The Discovery of Derived Traits Among Trilobites Representing Evolution

TRILOBITE TREE LAB 6

The Discovery of Derived Traits Among Trilobites Representing Evolution
Team: Chiton Crew
Members: Megan Thees, Jaena Bautista, Nhy Tran, Gretchen Janes

Introduction

In this lab, we constructed the phylogenetic tree of the Trilobites based on different fossils of the organisms. From looking at the generalized Trilobite structure, we can see how derived traits are used to construct a phylogeny tree as understanding Trilobite evolution.

Figure 1. A picture of a generalized trilobite, an image we used to distinguish traits of our collection of trilobites

Data Matrix

Organism	Eyes	Pygidium closed	Short Genal Spine	Pygidium short spike (2)	Long Genal Spine	Pygidium Long Spikes (2)	External Spikes (2)	Extended Axis	Cephalon Spikes
Callavia Broggeri (1)	1	0	0	0	1	0	1	1	0
Peronopsis interstricta (3)	0	0	0	0	0	0	0	0	0
Flexicalymene meeki (4)	1	0	0	0	0	0	0	0	0
Trimerus dekayi (5)	1	1	0	0	0	0	0	0	0
Olenellus clarki (6)	1	0	0	0	1	0	1	0	0
Paradoxides gracilis (7)	1	0	0	0	1	0	1	0	0
Calymene celebra (9)	1	0	1	0	0	0	0	0	0
Coronura aspectans (10)	1	0	1	1	0	0	0	0	0
Albertella helena (11)	1	0	0	0	1	1	0	0	0
Basiliella barrandei (13)	1	0	0	0	1	0	0	0	0
Dalamnites verrucosus (14)	1	0	1	0	0	0	0	0	0
Ogygopsis klotzi (16)	1	0	0	0	1	0	0	0	0
Crepicephalus towensis (17)	1	0	0	0	1	1	0	0	0
Trimerus delphinocephalus (18)	1	0	0	0	0	0	0	0	0
Odontopleura callicera (19)	1	0	0	0	1	0	1	0	1

Table 1 is a data matrix of traits we found compared across different fossils of Trilobites. The 1 represents the appearance of this trait and the 0 represents the absence of this trait

Trilobite Phylogeny Tree

Figure 1. Is a picture of the phylogeny tree we created based on traits that we believed were derived over time. Some of the traits we observed were the presence of eye, closed pygidium, short and long genal spines, short and long pygidium spikes, external and internal axis and cephalon spikes. The genus and species of the trilobites and their respective numbers are: Callavia Broggeri (1) Peronopsis interstricta (3) Flexicalymene meeki (4) Trimerus dekayi (5) Olenellus clarki (6) Paradoxides gracilis (7) Calymene celebra (9) Coronura aspectans (10) Albertella helena (11) Basiliella barrandei (13) Dalamnites verrucosus (14) Ogygopsis klotzi (16) Crepicephalus towensis (17) Trimerus delphinocephalus (18) Odontopleura callicera (19).  

Questions:

1. Which is the outgroup? Why?

We decided that Personopsis interstricta (3) would be the outgroup on our phylogeny tree because it looked the most different from all the other species and also the least complex so we believe that it was the closest to the common ancestor.

2. What is one ancestral characteristic? What is one derived characteristic?

Ancestral characteristic: Internal axis

Derived characteristic: Eyes

3. According to your tree, is the rear ‘spine’ of species 6 homologous or analogous to that of species 14? Explain

Homologous means having similarities in traits due to common ancestry whereas, analogous is having similarities in traits that are not due to common ancestry. According to our tree, the rear ‘spine’ of species 6 and 14 evolved in different periods independently (it may be due to their way of living and to survive in their current environment). Therefore, it is not due to common ancestry which means they are analogous.

4. Are there any traits that were lost but then evolved again independently? If so what are they and where do they occur?

Internal axis in Paradoxides gracilis (7) was lost but evolved again independently.

5. Describe one important difference between your tree and a tree estimated by a different lab group. Upon reflection, which tree seems better? Why?

Figure 2. A side by side comparison of Tommy, Shane, Jonathan and David’s tree (left) and ours (right).

Their tree is a cladogram where ours is a phylogeny tree, where both show the relationship between the Trilobites,  our phylogenetic tree has branches that represent evolutionary time and amount of change (hypothetical of course) and an undistinguished head or tail. Both of our trees show the outgroup being Peronopsis interstricta (3), we both agree that this is the outgroup because it seems to be the most basic Trilobite, lacking spinal rings unlike every other trilobite in this collection. Their cladogram seems to have more branches than our phylogeny tree making it more complex and more challenging to see the relationship between trilobites.  Both trees are well done, but we think that ours is easier to understand but maybe that is just because we created it!