Trilobite Tree Lab 6: Top Fifteen Trilobites Your Doctor DOESN’T Want You To Know About! (Beanboozld™) By Kaveena Ranaweera, Toqa Abosabaa, Leah Koung, Jennifer Chin

1. On our tree diagram, the outgroup is Peronopsis interstricta, the first species on the tree. We chose Peronopsis interstricta as our outgroup because this species had characteristics differing from other trilobites.

Figure 1. Phylogenetic tree, depicting the hypothetical evolutionary history of trilobites.

2. One ancestral characteristic of the trilobites is the pygidium, as the pygidium is found in every species on the tree. One derived characteristic is the pointed pygidium, seen in the trilobites from the fourth generation on.

3. The rear spine of species 6 is homologous to species 14 because both trilobites developed a rear spine, derived from a common ancestor.  
4. A trait that was lost but then evolved independently was the pointed pygidium. In the second generation, the Trimerus delphinocephalus developed a pointed pygidium, however, in the third generation, the pointed pygidium was lost, but then evolved again independently in the fourth generation.The Trilobites started out with a round pygidium but then it started to form a pointed pygidium after the Flexicalymene meeki. As time went on, the Basileilla barrandei formed a round pygidium in the fifth generation. Another trait that was lost was the pleuron. The pleuron was lost twice, once in the ninth generation and another in the tenth generation. The Paradoxides gracilis was the first to lose the pleuron in the ninth generation and then the Albertella helena lost the pleuron in the tenth generation.

Figure 2. Najib, Jung Soo, Tara, and Punyarith’s phylogenetic tree of Trilobites, showing a hypothetical evolutionary process.

The difference between our tree and Najib’s tree are the different characteristics used to reconstruct the phylogenetic tree of the Trilobites. We feel that our tree is better than Najib’s tree because it is simple, clean, organized, and has more synapomorphies.