Study: Earliest Trilobites Had Stomachs

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Exceptionally well-preserved trilobite fossils from the Cambrian Wulongqing Formation near Guangwei Village in southern Kunming, the capital city of Yunnan Province of China, contradict previous assumptions about trilobite digestive systems.

Palaeolenus lantenoisi showing evidence of crop, digestive glands, alimentary canal along thorax, and excreted waste posterior to pygidium. Scale bar - 5 mm. Image credit: Hopkins et al, doi: 10.1371/journal.pone.0184982.

Palaeolenus lantenoisi showing evidence of crop, digestive glands, alimentary canal along thorax, and excreted waste posterior to pygidium. Scale bar – 5 mm. Image credit: Hopkins et al, doi: 10.1371/journal.pone.0184982.

Trilobites are a group of extinct marine arthropods that form the class Trilobita.

They appeared in ancient oceans in the Early Cambrian Period, about 540 million years ago — well before life emerged on land, and disappeared in the mass extinction at the end of the Permian, about 252 million years ago.

They were extremely diverse, with about 20,000 species, and their fossil exoskeletons can be found all around the world.

Previous studies suggest that two body plans existed for trilobite digestive systems: (i) a tube that runs down the length of the trilobite’s body with lateral digestive glands that would have helped process the food; or (ii) an expanded stomach, called a ‘crop,’ leading into a simple tube with no lateral glands.

Until now, only the first type had been reported from the earliest trilobites.

Iron concentrations in digestive tract of Palaeolenus lantenoisi. Scale bars - 1 mm. Top right and left panels show elemental maps of Fe (yellow), Si (pink), O (green), and Al (cyan) concentrations in the areas indicated by black rectangles in the central photography. All other panels show elemental maps of just Fe concentrations (yellow) in the indicated areas. Note that here and in elemental maps in other figures, some areas where iron concentrations are expected but not evident are in ‘shadow’ due to the orientation of the specimen or because that part of the specimen is outside of the width of detection, so no elemental composition is available (compare the top two panels showing Fe, Si, O, and Al with those just below showing only Fe). Image credit: Hopkins et al, doi: 10.1371/journal.pone.0184982.

Iron concentrations in digestive tract of Palaeolenus lantenoisi. Scale bars – 1 mm. Top right and left panels show elemental maps of Fe (yellow), Si (pink), O (green), and Al (cyan) concentrations in the areas indicated by black rectangles in the central photography. All other panels show elemental maps of just Fe concentrations (yellow) in the indicated areas. Note that here and in elemental maps in other figures, some areas where iron concentrations are expected but not evident are in ‘shadow’ due to the orientation of the specimen or because that part of the specimen is outside of the width of detection, so no elemental composition is available (compare the top two panels showing Fe, Si, O, and Al with those just below showing only Fe). Image credit: Hopkins et al, doi: 10.1371/journal.pone.0184982.

Based on this, paleontologists had proposed that the evolution of the crop came later in trilobite evolutionary history and represented a distinct type of digestive system.

“Trilobites are one of the first types of animals to show up in large numbers in the fossil record,” said Dr. Melanie Hopkins, an assistant curator in the Division of Paleontology at the American Museum of Natural History.

“Their exoskeletons were heavy in minerals, and so they preserved really well. But like all fossils, it’s very rare to see the preservation of soft tissues like organs or appendages in trilobites, and because of this, our knowledge of the trilobite digestive system comes from a small number of specimens.”

“The new material — 270 specimens from the Wulongqing Formation — really expands our understanding.”

Spherical aggregates of iron oxide in crop of Redlichia mansuyi: (A) photograph of entire specimen; (B) SEM image of crop outlined in (A); (C) SEM image overlain by elemental mapping: Ti, Al (fuchsia), N, K (red), C (dark blue), Cu (pink), S (violet), P (light blue), Ca (blue-green), Na (yellow-green), Mg (orange), Fe (yellow), Si (cyan), O (green); (D) elemental map of just iron in area shown in B and C; (E) SEM image of area outlined in (B); (F) SEM image overlain by elemental mapping: Fe (yellow), Si (cyan); (G) elemental map of iron in area shown in E and F. Scale bar for (A) - 5 mm, for (B-D) - 1 mm, for (E-G) - 500 microns. Image credit: Hopkins et al, doi: 10.1371/journal.pone.0184982.

Spherical aggregates of iron oxide in crop of Redlichia mansuyi: (A) photograph of entire specimen; (B) SEM image of crop outlined in (A); (C) SEM image overlain by elemental mapping: Ti, Al (fuchsia), N, K (red), C (dark blue), Cu (pink), S (violet), P (light blue), Ca (blue-green), Na (yellow-green), Mg (orange), Fe (yellow), Si (cyan), O (green); (D) elemental map of just iron in area shown in B and C; (E) SEM image of area outlined in (B); (F) SEM image overlain by elemental mapping: Fe (yellow), Si (cyan); (G) elemental map of iron in area shown in E and F. Scale bar for (A) – 5 mm, for (B-D) – 1 mm, for (E-G) – 500 microns. Image credit: Hopkins et al, doi: 10.1371/journal.pone.0184982.

The new Wulongqing specimens are dated to the early Cambrian, about 514 million years ago. They belong to two trilobite species, Palaeolenus lantenoisi and Redlichia mansuyi, and about 20% of them show soft-body preservation.

Contradictory to the previously proposed body plans, Dr. Hopkins and co-authors identified crops in Palaeolenus lantenoisi and Redlichia mansuyi.

In addition, at least one specimen of Palaeolenus lantenoisi has both a crop and digestive glands — suggesting that the evolution of trilobite digestive systems is more complex than originally proposed.

“This is a very rigorous study based on multiple specimens, and it shows that we should start thinking about this aspect of trilobite biology and evolution in a different way,” Dr. Hopkins said.

The study is published in the journal PLoS ONE.

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