Neues
Jahrbuch für Geologie und Paläontologie,
Monatshefte
2002 Jerzy
Dzik
& Andrey Yu. Ivantsov Abstract.
Current controversy over the nature of biotic changes at the
Precambrian-Cambrian boundary centres on whether the Ediacarian organisms were
metazoans or not. A new dickinsoniid of the Dickinsonia
tenuis group from a recently discovered fossiliferous horizon at the White
Sea shore, Russia, exquisitely preserved in fine sediment, offers the needed
anatomical evidence. In several specimens which show the originally high
convexity of the body, the dorsal segmented unit (normally being the only
preserved structure in the dickinsoniids) was specifically deformed under the
sudden sediment load. In result, dense internal organs are reproduced as
furrows on the lower bedding surface, whereas those which easily collapsed as
oval elevations. This pattern corresponds to the distribution of probable
intestinal caeca in Dickinsonia costata
and probable gonads in the related dipleurozoan Yorgia. A pharyngeal structure is represented by a circular imprint
and the intestine by a wide axial furrow. Apparently, the new dickinsoniid
together with Spriggina and several
other Ediacarian genera represents a single clade, characterised by dorsally
located metameric hydraulic skeleton, serially arranged gonads, and metameric
intestinal caeca. |