Quirks & Quarks

Ancient sea creature sported a big fork on its head to toss away the competition, study suggests

Researchers have used 3D model reconstructions of a bizarre trilobite — an ancient shelled sea creature — to understand why it grew a trident as long as its whole body on its head.

Fights between horned trilobites are believed to be the earliest example of sexual combat

A three-dimensional rendering of a trilobite with a fork-like appendage at the front of its body.
A 3D model of a Walliserops trifurcatus trilobite, which sports a unique trident at the front of its head. (Alan D. Gishlick)

A species of ancient trilobites grew big forks on their heads to fight their opponents and impress potential mates, in what scientists say could be the earliest known example of ritualized combat. 

A new study published in the Proceedings of the National Academy of Sciences suggests that one trilobite species used a strange appendage on its head for sexual selection — and most likely for fighting other male trilobites. 

Though they've been extinct for 250 million years, trilobites were one of the most successful and diverse animal groups ever — dominating the ancient oceans for hundreds of millions of years. All trilobites shared certain traits — like a three-lobed body — and visually resembled modern woodlice or pillbugs, ranging in size from a few millimetres to the size of a sea turtle.

But a striking feature of one trilobite species, Walliserops trifurcatus, stands out from the rest: at the front of its head was a trident-like structure as long as the trilobite's body. This trident has made Walliserops a popular find among fossil collectors and an evolutionary puzzle for biologists. 

"The trilobite's fork most resembled the structures of animal weapons involved in a type of fighting behavior that's been characterized as shoveling," Alan Gishlick, assistant professor at Bloomsburg University and one of the study authors, told Quirks & Quarks

"You get your horn underneath your opponent, and then you lift him up and toss him away."

Fork as cutlery or a hunting weapon?

Trilobites are on display at the American Museum of Natural History in June 2013 in New York. Trilobite fossils are among the most common and beloved items for museums and private collectors alike. (Mary Altaffer/Associated Press)

An unusual fossil helped Gishlick and his colleague Richard Fortey solve this puzzle. Instead of the usual three prongs on its head appendage, this particular Walliserops specimen had four. 

Previously, the trident was thought to be used for either hunting or defense — to fork the prey or the opponent, Gishlick said. However, the appendage was not flexible or close enough to the creature's mouth to be useful as cutlery. 

The defense theory also relied on the trilobite's ability to wield the trident effectively, which the creature lacked. Plus, trilobites had a different method of defending themselves.

"They curl themselves up like a little pillbug into an armoured ball," Gishlick said.

The four-pronged trident in the unusual Walliserops fossil provided a big clue: the trilobite trident could have a significant variation from the species norm without getting in the way of the creature growing to maturity.

In fact, Gishlick said the specimen is slightly bigger than an average Walliserops.

"And yet its fork was clearly malformed in such a way that catching or stirring up prey would not have been as efficient, defending against predators would not have been as efficient," he said.

Excluding the functions of defense and hunting left one more possibility.

"There's another common explanation we tend to employ when it comes to extreme structures in living organisms and in fossils, and that's they have something to do with reproductive success," Gishlick said. "Organisms will devote a phenomenal amount of biological energy to structures that aid in their ability to gain mates."

These structures could be used for display, like a male peacock's vibrant tail feathers, or for ritualized combat with other males, like impressive crowns of antlers in deer and elk. 

Shoveling the competition

To figure out what exactly a fight between two Walliserops trilobites would have looked like, the researchers looked to modern-day animals that visually resemble the ancient creatures. Gishlick and Fortey created 3D computer models of the trilobite and compared its fork-like structure to the horns of several species of stag beetles. 

The closest match turned out to be the Japanese rhinoceros beetle, which also sports an impressive multi-pronged horn.

"If they dip their head down and get it underneath their opponent and lift rapidly, it just twitches and the other male is gone," Gishlick explained.

A rendering of a trilobite and a beetle side by side.
A side-by-side 3D model depiction of a W. trifurcatus trilobite and a Japanese rhinoceros beetle (Trypoxylus dichotomous) with a comparable anatomical weapon. (Alan D. Gishlick)

If this theory is correct, Walliserops tridents may be the earliest known example of sexual combat 400 million years ago. And to Gishlick, getting a glimpse of a creature's behaviour from millions of years ago is the coolest part. 

"The problem is, with fossils, we can't observe their behaviours and we are missing so much data," he said.

"So when you have a structure like this that … we can tie to a particular kind of sexual selection, particularly combat, which is harder to nail down, we're helping learn more about the past and realizing that the past isn't that different than the present." 

Written and produced by Olsy Sorokina


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