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RE: question about the Yixian formation

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] 
> On Behalf Of David Peters
> What is the explanation for all the quickly buried fossils 
> found in the Yixian formation? Is it volcanic ash? shallow waters?

It depends on what part of the Yixian you are talking about. The Lujiatun
Bed (which has the well-preserved 3D specimens, such as the type of Mei and
the parent-and-babies Psittacosaurus, etc.) does seem to be a tuff: an
ashfall deposit, and thus represents a quick mass mortality event rapidly
covered over. (Xu refers to it as the "Cretaceous Pompeii").

But most of the other fossil-bearing horizons (especially the ones with well
preserved integument) were not always rapidly formed. Instead, they are
mostly lake-bottom deposits where the mud is very fine volcaniclastic
sediments (volcanic ash broken down into mud). Like many lake bottoms, the
conditions were at least seasonally anoxic (lacking oxygen), such that decay
organisms and burrowers such as worms and clams could not live there.
Consequently, anything that died and settled to the bottom of these lakes a)
were not disturbed once they settled down and b) left a record of their soft
tissue in the extremely good medium of the mud.

Furisch (sorry, don't know how to umlaut this in ASCII) et al. (2007) looked
at 18 different fossil-bearing bedding planes over a 3 m section, and
proposed a model of summer anoxia and winter reoxygenation:

"Stage (1). During spring and early summer, the lake waters were well
oxygenated" with rich algal mat growth on the bottom.

"Stage (2). The rise in water temperature towards the summer, coupled with
consumption of oxygen by respiration processes and bacterial breakdown of
algal and plant material, gradually led to the establishment of dysoxia and
a corresponding deterioration of living conditions, which slowed down growth
of the organisms." And "oxygen depletion reached a lethal level for the
organisms that populated the lake and mass mortality occurred. The dead
bodies accumulated on the lake floor, where gradually the soft parts
decayed. However, due to the lack of oxygen, decay was too slow to degrade
the organic skeletons as well."


"Stage (3). During autumn and winter, an increased precipitation-induced
discharge of sediment-laden waters into the lake led to mixing of the water
masses, re-established oxic conditions, and deposition of one or several
thin layers of sediment. These sediment layers, often merely a veneer,
protected the organic skeletons from further decay."

Intersperesed with this system were flash-flood events which washed much
higher volumes of sediment into the lakes: these layers may actually be the
ones that covered over the vertebrate fossils most completely, and thus
preserved the best pterosaurs, dinosaurs, etc.

Fursich, F.T.  J. Sha, B. Jiang, Y. Pan. 2007. High resolution
palaeoecological and taphonomic analysis of Early Cretaceous lake biota,
western Liaoning (NE-China). Palaeogeography, Palaeoclimatology,
Palaeoecology 253:434-457.

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Fax: 301-314-9661               

Faculty Director, Earth, Life & Time Program, College Park Scholars
Fax: 301-405-0796

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA