Abstract
Tardigrades are a diverse group of charismatic microscopic invertebrates that are best known for their ability to survive extreme conditions. Despite their long evolutionary history and global distribution in both aquatic and terrestrial environments, the tardigrade fossil record is exceedingly sparse. Molecular clocks estimate that tardigrades diverged from other panarthropod lineages before the Cambrian, but only two definitive crown-group representatives have been described to date, both from Cretaceous fossil deposits in North America. Here, we report a third fossil tardigrade from Miocene age Dominican amber. Paradoryphoribius chronocaribbeus gen. et sp. nov. is the first unambiguous fossil representative of the diverse superfamily Isohypsibioidea, as well as the first tardigrade fossil described from the Cenozoic. We propose that the patchy tardigrade fossil record can be explained by the preferential preservation of these microinvertebrates as amber inclusions, coupled with the scarcity of fossiliferous amber deposits before the Cretaceous.
Highlights
Fossils have an important role in reconstructing the history of complex life through deep time
The fossil tardigrade Milnesium swolenskyi was described from New Jersey amber that is stratigraphically attributed to the Turonian (Upper Cretaceous), making it approximately 14 Myr older than Beo. leggi
Confocal fluorescence microscopy shows 9 that taxonomically important characters, the claws and bucco-pharyngeal apparatus, are preserved in the single specimen available. This imaging technique has yielded similar results for Mil. swolenskyi, emphasizing its use for studying the morphology of microinvertebrates preserved in amber, including that of the still problematic Beo. leggi [2]
Summary
Fossils have an important role in reconstructing the history of complex life through deep time. A notorious example are the tardigrades— known as water bears or moss piglets—a charismatic group of microscopic invertebrates that are famous for their survival after exposure to extreme conditions, such as the vacuum of space and ionizing radiation [1] Owing to their microscopic size and lack of heavily biomineralized body parts, the tardigrade fossil record currently consists of two described species regarded as members of the crown-group [2,3] and one Cambrian representative that may belong to the stem lineage [4,5]. To test the placement of the fossil relative to extant tardigrade superfamilies, we performed phylogenetic analyses using 28 morphological characters that can be grouped into four sets: body surface, claws, bucco-pharyngeal apparatus and egg morphology (electronic supplementary material, file S1).
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