Bivalves on mecochirid lobsters from the Aptian of the Isle of Wight: Snapshot on an Early Cretaceous palaeosymbiosis
Robin et al
Fossil symbioses (Bary 1879) that are not recognized in modern assemblages are rare. This holds true especially because fossil remains of interspecific association are sporadic and difficult to document. Thus, fossil associations illustrated on a large number of specimens are of particular interest. Numerous specimens of mecochirid lobsters (Glypheidea, Decapoda, Crustacea) from the Lower Cretaceous of the Atherfield Clay Formation (Isle of Wight, UK) seer small epibiotic bivalves on their exoskeleton. We propose to identify the post-mortem or syn-vivo nature of this association. To this end, we test and revise the systematic assignment of both crustacean and molluscan partners. A new genus name is proposed for the crustacean Meyeria magna M'Coy, 1849, as well as the new combination Atherfieldastacus magnus (M'Coy, 1849). To understand the nature of the association, a qualitative and quantitative study of the association is conducted on newly examined material (161 lobsters) looking at prevalence, density and abundance of the infestation (per anatomical region of the lobsters). Angularities and flat surface of exoskeletons were also evaluated. The distribution of the bivalves on both sides of the crustaceans (60% of the colonized lobsters), their preservation and their downward growth orientation suggests no post-mortem attachment of the mollusks. Hence, the association may be ascribed to a true palaeosymbiosis between these organisms. The non-homogeneous distribution of bivalves, that is to say an important colonization of the angular ridges of the carapace, is interpreted as selected sites by the mollusks for larval fixation, and may indicate possible half-burrowing posture of lobsters. This palaeosymbiosis has no modern equivalent, as cemented bivalve shells have never been reported on any population of decapod crustaceans. This fossil association may be ascribed to a local Aptian palaeoenvironment comprising swarming spats of anomiid bivalves.