1. Post a Picture of your tree. On your tree, which species is the outgroup? Why did you choose this species? Explain.
The specimen chosen as the outgroup was specimen 3, because of it’s very neutral and broad features, such as, having a round body, no spikes or frills, and no eyes. The lack of many distinct features leads us to believe that the other trilobites evolved from this one, by developing unique traits over time, to better suit their environment.
Figure 1. Phylogenetic tree was reconstructed by Robert, Nancy, Francis and Azar.
2. According to your tree, what is one basal or ancestral characteristic? One derived characteristic?
One ancestral characteristic, that is, a characteristic passed down through lineage over time, that both the older ancestors and newer ancestors express, would be the linear spinal feature. All the specimens seem to have a vertical, spine-like, column running the length of their bodies. The overwhelming presence of this trait would reinforce the idea that, this trait in particular, was passed down generation to generation, and remained a very prominent trait.
One derived trait, one that developed at some point in time, but is not shared throughout the ancestry of a specimen, would be the presence of head spikes. Only one specimen, specimen 19, has this trait, and nowhere in its past ancestry, no matter how far back we looked, was this trait observed in another organism. This lack of prominence shows as evidence that this trait developed very recently, and is unique to specimen 19.
3. According to your tree, is the rear ‘spine’ of species 6 homologous or analogous (homoplastic) to that of species 14? Explain.
According to our tree, the “rear” spine of species 6 is analogous to that of species 14. These species were different from the head, body and tail structures, especially the rear spine. Species 14 did not have pleural spike on its spine, while species 6 had very sharp pleural spikes on its spine.
4. Are there any traits that were lost but then evolved again independently? If so, what are they and where do they occur?
According to our tree, the dome structure first appeared on top of the heads of species 5, 9 and 4. The trait reappeared in species 10 and 14, after a hiatus which evolved round wide spiked head.
Figure 2. Phylogenetic tree was reconstructed by Shery, Na and Fatoumata.
5. Describe one important difference between your tree and a tree estimated by a different lab group (identify which group’s tree you used). Upon reflection, which tree seems better? Why?
Our first task was to find an outgroup, that way we can focus on more definite attributes of the rest of the population. While we focused on the head and spikes, the other group focused more on the shapes and spine. The better tree would be our group’s since it shows a higher level of parsimony. Parsimony means that we lumped more traits together with specimen. Shery, Na and Fatoumata’s tree has only two different trilobites attached to many attributes.
Resources Used:
Nguyen, N., Singhateh, F. & Said, S. (2017, January, 23). Trilobite Tree Lab 6: A Brief History of Trilobite Ancestry by Na Nguyen, Fatoumata Singhateh, & Shery Said [Blogger]. Retrieved from http://biol212-majorsanimalbiology-winter17.blogspot.com/2017/01/trilobite-tree-lab-6-brief-history-of.html
Shlichta, G., Hanson, C. & Mcfarland, J. (2017, Winter). Biol& 212 Major Animal Biology Laboratory Manual
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