Goofs

Despite the fact that Cephalaspis are stated and shown to be bad swimmers, they swim surprisingly well when they migrate through the sea. They should swim lower to the sea bottom in this scene.

The Hynerpeton is constantly making roar-like sounds. It is highly doubtful the animal was capable of emitting such noises in real life. Or even if it was, roaring non-stop would still be an unrealistic thing for an animal to do.

The Brontoscorpio molts on dry land. It was probably more likely to shed its outer shell in the water, since, being a water-going animal, it would risk dehydration on land.

Even some arthropods make strange, creaking sounds with their mouths. Most likely, these animals were silent, like modern day arthropods.

When the two Animalocaris fight, for a moment a strange, light-toned shape appears on the screen just over them, then disappears between frames.

As a Pterygotus swims past, above the camera's view, there is a thin line directly on the top of the screen, where you can see the background through the Pterygotus.

When the female Cephalaspis jets out of view, escaping from the Brontoscorpio, she starts to disappear before she reaches the edge of the screen. If you freeze-frame here (though the error is visible without freeze-framing), you can see that her upper portion has been "cropped off", and you can clearly see the background through her.

One shot has a couple of Cephalaspis swimming by who have no shadows. All the Cephalaspis that follow, however, do cast shadows on the sea floor.

When the scorpions are mauling the Cephalaspis, many of the fish jumping out of the sea aren't reflected in the water. Also in one shot, the reflection of the Brontoscorpio approaching the scene from the background is rendered in such a way, it makes it look like the animal's floating a feet or two above the ground.

The Cephalaspis actually lived in the Devonian, not in the Silurian period.