Vendobionts as giant protozoans

As mentioned above, the view that the largest, most common and most diversified organisms of the Ediacaran biota were giant protozoans (rather than metazoans) was based mainly on their allometric compartmentation. Partly overturned fronds show that quilting patterns were identical on upper and lower surfaces. The penetration of two infaunal Pteridinium specimens by another individual and their growth responses to this accident (Figure 10.1) are hard to reconcile with metazoans. So are the resting tracks left behind by Dickinsonia and Yorgia (Figure 10.2). They destroy the elephant-skin structure of the biomat and are too deep to have been produced simply by the weight of the organism; yet there are no scratches related to mechanical digging. Digestion of the biomat by microscopic pseudopodia best explains the morphology of these resting traces and the lack of a visible trackway connecting them.

Vendobionts, in the present definition, sometimes resemble arthropods and other 'Articulata' by their segmentation. During ontogeny, minute new modules were introduced at the generative pole and grew larger by secondary expansion. Yet, there are fundamental differences:

(i) Some vendobionts (e.g., the spindle-shaped form from Newfoundland; Figure 10.3) had a generative pole at either end. In others, the 'head' segment could become secondarily generative after a traumatic accident (Figure 10.1).

(ii) Secondary growth concerned mainly the long axes of the segments, while their diameters remained fairly constant (sausage-shaped quilts).

(iii) As there is no indication of molting, the flexible skin of these organisms must have been expandable. If the internal septa between quilts consisted of the same semirigid material, an arthropod-like ecdysis would have been mechanically impossible, particularly if quilts were further subdivided in a fractal mode.

Fig. 10.1. Infaunal vendobionts, such as Pteridinium, were immobile. When a newcomer (number 3) grew through resident individuals, the latter responded by growing upwards at the wrong ('head') end. Number 1 first broadened its lateral vanes in the proximal part. As it grew upward, the vanes swapped functions. The left lateral vane retained its relative position, but after having grown a pronounced fold it reversed dorsoventrality; the right vane turned smoothly into the new median vane; and the original median vane became the new right vane, again with a fold. Number 2 had only started to do the same before the whole colony was terminally smothered. As such overfolding by growth is rather common, it may have also been induced by sedimentation alone. (Modified from Grazhdankin and Seilacher (2002).)

Fig. 10.1. Infaunal vendobionts, such as Pteridinium, were immobile. When a newcomer (number 3) grew through resident individuals, the latter responded by growing upwards at the wrong ('head') end. Number 1 first broadened its lateral vanes in the proximal part. As it grew upward, the vanes swapped functions. The left lateral vane retained its relative position, but after having grown a pronounced fold it reversed dorsoventrality; the right vane turned smoothly into the new median vane; and the original median vane became the new right vane, again with a fold. Number 2 had only started to do the same before the whole colony was terminally smothered. As such overfolding by growth is rather common, it may have also been induced by sedimentation alone. (Modified from Grazhdankin and Seilacher (2002).)

(iv) Vendobiont 'segments' never carry a functional earmark, not even the terminal module that would have contained the anus in a metazoan interpretation. Nor was there rigorously determined growth or any fixed countdown programme.

(v) While segmental growth may have been controlled by Hox genes, there is no sign of a ParaHox cluster (Erwin, 2005) controlling dorso-ventral differentiation and the development of appendages.

There is, however, a major difficulty in considering the vendobionts as unicellular: their quilts are much wider than the chamberlets of large foraminiferans and the cells of metazoans, whose diameters (rather than volumes) were probably restricted by metabolic activities (mainly diffusive) of the contained protoplasm. The solution to this dilemma came from Xenophyophoria that happen to survive on present deep-sea bottoms.

Some members of the unicellular, but multinucleate, xenophyophores (e.g., Stannophyllum, Figure 10.4) resemble vendobionts by being foliate and consisting of negatively allometric, sausage-shaped chambers. These chambers, however, have agglutinated walls that cannot secondarily expand, as did the quilts of

Fig. 10.2. Some prostrate vendobionts were able to creep over the biomat (elephant-skin structure), leaving behind series of resting traces but no connecting trail. Their bumpy surface without scratch marks suggests a digestive mode of burrowing. Occasionally (star), the trace maker impressed its quilts before leaving. Note that the traces and their axes are not aligned. They also have larger diameters (broken line) than the negative hyporelief mask at the last station, suggesting that the organism expanded while feeding. (After Fedonkin (2003).)

Grazing Vendobiont, White Sea

Fig. 10.2. Some prostrate vendobionts were able to creep over the biomat (elephant-skin structure), leaving behind series of resting traces but no connecting trail. Their bumpy surface without scratch marks suggests a digestive mode of burrowing. Occasionally (star), the trace maker impressed its quilts before leaving. Note that the traces and their axes are not aligned. They also have larger diameters (broken line) than the negative hyporelief mask at the last station, suggesting that the organism expanded while feeding. (After Fedonkin (2003).)

vendobionts. Ediacaran xenophyophores (Figure 10.4), which also include strings of globular chambers, lived in shallower waters than do modern ones and were embedded in microbial mats. This is why they are found in situ as positive hyporeliefs on bed soles, reminiscent of trace fossils. These protozoans were also allometrically compartmentalized, but, as in vendobionts, the chambers were too wide for unicellular standards. In xenophyophores, however, the reason for this disproportion is known: rather than being filled by pure protoplasm, their chambers contain a softer fill skeleton (stercomare). Consisting of finer sediment particles taken up by the pseudopodia with the food, the function of the stercomare is to further subdivide the protoplasm into strands of permissible diameters (Tendal, 1972). As observed in the Nama style of preservation (Seilacher et al., 2003) and in large Charniodiscus (Figure 10.3), the chambers of Vendobionta appear to have been filled to about 50% with loosely bound sand grains that probably were ingested with food and may have served the same purpose as the stercomare.

Fig. 10.3. Vendobionts grew unusually large for unicellular organisms. While sharing a quilted foliate construction, they radiated into a variety of shapes and life styles on and below the ubiquitous Precambrian biomats. Note that Dickinsonia (as Yorgia, Figure 10.2) lived on top of the mat and were mobile, while Phyllozoon probably lived below the mat and was immobile (Figure 10.7). Among the anchored forms, Paracharnia was built for swaying in a current, while a stiff stem allowed other forms to stand upright in quieter waters. Note also that in Rangea the opposite branch of each quilt pair bent alternatingly into one of the two inner vanes and thereby reduced space problems. (Modified from Seilacher (2003).)

Fig. 10.3. Vendobionts grew unusually large for unicellular organisms. While sharing a quilted foliate construction, they radiated into a variety of shapes and life styles on and below the ubiquitous Precambrian biomats. Note that Dickinsonia (as Yorgia, Figure 10.2) lived on top of the mat and were mobile, while Phyllozoon probably lived below the mat and was immobile (Figure 10.7). Among the anchored forms, Paracharnia was built for swaying in a current, while a stiff stem allowed other forms to stand upright in quieter waters. Note also that in Rangea the opposite branch of each quilt pair bent alternatingly into one of the two inner vanes and thereby reduced space problems. (Modified from Seilacher (2003).)

In conclusion, it is reasonable to consider the Vendobionta as a coherent group of giant rhizopods that radiated into a variety of morphologies and life styles (Figure 10.3). The epinym 'unicellular dinosaurs' refers to their unusual size, ecological dominance, and morphological disparity, but also to their specialization, which made them more and more vulnerable in the face of global changes of any kind (Seilacher, 1998). The coming of macropredation and the elimination of tough microbial mats by bioturbators were such global events.

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