Trilobite Tree Lab 6: Phylogenetic Tree for Trilobite

Trilobite Tree Lab 6: Phylogenetic Tree for Trilobite

by Team Fossils Rock

Alondra Sotelo, Jinmei Wang, Radhika Dalal, Sharon Huynh

The purpose of this experiment was to classify 15 trilobites based on their morphologies into a phylogeny. The ensuing phylogeny and analysis follow:

PHYLOGENY:

Figure 1. Illustrated above is our Phylogenetic tree of Trilobites made up of 15 morphologically differing trilobites all rooted from a common ancestor.

QUESTIONS:

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

In our tree, trilobite three was the outgroup. The initial distinction between our species was the development of an attached or unattached dorsal area. What we mean by this was if the individual extensions from the spine were joined together or were distinct extensions. With regards to this characteristic, when surveying trilobite 3, we saw that its abdomen area was incomparable to that of the rest of the species. In terms of anatomy, the rest of the species that follow in the phylogeny also look remarkably more similar in contrast to trilobite three. This caused us to believe the rest of the species shared a more recent common ancestor than they did with trilobite 3. This is why trilobite 3 was named the outgroup.

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

According to our tree, one basal characteristic is the elongated pleura in trilobite 6, 11, and 19. However, as they evolve, trilobite 19 and 11 developed extremely long genal spines compared to trilobite 6; this makes the long genal spines the derived characteristic.

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 because they evolved from different common ancestors, or convergent evolution. The most recent ancestor of species 6 had long pleura, while the most recent ancestor of species 14 had normal-length pleura.

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 is lost but then evolves again independently is the number of tails on the trilobites. Trilobite 11 and 19 on the phylogeny have developed two tails. On the other hand, trilobite 17, which shares multiple common ancestors with trilobites with a single tail, redevelops this characteristic.

Figure 2. The image above is a phylogenetic tree constructed by team “Boys Who Cry” made up of Tommy, Shane, Jonathan and David.

5. Describe one important difference between your tree and a tree estimated by a different lab group.  Upon reflection, which tree seems better?  Why? We compared our phylogeny to Tommy, Shane, Jonathan, and David’s phylogeny. One main difference between our data and this group’s was that it seemed that they picked one characteristic to classify all of their trilobites by (tail shape/amount). On the other hand, our phylogeny used multiple different traits to categorize all of the organisms. The other group’s phylogeny was better because it was more parsimonious. Upon observing their phylogeny, it was evident that all the organisms with similar structures also shared a common ancestor in this tree rather than reappearing in another branch as they did in ours. Both of our phylogenies had an equal number of evolutionary changes, but because their phylogeny was better organized, I would say that their phylogeny was also better.