Is it an elephant or is it a shark?

I still remember when David first present elephant shark in a group meeting, I stopped him and asked naively: Excuse me, is it an elephant or is it a shark? :)

Of course, it's not an elephant. And now I know it's neither a shark.

Here is the today's Nature paper for the 'living fossil':

http://www.nature.com/nature/journal/v505/n7482/full/nature12826.html

It says

"Our functional studies suggest that the lack of genes encoding secreted calcium-binding phosphoproteins in cartilaginous fishes explains the absence of bone in their endoskeleton. Furthermore, the adaptive immune system of cartilaginous fishes is unusual: it lacks the canonical CD4 co-receptor and most transcription factors, cytokines and cytokine receptors related to the CD4 lineage, despite the presence of polymorphic major histocompatibility complex class II molecules. It thus presents a new model for understanding the origin of adaptive immunity."

Here is the taxa from the paper:

And I could just find the one I made in my PhD thesis :)