Chengjiang fauna found modern link animals

Recently, Professor Zhang Zhifei’s research group from the Department of Geology of Northwest University and Martin Smith of Durham University in the United Kingdom have achieved significant results in the study of Iotuba chengjiangensis in the early Cambrian Chengjiang fossil repository. The researchers found that the fossil is the earliest known reclusive link animal on Earth (a suborder of the link animal phylum Polychaete), and the discovery pushes the fossil record of the link crown group – stingosaurs by at least 200 million years.

In biological genealogy, the so-called “crown group” is the most recent common ancestor and descendants of all living group members). In contrast, there are “stem groups”, i.e. extinct groups of organisms that are outside the crown group but have a closer pedigree relationship with the crown group.

Simulated restoration of the Cambrian Chengjiang tube worm.Photo courtesy of Northwestern University

The study also shows that most types of some living animal phyla, such as link animals, may not thrive or live in the environment where specific fossils (that is, fossils are well preserved, and the organ structure and soft tissues of general organisms can be preserved). Therefore, the specific fossil library cannot fully reveal the historical process of the evolution of life on Earth, and other environmental fossils need to be constrained and supplemented, which provides new ideas and enlightenment for further exploring the fossil gaps of living animal groups.

The relevant research results were officially published in the Proceedings of the Royal Society B on February 1, London time.

According to reports, life on Earth has a history of 3.8 billion years. Long-term research believes that the Cambrian biological explosion gave birth to the basic framework of the earth’s animal tree, derived the roots of modern animal phyla, and 480 million years later, the Ordovician phyla of different animal groups appeared, resulting in the earth’s animal tree began to flourish. There are currently about 38 animal phyla on Earth.

Among them, link animals, as an important animal category in modern times, are very common in the sea and land, mainly the well-known earthworms, sand silkworms and leeches are the most common. Previous studies have suggested that link animals have appeared in the early Cambrian period, mainly belonging to link animal groups or paleolink animal subphyla. According to the latest research, the chain animal phylum exists or originated from the hermit ancestors, and the modern fan caterpillar (commonly known as the cagehead) belongs to this group, but the early fossil record is lacking.

Chengjiang Tubeworm was discovered in 1997 in the Chengjiang Fossil Reservoir in Yunnan, China. Due to its spiny-like structure in circles on the front of its body, Chengjiang Tubeworm is considered to be the earliest broom animal, but has lacked systematic description and research.

Based on the long-term accumulation of 15 specimens from the Institute of Early Life Research of Northwest University, Zhang Zhifei’s team re-studied the fossils of Chengjiang Tubeworm. The study found that the animal was worm-shaped, including a cylindrical torso and a retractable anterior area (i.e., the snout). There are long bundles of spines around the base of the snout, which are distributed in a ring around the anterior part of the trunk. The snout can contract and expand. When swollen, the surface has obvious nodules, which are usually not visible after contraction

The surface nodules of the trunk are stronger than the spines or hook-like structures on the surface of the snout, and the bulges are larger. The nodules on the surface of the body are generally triangular conical, with rapidly tapering at the top, either straight or curved. The body is hollow, there is a body cavity, there is a digestive tract in the body cavity, a funnel-shaped or scaly ball foregut area (pharyngeal cavity) is formed in front of the digestive tract, below the pharynx is the intestine, the intestine is downward, partially enlarged, tapered backward, the anus is located at the posterior end of the worm’s body, and linear glands develop on both sides.

Restoration of the trunk, bundle spines, snouts and gill signs of Chengjiang Tubeworm.Photo courtesy of Northwestern University

The new study has found that these thorns, unlike the tentacles of broom insects, are bundled and ctenoid, and can be stretched and rolled into the front end of the body, which develops a telescopic snout at the front end of the body and paired flap gills on the snout for feeding, which has the greatest similarity with modern fan caterpillars, Zhang said.

He told China Science News that modern fan caterpillars deposit and feed on the surface of the seafloor, and their intestinal distribution is also more consistent with the intestinal morphology of Chengjiang Tubeworm.

In addition to fossil morphological comparison, the study also integrates the morphological and molecular data of modern animals for clade analysis, combined with Bayesian analysis, maximum method and parsimotic method of analysis, although the results produced by different analysis methods are not exactly the same, but all analyses highly support the Chengjiang tube worm + fan caterpillar branch, indicating that the fossil represents the earliest known fan caterpillar (commonly known as the cage worm) on Earth.

“The analysis results of fan caterpillars show that Chengjiang Pseudotubulosis belongs to a highly evolved crown group of link animals, belonging to the fan caterpillar of the reclusive silkgill worm.” Zhang Zhifei said that because of the high position of the fan caterpillar system, other lineage groups should have been radiated before their appearance, so it is speculated that many taxa of link animals were highly diverse during the Chengjiang animal period.

Trunk and snout characteristics of Chengjiang Tubeworm.Photo courtesy of Northwestern University

Zhang Zhifei said that if it is speculated that the diversity of crowned animals in the Cambrian period was already high, but their fossils were very rare, this seems to be a great contradiction. One reason is that the environment in which animals in the crown group live is not suitable for the preservation of specific fossils. If so, it means that the types of fossils preserved in the Cambrian specific fossil library do not fully represent the entire Cambrian life community.

In addition, the study of Chengjiang Tubeworm pushed the radiation of link animals from the Ordovician period speculated by molecular biology research to the beginning of the Phanerozoic of the Earth, the Cambrian explosion period, indicating that link animal crowned protozoa had appeared in large numbers before the third order of the Cambrian, and this study pushed the ancestors of the more link animal subphylum forward to the early Cambrian for the first time. The study and findings further support the new hypothesis of the three-act formation of the Earth animal tree proposed by Shu Degan, an academician of the Chinese Academy of Sciences, that is, the three sub-realms and four supergates of the Earth animals appeared in the three acts of basal animals, protostomatal animals and post-mouth animals near the Precambrian-Cambrian boundary in 5.6-520 million years, showing the sudden and phased characteristics of the Cambrian explosion. (Source: Chen Bin, China Science News)

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