When we look at the fossil record, the Precambrian can easily be presented as a world without animals, particularly bilaterally symmetrical animals, before they rapidly appeared and diversified during the Cambrian period. Skeletal fossils – bits of shell and other hard parts – appear at the end of the Ediacaran period, then become much more diverse (and recognisable) during the early Cambrian. What we have from the Ediacaran period are mostly soft-bodied organisms, some of which resembled fronds sticking out of the sediment, whilst others rested on the surface, and yet others stuck in the mud. Although some of these organisms have been linked to modern animal groups, all have been cast into doubt at some point; there are very few which might be animals though they may be close to the base of the animal family tree. The little skeletal forms, such as Cloudina which resembles a stack of plastic cups, are thought to be no more complex than jellyfish at most.
When we look at the molecular data, it suggests that the major animal groups, including phyla, had evolved during the Precambrian. Bilaterian animals were supposedly quite diverse and even quite complex. The body fossil record does not line up with this data, nor does the trace fossil record (there are some putative bilaterian traces, but nothing definitive until close to the base of the Cambrian). This leads to an obvious question: where are all of the fossils?
In 2000, a fossil from Namibia was described and named Namacalathus hermanastes. These fossils were shaped like goblets on a stalk, with a large hole in the top of the goblet, and a number of windows around the sides. It lived sticking out of microbial mats, possibly filter-feeding in late Ediacaran reefs. It reproduced asexually, budding off little daughter versions of itself. This mode of reproduction led to its interpretation as a diploblastic animal, perhaps closely related to jellyfish and corals along with Cloudina.
A new study has suggested that Namacalathus is not a simple cnidarian, but a full-fledged bilaterian. Not only that, but a crown-member of a group called the Lophophorata, placing it close to the brachiopods and bryozoa. They analysed the structure of the shell of Namacalathus and found that it had features only found amongst lophophorates. They also found possible evidence of an organic-rich layer within the wall, a feature found in brachiopods, and they noted that the clonal budding occurs in a bilaterally symmetrical pattern. It is suggested that the soft parts of Namacalathus would have been bilateral, despite the hard parts possessing a different symmetry. This new interpretation may provide evidence for the existence of bilateral animals with skeletons in the Ediacaran period, but it is guaranteed to be controversial. Reconstruction of the living Namacalathus. 1, stem; 2, parental cup; 3, daughter cups; 4, hollow ciliated tentacles; 5, spines; 6, lateral lumen; 7 central opening; 8, inner skeletal layer—foliated with columnar microlamellar inflections; 9, internal (middle) skeletal later—organic rich; 10, external outer skeletal layer—foliated with columnar skeletal inflections (image copyright: J. Sibbick).
Zhuravlev, A.Yu., Wood, R.A., Penny, A.M. 2015. Ediacaran skeletal metazoan interpreted as a lophophorate. Proc. R. Soc. B 282: 18181. [Link]