Earth Linked Up

Pretend you are on a time machine. Imagine traveling back to the birth of multi-celled creatures. They looked awfully strange and unfit to living in any enviroment. Go further on and you see the Cambrian explosion, life diversifying at an unnatural speed. Looking in the shallow water, you see a tiny little worm with a spread tail and a line down it's back. It is none other than Pikaia, thought to be the ancestor of all vertebrates. You feel honored to see it, but it otherwise looks pretty boring and you start to leave.

But before you go, you notice something odd about it's shape. It's not moving side to side like an eel, but up and down like a whale. Upon closer examination, you see something messed up with it's noto-chord, too. It's somewhat flat, looking less like a spinal cord and more like a chain made of links. You check the time location on the dashboard and discover that you've accidentally gone down the wrong timeline. This is not our timeline, no. This is the world of Earth Linked Up.

Note: For simplicity, the evolution of invertebrates and other biota are mostly unchanged.

Ordovician
Fish evolve with fins, more or less. The flat, horizontal design of their planavertebra column makes a strong muscle configuration a bit difficult, so each planavertebra has four bone knubs, two on each side. Their body plan is similar to normal fish, just flipped on it's side. What would be the pectoral fins are now top and bottom fins that move side to side, with stabilizing fins running along their sides.