Editor’s note: We are delighted to present here Part 5 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.
Tribrachidium has made a quite chaotic journey across the tree of life. It has been considered as of uncertain affinity (Narbonne 2005), as a member of an extinct kingdom Vendobionta (McMenamin 1998), as a stem eumetazoan (Grazhdankin 2014), as a poriferan (Seilacher 1999, Seilacher et al. 2003), as a diploblastic grade coelenterate medusoid (Conway Morris 1990, Zakrevskaya & Ivantsov 2020), as a ctenophore similar to Eoandromeda (Tang et al. 2011, Budd & Jensen 2017, Giribet & Edgecombe 2020), as a scyphozoan cnidarian or scyphozoan-like stem cnidarian (Wade 1969, Keller & Fedonkin 1977, Fedonkin 1982, 1983, 1984, 1985c, 1986, 1987, 1992, Fedonkin & Cope 1985, Val’kov 1987, Valentine 1992, Gee 2000, Ivantsov & Fedonkin 2002, Fedonkin et al. 2007), as a relative of round worms (Dzik 2003), as a possible lophophorate (Glaessner 1962, 1984), as a stem echinoderm (Glaessner & Wade 1966, Paul 1979, Gehling 1987), or even as a fungus (Retallack 2013).
Rahman et al. (2015) remarked “that the similarities between Tribrachidium and these groups are superficial, and Tribrachidium is best understood as a multicellular eukaryote with uncertain relationships to crown Metazoa.” Likewise, Droser et al. (2017) suggested that “the triradialomorph clade went extinct during the late Ediacaran and that threefold symmetry was a failed evolutionary experiment in multicellular eukaryote body plans.”
We Have No Clue
I totally agree with these latter views, but still the fact that the same fossils can be attributed to at least six different phyla of marine invertebrates (as different as sponges, echinoderms, or jellyfish) as well as terrestrial fungi really should give reason to pause and to recognize humbly that we have no clue what these fossils were. The most recent review of the tri-radial Ediacaran organisms by Hall et al. (2018) therefore did not even attempt an identification of any modern relatives of this group, which they called a “short-lived but successful tri-radial body plan.” Instead, they honestly admitted:
Understanding the relationships among these enigmatic taxa, and, even more so, their relationship to modern clades, has proved to be a difficult task (Xiao & Laflamme, 2009). Clarifying fundamental biological properties, such as growth and development, and the ecological roles of these taxa, is the most realistic and plausible method we have for understanding what these organisms were and how they lived.
Sunday, “Unknown Biology of Trilobozoa.”