A piece of the Earth's history hidden in the intestine
FAU palaeontologists examine data on fossilised flatworms
Cat and dog owners regularly give their pets tablets to treat worms. But how long have such parasitic relationships existed – did the dinosaurs suffer from worms too? An international team led by researchers at FAU has analysed all documented occurrences of fossilised flatworms and examined their relationship with the evolution of their hosts. Their findings have now been published in the journal Advances in Parasitology*.
If a person becomes infected by worms, this is usually identified in a stool sample. This method also works using coprolites – fossilised faeces – from extinct species. The international team of researchers led by Dr. Kenneth De Baets from the Chair of Palaeoenvironmental Studies evaluated all documented fossilised flatworms from all over the world. The oldest evidence of parasitic worms is found in fossils of primitive fish species that lived during the Devonian period around 382 million years ago. Inside these fossils researchers have found evidence of over 75 fossilised attachment organs that the worms used to hold on in their hosts’ intestines. The worms are most likely monogenea – parasitic flatworms with simple life cycles – and possibly other parasitic worms, such as tapeworms or spiny-headed worms. The oldest examples of tapeworms with complex life cycles are contained in shark coprolites from the Permian Period and are around 259 millions years old. Unlike the simpler flatworms, these worms used several different species as hosts to allow them to spread as effectively as possible. Other samples of fossilised faeces from the Cretaceous Period which are around 126 million years old show that land-dwelling creatures such as dinosaurs were also already suffering from tapeworms in this period.
However, when the researchers’ analysis made it clear that tapeworms have been around for a very long time, they began to suspect that they existed as early as the Cambrian-Ordovician boundary around 488 million years ago. The oldest fossils of the vertebrate host species also date from this time. Furthermore, when the palaeontologists evaluated the artefacts that have been found so far, they discovered that the fossil record is consistent and shows evidence of a long-standing relationship between flatworms and hosts that stretched across millions of years. Nevertheless, the gaps in the information are still currently too large to say to what extent the evolution of the parasites was related to the evolution of their hosts or other environmental conditions.
For this reason, the researchers now want to use methods such as molecular clocks to find out more about how the flatworms developed. Molecular clocks are a way of determining the point in time when two species diverged. The DNA of the two species are sequenced and analysed to see how many differences there are. The more mutations are found, the further in the past the common ancestor lived. However, it is difficult to determine how fast a molecular clock ticks, as to do so the gaps in time between mutations must be pinned down. Dr. De Baets’s team of palaeontologists aim to calibrate the molecular clocks using geological and fossil data. In doing so they hope to arrive at a more reliable estimation of the time frame in which the flatworms evolved. Another experimental method that Dr. De Baets wants to use during a research stay at the renowned Natural History Museum in London is micro-computed tomography. This technique allows fossils to be examined without destroying them and enables 3D images to made to show reconstructions of missing pieces or the internal structures of coprolites.
* De Baets, K., Dentzien-Dias, P., Upeniece, I., Verneau, O., and Donoghue, P. C. J., 2015, Constraining the Deep Origin of Parasitic Flatworms and Host-Interactions with Fossil Evidence, Advances in Parasitology, in press.