Arthropods such as spiders, insects and crabs represent almost 80 percent of all animal species living today. Their evolutionary origins go back to the Cambrian, over 520 million years ago. However, the relationship between different fossil finds and their importance in the development of arthropods remains a mystery to researchers to this day. A newly discovered fossil species now provides an important link. Kylinxia zhangi shows features of preforms of the arthropods as well as features of later representatives of the animal tribe.
Arthropods, also known scientifically as arthropods, have achieved enormous biodiversity since the Cambrian over 520 million years ago. Numerous fossils provide information about the early ancestors of the species living today. But gaps in the line of ancestors make exact assignments difficult to this day. For example, it was unclear how the first real arthropods, from which today’s crabs and insects emerged, related to older ancestors, such as the more than one meter tall shrimp-like predator Anomalocaris, who was one of the top predators of the Cambrian Sea , and with the enigmatic five-eyed creature Opabinia regalis.
Features of various ancestors and descendants
Researchers led by Han Zeng from the Chinese Science Academy in Nanjing have now discovered the missing link. At the Chengjiang Fauna, a well-known archaeological site in southwest China’s Yunnan Province, they found six well-preserved fossils of a new species they named Kylinxia zhangi. The site offers particularly good conditions for the preservation of fossils. “This is why the Kylinxia fossils have extraordinary anatomical structures,” explains Zeng’s colleague Fangchen Zhao. “For example, nerve tissue, eyes and digestive system – these are soft parts of the body that we normally cannot see in conventional fossils.”
This enabled the researchers to classify the anatomy of the fossils more precisely. “Kylinxia is a very unusual chimeric species. It combines morphological features from different animals,” says Zhao. The researchers therefore chose the name of the new species based on the chimeric being Kylin from Chinese mythology, combined with the Chinese word for shrimp. Just like the real arthropods, also known as euarthropods, Kylinxia already has typical features of arthropods such as a segmented trunk, articulated legs and a head shield that has grown together. At the same time, the species also bears the hallmarks of older ancestors, including the conspicuous front claws of Anomalocaris, which were presumably used to carry prey to the mouth of the giant predator, and five large, stem-like eyes known from Opabinia.
Important link
The anatomical investigations on Kylinxia showed that the antennae of the euarthropods go back with high probability to the appendages of anomalocaris and did not develop independently of it. Kylinxia provides the crucial transition stage here. The antennae of today’s insects, in their most primitive form, did not serve as a sense organ, but rather helped to convey food to the mouth. “Kylinxia provides important insights into the relationships between early arthropods,” the researchers write in their publication. “It provides information about evolutionary adaptations and the origin of evolutionary innovations in this animal strain.”
According to co-author Maoyan Zhu, all of the results suggest that the evolutionary placement of kylinxia between anomalocaris and the true arthropods is correct. “Our findings thus reach the evolutionary roots of real arthropods,” he says. Zeng adds, “Kylinxia represents a critical transition fossil that is predicted by Darwin’s theory of evolution. It bridges the evolutionary gap from anomalocaris to true arthropods and forms an important ‘missing link’ in the origin of arthropods. “
Source: Han Zeng (Chinese Academy of Sciences) et al., Nature, doi: 10.1038 / s41586-020-2883-7