Self-paced
Until about 400 million years ago, fishes—the first true swimmers—were jawless. █████ ███████ ███████ ████ ███████ ██ ██████ ███████ ██ ███████ ████████ ██ ███████ ██ ████ █████████ ████ ██████ ████ ████ ███ ███████████ ██ ██████ █████ ████████ ███ ████ ██ ██████ ███████ █████████████ █████ ████ ███████ ████ ████████ ██ ██████ █████ ██ █████ ██ ██ █████ █████ ███ ██ ██████████ ██ ███████ █████ ██████ ███ █████ ██████ ████ █████████ █████ ███ ████ ██████ ███ ████████ █████████ ███ ███ ██████████ ███ ████████ ██████ ███ █████ ███ █████ ███████ ████ ██ ███ █████████ ████████ █████████ ████ ███ ██████ █████ ███████ ███ ████ ████████ ███ ███████ ██ ███ ██ █████████ ██████████ ███████ ███ ███████████████ ████ ██████ ████████ █████ ████ ████ ██████████ ██ ███ ███████
Stimulus Breakdown
The stimulus throws a bunch of information at us about fishes. To help identify which answer choice must be true, let's start by laying out what the stimulus says.
1) fishes were the first true swimmers
2) fishes first developed biting jaws about 400 million years ago
3) before developing jaws, fishes fed by eating surface plankton or by sucking in food particles from mud
4) biting jaws allowed fishes to pursue prey, bite it, and manipulate it with their teeth
5) jawed fishes developed in two evolutionary lines: one with cartilage skeletons, like sharks and rays, and the other with bone skeletons
6) the bone-skeleton group evolved into the teleosts, the most diverse vertebrate group, with about 21,000 species including barracudas and sea horses
Our goal is now to find the correct answer using all these fun fish facts. We're looking for an answer that must be true—it must logically follow from one or more of these facts. We should be able to identify which specific fact(s) guarantee the correct answer. On the other hand, incorrect answers could be false. They might be consistent with the stimulus, but they won't be guaranteed.
20.
If all of the statements ██ ███ ███████ ███ █████ █████ ███ ██ ███ █████████ ████ ████ ██ █████
a
Fish are the ███████ ████ ██ ███ █████ ███████
The stimulus doesn't tell us what jawed fishes prey on, just that they're capable of hunting prey. We don't even know for sure that jawed fishes eat fish, let alone that fish are their primary prey.
b
The jawless fishes ███ ███ ████ ████ █████ █████
We know that
c
Teleosts do not ████ ████ █████████ █████ ██ ██████ ████
We know that teleosts evolved from jawed fish, who had developed the capacity to hunt. However, we don't know if all teleosts use this capacity. It's entirely possible that some teleosts have returned to mud-sucking—think of catfish, for example.
d
Jawless fishes did ███ ████ █████████ ██ █████ ████████ █████████
All the stimulus tells us about skeletal material is that jawless fishes evolved into cartilage-skeleton and bone-skeleton lines. But the stimulus doesn't tell us the skeleton type of jawless fishes. So (D) could be true, but it could also be false.
e
Jawless fishes became ███████ █████████████ ███ ███████ █████ ████
Just because fish first developed jaws about 400 million years ago, that doesn't necessarily mean jawless fishes went extinct. It's possible they did, but they could also have survived as an offshoot on the evolutionary tree.