Trilobite Tree Lab 5: The Hypothesis of Trilobite Ancestry by Anh Nguyen, Brian Tran, Prach Techameena, and Justin Bisacky

Figure 1. Phylogenetic tree of fifteen Trilobites 

Figure 2. Phylogenetic tree of Trilobites. The eye trait appeared first, followed by the genal spine and the gain of tails.  

1. According to our tree, the out group was species [3] due to its distinct appearance compared to other Trilobites.

Figure 3. Out group in the Trilobite tree - Peronopsis interstricta.

It can be seen from the picture that species [3] did not have eyes, genal spine, or any tails. These morphological characteristics helped differentiate it from the rest of the Trilobites. Moreover, its overall traits were a lot different than other Trilobites. It was smaller in size compared to others. Also, its thorax was very short and narrow making it separate from other species in the group. 

2. As we can see from the tree, the ancestral character would be the eyes since it was the trait that the ancestors and all of the descendants have. One of the derived characteristics could be the possession of genal spine. The reason for this is because none of the more ancestral species ([18], [4], and [5]) had genal spines. The trait evolved later in the descendants.

3. According to our tree, rear "spine" or tail of species [6] was homologous to that of species [14] because this trait was inherited from a shared common ancestor. In other words, species [6] and [14] had tails because their common ancestor once had tails and passed this trait on to them rather than the trait had evolved independently. 

4. As seen from our tree, the pointy genal spine was analogous trait meaning that it evolved independently in species [16], [13], [6], and [1] as a result of convergent evolution. This is because species [16] and [13] and species [1] and [6] were not closely related. Pointy genal spine character in these species was not inherited from a common ancestor but evolved independently to adapt to similar environments.

5. For comparison, we used the tree produced by group “SAS – Serious About Science”. The trees are similar in that eye and general spine traits more ancestral characters close to the root. However, after those two common morphological characteristics, our group continued a tree based around the appearance of tails as a trait of focus while group “SAS” primarily used lots of traits such as the pleural spine, general spine, and spots. 

Figure 4. Phylogenetic tree by group "SAS - Serious About Science".

Parsimony seeks out the simplest solution to reconstructing the phylogenetic tree. We believe our reconstruction is more parsimonious. The group in comparison has nodes that do not contain multiple branches as time moves forward. This creates a demand for more nodes to explain the traits of the organisms. Our group’s phylogenetic tree was based around 4 main traits which could then branch out into multiple organisms. The traits chosen for our tree lead to more parsimony.