Trilobite Tree Lab 6: A Brief History of Trilobite Ancestry by Na Nguyen, Fatoumata Singhateh, & Shery Said

Figure 1. Phylogenetic tree of a group of Trilobites based in morphological characteristics. From left to right the species are: [3] Peronopsis interstricta, [18] Trimerus delphirocephalus, [5] Trimerus dekayi, [4] Flexicalymene meeki, [9] Calymene celebra, [13] Basiliella barrandei, [16] Ogygopsis Klotzi, [1] Callavia broggeri, [7] Paradoxides gracilis, [6] Olenellus clarki, [19] Odontopleura callicera, [11] Albertella helena, [17] Crepicephalus towensis, [10] Coronura aspectans, and [14] Dalamnites verrucosus. The tree shows which Trilobite species share a common ancestor, and which traits started showing at which point in time.

1. On our tree, Peronopsis interstricts (3) is the outgroup species. We chose this species because it is missing many of the characteristics and traits that are apparent in most of the other Trilobite species in our group such as axial rings and pleural spines.

2. According to our tree, one ancestral trait that showed up in all the Trilobite species is that they had segmented bodies. On the other hand, one derived characteristic that showed up later in the phylogenetic tree is the long tail spikes, which only showed up in a few of the Trilobites..

3. According to our tree, the rear spine/tail that was apparent in species 6 was analogous to that of species 14. The spine showed up as a derived trait that developed separately for each of the two species and was not inherited by both from a common ancestor.  This is due to the fact that there were many other morphological traits that made the two species seem different, so we chose to not place them in the same monophyletic group.

4. Sharp pleural spines is an example of a trait that was lost, but then evolved again separately. Sharp pleural spines initially developed after the 4th node from the root of the tree, but then were lost after the 5th node from the root of the tree, and then evolved again separately for species Albertella helena

5. For comparison, we looked at the tree of the group of Kaveena, Jennifer, Leah, and Toqa. One of the major differences between our tree and theirs is that many of the species that we believed shared an immediate common ancestor they believed that they did not, and that they evolved separately. Upon reflection, it seems like their tree is better, since it follows parsimony more closely. Our tree has a more complicated pattern where almost every taxon has a sister taxa, while theirs is more simplistic and does not involve a lot of unnecessary branching.