Trilobites were a diverse group of marine arthropods that lived throughout the p
ID: 81026 • Letter: T
Question
Trilobites were a diverse group of marine arthropods that lived throughout the paleozoic era before going extinct 250 million years ago. The photos show two fossil trilobites in the order Lichida found in the same rock formation. The right fossil is a species in the genus Actinurus, and the left is in the genus Acanthopyge. Imagine you found only Actinurus fossils in one layer of rock and then only Acanthopyge fossils in the next rock layer above the previous, making you wonder if Actinurus is the direct ancestor of Acanthopyge. In a well-constructed essay, discuss why it might be difficult to determine definitively that Actinurus is, in fact, the direct ancestor of Acanthopyge. Possible topics to include are fossil dating, biases and complications in the fossil record, gradualism vs punctuated equilibrium, and/or the origin of complex traits. You will be graded on content, writing style (grammar, word choice, sentence construction), and organization (logical flow, clear topic and concluding sentences). You can neatly hand-write in the space below or attach a typed answer to the end of this assignment.Explanation / Answer
Trilobites are one of the earliest known groups of arthropods. They were among the most successful of all early animals found in the oceans for over 270 million years. They finally disappeared in mass extinction at the end of Permian period. It is difficult to determine whether Actinurus is the direct ancestor of Acanthophyge because of several reasons.
Though fossil dating helps to determine the age of fossils, it cannot be considered as an accurate method in deciding the ancestry.
The remnants of trilobites include the preserved body and pieces of the exoskeleton which they shed during their life time. They molt off numerous outer skeletons into the fossil records before they die. This process of molting act as a bias because it shows more abundance of species than reality. Moreover, a fossil once formed on a rock may be destroyed by geological processes. Processes as simple as seeping may dissolve the remnants not giving the complete reading as expected. Time is another bias is in attainment of proper fossil records. Longer a fossil is preserved there is more chance of it getting destroyed. These complications in fossil records may be incomplete and misleading to come to conclusions.
The two ways in which evolution of species occur are gradualism and punctuated equilibrium. In gradualism, selection of species and process of variation happens gradually. Whereas as in punctuated equilibrium, changes are sudden. In this, there is a period of very little change and later a huge sudden change occur mainly due to mutations. These changes are then passed on to generations that follow. Such changes are then difficult to record accurately. Gradualism vs punctuated equilibrium is thus a limitation in studying the ancestry of Acanthophyge.
Trilobites have complex traits such as elaborately segmented body with jointed appendages and swimmerets, compound eyes, antennae, etc. Due to mutations happening over a period of time, these complex traits must have originated which might be difficult to explain.
Therefore, with all these limitations in the study, it is difficult to confirm that that Actinurus is the direct ancestor of Acanthophyge.
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