Monday, November 22, 2010

Palaeoporn 20

Pambikalbae hasenohrae

Frondose (frond-bearing) forms are the most complex and interesting of the Ediacaran fossils, but rare. This is because most of the organism lives above the sea floor, to which they are anchored by a round holdfast, and so are not preserved. A number of frondose forms have been found, but Pambikalbae is different.

Pambikalbae hasenohrae (a) complete block showing several specimens (upper right and lower left). Slab 45 cms. (b) same slab but tilted to show fossil is interlayers with the sandstone matrix (cracks on left side of slab). (c) part of the right hand frond in (a) showing a complex arrangement of chambers.

Pambikalbae was made up of numerous chambered vanes, making up a frond, supported by a tapering central stem and an anchoring stalk. Several series of chambers occured on the vanes, joined together at zigzag sutures, and were commonly filled with sediment on burial. However, the complexity is probably an artifact of complex composite moulding of various nearby chambers, one on the other. It appears that the chambers curve away from the central stem out to the free, or outer, margin. The chambers also appear to be set on the vane in a regular patter so as to limit overlap.

These chambers are big. Bigger than anything else so far found. Here's a reconstruction.

Pambikalbae is clearly not a pennatulacean or cnidarian 'sea pen' like Charnodiscus. However, it does contain characters that suggest an evolutionary grade of organisation comparable to known cnidarians. The configuration and size of the chambers seems ideal to house symbiotic algae or bacteria . . . however, here's some wild speculation.

There is a group of cnidarians which do share similarities with Pambikalbae, and that's the physophorid Siphonophorida. Here's one below (photo credit)

Interesting isn't it. The chambers are especially similar. However, there is one thing wrong. The physophorid is upside down. The chambers actually point downwards. Why? Because physophorids float, they are not anchored to the sea floor like Pambikalbae.

But it's not as problematic getting from Pambikalbae to a physophorid, as it would appear.

(a) inverted Pambikalbae. (b) a hypothtical ancestral 'calycophore' siphonophore. (c) generalised modern physonect.

Just change a water-filled bulb as an holdfast, for an air filled bulb for a float. Simple folding inwards of the holdfast could produce the physonect and ancestral calycophore pneumatophore. A futher point of comparison is that the vane of Pambikalbae has three serial rows of chambers, as in calycophore and physonect siphonophores.

So a frondose form with wanderlust as ancestral to the modern siphonophore cnidarians? Maybe. But is could also be a derived hyrozoan, or a sister group to the early Chondrophorina. Whatever it is, it ain't no pennatulacean!

Jenkins, R.J.F. and Nedin, C. (2007) The provenance and palaeobiology of a new multi-vane, chambered frodose organism from the ediacaran (later Neoproterozoic) of South Australia. In P. Vickers-Rich, P. and Komarower, P. (eds) The Rise and Fall of the Ediacaran Biota. Geological Society Special Publication 286, 195-222. doi: 10.1144/SP286.15

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