''Protomelission'' is a

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

fossil taxon of contested

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. It comprises

cataphract A cataphract was a form of armoured heavy cavalry that originated in Persia and was fielded in ancient warfare throughout Eurasia and Northern Africa. Historically, the cataphract was a very heavily armoured horseman, with both the rider and ...

arrays of box-like chambers, forming a club-shaped

thallus Thallus (: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. A thallus usually names the entir ...

surrounding a hollow central cavity. It was first described from phosphatized microfossils whose individual chambers were interpreted as housing the

zooid A zooid or zoöid is an animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooids can ...

s of a non-mineralized bryozoan, which would make it the only affinity representative of that phylum – implying that all animal

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

originated in the

period. The subsequent discovery of articulated macrofossils from the Xiaoshiba biota called into question the biological nature of the distal apertures, showing that the surface was instead covered with leaf-like triangular flanges. This has led to the reinterpretation of the fossil material as a "seaweed", strictly as a dasycladalean green alga.

History of description

''Protomelission'' was first described by Brock & Cooper in 1993 from limestones in Wirrealpa, Australia. A bryozoan affinity was dismissed on the basis that its walls were too thin, among other things. The fossil material was complemented by additional specimens from China, causing Brock and colleagues to revisit this earlier statement. A key line of evidence in favour of the bryozoan affinity was the regular array of openings exhibited by each chamber. However, these openings were later argued to have arisen taphonomically, i.e. by abrasion of an originally solid wall, or by enlarging a much smaller original hole. Where a bryozoan affinity would denote the presence of a stalked ring of tentacles emerging from each module, the recovery of macrofossil material with soft tissue preservation demonstrated that each chamber was instead associated with a tapering conical flange, better suited to photosynthesis. Authors of the original study are unconvinced by this reinterpretation, suggesting in media reports that the absence of tentacles may in turn be an effect of imperfect preservation.

Available fossil material

''Protomelission'' is known from two distinct modes of preservation: small shelly fossils from Australia and China, which are millimetric phosphatized fragments; and

Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fos ...

-type macrofossils, which preserve more delicate anatomy. The former are fully three-dimensional; the latter, squashed into a couple of planes of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

. These different preservational modes mean that the material from the two settings is not identical, and it is challenging to establish beyond doubt that the two forms of fossil belong to the same species, or even on a cynical view both to the same sort of organism.As emphasized by Paul Taylor in Whether or not the two types of fossil are

conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organism ...

, Yang et al. argue that the similarity in gross morphology is sufficient to show that a dasyclad interpretation is plausible for both sets of fossil material, and that the material cannot be considered a ''bona fide'' bryozoan.

History of description

Available fossil material

References

Further reading