[Annelida] Giant snail fossil created by a capitellid?
(by Geoffrey.Read from niwa.co.nz)
Thu Nov 17 18:18:22 EST 2011
A new explanation for a trace fossil named Dinocochlea ingens. Is the polychaete suggestion plausible ... or not? Noting the pitch of the original horizontal helix would have to be consistently a fairly large 100 mm or so, along up to 2000 mm. Burrows with occasional small spirals are known though for capitellids, and also for spionids and paraonids.
Taylor, Paul D.; Sendino, Consuelo 2011. A new hypothesis for the origin of the supposed giant snail Dinocochlea from the Wealden of Sussex, England. Proceedings of the Geologists Association 122(3): 492-500 When first described in 1922, Dinocochlea ingens, an Early Cretaceous (Valanginian) fossil from the Wadhurst Clay Formation (Wealden) of Hastings in Sussex, England, was regarded as the steinkern of a huge gastropod over 2 m in length and claimed to be the largest gastropod ever to have lived. Most subsequent researchers have doubted the identity of D. ingens as a gastropod, usually believing it to be a pseudofossil, but no convincing explanation has been proposed for its origin. Restudy of the type material leads to the new hypothesis that Dinocochlea originated as a horizontal, corkscrew-shaped burrow resembling Helicodromites that acted as a nucleus for concretion growth. Sectioned specimens of Dinocochlea are filled with concentrically banded sediment interpreted to be growth rings of a concretion. Modification of a computer simulation designed to illustrate gastropod shell growth shows how the development of a concretion around a corkscrew-shaped burrow could lead to the morphology seen in Dinocochlea. The most likely tracemaker for Dinocochlea was a capitellid polychaete ('threadworm') with a body just a few mm in diameter.
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