Abstract
Amber fossils provide snapshots of the anatomy, biology, and ecology of extinct organisms that are otherwise inaccessible. The best-known fossils in amber are terrestrial arthropods—principally insects—whereas aquatic organisms are rarely represented. Here, we present the first record of true crabs (Brachyura) in amber—from the Cretaceous of Myanmar [~100 to 99 million years (Ma)]. The new fossil preserves large compound eyes, delicate mouthparts, and even gills. This modern-looking crab is nested within crown Eubrachyura, or “higher” true crabs, which includes the majority of brachyuran species living today. The fossil appears to have been trapped in a brackish or freshwater setting near a coastal to fluvio-estuarine environment, bridging the gap between the predicted molecular divergence of nonmarine crabs (~130 Ma) and their younger fossil record (latest Cretaceous and Paleogene, ~75 to 50 Ma) while providing a reliable calibration point for molecular divergence time estimates for higher crown eubrachyurans.
Highlights
Transitions from marine to nonmarine habitats are infrequent in most metazoan groups, largely due to different physical and physiological requirements in saltwater and freshwater, competition with previously established residents, and exposure to new predators [1]
C. athanata is the first crab discovered in Mesozoic amber
The holotype is complete and articulated and preserves delicate features such as antennae, compound eyes, and mouthparts covered in minute hairs or setae (Fig. 2), which are rarely preserved in fossil crabs
Summary
Transitions from marine to nonmarine habitats are infrequent in most metazoan groups, largely due to different physical and physiological requirements in saltwater and freshwater, competition with previously established residents, and exposure to new predators [1]. Eubrachyurans are the most diverse group of crabs today in terms of anatomy, ecology, and species richness. They fall into two main categories based on the position of their sexual openings: the heterotremes (including primarily and secondarily freshwater crabs), with openings on the legs in males and thorax in females, and thoracotremes (including semiterrestrial and terrestrial crabs), with openings on the thorax in both sexes [4,5,6]. Two questions, (i) the phylogenetic position of the different freshwater crabs, and (ii) whether thoracotremes are a sister group to heterotremes or derived from within them, are still debated [7]
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