Trilobite Tree Lab: Tracing Phylogeny from Fourteen Fossilized Species

By Team Trilomorphy, aka Jungsoo, Tara, Punyanith, and Najib

1. The outgroup for our tree was Peronopsis interstricta (number 3, located in the upper left hand corner in Figure 1). We separated each fossil into two groups first, using easily distinguishable traits such as body shape and spines. P. interstricta exhibited traits that were drastically different from the other fossils, even missing essential characteristics like the presence of eyes. The body shape, which was pinched in the middle horizontally and spherically expanding in front and back, showed distinction from the other species.This organism may have originated from deeper sea level than the others because its eyes are a vestigial trait. This species might have evolved under an environment that does not need sight to produce offspring or keep a good level of fitness.

Figure 1. Phylogenetic reconstruction of trilobite evolution, done by Team Trilomorphy.

2. One basal characteristic found in the ancestral trilobite was the combination of the three structures: pygidium, thorax, and cephalon because all the species from the phylogenetic tree have the three structures. The presence of double tails is a derived trait. After the species were separated by “bumps,” two monophyletic groups had double tails while five monophyletic groups without bumps had the double tails. From this statement, we can conclude that double tails is not an ancestral trait, since not all of the species share this trait; it developed independently.

3. Based on the design of our tree, the rear “spine” of fossils is a homeoplastic trait of species 6 and 14. Although they share a common ancestor, that ancestor is not recent and probably did not have a rear “spine.” Therefore, the appearance of the spines would be due to convergent evolution, and the trait is analogous.

4. Based on our reconstructed tree, the double tail evolved twice, once in the common ancestor of species 17 and 11, and again in the bumpy species number 10. That characteristic appeared to be replaced by a single, spiny “tail” in both the bumpy and non-bumpy species. Another trait that was lost in one branch of the tree was the indentation, or smoothness of the pleural spine. It evolved twice, once in species number 4, and again in species number 13. After species 13, the trait was lost. This seems to indicate that the trait for a smooth pleural spine in species 13 evolved independently.

5. We compared our phylogenetic tree with the one done by John’s group (see Figure 2). John’s group contains many synapomorphies, losing and evolving traits such as “eyes” and “knobby head” several times. According to the principle of parsimony, fewer losses of traits are preferred. From that point of view, our tree had an advantage. However, John's group categorized more monophyletic groups, better showing the closer relationships of the trilobites. Their group also labeled more synapomorphies; our group had 8 characteristics compared with their 11. A combination of the two approaches would yield a better phylogenetic tree than either alone.  

Figure 2. The phylogenetic tree of our neighboring group (John’s group). The outgroup was the same as ours, but it must have fallen off the board.