Traces of an ancient immune system - how an injured arthropod survived 465 million years ago.

This report of a severely injured trilobite from the Middle Ordovician (~465 Ma) accords with a number of similar observations of healed lesions observed in trilobites. The uniqueness of the specimen described here is that the character of the repair-mechanisms is reflected by the secondarily built structures, which form the new surface of the ruptured compound eye. Smooth, repaired areas inside the visual surface advert to a clotting principle, rather similar to those of today, and the way in which broken parts of the exoskeleton fused during restoration seem to simulate modern samples. The irregularity and variance of newly inserted visual units indicate the severity of the injury, which, most probably, was caused by a predatory attack, presumably by a cephalopod; these were most likely, the top predators of the Ordovician. Furthermore, the state of the moulted cephalon tells the dramatic struggle of an organism that lived in the Palaeozoic, to survive. In sum the specimen analysed here is evidence of an ancient clotting mechanism not dissimilar to those of today, rapidly preventing any exsanguination and the breakdown of osmoregulation of this marine arthropod.