Early Diversification of Parasitism: Insights from Three-Dimensionally Preserved Tongue Worms (pentastomida) and Quantitative Morphology

Parasites represent one of the most successful lifestyles, with at least half of all extant organisms considered parasitic. They constitute a major component of modern-day ecosystems influencing biological interactions. Moreover, host-parasite interactions are a strong driver of diversification. The evolutionary history of parasitic metazoans is ancient, with the earliest evidence dating back to the Early Cambrian. However, the fossil record of parasitism remains scarce. Parasites tend to have a low preservation potential due to their usually small size and soft bodies. However, several localities have yielded exceptional preservation, sometimes including parasitic organisms, and are considered particularly promising localities for studying parasitism. Among these, Orsten is considered as one of these most promising localities. Yet, Orsten should be regarded as an exceptional type of preservation rather than a single Lagerstätte. Orsten-type preservation sensu stricto (i.e., only including Orsten-type deposits from Early Paleozoic) has been recovered from several localities worldwide and spanned from more than 50 Ma (from Lower Cambrian to Lower Ordovician). It typically preserves minute organisms in three dimensions with remarkably fine details. Orsten-type deposits have already yielded several organisms showing similarities with modern parasitic counterparts, including possible early representatives of Tardigrada, Pycnogonida, and Pentastomida. Nowadays, pentastomidans, or tongue worms, exhibit a highly modified body organisation resulting in a vermiform body, with only two pairs of appendages used for attachment. They infect a wide range of representatives of Tetrapoda, but mostly reptiles (including birds). Fossils of pentastomidans display a similar vermiform morphology, with two prominent appendages and sometime one or two pairs of vestigial trunk legs. The fossil record of tongue worms includes around 80 specimens recovered from Cambrian Orsten deposits. A reinvestigation of an Ordovician Orsten material from Sweden, has yielded more than 6500 fossils of tongue-worms. This Orsten material provides crucial insights into the early evolution of Pentastomida. Indeed, fossils of tongue worms present a greater morphological diversity than Cambrian ones, increasing the number of morphotypes from two to four. To better understand the evolution of tongue worms through time, we performed quantitative analyses of the morphology of different body parts. These fossils, combined with the quantitative analyses, provide important insights into the early diversification and evolutionary history of Pentastomida, as well as the early specialization of parasitism during the Early Paleozoic.