How trilobites adapted to survive 300 million years of climate change

Trilobites, those ancient marine arthropods which reigned over ocean floors for an astounding 300 million years, have long captured the interest of paleobiologists. The history of these early creatures not only predates but also outlasts the dinosaurs, surviving two prominent mass extinctions. 

Recent studies reveal the fascinating adaptations of a particular trilobite species, hinting at evolutionary traits that may have given rise to modern-day arthropods like spiders and lobsters.

Unusual discovery 

Most trilobites, with their exoskeletons distinctly divided into a head, a middle section known as the thorax, and a rigid tail, possess a specific number of segments in their mid-sections upon maturity. However, a recent discovery concerning the species Aulacopleura koninckii has revealed an unusual twist.

“My collaborators and I thought this species was weird. We couldn’t understand why Aulacopleura bodies varied and others living at the same time had a constant number,” said UC Riverside paleobiologist Nigel Hughes.

“Seeing trilobites with variable numbers of segments in the thorax is like seeing humans born with different numbers of vertebrae in their backs.”

Survival strategy 

But what could possibly drive such variation, and how did it influence the trilobite’s survival strategy?

Trilobites, akin to our contemporary pillbugs or “rollie pollies,” had a defense mechanism of curling up into a ball to shield themselves from menacing predators like the giant squids and fish of their era. 

This protective curl allowed them to tuck their tails under their heads, securing the vulnerable soft tissues behind their sturdy exoskeletons.

Focus of the study 

In a new study published in the Proceedings of the Royal Society B Biological Sciences, researchers used 3D modeling to investigate the Aulacopleura’s defensive tactics. 

The experts found that this protective rolling was effective for younger Aulacopleura with fewer than 18 thoracic segments. However, as the number of segments increased with maturity, a neat tuck of their tails became impossible.

“As the number of segments increased, the body proportions did not allow them to tuck their posteriors neatly under their heads and still be completely shielded, ” explained Hughes. 

Avoiding predators 

This raised a crucial question: why did Aulacopleura keep adding segments and how did they fend off their formidable predators?

Reconstructions suggest that mature Aulacopleura probably adopted a modified defense position. Instead of a neat tuck, they likely allowed their tails to slightly protrude past their heads, minimizing exposed regions. 

“Other possible defense maneuvers would have left gaps on the sides that exposed critical organs – highly unlikely,” said Hughes.

An evolutionary advantage 

The compelling reason behind this segment variation lies in an evolutionary advantage. 

“What is underneath these segments? Legs that serve as gills! The more segments, the more surface area for respiration,” Hughes noted.

This adaptation of growing additional gills might have equipped the Aulacopleura to endure decreasing oxygen levels in certain seafloor regions. While these hypoxic conditions drove predators away, the well-equipped Aulacopleura thrived, undeterred by threats.

The evolutionary choices of the Aulacopleura provide invaluable insights into the survival mechanisms of ancient species. Trilobites’ evolutionary trajectory could very well be the foundation upon which many modern arthropods, like insects and arachnids, have been built. 

Hughes sums up this trilobite’s survival lesson, stating: “It’s not so much that the meek will inherit the Earth, but the flexible.”

More about trilobites 

Trilobites are among the most captivating and well-known fossils, providing critical insights into the evolutionary history of life on Earth. Here’s a deeper dive into these ancient arthropods:

Origins and extinction

Trilobites emerged during the Cambrian Period, around 521 million years ago, and flourished for nearly 300 million years before they went extinct at the end of the Permian Period. Their tenure saw them survive two major extinction events before eventually succumbing to the largest mass extinction in Earth’s history.

Diversity and distribution

There were over 20,000 identified species of trilobites, making them one of the most diverse groups of extinct organisms. They inhabited a vast range of marine environments and were found on every ancient continent.

Anatomy

The name “trilobite” is derived from the creature’s three-lobed body plan – a central (axial) lobe flanked by two pleural lobes. Their bodies were also divided longitudinally into three major parts:

Cephalon (Head)

This part had a pair of compound eyes in many species, which were impressively sophisticated for such ancient creatures.

Thorax

Comprised of several articulated segments that allowed flexibility.

Pygidium (Tail)

A semicircular, shield-like segment.

Eyes

Trilobite eyes are an evolutionist’s delight. Some trilobites had no eyes, while others had large, crescent-shaped eyes. Their eyes were made of calcite (a mineral), which is a rare feature in the animal kingdom.

Behavior and ecology

Evidence from fossilized trilobite tracks suggests these creatures burrowed into the seabed, scavenged, and even swam in ancient oceans. Their hard exoskeletons provided protection against predators. When threatened, many trilobites could enroll, or curl up, to shield their soft undersides.

Evolutionary significance

Trilobites underwent several evolutionary changes during their existence, reflecting their adaptability to diverse ecological niches. Their extensive fossil record provides a window into evolutionary biology, bio-geography, and the effects of mass extinctions.

Fossil Record

Because of their hard exoskeletons made of chitin and calcium carbonate, trilobites fossilized well. These fossils are especially abundant in certain locations like Morocco, Wales, Russia, and parts of the United States.

Cultural Impact

Due to their unique appearance and abundant fossils, trilobites have become iconic in paleontology. Many cultures, even before understanding their ancient origins, used trilobite fossils as amulets or talismans.

In essence, trilobites serve as a crucial evolutionary benchmark. Their extensive and rich fossil record offers a detailed look at the marine ecosystems of bygone eras, helping scientists understand the intricacies of evolution, adaptation, and extinction.

Video Credit: Nigel Hughes/UCR

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