Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA.
Commun Biol. 2024 Aug 6;7(1):953. doi: 10.1038/s42003-024-06643-2.
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. 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.
缓步动物是一门多样性的微小动物门，以其极端的生存能力而闻名。分子钟表明，缓步动物在寒武纪之前就已经从其他泛节肢动物中分化出来，但它们的化石记录却非常稀少。只有化石缓步动物 Milnesium swolenskyi（晚白垩世）和 Paradoryphoribius chronocaribbeus（中新世）解决了分类地位问题，限制了用于估计这个门起源的校准点的可用性。在这里，我们使用共聚焦荧光显微镜修订了来自加拿大白垩纪琥珀的两个冠群缓步动物，揭示了解决其分类地位的关键形态特征。对 Beorn leggi 的正式形态重新描述表明它具有 Hypsibius 型爪子。我们还根据其独特的爪子特征描述了 Aerobius dactylus gen. et sp. nov.。系统发育分析表明，Beo. leggi 和 Aer. dactylus 属于 eutardigrade 超科 Hypsibioidea，为研究缓步动物起源增加了一个关键的化石校准点。我们的分子钟估计表明，冠群 Tardigrada 在早古生代就已经多样化，并强调了 Beo. leggi 作为一个校准点的重要性，它直接影响到浅节点的估计。我们的研究结果表明，eutardigrades 和 heterotardigrades 的独立陆地化分别发生在石炭纪末期和侏罗纪早期。这些估计还为隐生的趋同获得提供了最小年龄。
