Journal
of Morphology 202, (2002). Jerzy
Dzik Abstract. The Namibian Kuibis Quartzite fossils of Rangea
are preserved three-dimensionally owing to incomplete collapse of the soft
tissues under the load of instantaneously deposited sand. The process of
fossilization did not reproduce the original external morphology of the
organism but rather the inner surface of collapsed organs, presumably a system
of sacs connected by a medial canal. The body of Rangea
had tetraradial symmetry, a body plan shared also by the White Sea Russian
fossil Bomakellia and possibly some
other Precambrian frond-like fossils. They all had a complex internal anatomy,
smooth surface of the body, and radial membranes making their alleged colonial
nature unlikely. Despite a different style of preservation, the Middle
Cambrian Burgess Shale frond-like Thaumaptilon
shows several anatomical similarities to
Rangea. The body plan of the Burgess Shale ctenophore Fasciculus, with its numerous, pinnately arranged comb organs, is in
many respects transitional between Thaumaptilon
and the Early Cambrian ctenophore
Maotianoascus from the Chengjiang fauna of South China. It is proposed
that the irregularly distributed dark spots on the fusiform units of the
petaloid of Thaumaptilon represent a
kind of macrocilia and that the units are homologous with the ctenophoran comb
organs. These superficial structures were underlain by the complex serial
organs, well represented in the fossils of Rangea.
The Precambrian ‘sea-pens’ were thus probably sedentary ancestors
of the ctenophores. |