Biogeochemical Cycles & Geobiology

Cretaceous Amber Inclusions Illuminate The Evolutionary Origin Of Tardigrades

By Keith Cowing

Status Report

Nature via PubMed

August 19, 2024

Filed under Aerobius dactylus, Amber, Beorn leggi, Cretaceous, end-Carboniferous, eutardigrades, evolution, Fossil, heterotardigrades, Hypsibius exemplaris, invertbrate, Lower Jurassic, Milnesium swolenskyi, Miocene, Nature via PubMed, Paleontology, panarthropods, Paradoryphoribius chronocaribbeus, phylogeny, Pubmed, tardigrade

Cretaceous Amber Inclusions Illuminate The Evolutionary Origin Of Tardigrades — Phylogenetic relationships of Beorn leggi and Aerobius dactylus gen. et sp. nov. A Phylogenetic result of the total evidence approach using 36 morphological characters and 1774 bp 18S rRNA sequences. Numbers at nodes represent posterior probability supports. Fossils are highlighted in red bold texts. B Artistic reconstruction of the two fossil specimens by Franz Anthony. — Nature

Tardigrades are a diverse phylum of microscopic invertebrates widely known for their extreme survival capabilities. Molecular clocks suggest that tardigrades diverged from other panarthropods before the Cambrian, but their fossil record is extremely sparse.

Only the fossil tardigrades Milnesium swolenskyi (Late Cretaceous) and Paradoryphoribius chronocaribbeus (Miocene) have resolved taxonomic positions, restricting the availability of calibration points for estimating for the origin of this phylum.

Here, we revise two crown-group tardigrades from Canadian Cretaceous-aged amber using confocal fluorescence microscopy, revealing critical morphological characters that resolve their taxonomic positions. Formal morphological redescription of Beorn leggi reveals that it features Hypsibius-type claws.

Habitus of Aerobius dactylus gen. et sp. nov. (MCZ PALE-45862). Aerobius dactylus gen. et sp. nov. (MCZ PALE-45862) in ventral (A, D) and dorsal view (E, F). A Specimen photographed with transmitted light under compound microscope. B, E Specimen photographed with autofluorescence under confocal microscope at 639 nm; different colors indicate z-depth, with violet to red gradient representing the shallowest to deepest planes, respectively. C Schematic drawing. D, F Specimen and claws viewed in inverted greyscale to highlight autofluorescence intensity (darker—more intense, lighter—least intense). eo elliptical organ, Ln leg number, mo mouth. — Nature

We also describe Aerobius dactylus gen. et sp. nov. based on its unique combination of claw characters. Phylogenetic analyses indicate that Beo. leggi and Aer. dactylus belong to the eutardigrade superfamily Hypsibioidea, adding a critical fossil calibration point to investigate tardigrade origins.

Our molecular clock estimates suggest an early Paleozoic diversification of crown-group Tardigrada and highlight the importance of Beo. leggi as a calibration point that directly impacts estimates of shallow nodes.

Our results suggest that independent terrestrialization of eutardigrades and heterotardigrades occurred around the end-Carboniferous and Lower Jurassic, respectively. These estimates also provide minimum ages for convergent acquisition of cryptobiosis.

Ventral view of Beorn leggi (MCZ PALE-5213). A Specimen photographed with transmitted light under compound microscope. B Specimen photographed with autofluorescence under confocal microscope at 639 nm; different colors indicate z-depth, with violet to red gradient representing the shallowest to deepest planes, respectively. C Schematic drawing. Ln leg number. — Nature

Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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