Abstract
True crabs (Brachyura) are the most successful group of decapod crustaceans. This success is most likely coupled to their life history, including two specialised larval forms, zoea and megalopa. The group is comparably young, starting to diversify only about 100 million years ago (mya), with a dramatic increase in species richness beginning approximately 50 mya. Early evolution of crabs is still very incompletely known. Here, we report a fossil crab larva, 150 mya, documented with up-to-date imaging techniques. It is only the second find of any fossil crab larva, but the first complete one, the first megalopa, and the oldest one (other fossil ca. 110 mya). Despite its age, the new fossil possesses a very modern morphology, being indistinguishable from many extant crab larvae. Hence, modern morphologies must have been present significantly earlier than formerly anticipated. We briefly discuss the impact of this find on our understanding of early crab evolution.
Highlights
True crabs (Brachyura) are the most successful group of decapod crustaceans
Most extant species are marine, but some 1,300 species are known from freshwater habitats[4], and some species spend most of their life on dry land[5]. This species richness and ecological diversity is even more astonishing when considering the age of this group
Insights into the early evolution of crabs are hampered by two difficulties: (1) the true identity of their earliest representatives is a subject of debate[8,9], and (2) early crab fossils are usually exclusively remains of the carapace[10] and of limited value in understanding the biology of these early forms
Summary
True crabs (Brachyura) are the most successful group of decapod crustaceans. This success is most likely coupled to their life history, including two specialised larval forms, zoea and megalopa. The first representatives of Brachyura appear comparably late in the fossil record, in the Jurassic (about 180 mya), and start to diversify in the middle Cretaceous (about 100 mya) with a later increase of species richness in the Eocene (about 50 mya)[6,7]. Despite this somewhat late origin (as compared to other groups of decapod crustaceans), brachyuran crabs diversified extremely rapidly both morphologically and ecologically. These incomplete fossils lacked appendages and a pleon, consisting only of the zoeal carapace and eye[20,21]
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