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Battle Bugs: Trilobite Fossils with Weapons?

Trilobites and Rhinoceros Beetles: Fig. 1 a. Psychopyge termierorum. AKA Psycho. Fig. 1b. Walliserops tridens. AKA Wally Photos from Houston Museum of Natural Science HMNS Fig. 1c. The Japanese rhinoceros beetle http://www.sciencedirect.com/science/article/pii/S0003347213003850

Authors

Aaron Shunk.

Abstract

Wally and Psycho are trilobites (Fig. 1a, 1b) that lived in the shallow marine seas that existed in what is now Morocco during Devonian Period at about 400 million years ago. They are both phacopid trilobites that have elaborate head gear that was likely used as weapons. All trilobites have been extinct since the Permian mass extinction at about ~250 million years, but fortunately they left behind abundant fossil record for science to learn from. Here we compare Trilobite fossils with diverse head gear to living rhinoceros beetles (Fig. 1c). We conclude it is plausible that trilobites used headgear as a weapon for bug-to-bug battle, and that the large diversity in trilobite head gear may represent natural experiments to find the optimal “Battle Bug”. 

Content

•In modern insects most ornaments and weapons typically relate to sexual selection theory. Sexual selection is the part of natural selection involving mate choice. The quality of the mate is thought to relate to quality of the ornaments (like peacock tail feathers) or weapons that it possesses. However, there is a potential cost for the individual to build the elaborate weapon or ornament. The cost makes it where not every individual is able to get enough food and resources to build the feature and the structure itself may even get in the way of other parts of life- like escaping predators. For selection to occur, the cost should be less than the advantage. Those with the optimal body forms win at the mate selection game.  A weapon is commonly used for bug-to-bug combat. The bug battle for giant rhinoceros beetles is for turf and the associated resources and rights to mate. This National Geographic video shows a real life bug battle (https://www.youtube.com/watch?v=EjvLOAIxbNQ) – Spoiler alert, to me it looks like sumo match that ends with a body slam.  

•Trilobites were arthropods and a distant relative to modern insects. Obviously there are significant differences between a trilobite and an insect (or other living arthropod like a shrimp or horse shoe crabs), but still modern insect ecology may provide clues about ancient fossils and paleo-ecology. There is uncertainty about why the phacopid trilobites like Psycho and Wally had such ornamentation and weapons, but scientists can use modern relatives to make educated guesses about the function of ancient features. We can learn from modern Rhinoceros Beetles since they have similar ornaments and weapons that are used for mate selection. The better the bug in battle the more other bugs like him for a mate. It was traditionally assumed the elaborate weapons were costly for the animal to build, and selection would have to weigh the cost and benefits of having large weapons. However, research on the beetles found no evidence for a cost for building the horns, which may explain the high diversity of beetle horn types (Erin L.McCullough and Douglas J.Emlen, 2013). By analogy in one group of trilobites a number of lines of evidence support sexual selection as an explanation for the evolution of some types of ornaments and weapons: their ontogeny, their diversity, their growth patterns, and the presence of possible sexual differences (Robert J Knell and Richard A Fortey, 2005). The phacopid trilobites also have a large diversity of horn types and ornaments. So it is possible the same principle of low-cost manufacturing for beetles can be applied to trilobites. The analogy serves as a hypothesis for why the group experimented with so many designs. Perhaps, the horns were inexpensive weapons to make, so nature experimented greatly looking for the best design for a bug-to-bug battle.

Data/ Methods

Compared modern Rhinoceras Beetles patterns described (McCullough and Emlen, 2013) to the diverse assemblage of phacoid trilobites fossils available at the Houston Museum of Natural Science (Fig. 1). Also, compared the paper "Trilobite spines and beetle horns: sexual selection in the Palaeozoic" about Raphiophorid trilobites to the Phacopid trilobites. 

 

Meet Wally.

Family: Acastidae

Genus species: Walliserops tridens

Generic Name: Phacopid trilobite

Geologic Age: Lower to Middle Devonian age rocks (~385 to 415 million years ago)

Geographic location: from Atlas Mountains of Oufaten, Morocco.

 

Meet Psycho Bug.

Family: Acastidae

Genus species: Psychopyge termierorum

Generic Name: Phacopid trilobite

Geologic Age: Lower to Middle Devonian age rocks (~385 to 415 million years ago)

Geographic location: from Atlas Mountains of Morocco.

 

 

REFERENCES

Robert J Knell and Richard A Fortey, 2005. Trilobite spines and beetle horns: sexual selection in the Palaeozoic? Biol Lett. 2005 Jun 22; 1(2): 196–199. Published online 2005 May 27. doi:  10.1098/rsbl.2005.0304

Houston Museum of Natural Science: http://www.hmns.org/exhibits/permanent-exhibitions/the-morian-hall-of-paleontology/

Read More about Rhinoceros Beetle: http://www.sciencedirect.com/science/article/pii/S0003347213003850

National Geographic Video: https://www.youtube.com/watch?v=EjvLOAIxbNQ

McCullough and Emlen, 2013. Animal Behaviour Volume 86, Issue 5, November 2013, Pages 977-985 http://www.sciencedirect.com/science/article/pii/S0003347213003850



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