Palaeo biologists at the University of Kansas have discovered a ciliate fossil embedded in the wall of a fossilised leech cocoon, itself embedded in rock that has been dated back to 200 million years ago. 

The ciliate is a member of the Vorticella family, commonly known as “bell animals” and is a single celled protozoan that moved in water through the use of cilia that surrounded its body. The specimen would normally be impossible to fossilise. 

“Soft bodied organisms can become fossilised only under exceptional circumstances. This discovery forms the first fossil record for the Vorticellidase family, and only the second fossil record for the ciliate subclass Peritrichia,” Benjamin Bomfleur, lead author of the findings that are published in PNAS, told Laboratory News. 

The specimen was discovered in rock samples taken from Timber Peak in the Eisenhower Mountain Range in Antarctica. The team extracted the fossil from the rock using a technique called cuticular analysis where scientists take a sample of suitable, plant-bearing deposit and digest the rock away using a combination of acids. 

“What remains is the bulk or organic content of the rock. This can be mounted on slides and analysed under a microscope as they show a wealth of micromorphological features that are important for classification. I used this technique with our rock samples from Antarctica and found the cocoon fossils among the organic debris that we isolated from the rock,” Bomfleur added. 

Leeches cocoon themselves completely just before laying their eggs. The eggs –still in the cocoon- are then deposited on nearby surfaces, such as rocks where they are cared for by the parent. The team believe the ciliate become stuck in the wall of the cocoon and then become fossilised. 

This surprising find indicates that researchers have another avenue to explore in the hunt for fossils. Like amber, ancient cocoons could be considered conservation traps. 

“That is probably one of the most exciting aspects of this discovery; we expect that if we start looking closer for such cocoon fossils, we might open up a real treasure trove for fossil microorganisms. Such soft-bodied microorganisms form an essential component in the functioning of the biosphere today, yet we know hardly anything about the evolutionary history of many of these life forms” Bomfleur told Laboratory News.