Hidden diversity in Early Palaeozoic parasites: new indication from Orsten-type Ordovician pentastomidans (tongue-worms)

Orsten-type Lagerstätten sensu stricto are represented by several localities around the world, delivering a unique window into the meiofauna of the Early Palaeozoic. They are providing beautifully three-dimensionally preserved, phosphatized organisms, mostly articulated arthropodans. Among them, Pentastomida (tongue-worms) is an intriguing group of parasites. Today, they infect almost all groups of vertebrates, including humans. They are most commonly found parasitising the respiratory tracts of squamates and crocodilians. Pentastomidans are characterised by an extremely simplified worm-like body, two pairs of head hooks, and a mouth. Their fossil record is scarce and to date limited to 81 specimens preserved in Orsten-type Lagerstätten, mostly from the Upper Cambrian of Sweden. Here, we reinvestigate an Ordovician assemblage from the island of Öland, Sweden, first briefly reported by Dietmar Andres in 1989. This assemblage contains more than 6500 fossil specimens of Pentastomida preserved in three dimensions through Orsten-type phosphatisation. Our study provides the first detailed description of two new morphotypes of fossil pentastomidans, increasing the total number of known morphotypes from two to four. The assemblage reveals a pronounced morphological diversity among three of these morphotypes and offers unique insights into the evolutionary history and early diversification of tongue-worms during the Early Palaeozoic.