Mesa Chelonia, 25km ENE of Shanshan, Turpan Basin (Jurassic to of China)

Where: Xinjiang, China (43.0° N, 90.5° E: paleocoordinates 43.4° N, 98.4° E)

• coordinate estimated from map

• outcrop-level geographic resolution

When: Qigu Formation, Bathonian to Bathonian (168.3 - 163.5 Ma)

• The turtle bone bed is situated at the transition between a rock unit consisting of red and green striped sediments below and a massive unit consisting of deep red sediments above. Based on the above-mentioned sedimentological characteristics, we interpret the turtle bone bed as being situated in the lower part of the Qigu Formation in the Turpan Basin, because this interpretation is consistent with previous stratigraphic schemes. We explicitly note, however, that the classification of sediments in the Turpan Basin is still in its infancy and that more detailed stratigraphic work may arrive at other results in the future. All fossil turtle material is concentrated in a single horizon that is only 10–20 cm thick and laterally restricted to less than 100 m. The stratum dips with approximately 60° to the north and is exposed on the west and east sides of a mesa that is capped by Pleistocene alluvial deposits.

•Published reports on the geology and stratigraphy of the Turpan Basin are sparse (Shao et al. 1999, and references therein) and many uncertainties still remain regarding the absolute age of formations and their correlation with similar units in adjacent basins. The Jurassic sediments are divided into the Early Jurassic Sangonghe Formation, the Middle Jurassic Xishanyao, Sanjianfang, Qiketai, and Qigu Formations (the latter was recently dated in the Junggar Basin with 164.6 Ma±1.4 Ma, Wang and Gao 2012), and the Late Jurassic Karaza Formation (Dong 1997).

Environment/lithology: alluvial fan; quartzose, gray, green, muddy, sandy siltstone and siltstone

• Intermittent tectonics significantly influenced deposition across northwestern China throughout the Jurassic (Eberth et al. 2001). The petrologic immaturity and poor rounding of the sand and silt-sized grains contained in the turtle layer is typical for alluvial systems near newly uplifted orogens. The depositional environment in the Jurassic Turpan Basin can be characterized as a braidplain channel system with episodic flow conditions. Vertisols were deposited in a climate with alternating wet and dry seasons or climate cycles. Small sized, shallow channels, much wider than deep, as well as matrix-supported clasts with a diameter of 1–2 cm, sometimes arranged in arc-shaped bands, indicate bed load transport adjacent to major channels with alternating upper and lower flow regimes.
• The turtle bone bed is situated at the transition between a rock unit consisting of red and green striped sediments below and a massive unit consisting of deep red sediments above...All fossil turtle material is concentrated in a single horizon that is only 10–20 cm thick and laterally restricted to less than 100 m.

•A reddish-brown vertisol horizon indicates influence of pedogenesis in the exposed section below the turtle-rich layer. The horizon has a lumpy texture and faintly shows light-colored, centimeter-thick layers and subvertical knoll

•lines. The vertisol is overlain by a light-colored horizon of fluvial siltstone with arc-shaped bands of lithoclasts, small-scale cross bedding structures, and obscured, internal small-scale ripple marks. There is no erosion to the lower bedding plane of the siltstone layer. However, the upper surface of the partially eroded siltstone layer situated directly below the turtles shows a high relief and is plastically deformed (ESM Fig. 2). The fossiliferous stratum is a grayish-green siltstone with a fine sand fraction that contains subrounded mudstone lithoclasts with a diameter of 1–10 mm, which are especially abundant in the highly fossiliferous zone. The thickness of the stratum, clast size, and turtle bone density rather abruptly decrease towards the marginal zone and gradually decrease toward the margins of the fossiliferous layer. The lower contact of the high concentration zone reveals that the fossiliferous layer is highly irregular in thickness and that it locally penetrates deeply into and/or mingles with the underlying siltstone. In addition to its erosive base, the turtle layer consistently shows irregular bedding planes. Contorted portions of the turtle layer are only a few centimeters wide and contain vertically embedded turtle shells. The turtle shells have a carapace length of 15–20 cm. Numerous carcasses are disintegrated, but abrasion or sorting of bones is not evident. A standard petrographic thin section (ESM Fig. 3) of the turtle layer reveals a poorly sorted siltstone that contains a large number of subangular quartz grains ranging from 20 to 100 μm in diameter. Rounded mudstone clasts from 0.1 to 10 mm in diameter are suspended in the siltstone matrix.

•The immediate top of the turtle layer consists of intercalated yellow-reddish fine-grained mudstones interpreted as overbank deposits. Lumpy weathering and red-colored areas, often void of internal structures, indicate renewed pedogenetic processes. Several detailed profiles along both sides of the mesa (Fig. 2) reveal that lithoclastic conglomerate bands occur in lenses above and below the fossiliferous layer that are limited in lateral extent to several meters. No root structures, invertebrate traces, or evidence of subaerial exposure (e.g., desiccation cracks, caliche) are present in the entire sequence.

Size class: macrofossils

• The prepared block of fossil turtles (dimensions: 1.0 m×0.6 m×0.1 m) was quarried from an exposed portion of the inner zone that was surficially weathered and therefore only included the stratigraphically deeper portions of the fossiliferous layer. The block contained the remains of at least 18 turtles, which were embedded in a weathered, soft matrix (Fig. 3). Several specimens were preserved with limbs and tails preserved in articulation, although the advanced degree of weathering and their fragile nature did not allow preserving these under the available conditions. The remains of at least four skulls were found during preparation. In parts of the block, turtle shells were stacked directly on top of each other, whereas in other portions, turtle shells are stacked against and next to each other. The turtles were therefore buried in a dense stacking pattern.

Collected by Sino-German Cooperation Project in 2008, April 2009, Ocotber 2011

Collection methods: quarrying, surface (in situ),

• The turtle Konzentratlagerstätte described herein was discovered in April 2008 by a team of the Sino-German Cooperation Project, but limited time did not allow for the excavation of fossils or study of the site. In April 2009 and October 2011, subsequent field teams revisited the locality, identified and documented the extent of the fossiliferous layer, and quarried several large turtle-rich blocks of matrix. The quarried fossils are currently being stored in the nearby city of Shanshan, Xinjiang, and will eventually be integrated into the municipal museum that is currently under construction. All specimens have been assigned Sino-German Cooperation Project numbers, which will be deposited with the specimens once the museum is operational. The detailed coordinates of the locality will be archived at the museum as well and will be disclosed to qualified researchers interested in studying the site.

Primary reference: O. Wings, M. Rabi, J. W. Schneider, L. Schwermann, G. Sun, C. Zhou, and W. G. Joyce. 2012. An enormous Jurassic turtle bone bed from the Turpan Basin of Xinjiang, China. Naturwissenschaften 99(11):925-935 [R. Benson/R. Benson/P. Mannion]more details

Purpose of describing collection: taxonomic analysis

PaleoDB collection 135364: authorized by Roger Benson, entered by Roger Benson on 23.10.2012

Creative Commons license: CC BY (attribution)

Taxonomic list

• The extremely high density of fossil turtles at Mesa Chelonia and the absence of other fossils with the exception of some small, isolated ganoid fish scales are puzzling. Neither any plant remains nor other vertebrate groups appear to be present in this layer. This may be due to the consumption of other vertebrates (i.e., fishes) as a food-resource by the turtles, the escape of more amphibious vertebrates such as crocodilians, or —less likely—may indicate that the habitat was too ephemeral to support a more diverse aquatic fauna.
Reptilia
 Testudines - Xinjiangchelyidae
Annemys sp. Sukhanov and Narmandakh 2006 turtle
Including SGP 2009/4, SGP 2009/9, SGP 2009/ 14, SGP 2009/18. three partial shells with various degrees of lateral crushing and an isolated, uncrushed skull.