Sponges: the hexactin problem and its importance for animal origins

Lots of interesting work currently going on at the base of crown group Metazoa.

Sponges (Porifera) have a very long and complex evolutionary history, and hexactinellids (glass sponges) are among the most intriguing. Traditionally, though, they have been identified by the presence of the spicule form known as hexactins. The problem is that hexactins are now appearing in the strangest and most not-hexactinellid-like fossils, and this has serious implications for interpreting and understanding the early evolutionary history of the phylum. Were hexactins evolved numerous times, or were they originally shared across the classes..? The answer affects what we expect from the earliest fossil record of sponges going back into the Ediacaran... and at least one scenario throws up some major challenges to our understanding of Ediacaran life.

The YT channel Life through Time focuses (mainly) on pre-mesozoic palaeontology through the perspective of Joseph Botting, a leading researcher in the field. He presents not just the raw material but also a (usually) highly digestible step-by-step guide through his analytical processes. I find it fascinating.