A mysterious fossil from Kansas turns out to be a large shark

[Halichoeres]

A fossil from the Niobrara chalk was always assumed to be calcium carbonate, which is part of invertebrate and algal skeletons. It was named Platylithophycus ("flat stony seaweed") because it was assumed to be a macroalga. Then it was reinterpreted as part of a mollusk. It turns out instead to be composed of calcium phosphate, and with a morphology just like the tesserated cartilage of sharks. In fact it looks like the gill arches of a large filter feeding shark (the fossil is almost half a meter long):



Write-up:
https://www.sciencedaily.com/releases/2018/04/180416155605.htm

Paper (paywall): https://www.cambridge.org/core/journals/journal-of-paleontology/article/resolving-the-identity-of-platylithophycus-an-enigmatic-fossil-from-the-niobrara-chalk-upper-cretaceous-coniaciancampanian/3890CF68F096178083D420EC31D763E4