Lab #5: Family Tree-lobites by Caleb Smith, Kevin Gomez, Natalie Durland, and Bethel Berhanu

Figure 1. Phylogenetic tree of trilobites based on morphological characters. Refer to the numbered trilobite cards.

Figure 2. Similarity matrix of trilobite characters. 0 indicates the absence of the character in question and 1 indicates the presence of the character. In the case of pygidial spines, 2 indicates the presence of 2 pygidial spines and 3 indicates any number greater than 2, while in the case of genal spines 2 indicates the presence of both common genal spines and a row of spines at the end of the cephalon opposite of the thorax. Refer to the numbered trilobite cards.

We chose species 3 (Peronopsis interstricta) as the outgroup because it is the most morphologically distinct of the given trilobites. Species 3 differs primarily in its lack of visible eyes and small thorax relative to its cephalon and pygidium. These differences suggest that species 3 diverged from a common ancestor at an earlier point than any of the other trilobites.

We determined that one basal trait of the trilobites is the presence of an axial lobe. While this structure is most prominent in species which diverged from a common ancestor more recently, it is visible even in species 3, 5 (Trimerus dekayi), and 18 (Trimerus delphinocephalus). The presence of pleural spines is a more recently derived trait. These structures appear relatively complex and specialized, and the given sample of trilobites seems to indicate a progression from closely packed pleural segments, to separated pleural segments, to curved pleural segments, and finally to pleural spines.

Our trilobite phylogeny led us to the conclusion that the pygidial spine of species 6 (Olenellus clarki) is analogous to that of species 14 (Dalamnites verrucosus). The spine in species 6 appears to be an extension of the axial lobe, while in species 14 it seems to have formed from the pygidial segments surrounding the axial lobe. Species 6 most likely derived this trait by building on the exposed axial lobe, which extends past the pygidium, present in species 1 (Callavia bröggeri) and 7 (Paradoxides gracilis). In contrast, species 14 possesses a bump-covered body, fused pygidium, and short genal spines indicative of recent common ancestry with species 10 (Coronura aspectans). Therefore, both pygidial spines most likely evolved independently, rather than being passed down by a common ancestor of species 6 and 14.

The trait of a fused pygidium was lost at the common ancestor of species 1 and 7, then resurfaced independently in species 11 (Albertella helena).

The phylogenetic tree created by Cammie, Thea, Mark, and Mackenzie differed from ours in that it did not use pygidial spines as evidence of common ancestry. We chose to base our phylogeny on pygidial spines because the spines formed by extended axial lobes in species 1, 7, and 6 appear homologous, and seem to demonstrate an increase in complexity over time. After considering both phylogenetic trees, we decided that our own tree better represents the true evolutionary relationships among trilobites; the other group's phylogeny violates parsimony by requiring the extended axial lobe to be derived independently in species 1 and in the common ancestor of species 6, 7, and 11. Their phylogeny also requires the bumps present in species 10, 14, and 19 (Odontopleura callicera) to be independently derived for all 3 species.