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Methods of sampling and study

In the field

It is intriguing that in deposits known for nearly a hundred years the first bird was not described until 1995 and yet a single fossil quarry at Sihetun has since yielded over 1000 confuciusornithid specimens alone. This results partly from the nature of the sediments and is also testament to the employment of thorough and detailed excavation methods.

The sediments are generally poorly indurated and often crumble on exposure. In order to expose the fossil samples, excavation takes place layer by layer until an impression of a buried specimen is visible on overlying layers.

The sample is then often encased in plaster before cutting the slab free from the rock mass.

[Right] IVPP Excavation site in 2001 at Zhangjiagou, Beipiao, Liaoning. Note large plaster-encased slab awaiting extraction.

In the laboratory

The slab is then transferred to a preparation laboratory where skilled preparators remove the overlying layers, chip by chip using needles, scalpels and drills to reveal the skeletal remains. At the IVPP in Beijing, as many as twenty preparators are working on these priceless fossils.

Slabs showing a fossil reptile before (left) and after (right) preparation.

The removed chips usually will not contain any skeletal material. However, they frequently contain the remnants of any integument including skin impressions or feathers. Historically much of the removed material has been discarded; but ideally the location of each chip is noted and recorded as part of the sample. These chips form the basis of much of the studied material in which melanosomes have been observed.

Several hundred chips may be collected from a single specimen and whilst individually of little visual impact, collectively they may hold the information necessary to map the entire colour of a bird.

Mounted sample chips from a Confuciusornis specimen (IVPP V13171) - in total over 300 chips from this bird have been examined.

Chips are mounted onto stubs for optical observation and initial SEM screening performed without a conductive coating to preserve the pristine state. The SEM has to be operated in Variable Pressure (VP-SEM) mode, and observations are restricted to backscatter electron imaging. However this is sufficient for identifying key components of the sample and selecting individual chips for further higher resolution analysis.

Optical (left) and BSE (right) images of small vaned feather specimen from IVPP V13171 Confuciusornis. The bright rachis (quill) indicates preservation in calcium phosphate.

The high-resolution SEM analyses are performed once a thin gold coating has been applied.

[Left] Focussed Ion Beam cut trench (20Ám x 5Ám deep) revealing the ultrastructural detail of a feather sample.

In some cases further analysis using a Focussed Ion Beam (FIB) instrument can cut trenches into the sample to analyse minute cross sections only a few microns wide through the material to investigate the ultrastructure and even extract wafers for Transmission Electron Microscopy (TEM) analysis.

Dicynodon Illustration courtesy of John Sibbick.
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