Editor’s note: We are delighted to present here Part 3 of “A Precambrian House of Cards,” a series by Dr. Bechly. Find the full series at this link. An extensive References section will follow at the end of the series.
The only one of the four examples selected by Evans et al. (2021) that I have not yet discussed in my articles is Tribrachidium. So, let’s use this opportunity to have a brief look at this strange fossil organism and its fossil relatives.
Tribrachidium is a 3-38 mm wide, disc-shaped fossil with three large spiral ridges (antimeres) in the center and numerous radiating fine grooves that are especially prominent along the outer margin. Several scientists (e.g., Gehling 1991, Seilacher 1999, Fedonkin et al. 2007, Ivantsov & Leonov 2008) have suggested that the fossil discs represent a collapsed body of a hemispherical organism, which lived lying on the sea floor like a bathtub mat. Tribrachidium heraldicum was originally described from the Ediacaran of Australia by Glaessner & Daily (1959), where it is a relatively common fossil, but it also occurs at other localities from this period (Hall et al. 2015). Together with some similar fossil genera (i.e., Albumares, Anfesta, Hallidaya, Rugoconites, and Skinnera) it has been attributed to a separate class or phylum called Trilobozoa (Fedonkin 1985c, 1987), later unnecessarily renamed as Triradialomorpha (Erwin et al. 2011, Laflamme et al. 2013), Tribrachiomorpha (Grazhdankin 2014), or Trilobozoida (Just et al. 2014). Apparently, Runnegar (1989) was the first to suggest a status as separate phylum.
The name-giving and distinctive feature of this group is a consistent “triradial (threefold) symmetry, which is entirely unknown among living animal phyla” (Rahman et al. 2015), with a few exceptions among living cnidarians and as a secondary phenomenon in some nematodes and gastrotrichs (Fedonkin 1992). Hall et al. (2018) reviewed all the genera, revised the composition of the Trilobozoa, and established this group as a clade in the Hennigian sense of phylogenetic systematics or cladistics, thus as a strictly monophyletic group that includes all the descendants of a single ancestral species.
And there is another point worth mentioning: Trilobozoans are unique to the Ediacaran biota; they appeared suddenly 560 million-years-ago in the fossil record without any precursors in the older layers, and likewise suddenly disappeared 550 million years ago with the beginning of the Cambrian (Grazhdankin 2014: Fig. 7, Hall et al. 2018). Their distinct body plan appeared out of nowhere and did not change during their existence. Thus, trilobozoans as such are hardly a good example of Darwinian evolution.
Tomorrow, “Potential Other Trilobozoans.”