Palaeocorystid Crabs are widespread in sedimentary rocks from the Middle-to-Late Cretaceous across the world. They are interpreted as having been specialist back-burrowers, a lifestyle that requires several modifications to the bodyplan of normally sideways-moving Crabs, particularly narrowing of the body (which is wider than it is long in most Crabs) and reduction of the surface area of the posterior part of the body.
In a paper published in the journal Zootaxa on 20 August 2013, Barry Van Bakel of the Oertijdmuseum De Groene Poort and the Naturalis Biodiversity Center describes a new species of Palaeocorystid Crab from the Late Cretaceous Carrière du Mas Quarry, 3 km northeast of Roullet-Saint-Estèphe in Poitou-Charentes in southwest France. The beds that produced the Crabs are silty-to-sandy blue-grey mudstones that have previously produced numerous Oysters, Lingulid Brachiopods, Shark’s teeth, Fish remains and Decapod Crustaceans in phosphatic nodules.
The new species is placed in the genus Joeranina, and given the specific name houssineaui, in honour of Romain Houssineau, who collected the specimens from which the species is described. The species is described from four specimens, with the best preserved being 14 mm in length and 9 mm in width. The Crabs are known from their bodies only, with no limbs preserved, and are subhexagonal in shape.
Joeranina houssineaui, upper Cenomanian, ‘Carrière du Mas’, Roullet-Saint-Estèphe, Poitou-Charentes, southwest France. (A) First specimen, dorsal view of carapace. (B) Second specimen, dorsal view of carapace. (C) Third specimen, dorsal view of carapace. (D, E)F ourth specimen, left lateral and dorsal views of carapace. All specimens whitened with ammonium chloride sublimate prior to photography. Scale bars 3 mm. Van Bakel (2013).
Unusually for a fossil Crab, which like most marine Arthropod fossils most often are formed by the shed carapaces of growing animals (the rigid exoskeletons of Arthropods do not grow with the animals, and instead must be shed at regular intervals), the first specimen of Joeranina houssineaui shows preservation of much of the internal structure of the living animal. This enables the internal pleurites (tail segments of the body, usually hidden in a Crab but exposed in life in some highly derived Raninoideans) to be seen, with pleurites5 and 6 exposed on the right hand side and pleurites 4-7 exposed on the left. This enables the observation of a radial structure in the arrangement of the pleurites, with pleurite 7 being small and horizontally positioned, and pleurite 8 further reduced and hidden from view (this radial structure is caused by the tail of the Crab being permanently coiled up within the shell).
Joeranina houssineaui, upper Cenomanian, ‘Carrière du Mas’, Roullet-Saint-Estèphe, Poitou-Charentes, southwest France.(A)Left lateral view; (B) ventral view; (C) anterior/frontal view; (D) posterior view; (E) right lateral view. 1–8, thoracic sternites 1–8; a1, abdominal somite 1; aP1–aP5b, arthrodial cavity of P1–P5; ep, epistome; en, endostome; i, intestine; o, orbit; om, oxystomian mouth; pl4–pl7, pleurite 4–7; pt, pterygostome. Specimen whitened with ammonium chloride sublimate prior to photography. Scale bar 3 mm. Van Bakel (2013).
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