A virtual autopsy reveals secrets of the head of an arthropod 500 million years ago

Arthropods are the most diverse species on Earth since the birth of Zezhou. Since the Cambrian period, they have been active on the planet, and large appendage real arthropods are one of the most important categories. But all along, there have been many unknowns about how the large appendage real arthropods evolve and the relationship with their living distant relatives.

A virtual autopsy reveals secrets of the head of an arthropod 500 million years ago

Chengjiang Biome Fascinating Lincholi larvae Specimens and CT 3D Images RespondentSIs

Recently, the research team of Liu Wei, Hou Xianguang, Yu Daando and Harvard University Associate Professor Javier Ortega-Hernandez of Yunnan Institute of Paleontology, Yunnan University’s Institute of Paleontology, and other researchers, published a collaborative paper on “The Incomplete Development of The Heart plate in Cambrian large exopods of real arthropods” published online in Current Biology, opening a new window for the study of developmental biology to reveal the leg structure of the Cambrian Jiangriver arthropods.

The fossils of Chengjiang biome contain the evolutionary history of arthropods

In the long history of the earth, due to various reasons, most of the arthropod species that once lived on the earth have been extinct, the survivors have gradually evolved into today’s spiders, scorpions and other cessions, moths, horses and other polypods, as well as shrimp, crabs, insects and other crustaceans.

Near Kunming, Yunnan, a large number of specially buried fossil specimens have been preserved in the formation of the Cambrian Chengjiang biome, which paleontologists are obsessed with. These seemingly ordinary “stones” record the Cambrian outbreak 518 million years ago, arthropods and many other descendants of animals have had a prosperous scene.

In July 1984, Researchers at Hou Xianguang discovered that these layers of ancient fossils provided extremely valuable evidence to reveal the Cambrian eruption more than 500 million years ago. The Chengjiang biota, which has been selected as the World Natural Heritage, has been hailed as “one of the most amazing scientific discoveries of the twentieth century”, opening up an important innovation field for early life science research.

Over the years, the researchers’ morphological observations of Chengjiang fossils have been limited to the use of traditional imaging techniques such as optical microscopes to observe the two-dimensional structures preserved on the fossil surface, while the information on the animal’s body structure, which is kept inside the fossil specimens, can only be studied by means of time-consuming and destructive steel needle repairs.

Because of this, the researchers can only speculate on the scientific question of whether many important fine structures of the body of early arthropods, such as whether the plate structors really exist, can only be speculated through branch analysis and other methods. Even if the speculation is accurate, the researchers were unable to demonstrate the true morphology of such fine structures.

The mouth plate of the charming Lincholi larvae may not be fully developed

Following a 2mm-long, fascinating Lincholi larvae, published in the Proceedings of the National Academy of Sciences in 2016, the U.S.-China team set its sights on a new approach, microCT technology, which greatly broke through the limitations of previous observations of the size and morphology of back-armor structures in the development of individual animals.

In order to achieve high-precision restoration and observation of fossils with out-of-nosed and any angle, with the help of high-precision micro-CT, they modeled the fine structure of Chengjiang arthropod stocland stoctors through computer 3D software “magic hands” and isolated the structure segregated parts of the body. For the first time, the team has clearly revealed the fine structures of the Cambrian large appendage arthropod, the mouth plate that is preserved on the head of the charming Lincholi worm. The study involved two three-dimensionally preserved larvae fossils of the species, which were only 5 mm long and 7 mm long.

The study found that the charming Lincholi worm in the body length of only 5 mm of larvae, the eyes grow a distinct plate, the surface of the mouth plate also has a pair of small protrusions. Using open-source computer 3D restoration software, the researchers erased the large appendage of a 7mm-long fossil of the fascinating Lincholi worm larvae, and found that the proportion of the mouth plate in the head decreased, indicating that there may be incomplete development of the plate structure during the individual development of the species. “In the past, traditional means have been impossible to observe and study this fine structure. Liu said.

In addition, they performed a longitudinal digital cut on the animal’s head and found narrow pharynx passages extending under the plate structure into the body, consistent with what was observed on the species’ adult specimens.

Subsequently, they compared the fossil data with the embryonic development data of living arthropods, and found that the charming Lincholi worm, as the representative of arthropods in the Cambrian period, had evolved a distinct structure that was homologous with the mouth plates of the existing spiders, scorpions and other arthropods.

This protruding plate structure, which is common in living arthropods, has been found in the dry group of slugs 518 million years ago. This analysis provides direct fossil evidence for paleontology and evolutionary biology to understand the long-suffering arthropod head problem.

The researchers also discussed the body structure of toothed animals such as Cambrian “star” prawns, which may have the same structure as the mouth plate of the real arthropod. “Whether the large claws of toothed animals are the homologous structure of the mouth plates of real arthropods is a scientific problem that we will be addressing in the next stage. Liu said.