Link animals (body segmented worms) are an important phyla with a high diversity of species in the animal kingdom today, widely distributed and playing a pivotal role in a variety of terrestrial and marine ecosystems, such as the common earthworm, leech and sand silkworm. Polychaetes are the most abundant and evolutionary group of link animals, with pairs of bristled warts on their bodies, living mainly in the ocean. The study of early polychaetes has been one of the research hotspots that reveal the origin and early evolution of link animals.
On June 11, 2020, Hou Xianguang and Ma Xiaoya of Yunnan University jointly published a research paper titled "A Cambrian crown annelid reconciles phylogenomics and the fossil record" on Nature Online, which found a new polychaete in the Cambrian strata rich in soft body fossils in eastern Yunnan Province, China, about 514 million years ago. The study shows that the animal is a well-lived, belonging to the living polychaetaceae of the long-handed sandworm family (Magelonidae), which is the earliest known fossil record of the animal branch of the living link, which provides important evidence for understanding the early evolution of the link animal, and also reveals the ecological and morphological diversity of the Cambrian link animal.
In addition, on July 16, 2013, Hou Xianguang of Yunnan University and Nicholas J. Strausfeld of Arizona State University published a research paper entitled "Brain structure resolves the segmental affinity of anomalocaridid appendages" on Nature Online. The study identified well-preserved brains that lived in the Lower Cambrian, the world's earliest known fossil carnivore, about 520 million years ago. The findings revealed that the animal's brain was surprisingly simple, far less complex than the brains of some of its prey. The results support the hypothesis that true arthropods originated in lonopods, end the scientific debate on the homology of large-clawed limbs of odd shrimp, and reject the hypothesis that odd shrimp is the ancestral taxa of cheetus. The new research provides key clues to major scientific questions such as the origin of arthropods and the origin of complex ecosystems.
On October 10, 2012, Hou Xianguang and Ma Xiaoya of Yunnan University published a research paper entitled "Complex brain and optic lobes in an early Cambrian arthropod" on Nature Online, which is the oldest fossil record of animal brain structure to date, and its exquisite fossil preservation allows scientists to conduct in-depth research and direct comparison with the brain structure of living animals. The study's first featured study reported on the perfectly preserved brain and other neural and sensory structures, which are extremely rare in all paleontological studies, and opened up a new field for the study of fossil fauna specifically preserved in the Cambrian period. The Fuxian Lake insect in the Chengjiang fossil group is a relatively simple and primitive Cambrian arthropod, and the study surprised scientists that this ancient arthropod already has a very complex brain structure, which is very similar to the brain structure of living insects and crustacean arthropods. Their brains are composed of three parts: forebrain, midbrain and hindbrain, each with a pair of nerve bundles connected to the stem-like eye, antennae and a second pair of appendages. The paper also reports in detail on the stem-like compound eyes of the Fuxian Lake insect, and fossil evidence shows that the animal's visual system is also more complex, and its eyes can be flipped to a certain extent. Therefore, the animal's complex neural structure and its complex sensory structure are complementary to each other, and these studies further support that the Fuxian Lake worm may be a predator in the Chengjiang fauna. The article points out that according to the brain structure characteristics of Fuxian Lake insects, arthropods already had complex brain structures during the Cambrian explosion of life, so the relatively simple brain structure of living gill-footed arthropods should be formed by secondary degeneration, rather than the ancestral type of arthropod brain structure.
On October 10, 2008, Hou Xianguang's team at Yunnan University published a research paper entitled "Collective Behavior in an Early Cambrian Arthropod" on Science Online, which demonstrated that the origin of animal collective behavior coincided with the time of the emergence of animal explosions in the Cambrian explosion of life. The collective behavior characteristics of a new species of shrimp-like arthropods have been found in the world-famous early Cold Wushi Yunnan Chengjiang fauna (about 530 million years ago), which is also the oldest fossil indicating the collective behavior characteristics of animals. Among the more than 20 specimens showing collective behavior characteristics, one of the specimens shows that the heads and tails of twenty individual animals are connected to each other, that is, the tail of the former animal is embedded in the front of the shell of the latter animal individual, forming a strong and inseparable chain. The collective behavioral characteristics of individual animals arranged one after another in a chain-like connection are very unique in arthropods, and have not been found in any modern arthropods, and are difficult to compare with other animal behavioral characteristics. The animal floats or swims in a chain of waters in the ocean, and its collective behavior may be to more effectively resist predator attacks or to migrate quickly and efficiently to escape the harsh environment of the time. The research achievements have won the first prize of the National Natural Science Award, the Special Prize of Natural Science of Yunnan Province, the He Liang Heli Foundation Science and Technology Progress Award and other important awards, and on July 1, 2012, the Chengjiang Animal Fossil Conservation Area was approved as a World Natural Heritage Site by the United Nations Heritage Center.
The eastern part of Yunnan Province is rich in Cambrian strata, rich in cambrian animal fossils with unique preservation of soft bodies. Major fauna have been widely present in these specifically buried Cambrian rocks, providing important fossil evidence for understanding the early biodiversity explosions, known as the "Cambrian explosions".
However, although link animals are a very important animal phyla in today's ecosystems, their fossil record in the early Cambrian strata is very rare, which leads scientists to often lack strong fossil evidence in the process of studying the origin and early evolution of link animals.
Restoration of Dannychaeta tucolus of the early Cambrian dannychaeta tucolus in Yunnan Province (painted by Robert Nicholls)
The study describes a new cambrian genus called Dannychaeta tucolus. Studies have shown that it belongs to the family Magelonidae ( Malonidae ) in living polychaetes , with a typical shovel-headed head and a pair of elongated palps . This is the earliest fossil record of the animal branch of the live link, indicating that the danny caterpillar and the live link animal evolved from the same recent common ancestor, collectively known as the link animal crown group. The cambrian link animals that have been reported before are the primitive ancestors of the link animals.
In addition, unlike other cambrian animals that have been reported, studies have shown that the Danny Caterpillar camp is fixed in life. Many of the living polychaetes live in a managed or fixed burrowing mode of life to avoid predators or hide themselves during predation, often referred to as fixed link animals. Modern biologists, through phylogenetic genomics, believe that the fixed link animals should represent a relatively old branch of the link animal evolution tree, but until now, the fixed life has not been found in the early link fossils.
Dannychaeta tucolus, a salmonid animal in the early Cambrian period of Yunnan Province (courtesy of Chen Hong)
Chen Hong, a co-first author at Yunnan University, said: "We were surprised to find a polychaete worm from 514 million years ago that lives in a tube and is very similar to a species that is still alive today. ”
Co-first author Dr Luke Parry of the University of Oxford said: "Live animals operate in a variety of life patterns in the modern ocean, including entrenched filter eaters or ambush predators. The Cambrian animals we previously knew were probably crawling on the ocean floor, and what we saw in The Danny's caterpillars was completely different. The discovery of Danny's caterpillar tells us that even in very early ecosystems, ancient link animals have occupied many different niches. ”
Researcher Ma Xiaoya, co-corresponding author of Yunnan University, said: "This is the earliest fossil evidence of fixed link animals, and it is also the first appearance of animal branches in the fossil record of the living link. Because animal fossils from the early Cambrian period are very rare, we were very surprised and delighted by this discovery. This finding once again shows that the Cambrian explosion of life is not only an important evolutionary event for biodiversity but also for the dramatic increase in biodiversity. ”
The project has been co-funded by the National Natural Science Foundation of China and the Science and Technology Foundation of Yunnan Province.
Note: Some of the analysis is from the official website of Yunnan University.
Resolve links:
http://www.news.ynu.edu.cn/info/1101/26350.htm
Informational messages:
https://www.nature.com/articles/s41586-020-2384-8