Dinosaurs were pestered by bites from blood-sucking giant fleas that left them in serious pain, according to fossil records found and studied by mainland and overseas scientists. Unlike the annoying insects today that jump around and give dogs fits, the Jurassic-era fleas of 150 million years ago were huge by comparison and had serrated mouths. And their bites were a source of anxiety even for a Tyrannosaurus. In a paper published in science magazine Nature, Professor Huang Diying and his colleagues describe the fossils of giant fleas they unearthed in Inner Mongolia and Liaoning in recent years. These oldest and 'definitive' fleas were found to have lived among dinosaurs in the middle-Jurassic to early-Cretaceous periods, and were 10 times bigger than the fleas of today. The largest of them reached 2.1cm in length, barely crushable with a thumb, compared with modern fleas that are just 1mm to 3mm long, said Huang, a researcher with the Nanjing Institute of Geology and Paleontology under the Chinese Academy of Sciences. 'Whomever they clung to could feel pain beyond imagination,' he told the South China Morning Post yesterday. 'I am glad they are no longer with us.' Huang said their mouths looked almost identical to those of modern fleas, except for their much larger size and saw-like edges. The short antennae and backwards-pointing hairs on their body also confirmed their flea lineage. But these prehistoric giants differed from their modern descendants in a very noticeable way - they crawled. The absence of jumping hind legs has given rise to scientists' speculation that they were ambush specialists. Michael Engel, a palaeoentomologist at the University of Kansas, in the US, and a co-author of the paper, told Nature that the fleas fed by 'hiding in the periphery and then scrambling onto the host for brief periods to feed before bolting again'. Huang said they had not yet found direct evidence that the parasites fed on dinosaurs, but the structure of their mouthparts indicated that the fleas were used to dealing with animals with very thick hides or feathers. 'The saw-like, backwards-pointing hooks in their mouths made it very painful, if even impossible, for their hosts to get rid of them once bitten,' he said. 'So if a dinosaur was bitten it might have been incapable of doing anything but letting the fleas suck at will. I think the fleas knew when to stop, or they would have eventually killed their hosts.' Scientists took up the research to find out more about the evolutionary origins of fleas. But the task proved difficult, as the parasites left behind few fossil records, Huang said. Unlike other small creatures with wings, such as dragonflies, the ancient fleas stayed far away from water if they could help it, and were therefore unlikely to be buried and preserved in mud. The almost perfectly preserved fossils in the study were found in Ningcheng county of Inner Mongolia and Chaoyang city of Liaoning province. These two sites have featured China's most productive fossil beds in recent years, with earlier finds ranging from the first dinosaurs with feathers to the earliest birds. In the 1980s, Australian scientists discovered a flea fossil dating back 120 million years. But the sample was incomplete, with many crucial details missing. Huang said he came across a flea fossil at a fossil market in 2008. The sample wasn't of high quality, but it started his interest. In 2009, he found a 2.1cm flea in the fossil bed in Inner Mongolia. The site used to be a lake surrounded by forest and frequented by animals. Some volcanic ash had sunk to the bottom of the lake, helping preserve the site, he said. 'Palaeoentomologists are constantly surprised and delighted by what they find here,' he said. The fleas and their hosts were found to have evolved together. The hosts developed new ways to get rid of the fleas, and the fleas developed new ways to stay on their hosts, he said, adding: 'We are as vulnerable as the dinosaurs.'