A Triassic Giant Amphipod from Nevada, USA
ABSTRACT The fossil giant amphipod Rosagammarus minichiellus n. gen., n. sp. occurs in Triassic limestone (Luning Formation, west-central Nevada) in association with giant ichthyosaurs (Shonisaurus sp.) and the deep-water trace fossil Protopaleodictyon ichnosp. Fossil pereion and pereiopod morphology suggest affinities with the Acanthogammaridae, a freshwater amphipod family largely endemic to Lake Baikal. The large size (17 cm) of the Triassic amphipod shows that supergiant, deep marine amphipods comparable to modern Alicella gigantea Chevreux, 1899 were extant during the early Mesozoic. By analogy with Alicella gigantea, Rosagammarus minichiellus n. gen., n. sp. was a necrophagous, benthopelagic scavenger that fed on ichthyosaur and other sea floor carcasses. Rosagammarus minichiellus n. gen., n. sp. appears to be the oldest known fossil amphipod. This discovery extends the known geological range of Amphipoda by at least 170 million years.
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ABSTRACT: Arthropods are a major component of the marine zooplankton, functioning as intermediates between primary producers and tertiary consumers in pelagic food webs. They have likely occupied the water column ecospace since the early Cambrian, co-evolving with several major plankton groups through the Phanerozoic. Analysis of the fossil record of arthropods indicates that a zooplanktonic lifestyle has arisen independently across several arthropod groups, and that the arthropod body plan has shown adaptability to fundamental environmental change. Key anatomical adaptations and reproductive strategies that are compatible with and/or facilitate a zooplanktonic lifestyle include well-developed swimming appendages, buoyancy, an active metabolism supported by efficient respiration, visual or photoreception organs for use in detecting mates, food and predators, and the nurturing of young within the exoskeleton. Many of these characters are apparent in the earliest record of arthropods. The development and diversification of arthropod zooplankton into the food chain helped enable large secondary and tertiary consumers to enter the water column and also contributed to the export of organic matter to the seabed via the faecal stream. Although the fossil record of arthropod zooplankton is extremely poor for most of the Phanerozoic, their position in marine food webs suggests they have been fundamental to rebuilding marine trophic structure following major extinction events, and to maintaining marine plankton diversity through Phanerozoic time.Earth-Science Reviews 02/2015; DOI:10.1016/j.earscirev.2015.02.003 · 7.14 Impact Factor
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