THE TASK

The reading Agnostids were a group of marine animals that became extinct about 450 million years ago. Agnostid fossils can be found in rocks in many areas around the world. From the fossil remains, we know that agnostids were primitive arthropods-relatives of modern-day insects. However, the fossil information does not allow paleontologists to determine with certainty what agnostids ate or how they behaved. There are several different theories about how agnostids may have lived. Free-Swimming Predators First, the agnostids may have been free-swimming predators that hunted smaller animals. It is known that other types of primitive arthropods were strong swimmers and active predators, so it is reasonable that the agnostids may have lived that way as well And while the agnostids were small, sometimes just six millimeters long, there were plenty of smaller organisms in the ancient ocean for them to prey on. Seafloor Dwellers Second, they may have dwelled on the seafloor. Again, there are examples of other types of primitive arthropods living this way, so it is possible that agnostids did too. On the seafloor they would have survived by scavenging dead organisms or by grazing on bacteria. Parasites Third, there is the possibility that the agnostids were parasites, living on and feeding off larger organisms. One reason that this seems possible is that there are many species of modern-day arthropods that exist as parasites, such as fleas, ticks, and mites. The agnostids might have lived on primitive fish or even on other, larger arthropods.

Answering questions (Main points of reading) 1. There are three theories about how agnostids, which were a group of marine animals that became extinct about 450 million years ago, may have lived. 2. First, it is reasonable to conclude that the agnostids may have lived the way just like other types of primitive arthropods as well, which are strong swimmers and active predators though they were small. 3. Second, it is also reasonable to conclude that agnostids may have dwelled on the seafloor just as other types of primitive arthropods. 4. Third, there is the possibility that the agnostids were parasites that are living on and feeding off larger organisms since many species of modern-day arthropods exists as parasites. (Main points of the lecture) 1. The statements below proved the theories are not convincing. 2. First, the eyes of agnostids are too small, and there is no evidence of fossil record of their special sensory organs to catch up prey. 3. Second, many agnostids species dwelled on multiple geographic areas because seafloor dwellers move slowly across the seafloor and remain living in localized areas. 4. Third, populations of parasites are not very large. Although too many parasites would kill off the host organisms they live on, the populations of many agnostids were in fact very large.

TEST TAKER'S RESPONSE 1.

TEST TAKER'S RESPONSE 2.