oval but others are more triangular with a bi-lobed posterior margin. They resemble trackways that have been named Caririchnium in North America, which may have been made either by bipeds or quadrupeds. Similar tracks from Europe have been named Iguanodontipus (Sarjeant et al., 1998). The axis of the foot (digit III) shows pronounced inward rotation up to about 30. The step is also long ranging from 95e100 cm. The tracks are about as wide as long (28 by 28 cm in the case of the trackway based on tracing T 679). A well-preserved ornithopod track cast in the RCPG collections (Fig. 12) is somewhat smaller and more elongated (length:width ¼ 23:18 cm). The four clear trackways at the ornithopod herd site are parallel and equally deeply impressed. This probably indicates a group heading WSW (towards azimuth 240 approximately) rather than separate individuals passing at different times. Based on footprint dimensions all the animals were about the same size. The inter-trackway spacing is fairly regular at about 1.3 m. Thus, only about 4 m separates the four trackways Pterosaur tracks Fig. 9. Theropod tracks from main site. AeC morphotype 1. A, trackway (T1) segment of largest track maker (corresponding to footprint C). B, second largest track (T2). DeE, morphotype 2, intermediate-sized tracks, note low divarication angle for D. F, morphotype 3, small track. tracing three consecutive manus pes sets (T 707). The mean pes length and width is 64 and 52 cm, respectively, and corresponding manus measurements 34 and 43 cm (N ¼ 3 in all cases). Outer trackway width ranges from 134e166 cm (mean 150) and inner trackway width averages about 33 cm. Small sauropod tracks and trackways are associated with the ornithopod herd site. Most are somewhat indistinct. However, one clear trackway segment consisting of three consecutive manus-pes sets is clearly discernible. The pes tracks average about 22 by 16 cm (length:width) and the manus tracks 11 by 16 cm (N ¼ 3). The step is about 55 cm, the stride 88 cm, the pes and manus pace angulation about 110 and 95, respectively, and the inner and outer trackway widths are 15 and 55 cm, respectively. The pes tracks lack claw impressions and appear to show minimal outward rotation of the long axis. The manus tracks show outward rotation and are situated somewhat further from the trackway axis than the pes Ornithopod tracks Ornithopod tracks are preserved as in situ trackways (Figs. 7, 8) and as natural casts (Fig. 12). The track makers were evidently bipeds (i.e., without manus impressions) with a typical Pterosaur tracks discovered from this site are the first known from China (Li et al., 2002b) and only the second example known from Asiadthe first having been reported by Lockley et al. (1997) and Hwang et al. (2002). A photograph (cf. Fig. 13) of a manus pes set was published by Lockley and Peterson (2002, p. 81). The map of the trackway is published here for the first time (Fig. 14). The single trackway consists of 12 manus-pes sets in unbroken sequence. The manus and pes impressions average between about 13 and 14 cm long by about 3 cm wide. The whole trackway was traced (T 680). It is fairly straight but curves slightly to the right (clockwise). This causes the distance between the right and left manus impressions (i.e., from the inner to the outer side of the curve) to be consistently slightly longer than the distance between left and right (i.e., from outer to inner side of the curve). Likewise the left-toleft stride distances, on the outer circumference, are slightly longer than the right-to-right distances on the inner circumference. Manus prints are situated slightly outside the pes prints. Thus, pes-pes angulation values are higher (122e145 ) than manus-manus values (95e120 ). Three manus pes sets were replicated and assigned the numbers CU , and As is typical of many pterosaur trackways the manus impressions are deeper than the pes impressions (Lockley et al., 1995). A few pes impressions, however, show impressions of four claws (Fig. 14) and traces of digital pad impressions. Morphologically the tracks are similar to Pteraichnus, an ichnogenus that dominates the Late Jurassic pterosaur track record. The Gansu tracks, however, are larger than any named Pteraichnus ichnospecies.

7 50 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 Fig. 10. Photographs of sauropod tracks. A, general view of trackway S1. B, detail of manus-pes set from trackway S1. CeD, individual manus and pes, respectively, from trackway S4. Large scale bar represents 50 cm, small scale bar, 20 cm. Compare with Fig. 3.

8 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 51 Table 1 Measurements for a large sauropod trackway Pes # and size (l:w) Manus # and size Pes-pes stride (cm) Manus-manus stride RP1 e RM1 43:56 RM1-RM2 285 LP1 88:70 LM1 45:52 LP1-LP2 270 LM1-LM2 272 RP2 85:73 RM2 43:56 RP2-RP3 285 RM2-RM3 285 LP2 90:71 LM2 44:50 LP2-LP3 285 LM2-LM3 285 RP3 90:80 RM3 40:60 LP3 90:75 LM3 50:50 Mean 88.6: : Bird tracks At least two slabs with small bird tracks were recovered from loose material removed as overburden from strata overlying the main site. The larger specimen (Fig. 15) designated as Research Center of Paleontology, Gansu (RCPG) No. 001 was described as Avipedidae gen. et sp. indet. (Li et al., 2002a). The tracks were described as being similar to Koreanornis (Kim, 1969) and Aquatilavipes swiboldae (Currie, 1981). We are inclined to agree with the latter comparison, based on size and the absence of a hallux impression. As shown by Lockley et al. (1992), Koreanornis is a very small track, rarely exceeding 2.5e3.0 cm in length or width, which sometimes shows a hallux impression. 5. Paleoecological and paleoenvironmental synthesis The Gansu track sites are arguably the most visually and scientifically spectacular currently known in the Cretaceous Fig. 11. Scale diagram drawn from tracing (T 707) of the smaller of two widegauge sauropod trackways (S3) showing clear separation of manus-pes impressions. Compare with Fig. 3. Fig. 12. Photograph of ornithopod track cast. of China. In addition to yielding the first known Chinese pterosaur tracks, they have produced the longest and best-preserved sauropod trackways, and the first documented example of multiple parallel ornithopod trackways. The diversity of trackways is also striking. A preliminary list of track types suggests up to nine different track makers as follows: (1) Theropod morphotype 1; (2) Theropod morphotype 2; (3) Theropod morphotype 3; (4) Large wide-gauge sauropod; outwardly rotated pes (Brontopodus); (5) Small wide-gauge sauropod; minimal pes rotation; (6) Bipedal ornithopod (cf. Caririchnium); (7) Unknown vertebrate (possibly a quadrupedal ornithopod); (8) Pterosaur (Pteraichnus); (9) Bird (Aquatilavipes). We are not certain that the small and large sauropod trackways represent different track makers. Similarly the diversity represented by the theropod track types is uncertain pending detailed morphometric and ichnotaxonomic analysis. Nevertheless, a cautious survey of track types based on a count of well-preserved trackways recorded on the three surfaces mapped during this study as well as those on the unmapped surface overlying the main site and those in the collection of isolated footprints, which yielded isolated bird and ornithopod tracks, provides a preliminary census of 34 individuals (Table 2; Fig. 16). This census is representative of the number of vertebrate individuals estimated from trackways in This number will certainly increase in the future. It shows that saurischians dominated the ichnofauna. However, ornithopods were also a significant component of the ichnofauna. Future studies are likely to significantly increase the number of trackways known at these sites.

9 52 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 Fig. 13. Photographs of pterosaur tracks. A, pterosaur trackway runs from top to bottom of photo, parallel to scale bar (50 cm) and in the opposite direction to the clear theropod trackway T1. B, detail of manus-pes sets 9e11 with scale bar in same position as in photograph A. Manus 9 and 11 are close to theropod tracks in the upper and lower right of picture, respectively. Pending the results of further studies, the following general paleoichnological inferences are possible. The ichnofauna was saurischian dominated, as is typical of most Cretaceous vertebrate ichnofaunas from inland China. Such ichnofaunas appear to be associated with red-bed depositional sequences deposited in inland basins with semi-arid climatic regimes. The Yanguoxia ichnofauna is similar to that recorded in the late Barremian to Aptian Jing Chuan Formation of Nei Mongol (Lockley et al., 2002), except for the presence of ornithopod and pterosaur tracks. Currently the only other well-documented East Asian sites where ornithopod and sauropod tracks co-occur in significant abundance are found in South Korea (Lim et al., 1994). Our preliminary observations suggest that the Gansu ornithopods were morphologically similar to those found in Korea, and evidently progressed bipedally. Ornithopod tracks, including some that represent quadrupedal progression (You and Azuma, 1995) also occur at a few sites in northeastern China that are associated with humid, coalbearing paleoenvironments situated close to the Cretaceous continental margins. But in such settings sauropod tracks have not been recorded. Recent studies have shown that bird tracks are very common in the Cretaceous of East Asia (Lockley et al., 1992, 2002, 2003; Zhen et al., 1995; Azuma et al., 2002; Li et al., 2002a,b). Without exception these tracks are indistinguishable from those of modern shorebirds such as sandpipers and plovers, though this does not necessarily imply that they belonged to these modern clades. Some, like those from Gansu, Sichuan, and Japan lack a hallux and have been assigned to Aquatilavipes. Others such as Koreanornis and Jindongornipes have a hallux, and yet others such as Uhangrichnus and Hwangsanipes reveal web impressions. Thus, the ichnogeneric diversity of East Asian bird tracks is moderately high, and greater than that recorded for any other region. The implications are that the Cretaceous avifauna of shorebirds or shorebird-like forms was well developed by the Early Cretaceous. This evidence, which comes almost exclusively from the track record associated with lacustrine deposits, compliments the body fossil record of other non-shorebird, or non-shorebird-like clades from the famous Yixian Formation. Like birds, pterosaurs as fliers were capable of widespread distribution. However, the East Asian track record is currently

10 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 53 Table 2 Distribution of main vertebrate tracks types at three mapped sites and in collections (designated as other ) Track type Main site Sauropod site Ornithopod site Other Theropod 6 4 e 1 Sauropod Ornithopod e e 4 2 Pterosaur 1 e e e Bird e e e 2 Unknown taxon e 1 e e pterosaur track occurrences are associated with bird tracks, these two flying forms appear to have coexisted in the same habitat. However, the large size difference may indicate that they occupied different niches. In the Late Jurassic, small pterosaur tracks are very abundant in certain regions and true avian tracks are unknown. In the Cretaceous the pattern reverses, as small pterosaur tracks appear rare in comparison with larger pterosaur tracks and relatively abundant tracks of small birds. It may be that the apparent rarity of small pterosaur tracks in the Cretaceous is a function of inadequate sampling. However, this suggestion, based on negative evidence is not convincing. Cretaceous pterosaur tracks are mostly large in comparison with Jurassic ichnites and so appear to be an accurate reflection of the trend towards increased size during pterosaur evolution. Fig. 14. Map of pterosaur trackway with detail of best manus-pes set (upper left). sparse, consisting only of the Gansu Pteraichnus occurrence, described herein, and Haenamichnus, the largest known pterosaur tracks from the upper Cretaceous of Korea (Hwang et al., 2002). It is interesting to compare the high density of small shorebird-like tracks with the low density of large pterosaur tracks in the Cretaceous of East Asia. Given that both 6. Discussion of unusual trackways A number of unusual trackway configurations from the Gansu site lack obvious analogs in the ichnological literature. These include the pes-only accelerating sauropod trackway and a possible manus-only ornithopod trackway. The irregular pes-only sauropod trackway is here interpreted as evidence of an animal walking on an overlying surface, now removed. The track maker dug its longer claws (pes Fig. 15. Bird-track casts (cf. Aquatilavipes). A, slab with four tracks (cf. Azuma et al., 2002, fig. 3). B, slab with a single track. Both slabs are reversed to show positive top view. Fig. 16. Pie diagram of census of main track types from the Yanguoxia site. See text for details.

11 54 J. Zhang et al. / Cretaceous Research 27 (2006) 44e55 digits IeIII) into the soft underlayers. Assuming the overlying layer to have been soft, as indicated by the depth of the claw impressions in the layer beneath, the irregular trackway may have been the result of the large animal varying its gait to negotiate a difficult and potentially dangerous surface where it may have run the risk of slipping or getting stuck. It is known that sauropods sometimes perished in quagmires (Ayer, 1999). Thus, they presumably avoided such dangers wherever possible. One way to do this would be to move quickly away from substrates that threatened to bog them down. The irregular trackway, with traces of spread or projecting claws could be interpreted as evidence of such a reaction to an unstable substrate. Lockley et al. (1994b) reported somewhat similar ichnological evidence from a Cretaceous site in Texas where a sauropod trackway consists only of underprints of the two largest pes claws and the associated manus impressions. At this site, however, the underprints are shallow, suggesting a firmer substrate, and the trackway is more regular than the Gansu example. Normally, incomplete sauropod trackways preserved as underprints are dominated by manus impressions. Pes-only sauropod trackways may theoretically be the result of overprinting of the manus by the pes. However, this cannot be determined in this case, where only the pes claw traces are preserved on the exposed surface. Given the long history of debate about swimming sauropods (Lockley and Rice, 1990), it would be an oversight to ignore the theoretical possibility that the Gansu trackway was made by a swimming sauropod. However, given the presence of normal walking trackways on the same surface, we consider any swimming sauropod interpretation speculative. The unknown trackway may provide evidence to support the idea that the accelerating sauropod trackway site was overlain by soft sediment, sometime after the shallower tracks were made on the sandstone surface that is now exposed. We have suggested that the unknown trackway (Fig. 6) might represent a manus-only ornithopod trackway. In order for this to occur the small, peg-like front feet would have had to sink deeper than the larger, broader hind feet, as often occurred with sauropods (Lockley et al., 1994b). This scenario could have occurred on a soft substrate. As a facultative biped, an ornithopod might have switched to quadrupedal progression if it began sinking too deeply into the substrate. Although this interpretation is also conjectural, we must bear in mind that the trackway pattern is close to that recorded for the manus portion of trackways made by ornithopods during quadrupedal progression. 7. Conclusions The Yanguoxia site provides a paleoichnological glimpse of diverse dinosaur-pterosaur-bird faunas in northwest China during the Early Cretaceous. In addition to being the most visually spectacular site currently known in this region, systematic excavation is progressively increasing the number of known trackways. Already an estimated diversity of about nine vertebrate ichnotaxa is inferred, which is considerably more than any other contemporary site in East Asia. The following characteristics of the site ichnofauna are noteworthy: (1) This is arguably the most diverse vertebrate track site known from the Mesozoic of Asia. Diversity estimates suggest about nine ichnotaxa including Brontopodus, cf. Pteraichnus, Aquatilavipes, cf. Caririchnium, and various unnamed theropod morphotypes. (2) This is the first pterosaur track site reported from China. (3) This is first well-documented Asian co-occurrence of Cretaceous sauropod and ornithopod tracks outside South Korea. This may suggest mixing of low-latitude sauropod and higher latitude ornithopod ichnofaunas near paleolatitude 30. (4) The saurischian-bird component of the ichnofauna is similar to that reported from deposits of the same age in Nei Mongol. (5) The site reveals a rare pes-only example of an accelerating sauropod. (6) A possible manus only ornithopod trackway may be the first on record. 8. Note added in proof In addition to the two Asian pterosaur track occurrences noted in Section 5, a third discovery was recently reported (Kim et al., 2006). Acknowledgements We are grateful to the Provincial Government of Gansu and the staff of the Bureau of Geology and Resource Exploration of Gansu Province, at Lanzhou, for their hospitality and assistance, without which the excavation of the site would not have been possible. Travel for Martin Lockley was supported in part by the fund of the Grant-in-Aid for Scientific Research of the Japan Society for the Promotion of Science (Matsukawa, no , 1999e2000), and the Grant-in-Aid for University and Society Collaboration of the Japanese Ministry of Education, Science, Sports and Culture (Matsukawa, no , 1999e2001). Travel from Beijing to Lanzhou for Jianping Zhang and Martin Lockley was supported by the Project of Geoheritage Protection of the Ministry of Science and Technology, People s Republic of China. We also thank the China University of Geosciences at Beijing for providing the accommodation for M. Lockley in Beijing. References Ayer, J., The Howe Dinosaurs. 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