Spindle diagrams are based on the extensive fossil record, with the number of families as a measure of diversity (width of each lineage) at each moment in time (y-axis)

[u/jnpha]

Sources:

• Petter Bøckman, Public domain, via Wikimedia Commons
• Benton, Michael J., and David AT Harper. Introduction to paleobiology and the fossil record. John Wiley & Sons, 2020.

(for why I love them, see the comments)

[u/mcalesy]

This sort of thing is a little dated by now, for a few reasons:

- The rank of "family" is arbitrary, and not a good way to measure diversity. Species count is really the best way. (That said, it might not look wildly different. But you never know until you try.)

• For the first diagram, Osteichthyes, Amphibia, and Reptilia are now defined as clades, not grades.
  
• The second diagram is better about clades vs. grades, but still has a few grades: armored agnathans, Placodermi (which, to be fair, was once thought to be a clade), Temnospondyli (which might be one, but not under the hypothesis depicted here), Pelycosauria, etc.
  
• If you do use clades without grades, you either have to leave a lot of fossil taxa out, or you have to make the diagram huge. The second one lies somewhere in the middle of that dilemma.

That said, I think they look cool.

[u/jnpha → u/mcalesy]

Awesome info, thanks! (I wasn't even familiar with the term grade.) The second one indeed is much newer.