Interpreting Late Precambrian Microfossils

Chia-Wei Li et al. describe microfossils found in Doushantuo phosphate rocks from Guizhou Province, South China that are about 580 million years old (1). Well-preserved macroscopic multicellular fossils associated with prokaryotic and eukaryotic microfossils from the same source rocks have been documented in several studies (2– 6). The Doushantuo fossils are outstanding in both preservation quality and diversity, showing some details of cellular and tissue structures, and thus providing an exceptional opportunity to understand the evolution and diversification of early plants and animals just before the Ediacaran metazoan radiation. Interpretation of these fossils, however, should not be based only on a superficial morphological comparison between the fossils and some living forms. Taphonomic changes of the macroand microfossils embedded in the Neoproterozoic phosphorites have to be taken into consideration. Postmortem degradation and diagenetic changes of the biological structures, which led to the present morphology of the fossils, are important for judging the biological nature of the ancient organisms (7). We have studied the Doushantuo phosphorite fossils for more than a decade, and we disagree with the interpretation of Li et al. that some of the submillimeter microfossils were parenchymella larvae and amphiblastula embryos of sponges (1). The putative parenchymella larvae of sponges, with a shoe-shaped morphology and dense peripheral flagella, as described by Li et al., are actually acanthomorphic acritarchs. Acritarchs are organic-walled microfossils that cannot be placed confidently into any existing classification categories, but most are comparable to remains of cysts of planktic eukaryotic algae (8). The acanthomorphic acritarchs (an acritarch group with distinct peripheral processes) are common and abundant in Neoproterozoic and Paleozoic deposits. Each individual acanthomorphic acritarch consists of an ovoid body (vesicle) surrounded by processes (Fig. 1, A and B). The processes of different taxa are varied in shape and size: mammillate, spine-form, hair-like, or flagellum-like. The specimen illustrated by Li et al., figure 2E of their report (1), is a deformed individual, with a shrunk vesicle and hair-like (or spine-like) processes. This specimen is morphologically comparable to those of the acritarch genus Ericiasphaera, which was set by Vidal in 1990 (9) and was redescribed to apply the large sphaeroidal acritarchs bearing numerous regularly arranged, solid, flexible, or rigid processes (6). Three species from the Doushantuo Formation have been described and assigned to this genus (6). We have observed many acanthomorphic acritarch specimens in the Doushantuo phosphorites (4, 6, 10), which display a wide range of morphological variation resulting from taphonomic changes: from a relatively intact specimen (Fig. 1, A and B), to the shrunk forms at different degradation states (Fig. 1, C and D). Some of these specimens are similar to those illustrated by Li et al. (figure 1C in the report). The granular and variable structures within the phosphorites have been interpreted as sclerocytes (sponge skeleton-setting cells) and amoebocytes, respectively, by Li et al. (1). However, as the figures in their report show, these structures have no distinct boundary membranes. Instead, they consist of amorphous kerogen. We have seen such structures in the vesicles of acritarchs (Fig. 1B) or in fossil remains (Fig. 1, E and F).