Kimberella as a stemgroup mollusc

As stated previously, the presence of truly multicellular animals in the Late Proterozoic is documented by various kinds of fossils. While 'worm' burrows and

ill-'VENDIAN SHALLOW-MARINE BIOMATSglllS:
Sea Origin Life

Fig. 10.4. Because they survive in the present deep sea, xenophyophores are known to be giant rhizopods. Their chambers resemble vendobiontan quilts in size, shape, and allometric diameter, but walls are agglutinated and non-expandable. As a silty fill skeleton (stercomare) further subdivides the protoplasm, chambers can be wider than in foraminiferan shells. Shallow-marine Ediacaran representatives, long considered as trace fossils, probably lived embedded in biomats. (Modified from Seilacher et al. (2003); enlarged cross section from Tendal (1972).)

Fig. 10.4. Because they survive in the present deep sea, xenophyophores are known to be giant rhizopods. Their chambers resemble vendobiontan quilts in size, shape, and allometric diameter, but walls are agglutinated and non-expandable. As a silty fill skeleton (stercomare) further subdivides the protoplasm, chambers can be wider than in foraminiferan shells. Shallow-marine Ediacaran representatives, long considered as trace fossils, probably lived embedded in biomats. (Modified from Seilacher et al. (2003); enlarged cross section from Tendal (1972).)

embryos are difficult to affiliate, Kimberella (originally from South Australia and described as a cubozoan medusa) was clearly a mollusc-like creature. It is more common and better preserved in the White Sea region of Russia (Figure 10.5). Although Kimberella certainly lived above the sediment, death masks on sole faces from the White Sea preserve the ventral side. They provide the first earmark of a mollusc: the mucus secreted by the flat foot probably served as a matrix for the bacterial mask makers. Another clue is the morphology of this foot.

Early Molluscs:

Death Masks and Traces

Ediacaran, White Sea mmb: m^^mm &

Kimberella 5 cm ! Mpñr Radulichnus Vi-

(ventral death masks)

Radulichnus x,g ty^fyf (hypichnial undertraces) 1^' '

Fig. 10.5. Ventral death masks and associated radula scratches (Radulichnus) identify Kimberella as a stem-group mollusc. Grazing traces of juveniles are not preserved, because they did not reach the sole of the biomat. Kimberella also differed from later chitons and gastropods by grazing with an expandable proboscis in a stationary mode rather than in continuous meanders. (From Seilacher et al. (2003).)

It shows fine transverse wrinkles in the central area and a coarser crenulation along the circumference, related either to gills or to longitudinal and circular muscles that contracted upon death. One also recognizes the marginal impression of a limpet-like hood. As can be seen from occasional deformations, this hood was flexible and possibly covered by small sclerodermites, as in Cambrian halkieriids (Conway Morris and Peel, 1990). The ventral mask of Kimberella must have formed before the decay of the intestines, which caused an upward collapse in the centre. All these features suggest an animal similar to Cambrian halkieriids or to modern polyplacophorans. However, it had no real shell and grazed algae not on rock surfaces, but on the biomats that were ubiquitous on Precambrian sea bottoms.

To these biomats we also owe a detailed record of feeding habits of Kimberella: radular scratch patterns (Radulichnus; Figure 10.5). The radula is a feeding apparatus found in all modern mollusc classes except bivalves (where it probably has been lost with the transition to filter-feeding). The chitinous radula teeth scrape

Bergaueria

Fig. 10.6. In Ediacaran times, metazoan burrowers did not penetrate more than a few millimetres into the sediment. A. In actinian resting traces (Bergaueria sucta), slight lateral movements are expressed by concentric lines. B. Palaeophycus sp. records the horizontal movement of a worm-like undermat miner that avoided a possible sand sponge (Tribrachidium). C. The 'worm' burrow Nenoxites appears to have been lined with fecal pellets. D. Aulichnites is probably the backstuffed burrow of a slug-like creature. (A and B from Seilacher et al. (2003); C and D after Fedonkin (2003).)

Fig. 10.6. In Ediacaran times, metazoan burrowers did not penetrate more than a few millimetres into the sediment. A. In actinian resting traces (Bergaueria sucta), slight lateral movements are expressed by concentric lines. B. Palaeophycus sp. records the horizontal movement of a worm-like undermat miner that avoided a possible sand sponge (Tribrachidium). C. The 'worm' burrow Nenoxites appears to have been lined with fecal pellets. D. Aulichnites is probably the backstuffed burrow of a slug-like creature. (A and B from Seilacher et al. (2003); C and D after Fedonkin (2003).)

algal films from rock surfaces or an aquarium wall. On soft sediment, however, their characteristic scratch patterns are wiped out when the mollusc's foot crawls over them.

How could such scratches be preserved on Ediacaran soft bottoms? Precambrian sea bottoms were covered by biomats sufficiently tough to carry an animal the size of Kimberella without leaving a trail. Radular teeth, on the other hand, penetrated deep enough to produce sharp undertraces at the mat's base, which is commonly marked by distinctive 'elephant-skin' structures (Figure 10.5). Only the grazing activities of juveniles fail to be recorded, because their teeth did not penetrate deep enough to produce an undertrace.

In fact, Kimberella would not have wiped out its own raspings anyway, due to a very unusual mode of grazing. Modern molluscs (and cows) move their heads to the left and right while grazing and crawl a step forward after every swing. Kimberella, however, stayed in place and foraged with the mouth at the tip of a retractable trunk.

Therefore the bipartite scratches are not arranged in continuous guided meanders but in concentric arcs (Gehling, personal communication, 1995). As the width of the swing increased automatically the further the trunk extended, the scratch field produced during each meal was conical, rather than a continuous band of meanders. Obviously, less energy had to be spent in stationary than in mobile grazing; but an extended trunk was probably too vulnerable after the onset of macropredation in the Cambrian ecologic revolution.

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