Death of Dinosaurs in the Early Cretaceous of China (Lujiatun Unit of the Yixian Formation)

At Everything Dinosaur we define science as the “search for truth” and one of the fundamental principles of scientific working is the examination and assessment of evidence which leads to conclusions being drawn and theories put forward.  However, different scientists can examine the similar evidence and come to contrasting conclusions.  Let’s illustrate this point by looking at two scientific papers published recently that both seek to explain the remarkable degree of fossil preservation seen in a sequence of Lower Cretaceous strata laid down in north-eastern China.  Let’s explore the mystery of the “Chinese Dinosaur Pompeii”.

Chinese Pompeii

Last year, a team of international researchers led by Associate Professor of Palaeontology and Stratigraphy at Nanjing University, Baoyu Jiang published a paper that concluded the remarkably well-preserved dinosaur, bird and mammal fossils that form part of the Jehol Biota were created “Pompeii-style” by pyroclastic flows.  A pyroclastic flow is an immense, fast-moving cloud of extremely hot gas and dust that can occur with some types of volcanic eruption.  It would have swept everything before it and killing instantly any unfortunate animal or plant that was in the way.

The research team cited evidence such as criss-crossed cracks on the edges of fossilised bones, evidence of heat stress, microscopic debris showing plant remains that had been blackened by being exposed to very high temperatures prior to fossilisation and hollow bones filled with fine quartz grains, tell-tale signs of a pyroclastic flow.  Although the fossils are some 120 million years old, the same evidence can be found in the bodies of citizens of Pompeii who perished when this Roman town was engulfed by a pyroclastic flow which erupted from Vesuvius back in 79 AD.

Evidence of Sudden and Dramatic Death – Caught in Pyroclastic Flows

Evidence for pyroclastic flows from the Jehol Biota.

Picture credit: Baoyu Jiang

The picture above shows photomicrographs (photographs of images produced under a microscope), showing thin sections of fossilised bone of two relatively common vertebrate fossils from the strata that was investigated.

Psittacosaurus and Confuciusornis

The pictures show a dinosaur, Psittacosaurus and a thin section of the bone fossil from an ancient bird, Confuciusornis (top Psittacosaurus spp. and bottom Confuciusornis spp.).  The white arrows indicate missing bone material and cracks can be seen at both the dorsal and ventral edges of the bone.  This evidence supports the idea that the bones were subjected to intense heat, such as that found in volcanic pyroclastic flows.

Victims of a Pyroclastic Flow?

a = Psittacosaurus, b and c = Confuciusornis fossil material

Picture credit: Baoyu Jiang

Note the position of the limbs in the photographs of the fossils (above), particularly those fossils representing the bird Confuciusornis.  The pose is like that of a boxer.  This pose results from the shortening of muscles and tendons that occurs postmortem and this boxer-like box has been cited as further evidence to support the idea of mass mortality as a result of a pyroclastic event.

Conflicting Views as to How these Fossils were Formed

Associate Professor Baoyu Jiang and his colleagues have studied the flora and fauna preserved in the Lower Cretaceous deposits for many years.  It had been known for some time that volcanoes were active in the area at around this time in the Cretaceous, testament to the frequent eruptions were the many layers of fine, volcanic ash that could be identified in the rock layers.

The paper citing pyroclastic flows as the reason for the remarkable, often three-dimensional preservation of vertebrates led to considerable debate amongst scientists at the time of its publication.  Now another paper has been written, which argues that the fossils of the Lujiatun Member of this Formation do not owe their existence to violent clouds of hot ash, rocks and dust travelling at hurricane speeds, but are the result of slightly more gentle, (but equally dramatic), deposition forces.

Choked with Volcanic Ash

A team of scientists from Bristol University in association with the IVPP (Institute of Vertebrate Palaeontology and Palaeoanthropology – Beijing) and University College, Dublin have reassessed the “Chinese Pompeii” deposits and their fieldwork suggests that the fossils were transported in water which was choked with volcanic ash, rather than have the fossils forming as a result of sudden airborne ash fall.

A New Study Suggests Vertebrates such as Psittacosaurus were Buried by Ash that was Deposited by Water

Overcome by ash carried in water flows not pyroclastic flows.

Picture credit: Bristol University Press Release

The fossils of the Jehol Biota come from the Lower Cretaceous Yixian and Jiufotang Formations.  Both freshwater and terrestrial creatures are found in the same horizons and some scientists have interpreted these deposits as evidence for mass mortality events.  The research group that included the Bristol-based team, set out to explore the events and mechanisms that led to the exceptional preservation.  By analysing in microscopic detail the sediments and residual fossils from the Lujiatun Member (the vicinity of Lujiatun village) and comparing the strata to fossils in the collections of Chinese museums, the scientists concluded that the beautifully preserved specimens of the Lujiatun Unit are not the result of one single, massive catastrophe caused by a volcanic eruption.

Fossil-bearing Sediments were Remobilised and Deposited by Water

Their study suggests that the fossil-bearing sediments were remobilised and deposited by water.  If this is the case, the psittacosaurs, other dinosaurs, primitive mammals and birds for example, were not wiped out by one huge, airborne delivery of volcanic ash, but in multiple flood events which carried very high loads of ash and other debris from volcanoes sweeping all before them and burying the unfortunate animals and plants.

One of the problems that occurs when trying to conduct a study such as this, is that many of the fossils in museum collections have been found by local farmers who then sell on the fossil material.  Not very accurate excavation records are kept and therefore it is often extremely difficult to match up a museum specimen with the actual horizon from which it originated.

Buried by Sediments

Commenting on the research, lead author of the scientific paper that has just been published in the journal of “Palaeogeography, Palaeoclimatology, Palaeoecology”, PhD student Chris Rogers of Bristol University said:

“Without stratigraphic information of the fossils in the field, it was impossible to accurately establish a mode of death for these animals.  Once we established proper placement of these fossils in the sedimentary sequence it became clear that these animals had been buried by sediments that were deposited by water and not by volcaniclastic flows.”

It is likely that the debate over the nature of the Jehol Biota will rumble on (just like a pyroclastic flow), this is an example of groups of scientists building on each other’s work to better understand how certain fossils are formed.  However, they were formed, the Jehol Biota provides palaeontologists with a unique insight into the flora and fauna of this part of the world back in the Early Cretaceous, a time when the Aves were rapidly diversifying and there were important revisions undergoing in both the Mammalia and Reptilia.

For models and replicas of Confuciusornis and other prehistoric creatures: PNSO Age of Dinosaurs Models.