Proarticulata, the largest mobile animals of the Late Precambrian, inhabited shallow marine basins of normal salinity in Baltica (eastern and northern Europe) and East Gondwana (Australia) in the Late Ediacaran. They represented a substantial part of benthic palaeocommunities of macroorganisms inhabiting microbial mats, and at least some of them fed on the upper lamina of such a mat, absorbing the nutrition by their ventral side. Moulds attributed to the Proarticulata were formed by sub-bilateral organisms, divided into two transverse rows of half-segments (isomers). It was established that these moulds did not represent the entire body, but only a part, that was relatively resistant to decomposition. This sack-like structure, divided into transverse elements, enveloped the dorsal and ventral sides of the body but did not spread to its inner parts. It was dense and probably served as a resilient support. On the dorsal side, this integument created a continuous shield that showed no signs of segmentation. The outer surface of the dorsal side of several genera of the Proarticulata was covered with numerous evenly distributed tubercles. In a single specimen, the tissue at the posterior end of the body formed a pair of long filamentous outgrowths. Based on the feeding traces, it was inferred that the ventral side of the body was segmented externally similar to the supporting structure. The cover tissue on the ventral side probably bore cilia and furrows of suggested ciliary sweeping were preserved on the traces. The integument and segmented structure are interpreted as an epidermal tissue and a basal matrix underlying it. A tissue organization in combination with the anteroposterior and dorsoventral polarity of the Proarticulata body allow us to assign them to the Eumetazoa and
We describe traces of macroorganisms in association with the body imprints of trace-producers from Ediacaran (Vendian) deposits of the southeastern White Sea region. They are interpreted as traces of locomotion and are not directly related to a food gathering. The complex remains belong to three species: Kimberella quadrata, Dickinsonia cf. menneri, and Tribrachidium heraldicum. They were found in three different burials. The traces have the form of narrow ridges or wide bands (grooves and linear depressions on natural imprints). In elongated Kimberella and Dickinsonia, the traces are stretched parallel to the longitudinal axis of the body and extend from its posterior end. In the case of the isometric Tribrachidium, the trace is directed away from the margin of the shield. A short length of the traces indicates that they were left by the organisms that were covered with the sediment just before their death. The traces overlaid the microbial mat with no clear signs of deformation under or around the traces. A trace substance, apparently, differed from the material of the bearing layers (i.e., a fine-grained sandstone or siltstone) and was not preserved on the imprints. This suggests that the traces were made with organic material, probably mucus, which was secreted by animals in a stressful situation. The mucus traced the movements of the organism before death. The discovered traces of locomotion are direct evidence of the ability of some Ediacaran macroorganisms to move independently.
Several specimens of Dickinsonia cf. D. menneri, originating from a single burial event at the Lyamtsa locality of the late Ediacaran (Vendian) in the southeastern White Sea area, Russia, represent deviations from normal morphology: a reduction in the total length of the body; the loss of portions of the body; various deformations of the transverse elements, called isomers; and splitting of the longitudinal axis with the formation of two posterior ends. It is assumed that these deformations were formed as a result of non-lethal damage, which occurred long before the burial event, and the response of Dickinsonia to them. The progress of the regeneration process at the damaged areas, and especially its deviations, indicates that the growth zone was located at the posterior end of the Dickinsonia body. The cause of non-lethal damage to Dickinsonia could not be established, but the local distribution of deformed specimens preserved in the same burial event alongside cyanobacterial colonies, and the presence of weak deformations, expressed only in shortening of the length of some isomers, lead to the conclusion that damage resulted from short episodes of physicochemical impact, rather than occasional attacks by a hypothetical macrophage.
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