Some unusual marine organisms don’t seem to fit anywhere in the animal classification scheme.
The earliest chordate in the Lower Cambrian is now shown to be a full-gilled vertebrate fish.
The frond-like Ediacaran fauna known as Rangeomorphs had simple colony-like body plans consisting of self-similar branching patterns of tubular units.
Phylum Placozoa contains one member, but it has a body plan, six cell types, and a complex genomic toolkit.
A news article at Nature, "Oxygen fluctuations stalled life on Earth," is remarkable for skirting the main issue, the primary enigma.