Wait a minute, where's the dorsal fin...in fact where's the other fins, besides the tail fin?

Samdaman said:

Wait a minute, where's the dorsal fin...in fact where's the other fins, besides the tail fin?

Apparantly sacabambaspis needs no fins... or jaws. It existed before they had evolved.
https://en.wikipedia.org/wiki/Sacabambaspis

Samdaman said:

Wait a minute, where's the dorsal fin...in fact where's the other fins, besides the tail fin?

No need to be fast or nimble when all your competitors are just as slow, if not slower.

Agemmata said:

No need to be fast or nimble when all your competitors are just as slow, if not slower.

Until Mr. Dunkleosteus and friends appeared.

[deleted]

Shinjidude said:

Until Mr. Dunkleosteus and friends appeared.

Which was ironically, iirc, driven to extinction by faster, more nimble, unarmored fish that out-competed it.