A spectacular new terrestrial Konzentratlagerstatte is introduced from the Turpan
Basin of Xinjiang, China that probably belongs to the late Middle Jurassic Qigu Formation.
It contains a mass accumulation of "xinjiangchelyid" turtles preliminarily identified
as Annemys sp. In the zone with the highest turtle concentration, complete and articulated
turtle skeletons are tightly packed at a density of up to 36 turtles per square meter.
The fossiliferous layer is thickened here and shows an erosional base. This high concentration
zone outcrops approximately 10 m in length and shows no decrease in turtle density
after exposing 2 m of the layer into the hillside. Adjacent is a more expansive zone
of at least 10 m by 30 m. In this region, the fossiliferous layer is evenly thick,
and approximately five, fully disarticulated turtles are present per square meter.
A conservatively estimated 1,800 turtles may, therefore, have been deposited at this
site. It is likely that these aquatic turtles gathered in a retreating water hole
in a riverine environment during a drought, much as some aquatic turtles will do today,
but perished when the habitat dried up completely. A following catastrophic rainfall
event caused a debris flow, possibly channelized in a dry river bed, which transported
complete turtles, disarticulated turtles, and mudstone clasts and deposited them after
a short distance. This taphonomic model is consistent with previous environmental
reconstructions of the Turpan Basin during the late Middle Jurassic in predicting
the episodic breakdown of regional monsoonal circulation resulting in a seasonally
dry climate with severe episodic droughts.
