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Meanwhile, it enriches the inclusions biodiversity of Mexican amber, and suggests the distribution of Palaemon in southeastern Mexico before Early Miocene. The shrimp in this study represents the first and oldest definite record of the Caridea species preserved in amber all over the world.
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Tides thus played an important role in the embedding of more ancient aquatic organisms in Mexican amber, in which Decapoda species are rare relative to ostracods, copepods, tanaidaceans, amphipods, and isopods, and only crabs have been reported so far in Mexican amber 18. From Cretaceous to Miocene, the coastal areas, wetland and continental lowlands were often flooded, so some marine organisms failed to return to the sea and had to gradually adapt to estuaries and freshwater environment, which was one of the main reasons for the high species diversity of this region 2. Ever since then, amber has deposited along the Gulf of Mexico coast 16.
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The amber, collected from Simojovel de Allende, Chiapas, southeastern Mexico, most possibly formed by the resin secreted from the Hymenaea in Early Miocene 15, 16, 17. Here, we first report a Palaemon shrimp preserved in Mexican amber which is famous for rich inclusions, such as fungi, flowers, seeds, pollen, leaves, arachnids, insects, vertebrates, and especially Crustaceans 14. To date, no record of caridean shrimp preserved in amber has been reported. There have been only three definite fossil species included in the Palaemon, all preserved in rock impressions 12, with the earliest known record originating from Lower Cretaceous in Italy 13. Reliable fossil records can provide us with evidences for the origin, phylogeny and separation events between these lineages, but available palaeontology materials are extremely scarce most likely due to the aquatic environment and the relatively weak calcification of the exoskeleton of the shrimps 11. The genus has a worldwide distribution, and the most reasonable explanation for the distribution is probably due to dispersion and colonization events 9, 10. The 87 extant species of Palaemon are found in various habitats, such as marine, brackish and freshwater 7, 8. Palaemon Weber, 1795 is the second most species-rich genus besides the Macrobrachium Spence Bate, 1868 in the Palaemonidae 4, 5, 6. It is now widely believed that it originated from the marine environment in the indo-western Pacific warm waters, and has successfully adapted to non-marine environments, such as estuaries and limnic environments 2, 3, 4. Palaemonidae Rafinesque, 1815 is the largest shrimp family within the Caridea, with world-wide distribution 1.