LAB #5: SYSTEMATICS By Joseph Yau, Azeem Jimoh, Severin Robins and Jason Hall

BIOL&212 Systematics Lab - Trilobite tree diagram

Blog post for results to the trilobites lab by Joseph Yau, Azeem Jimoh, Severin Robins and Jason Hall

Figure 1:

Figure 1: Depiction of 15 trilobites on a phylogenetic tree, based on the traits scored in the similarity matrix.

Questions

(Q1).In our tree, the species chosen for our outgroup was Peronopsis interstricta or trilobite number 3. The reason why we chose trilobite 3 as our outgroup was its physical differences. Trilobite 3’s lack of visible pleura, horizontal symmetry as opposed to vertical symmetry, and overall different shape made us choose it as our outgroup.

(Q2).According to our tree, one ancestral trait you can see is the lack of a genal spine. Assuming the most recent common ancestor did not have a genal spine, we can see that the trait was passed on to the future species as seen in trilobites 3, 5, 4, 18, and 9. Then by this logic we can see say the the presence of a genal spine is a derived trait as after the split where trilobite 9 is, we can see that all other species of trilobites had developed some form of genal spine.

Table 1:

Trilobite Number	Genal Spine (have=1)	Round Bottom (have=1)	Rough Texture (Have=1)	Pointed Pleurons (have=1)
1	1	0	0	1
3	0	1	0	0
4	0	1	0	0
5	0	1	0	0
6	1	0	0	1
7	1	0	0	1
9	0	1	0	1
10	1	0	1	1
11	1	1	0	1
13	1	1	0	0
14	1	0	1	1
16	1	1	0	1
17	1	0	0	1
18	0	0	0	0
19	1	0	1	1

Table 1: This table is a similarity matrix, showing the traits we selected and which trilobites possessed those traits and which did not possess them.

Table 2: This table is another similarity matrix which shows the number of different traits

by comparing each trilobite with each other

(Q3). According to our phylogenetic tree, the rear ‘spine’ trait appears to be analogous between trilobite 6 and 14. This means that this similarity in the species occurred, not because of a common ancestor but, because there was a similar need for it in both environments which may or may not be similar.    

(Q4). There appears to be on spot on our tree where the loss of having a round bottom is lost for one species but does come back in later species. To be specific if you look at trilobite #9, the bottom of the trilobite appears to point rather than display a smooth, ‘round’ look as generations before it have displayed, specifically the outgroup. However, going to later generations the trilobites all regain their rounded bottoms in that specific branch of the phylogenetic tree

(Q5). There is a main difference in our tree and the tree estimated by Hannah’s group, which is the use of traits. In their group, traits are used to differentiate from other trilobites and use them to estimate the tree. The traits used are too specific that they have a bigger chance to be homoplasy. Some of the traits used is a bit similar too. For example, there are ‘long pleuron spikes’ and ‘pointy pleurons’. These traits confuse the grouping of trilobites grouped under these two traits. If the trait used as synapomorphies appears in the trilobites outside of the group, it would be unclear. In our group, although we are using less traits, there is a clearer cut between branches. To sum up, we like our tree more =]

Figure 3: A phylogeny tree showing the traits developed and where. Based of  logic and the matrix shown in table 1 and 2