- Copyright © 2016, SEPM (Society for Sedimentary Geology)
In 2015, a group of small predominantly tridactyl tracks was discovered in the lower Shawan Member of the Lufeng Formation during an expedition to the Dalishu area, Lufeng County, Yunnan Province, China. The tracks are attributable to Anomoepus, and although mostly tridactyl, they include a few examples with characteristic tetradactyl morphology. Although considered a characteristic index ichnotaxon of a footprint-based Lower Jurassic biochron, Anomoepus has often been overlooked in assemblages dominated by theropod tracks. This is one of the earliest Anomoepus records from the Jurassic of China, and the fifth report of Anomoepus from China at all. To date, four reports represent sites inferred to be Lower or Middle Jurassic in age, with one dated as Upper Jurassic. It is an important component of Early Jurassic ichnofaunas because it points to the presence of ornithischian trackmakers, which are often rare or missing in the local skeletal faunas (Lufengosaurus faunas).
Anomoepus is a classic dinosaur ichnogenus. The first specimens were discovered from the Early Jurassic Newark Supergroup rift strata of the Connecticut Valley (Hartford and Deerfield basins) and were named by Hitchcock in 1848. Abundant well-preserved materials allowed the Anomoepus trackmaker to be recognized as an early ornithischian (Olsen and Rainforth 2003; Lockley and Gierliński 2006) and provided detailed information on morphological diagnostic characters such as manus impressions (e.g., Hitchcock 1858). Currently, the ichnogenus is known from eastern and western North America, Europe, southern Africa (Kent et al. 1995), and China (Lockley and Matsukawa 2009). The Anomoepus–Grallator (Eubrontes) Zone is considered a track assemblage typical of Liassic Age (Haubold 1984, 1986; Lucas 2007; Lockley et al. 2013).
In China, Anomoepus tracks are primarily found in Lower–Middle Jurassic formations in the southwest of Sichuan Province (Lockley and Matsukawa 2009) and in the Ordos Basin, including Jiaoping (Young 1966; Xing et al. 2015), Shenmu (Li et al. 2012), and Zizhou (Xing et al. 2015). Some tracks are also known from the Hailiutu Basin, near the Ordos Basin (Li et al. 2010). In China, Anomoepus tracks usually co-occur with Grallator–Eubrontes–Kayentapus and Deltapodus (Xing et al. 2015). The Upper Jurassic Nan'an site in Chongqing municipality, southwestern China, also preserves Anomoepus tracks (Xing et al. 2013).
In 2015, a fossil hunting expedition was launched into the Dalishu area in Lufeng County, in accordance with the “2013 National Project for Preservation of Geologic Relics of Dinosaurs in Lufeng” and funded by the Ministry of Land and Resources. Authors LX, TW, and ZW of this report were part of this expedition and discovered a group of tridactyl tracks approximately 600 m northwest of the main Dalishu tracksite, where large ornithischians and theropod tracks were discovered (Xing et al. 2016). The location was catalogued as Dalishu tracksite III (GPS: 24°56′30.65"N, 102° 0′23.26"E, WGS84) under the abbreviation DLSIII (Fig. 1A, 1B).
In the following we give a detailed description of this assemblage, which is important because it documents the presence of small ornithischians as a significant component of the Lower Jurassic Lufeng Formation, not reflected in the skeletal record thus far.
The 3D photogrammetry image was compiled from 32 images (average elevation from subject was 1.01 m) taken with a Canon EOS 5D Mark III (Focal Lengths of 67, 70, 84, and 85 mm, resolution 3840 × 2560, pixel size 9.65398 μm) using Agisoft Photoscan Professional (v.1.0.4) model error 0.147 pix and Cloud Compare (v.2.5.3). Outline tracings of tracks on transparency film were taken from original specimens and digitized with vector-based drawing software. Measurements are based on standard parameters such as maximum length of footprint (ML), maximum width of footprint (MW), ML/MW ratio, divarication angle between digit traces II–IV, mesaxony value based on length/width of anterior triangle (AT) using the method of Weems (1992), pace length (PL), stride length (SL) and pace angulation (PA) (Table 1).
The exposed Red Beds in the Lufeng Basin are from the Lufeng Series, which was previously divided into the Lower Jurassic Lower Lufeng Formation (dark purple beds) and the Middle Jurassic Upper Lufeng Formation (deep red beds) (Bien 1941; Sheng et al. 1962). Fang et al. (2000) subsequently studied the stratigraphic section at Lao Changqing–Da Jianfeng, in the Chuanjie Basin, and restricted the name “Lufeng Formation” to what was previously known as the lower Lufeng Formation. They divided the redefined Lufeng Formation into the Shawan and Zhangjia'ao Members (Fig. 1C). The Lufeng Formation is a succession of mudstone, siltstone, sandstone, and limestone deposited in piedmont plain, lake and fluvial environments (Luo and Wu 1995; Tan 1997).
Dalishu tracksite III is within the protection of Lufeng Dinosaur National Geological Park, Yunnan Province. It is located about 50 m west of the bonebed where the coelophysoid theropod dinosaur Panguraptor was found (You et al. 2014). The tracks are preserved in a fluvial sandstone layer within the lower Shawan Member. Biostratigraphic correlations (Luo and Wu 1995) indicate an early–middle Early Jurassic (Hettangian–Pliensbachian) age, whereas magnetostratigraphic analysis indicates a late Sinemurian–?Toarcian age (Huang et al. 2009).
Invertebrate Tracescf. Gordiaichnosp
Burrows (3–4 mm thick) preserved in convex hyporelief on siltstone beds in lacustrine deposits of the Lufeng Formation. The burrows are smooth, loosely winding, with a marked tendency to cross. Long diameter of a loop is up to 8.5 cm (Fig. 2).
Although the specimen resembles Gordia arcuata Ksiazkiewicz 1977, it is incomplete, and therefore identified only tentatively. The ichnogenus Gordia was originally defined based on its resemblance to Gordius, a freshwater worm usually called the hair-worm (Emmons 1844). Gordia is a facies-crossing form and ranges from Vendian (Narbonne and Hofmann 1987) to Holocene (Ratcliffe and Fagerstrom 1980).Palaeophycus tubularisHall 1847
Straight to slightly curved, generally smooth, horizontal, commonly collapsed cylindrical burrows, 2–3 mm in diameter and up to 30 cm long. Burrow density is more or less high. Wall-lining is thin, less than 1 mm. Burrow-fill is similar to host rock and typically massive. Burrow surfaces are typically smooth, but may be rugose in places. The burrows are observed in convex hyporelief on the surfaces of siltstone beds (Fig. 2).
The thin wall-lining of Palaeophycus tubularis is distinct from that of other ichnospecies of Palaeophycus, P. alternatus, P. herberti, P. striatus, and P. sulcatus. Palaeophycus is a eurybathic form ranging from late Precambrian (Narbonne and Hofmann 1987) to Pleistocene (D'Alessandro and Bromley 1986).
Palaeophycus-Gordia assemblages represent a nearshore environment, belonging to the Cruziana ichnofacies (Zhang 1991). Based on the general chemical and trace element composition of its deposits, the lower part of the Lufeng Formation includes shallow lacustrine strata (Tan 1997).
SYSTEMATIC PALEOICHNOLOGYOrnithischia Seeley 1887 ?Thyreophora Nopcsa 1923 Ichnofamily Anomoepodidae Lull 1904 Anomoepus Hitchcock 1848
(After Olsen and Rainforth 2003). Small tracks (pes imprints < 20 cm long) of a facultative biped with wide digit divarication and weak mesaxony. Pes imprints tetradactyl, but functionally tridactyl. Sitting tracks with metatarsal and pentadactyl manus impressions. Metatarsal-phalangeal pad of digit IV almost directly in line with axis of digit III in walking tracks. Pedal digit I (hallux) relatively long and often at least partially impressed, especially in sitting tracks. In the manus, digit III longest with other digits decreasing in length symmetrically away from that digit. Tail trace usually present in sitting tracks. Fully bipedal as well as quadrupedal walking trackways occur.
Anomoepus scambus Hitchcock 1848. Anomoepus isp.
All tracks are preserved as concave epireliefs and are distributed on two adjacent sandstone surfaces. Fifteen natural molds (14 in lower surface I, one in higher surface II) of small (9–13.5 cm in length) tri-tetradactyl footprints of bipedal dinosaurs from surface I of Dalishu tracksite III have been cataloged as DLSIII-O1-R1–R2, O2-L1–R1, and DLSIII-OI1–11 (Fig. 3; Table 1). Surface II is slightly higher and preserves only one track, which has been cataloged as DLSIII-OI12. DLSIII-OI12 was collected and, stored at the museum of Lufeng Dinosaur National Geological Park. All other tracks remain in the field.
Description and Comparisons.—
Trackway DLSIII-O1 consists of three pes imprints and DLSIII-O2 of two pes imprints that are rotated toward the midline (as measured by inward rotation of the trace of digit III). Manus and tail traces are absent.
DLSIII-O1 tracks (Figs. 4, 5) are tetradactyl and the length/width ratios of these tracks range from 1.0 to 1.1. A soft substrate may explain the sub-optimal morphological features in the DLSIII-O1 tracks, which include the raised middle part and indistinct digital pads. However, all imprints preserve digit I (hallux) traces. The digit I traces are relatively long and clearly oriented backward and towards the median axis of the trackway. Digit III of DLSIII-O1-L1 shows inward displacement. Trackway DLSIII-O1 is narrow (pace angulation about 152°) and is characterized by comparatively short stride lengths (57 cm on average), given a mean footprint length of 10.8 cm.
Other tracks from surface I are tridactyl but, because they are relatively shallow, they have no clear digit pads and no distinct claw traces (Figs. 6, 7). However, they share the same overall morphology of the digit group II–IV with wide digit divarication angles (77°–115°) and weak mesaxony (0.41). Furthermore, the proximal region of digits II and IV form a U-shaped metatarsophalangeal region that is aligned with the axis of digit III. These features are diagnostic and support an assignment to Anomoepus.
DLSIII-OI12, from surface II, is the most representative of the track morphology (Fig. 6). The length/width ratio is 0.8; digit III projects the farthest anteriorly. Pads are preserved with the proximal one of digit III being well-rounded and nearly circular in shape. A phalangeal pad formula (including metatarso-phalangeal pad IV) of x-2-3-4-x is recognized. The claw traces are relatively blunt. The metatarsophalangeal pad of digit IV is located in line with the axis of digit III. The divarication angle is wide (111°), and the anterior triangle length/width ratio is 0.48.
The morphology of the small tridactyl tracks from DLSIII strongly resembles that of the ichnogenus Anomoepus, being similar in size and having wide divarication angles, weak mesaxony, and short stride lengths. Thus, the DLSIII tracks are assigned here tentatively to Anomoepus isp. The preservation does not allow a distinct ichnospecies determination.
ANOMOEPUS FROM CHINA
In China, most Anomoepus tracks are found in Lower–Middle Jurassic formations (Table 2). The first discovery was made by Lockley and Matsukawa (2009) at the Middle Jurassic Jinlijing site in Sichuan Province. The digit pads of Jinlijing Anomoepus isp. are indistinct as they are poorly preserved, but wide divarication angles and weak mesaxony together with inward rotation indicate that they belong to Anomoepus. Size differences demonstrate local Anomoepus trackmakers of variable size. In this same study these authors also recognized tetradactyl Anomoepus among specimens in the Chongqing Natural History Museum that were rescued from a site in the city since destroyed by construction (Xing et al. 2013).
In northern China, Anomoepus tracks are found at the Lower Jurassic Wulatezhongqi site in Inner Mongolia (Li et al. 2010), the Lower Jurassic Lijiananwa site in Shaanxi Province (Li et al. 2012), and the Middle Jurassic Huo and Wang sites in Shaanxi Province (Xing et al. 2015). Xing et al. (2015) assigned Shensipus tungchuanensis Young 1966 from the Middle Jurassic Jiaoping Coal Mine site, Shaanxi Province, to Anomoepus tungchuanensis n. comb. All these tracks, except specimens from the Lijiananwa site, which have yet to be described in detail, have characteristics of Anomoepus, such as wide divarication angles (80°–91°) and weak mesaxony (0.40–0.60). WLT T7-12 from the Wulatezhongqi site has a metatarsal pad and a relatively long digit I trace (Fig. 8). Li et al. (2012) referred the Wulatezhongqi specimens to Anomoepus intermedius Hitchcock 1865. However, Olsen and Rainforth (2003) consider A. intermedius to be a subjective synonym of A. scambus. Generally, these records suggest that Anomoepus type trackmakers had a relatively wide distribution in the Ordos Basin and the nearby Hailiutu Basin during the Early–Middle Jurassic.
cf. Anomoepus found at the Upper Jurassic Nan'an site of Chongqing municipality, in southwestern China, is larger in size (more than 20 cm) (Xing et al. 2013) and has an anterior triangle length/width ratio of 0.58, both of which differ from all Lower–Middle Jurassic Anomoepus known from China.
DLSIII Anomoepus isp. shows a length/width ratio and mesaxony similar to most Anomoepus from China, while its divarication angles between digits II and IV are larger. This discrepancy may reflect a preservation artifact or regional divergence in trackmaker types.
The records of both skeletal and track fossils suggest that the Lufeng Basin and the Sichuan Basin had similar dinosaur faunas during the Jurassic (Peng et al. 2005). Though the Sichuan Basin record lacks ornithischians from the Early Jurassic, it shows diversified small Neornithischia from the Middle Jurassic, including Xiaosaurus Dong and Tang 1983, Agilisaurus Peng 1990, and Hexinlusaurus Barrett et al. 2005. All these taxa are 1.4–2 m long (Peng et al. 2005). Estimates from footprints are based on the hip height of small ornithopods, using 4.8× the maximum length of the pes trace as the hip height conversion factor (Thulborn 1990), and the average ratio of hip height to body length of 1:2.63 (Xing et al. 2009). The trackmakers of DLSIII Anomoepus isp. are thereby estimated to have been 1.3 m long, similar to those of Neornithischia from the Sichuan Basin.
Although Anomoepus was originally discovered in New England in the eastern United States (Hitchcock 1848) and has been repeatedly reported and described or re-described from that area (Olsen and Rainforth 2003), it was not widely recognized or described from other regions until quite recently, probably because it was often mistaken for a small theropod track. For example, despite abundant reports of classic Lower Jurassic Grallator and Eubrontes dominated assemblages in the western United States (Lockley and Hunt 1995; Lucas 2007), Anomoepus occurrences were not described or known in much detail from this area until 2006 (Lockley and Gierlinski 2006). Although Anomoepus is considered a characteristic track type of the Lower Jurassic ichnofaunal biochron (Lucas 2007), it also occurs in the Middle and Upper Jurassic of China and North America (Lockley and Gierlinski 2006). To date, Anomoepus tracks are poorly known from the Jurassic of Europe where they have mostly been reported from Poland as a Lower Jurassic record (Gierlinski and Pienkowski 1999; Gierlinski et al. 2004). Purported Triassic (?late Norian–Rhaetian) occurrences from Poland and Slovakia (Niedźwiedzki 2011) are doubtful. In China, Anomoepus was not recognized as a significant component of Jurassic ichnofaunas until 2009. Since then it has also been recognized as a component of Middle and Upper Jurassic assemblages in China and elsewhere (Lockley and Gierlinski 2006).
Based on its original occurrence in the Lower Jurassic of New England (Hitchcock 1848) and recurrence in other Lower Jurassic assemblages, Anomoepus was considered as a characteristic ichnogenus of the Lower Jurassic biochron of Lucas (2007). According to this author, it is the first ichnologically based biochron with a global distribution. This is confirmed by the aforementioned Lower Jurassic occurrences in Europe, Asia and North America, but it should be noted that Anomoepus also occurs in the Middle and Late Jurassic, for example in the Late Jurassic of Spain (Lockley et al. 2008). Therefore, it is not an exclusively Lower Jurassic ichnogenus. In this regard we also note that Anomoepus is not a mono-ichnospecific ichnogenus as claimed by Olsen and Rainforth (2003); see Lockley and Gierlinski (2006) for discussion.
Some of the tracks exhibit a morphology associated with soft sediment collapse around digits. This is particularly evident for DSLIII-O1-R1 (Fig. 5). This track displays a raised area in the center of the track, at the convergence of the digit impressions. The raised portion within the track possesses two raised areas (Fig. 5A), and an inverted ‘v' shape at the posterior end, a morphology highly similar to theropod tracks made in soft mud described by Gatesy et al. (1999, fig. 1c, d), and by Falkingham and Gatesy (2014, fig. 4) in simulated dry granular media. These were shown by those authors to be produced as the foot exits the sediment. The presence of this morphology suggests that the infill is likely to be part of the original track, and that the surface currently exposed may not be the original tracking surface.
Small predominantly tridactyl tracks from the lower Shawan Member of the Lufeng Formation in the Dalishu area, Lufeng County, Yunnan represent the fifth report of Anomoepus from the Jurassic of China. A few examples show a diagnostic tetradactyl morphology.
Most reports of Anomoepus from China are associated with sites dated as Lower or Middle Jurassic. This stratigraphic occurrence is consistent with the occurrence of the ichnogenus in the globally widespread Lower Jurassic biochron of Lucas (2007).
Anomoepus appears to be less common in the Jurassic than theropod tracks, which are ubiquitous in most regions. For this reason they may have been overlooked, and the increase in reports of Anomoepus in recent years may reflect improved ability by ichnologists to identify this ichnogenus.
The occurrences reported and reviewed here show that Anomoepus is an important component of Early Jurassic ichnofaunas indicating the presence of ornithischian trackmakers in biotas which locally lack or contain only rare ornithischian skeletal remains.
The authors thank Xing Xu (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences) for his critical comments and suggestions on this paper. This research was supported by the 2013 and 2015 support fund for graduate students' science and technology innovation from China University of Geosciences (Beijing), China, and the World Dinosaur Valley Park, Yunnan Province, China. Reviewer B. Breithaupt, three anonymous reviewers, and editors A. Fiorillo and T. Olszweski are thanked for comments and suggestions that improved the paper.
- Received September 30, 2015.
- Accepted September 25, 2016.