The trilobites' guide to surviving environmental change

Scientists have worked out how one unusual species of trilobite -- an ancient, sea-dwelling relative of spiders and lobsters -- was able to defend itself against predators and survive a bumpy ride as Earth's oxygen levels fluctuated.

The seas were filled with trilobites for nearly 300 million years starting in the Cambrian Period, some 520 million years ago. During their time on Earth, which lasted much longer than the dinosaurs, they survived two major episodes of mass extinctions and dominated ocean floor ecosystems.

Their armored bodies are divided into three sections: a head, a thorax or middle section, and a rigid tail. There are more than 20,000 known trilobite species and, when mature, most of them have a very specific number of segments in their mid-sections. However, in Aulacopleura koninckii, scientists discovered something unusual.

Though each early growth stage showed little variation in size and shape, mature Aulacopleura developed anywhere between 18 and 22 mid-section segments.

"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 Nigel Hughes, UC Riverside paleobiologist and corresponding author of a new study about this trilobite.

"Seeing trilobites with variable numbers of segments in the thorax is like seeing humans born with different numbers of vertebrae in their backs," Hughes said.

The researchers had questions about this anomaly, how it affected the animals' ability to protect itself, and why it might have developed in this way. These questions are answered in a new study published in the Proceedings of the Royal Society B: Biological Sciences.

Like modern pillbugs or "rollie pollies," trilobites curled up into a ball shape to protect themselves from large squid-like creatures, fish, and other predators. When rolled up, they could tuck their tails neatly under their heads, so the soft tissues were protected by their hard exterior skeletons. In the case of Aulacopleura, 3D modelling showed that protection during rolling up was restricted to smaller, immature forms with less than 18 segments in the middle.

"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," Hughes said. "So, why did this species keep adding segments anyway, and how could it survive the nasty predators?"

Based on their virtual reconstructions, it seems highly likely that when Aulacopleura with a large number of mid-segments felt threatened, they would roll up like their relatives and simply let their tails extend past their heads, minimizing the exposed gap.

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

As to the question of why this trilobite varied in the number of mid-section segments, the researchers turned to their earlier work. "What is underneath these segments? Legs that serve as gills!" Hughes said. "The more segments, the more surface area for respiration."

Growing additional breathing apparati likely gave these animals the ability to tolerate dips in local seafloor oxygen levels that excluded other species, such as those that preyed on larger Aulacopleura. Parts of the sea floor becoming anoxic forced predators to retreat to sites where oxygen remained sufficient. But larger Aulacopleura, with their extra gills, could stay put, predator-free.

Learning how this species adapted to both biological and physical pressures gives researchers a better understanding of how survival strategies evolve. The way trilobites developed holds clues to how the common ancestor to major groups of modern arthropods, including insects and arachnids, first evolved.

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Trilobites' growth may have resembled that of modern marine crustaceans

Trilobites -- extinct marine arthropods that roamed the world's oceans from about 520 million years ago until they went extinct 250 million years ago, at the end of the Permian period -- may have grown in a similar fashion and reached ages that match those of extant crustaceans, a new study has found.

In a paper published in the journal Paleobiology, researchers from the University of British Columbia and Uppsala University show that the Ordovician trilobite Triarthrus eatoni, some 450 million years ago, reached a length of just above 4 cm in about 10 years, with a growth curve very similar to that of small, slow-growing crustaceans.

"T. eatoni lived in low-oxygen environments and, similarly to extant crustaceans exposed to hypoxic conditions, exhibited low growth rates compared with growth under more oxygenated conditions," said Daniel Pauly, principal investigator of UBC's Sea Around Us initiative and lead author of the study. "Low-oxygen environments make is more difficult for water-breathers to grow, and add to the difficulties of breathing through gills, which, as 2D surfaces, cannot keep up with the growth of their 3D bodies. Thus, under hypoxic conditions, they must remain small if they are to maintain the rest of their body functions."

In the case of trilobites, their exopods -- external branches on the upper part of their limbs -- functioned as gills. Thus, these ancient animals had similar growth constraints to those of their modern counterparts.

To reach these conclusions, Pauly and his colleague from Uppsala University, paleontologist James Holmes, resorted to the analysis of length-frequency data, a method developed within fisheries science and marine biology for studying the growth of fish and invertebrates lacking the physical markings that indicate their age.

The information to perform their analysis was obtained from an earlier publication with information of the length frequency distribution of 295 exceptionally-preserved trilobite fossils collected at 'Beecher's Trilobite Bed' in New York State.

After estimating the parameters of a growth model widely used in fisheries science, the von Bertalanffy growth function, the researchers compared their results with published data on the growth of extant crustaceans. They found that the growth parameters they estimated for Triarthrus eatoni were well within the range of recent, slow-growing crustaceans.

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Primordial ‘hyper-eye’ discovered

An international research team has found an eye system in trilobites of the suborder Phacopina from the Devonian (390 million years B.P.) that is unique in the animal kingdom: each of the about 200 lenses of a hyper-facet eye spans a group of six normal compound-eye-facets, forming a compound eye itself. In addition to the hyper-facetted eyes, the researchers, led by zoologist Dr. Brigitte Schoenemann at the University of Cologne's Institute for Didactics of Biology, identified a structure that they believe to be a local neural network which directly processed the information from this special eye, and an optic nerve that carried information from the eye to the brain. The article, 'A 390 million-year-old hyper-compound eye in Devonian phacopid trilobites,' has been published in Scientific Reports.

Trilobites are arthropods that once inhabited the world's oceans and became extinct about 251 million years ago. The discovery was made when Schoenemann and her colleagues examined X-ray images taken by radiologist and amateur paleontologist Wilhelm Stürmer in the 1970s. Stürmer had already believed the filaments under the trilobite eyes to be nerves, or a light guiding system. Schoenemann also found markings by Stürmer on the images designating the six subfacets. However, scientists at the time did not believe his interpretations. Now, however, the re-examination of the images and verification with modern computed tomography succeeded in confirming his conjectures.

Most trilobites had compound eyes similar to those that are still found in insects today: a large number of hexagonal facets form the eye. There are usually eight photoreceptors under each facet. Comparable to the image of a computer screen, which is built up from individual pixels, an image is built up from the individual facets. In dragonflies, there are up to ten thousand individual facets. In order to produce a coherent image, the facets must be very close together and connected by neurons. However, in the trilobite suborder Phacopinae, the externally visible lenses of the compound eyes are much larger, up to 1 mm in diameter and more. In addition, they are set farther apart. Until now, scientists had not been able to explain this because space is wasted where light could be captured. Since a small cup sits under the lens, they assumed that at the bottom of the capsule was a small retina comparable to that of humans.

Dr Schoenemann's analysis of Wilhelm Stürmer's 40-year-old X-ray archive now suggests a different interpretation: a hyper-compound eye. Each phacopid had two eyes, one on the left and one on the right. 'Each of these eyes consisted of about 200 lenses up to 1 mm in size,' said Schoenemann. 'Under each of these lenses, in turn, at least 6 facets are set up, each of which together again makes up a small compound eye. So we have about 200 compound eyes (one under each lens) in one eye.' These sub-facets are arranged in either one ring or two rings. 'Underneath sat a foam-like nest that was probably a small neural network to process the signals,' the zoologist added. The filaments Stürmer found in fact did turn out to be nerves leading from the eyes to the trilobite's brain. Further examination with modern computer tomography confirmed these structures.

Wilhelm Stürmer was the head of the X-ray department at Siemens and an avid paleontologist. With a VW bus equipped as an X-ray station, he drove from quarry to quarry to X-ray fossils. Among other things, he discovered structures called filaments under the animals' eyes, which he thought were fossils of soft tissues, especially optic nerves. 'At that time, the consensus was that only bones and teeth, the hard parts of living things, could be seen in the fossils, but not the soft parts, such as intestines or nerves,' Schoenemann explained. Stürmer's heir gave the zoologist his archive. But the hobby-paleontologist had not only correctly identified the optic nerve, she notes: 'On an X-ray negative, there was an arrow in red pen pointing to the structure of the six lower facets under a main lens. This probably indicated that Stürmer had already recognized the hyper-compound eye.' At the time, however, scientists assumed that nerves did not fossilize, nor that light guides existed in natural optical system. Optical fibres were not discovered until the 1980s in the compound eyes of a deep-sea crab.

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450-million-year-old sea creatures had a leg up on breathing

 A new study has found the first evidence of sophisticated breathing organs in 450-million-year-old sea creatures. Contrary to previous thought, trilobites were leg breathers, with structures resembling gills hanging off their thighs.

Trilobites were a group of marine animals with half-moon-like heads that resembled horseshoe crabs, and they were wildly successful in terms of evolution. Though they are now extinct, they survived for more than 250 million years -- longer than the dinosaurs.

Thanks to new technologies and an extremely rare set of fossils, scientists from UC Riverside can now show that trilobites breathed oxygen and explain how they did so. Published in the journal Science Advances, these findings help piece together the puzzle of early animal evolution.

"Up until now, scientists have compared the upper branch of the trilobite leg to the non-respiratory upper branch in crustaceans, but our paper shows, for the first time, that the upper branch functioned as a gill," said Jin-Bo Hou, a UCR paleontology doctoral student who led the research.

Among the oldest animals on earth, this work helps situate trilobites on the evolutionary tree more securely in between older arthropods, a large group of animals with exoskeletons, and crustaceans.

The research was possible, in part, because of unusually preserved fossil specimens. There are more than 22,000 trilobite species that have been discovered, but the soft parts of the animals are visible in only about two dozen.

"These were preserved in pyrite -- fool's gold -- but it's more important than gold to us, because it's key to understanding these ancient structures," said UCR geology professor and paper co-author Nigel Hughes.

A CT scanner was able to read the differences in density between the pyrite and the surrounding rock and helped create three-dimensional models of these rarely seen gill structures.

"It allowed us to see the fossil without having to do a lot of drilling and grinding away at the rock covering the specimen," said paleontologist Melanie Hopkins, a research team member at the American Museum of Natural History.

"This way we could get a view that would even be hard to see under a microscope -- really small trilobite anatomical structures on the order of 10 to 30 microns wide," she said. For comparison, a human hair is roughly 100 microns thick.

Though these specimens were first described in the late 1800s and others have used CT scans to examine them, this is the first study to use the technology to examine this part of the animal.

The researchers could see how blood would have filtered through chambers in these delicate structures, picking up oxygen along its way as it moved. They appear much the same as gills in modern marine arthropods like crabs and lobsters.

Comparing the specimens in pyrite to another trilobite species gave the team additional detail about how the filaments were arranged relative to one another, and to the legs.

Most trilobites scavenged the ocean floor, using spikes on their lower legs to catch and grind prey. Above those parts, on the upper branch of the limbs, were these additional structures that some believed were meant to help with swimming or digging.

"In the past, there was some debate about the purpose of these structures because the upper leg isn't a great location for breathing apparatus," Hopkins said. "You'd think it would be easy for those filaments to get clogged with sediment where they are. It's an open question why they evolved the structure in that place on their bodies."

The Hughes lab uses fossils to answer questions about how life developed in response to changes in Earth's atmosphere. Roughly 540 million years ago, there was an explosive diversification in the variety and complexity of animals living in the oceans.

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