BIOSQUAD

Baily, Matthew, EJ, Lia

1. Post a Picture of your tree. On your tree, which species is the outgroup?   Why did you choose this species?  Explain.

Figure 1. Reconstruction of trilobite phylogenetic tree based on morphological traits.

In our group, as seen in Figure 1, we determined the outgroup to be Peronopsis interstricta. An outgroup is the taxon that is most distantly related to the other taxa being studied. All members of the group are more closely related to each other than they are to the outgroup. Peronopsis interstricta lacks many of the characteristics that the other Trilobites have, such as eyes and axial rings. It’s physical appearance is substantially different than all other tree members.

2. According to your tree, what is one basal or ancestral characteristic? One derived characteristic?

An ancestral trait is one that appears in a common ancestor. In this case, the genal spine is an example of an ancestral trait due to the fact that it’s present in Coronura aspectans and all species after that in the tree. A derived trait is one that was not present in a common ancestor. The two tail spines on Crepicephalus towensis and Albertella helena is an example of this. This characteristic is seen only in these species and not in a common ancestor.

3. According to your tree, is the rear ‘spine’ of species 6 homologous or analogous (homoplastic) to that of species 14?  Explain.

The rear spine in species 6 are analogous to species 14 because they aren't from the same evolutionary origins. They both acquired this spine on there rear  through evolution and natural selection making a spine better their chances to survive and was probably used as a defence mechanism.

4. Are there any traits that were lost but then evolved again independently? If so, what are they and where do they occur?

Yes. One trait that independently evolves between two species is the bumps on the glabella. This occurs in two places. It occurs before the evolution of the genal spine with the common ancestors of the Flexicalymene meeki (4) and the Calymene celebra (9). It also occurs after the genal spine is derived, as seen in the Odontopleura callicera (19). Another trait that evolved twice was the tail of the trilobite. It occurs in the common ancestor of the Odontopleura callicera (19), Dalamnites verrucosus (14), and Coronura aspectans (10). It then reappears in the common ancestor of the Olenellus clarks (6) and the Paradondes gracillis (2).

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?

Figure 2: Trilobite phylogenetic tree reconstruction by the BioBeads group.

Upon analyzing a tree reconstruction by the BioBeads (Figure 2), one significant difference between this tree and our group’s is how the spines were distinguished. Our group identified the development of any genal spine as arising early in the tree, and then classifying several trilobite species based on the spikiness of their pleural spines and growth of tail spines. BioBead’s tree identifies genal spines as an early characteristic, then further groups species based on the length of their genal spines. BioBead’s tree is more parsimonious in identifying distinguishing characteristics, though I feel by visually indicating that we used the spikiness of pleural spines or growth of tail spines as an identifying characteristic, it implies a more deliberate categorization to the viewer. Our group’s rationale was that some species evolved spikier spines as self-defense.