31 Opisthokonts
There are two main opisthokont groups: the animals and the fungi. The Proterozoic fossil record of animals is worthy of an extensive review in its own right; I will not discuss it here except to note that the earliest well accepted evidence for animals are ~580 Ma phosphatized embryos from the Doushantuo Formation, China (Xiao et al., 1998b; Xiao and Knoll, 2000; Condon et al., 2005). See papers by Jensen et al. and Bottjer and Clapham, both in this volume, for further information on Proterozoic animals.
The presence of fungi in the Proterozoic Eon is much more controversial. Several authors have noted similarities between certain microfossils and modern fungi, but in none of these reports has a convincing case been made (e.g., Schopf and Barghoon, 1969; Darby, 1974; Timofeev, 1970; Allison and Awramik, 1989; Schopf, 1968). Some Ediacaran taxa have also been interpreted to be fungal. Retallack (1994), for example, argued that because vendobionts exhibit minimal compaction, they cannot represent soft bodied animals like worms or jellyfish, and instead may be fossilized lichens (a symbiotic association between a fungus and an alga). Minimal compaction has been observed in some softbodied animals, however (e.g., Hagadorn et al., 2002), and, at least in the Ediacaran biota, could be attributed to unusual "death mask" preservation where early diagenetic minerals form a resistant crust (e.g., Gehling 1999). More recently, Peterson et al. (2003) argued that Ediacaran fossils from Newfoundland, including Aspidella, Charnia, and Charniodiscus, may represent stem-group fungi. Their argument is based primarily on a process of elimination: the fossils are found in sediments deposited below the photic zone and thus cannot be algal; the fossils do not exhibit evidence for escape or defouling behavior despite having been smothered by a thin layer of ash and thus cannot be animals; and the fossils lack evidence for shrinkage—observed in other Ediacaran taxa— inconsistent, again, with an animal interpretation. As the authors admit, however, there is little positive evidence in the form of fungal-specific characters to support a fungal affinity.
Fungi have also been reported from the 551-635 Ma Doushantuo Formation (Yuan et al., 2005). Filaments interpreted to be fungal hyphae occur in lichen-like association with clusters of coccoidal, probably cyanobacterial unicells. A fungal interpretation is based on a combination of characters—dichotomous branching, pyriform terminal structures, absence of sheaths, and narrow diameter (<1pm)—not seen in other filamentous organisms like cyanobacteria, but comparable to features observed in hyphae of glomalean fungi (Yuan et al. 2005).
Even earlier evidence for possible Proterozoic fungi comes from organic-walled microfossils preserved in the 723-1077 Ma Wynniatt Formation, Shaler Supergroup, arctic Canada (Fig. 2A; Butterfield, 2005). These beautifully preserved fossils consist of a large central vesicle with branching, septate, filamentous processes apparently capable of secondary fusion (Figs. 2A-B). Secondary cell-cell fusion is found in both the fungi and the red algae (Gregory, 1984; Graham and Wilcox, 2000), and possibly in the brown algae as well (Butterfield, 2005, and references therein). Because the processes are similar to fungal hyphae, however, Butterfield (2005) specifically compared the Wynniatt fossils with fungi, noting that hyphal fusion is a synapomorphy of the basidiomycetes+ascomycetes (Fig. 2C; Gregory, 1984). Butterfield (2005) referred the Wynniatt fossils to the genus Tappania, noting similarities with Tappania species from the ~1450 Ma Roper Group, Australia (Javaux et al., 2001), and the Meso-Neoproterozoic Ruyang Group, north China (Yin, 1997). Secondary fusion has not been reported in Tappania, however, and it is not obvious that the younger and older populations are related.
An additional opisthokont group, the unicellular choanoflagellates, produce siliceous 'baskets' ~10-20 pm in size, and thus, could, in principle, have a fossil record (Leadbetter and Thomsen, 2000). No fossil choanoflagellates have been reported, however, from either Proterozoic or Phanerozoic rocks, although this may reflect a lack of search image as much as a lack of preservation.
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