Tuesday, 9 July 2013


The giant, 165 million year old pseudopulicid flea
About a year ago, I posted on the fossil discovery from China of massive fleas known as the pseudopulicids. At two centimetres in length, these 165 million year old vampires were hundreds of times larger than their modern contemporaries. Based on their long, thick probosces and sharp, saw-like teeth, these creatures were adapted to feed on large prey such as dinosaurs, whose fossil remains were found in the same formation.

Later examples of fleas from the fossil record are far smaller. The question was why did these bloodsucking arthropods reduce their size over millions of years? Size generally gives a species an advantage.

A possible theory is that they adapted to the atmospheric conditions of the time. During the Carboniferous period some 300 million years ago, oxygen levels in the air were around 30% in comparison to the 20% we have today.

This allowed arthropods to grow to gargantuan sizes. The geological record shows that oxygen levels 165 million years ago were also higher than today. As a result, in the transition from high to low oxygen levels fleas could have reduced their body size. Yet large arthropods still exist. If oxygen levels dropped fast enough over enough of a range to affect fleas, then a similar trend should be seen in other groups which it is not necessarily the case.

Now a more accurate explanation for the physical change in fleas has been put forward in response to new fossil evidence from China. The fossils of the giant flea (incidentally the oldest yet discovered of the group) were found by Chungkun Shih in a formation of ancient lake sediments in north west China. This year, from a younger part of the formation, Shih and colleagues uncovered the remains of a younger, 125 million year old flea.

They gave it the name of Sauropthyrus exquisitus. What makes it important is its morphology. Not only was it half the size of the older pseuodopulicids, coming in at just one centimetre in length, it had a shorter, thinner proboscis and lacked teeth. Other differences were also apparent: the 165 million year old male fleas had completely external genitalia, a feature not seen in the modern flea.

Two fossils of the 125 million year old flea Sauropthyrus exquisitus
Sauropthyrus appeared to occupy a middle ground with partially internal genitalia. A similar trend was also seen in its legs which were longer than those of the older flea but still shorter in comparison to modern examples of the group. Finally, unlike its giant and toothy ancestor, Sauropthyrus had a body covered in thick strong bristles. The researchers hypothesised that fleas originally evolved to feed on dinosaurs and their relatives.

The major challenge was to pierce the thick skin and feed upon the blood vessels beneath, which would explain the saw-like teeth and thick, long proboscis seen in the 165 million year old pseudopulicid. Dinosaurs with thick skin, such as the sauropods, certainly lived alongside the giant flea. Yet as time progressed, large dinosaurs became less common, giving way to a diverse wave of smaller dinosaurs, birds and pterosaurs.

Shih and his colleagues suggest that fleas adapted to a changing range of prey whose skin was thinner. They became smaller, developing thinner probosces and losing their teeth altogether so that they could feed without causing painful bites and risk detection from the host. Of course smaller prey meant that the fleas would have to find new hosts.

A flea from Robert Hooke's Micrographia. Due to their small
size, Hooke had to use a microscope to observe the specimen. 
As they were unable to fly, they had to find another way of getting to a new food source, namely jumping which explains the long legs seen in Sauropthyrus and the bristles which would have helped it grip onto its host. The partially internal genitalia would have aided it, by reducing air resistance during the jump. 'You can imagine if you have something sticking out, it's hard to move around,' said Shih.

'The new discovery suggests the parasites co-evolved with their hosts in a way to balance their bloodsucking and hiding abilities.' 

Observing a habitat at the largest level is easy: a deer eating grass or a lion devouring prey. What we often forget is that entire worlds exist yet remain hidden from us. Fleas are a prime example. Quietly adapting to the world around, they are invisible and soundless.