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Exceptionally preserved cryptoniscium larvae - morphological details of rare isopod crustaceans from French Cretaceous Vendean amber

Abstract : Epicaridea is an ingroup of Isopoda that comprises only parasitic crustaceans. Within parasitic isopods, epicarideans represent a special case: throughout their ontogeny they switch from a small intermediate host (copepod) to a final host (various larger crustaceans), and develop through distinct larval phases (epicaridium, microniscium and cryptoniscium). Young males of some species retain a larval morphology. Recent findings of fossil epicarideans in amber from the Miocene of Mexico consisted in the only epicaridean body fossils, until one specimen has been figured from Cretaceous amber from France. Here we provide a detailed analysis of this specimen and 20 more specimens from the same locality. The presented specimens represent the oldest occurrence of epicaridean body fossils, extending their fossil record by 67 million years. The fossils are exceptionally well preserved and, despite their small size of less than 0.5 mm, reveal even fine morphological details. The specimens correspond either to cryptoniscium larvae or males that have retained their larval morphology. There are no morphological features in the fossils that argue against conspecifity of all specimens. All character states found in the fossils are also present in extant species. Given the displayed combination of character states and the age difference, it is unlikely that the specimens are conspecific to any extant species nor to much younger fossils from the Miocene of Mexico. The species Vacuotheca dupeorum gen. et sp. nov. is described and interpreted as an epicaridean of uncertain affinities, but that is not part of the epicaridean ingroup Dajidae. Furthermore, multiple aspects of the evolutionary history of parasitic isopods and epicarideans in particular are discussed. This includes possible scenarios for host changes that could have led to the life cycle of modern epicarideans and the evolution of size within epicaridean larvae
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Mario Schädel, Vincent Perrichot, Joachim Haug. Exceptionally preserved cryptoniscium larvae - morphological details of rare isopod crustaceans from French Cretaceous Vendean amber. Palaeontologia Electronica, Coquina Press, 2019, 22, pp.22.3.71. ⟨10.26879/977⟩. ⟨insu-02369905⟩

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