Yunnan Netnews (Reporter Gao Yimeng Correspondent Wang Wei) On September 3, Commonus Biology, a subsidiary of Nature Publishing Group, published online a research team by Hou Xianguang, a key laboratory of paleontological research in Yunnan Province, and colleagues from the University of Leicester, Oxford, the University of Manchester, and the Chinese Academy of Sciences, entitled "Differences in the attachment structure of different hypermycosms in the early Cambrian Period Reveal the Diversification of The Body Patterns of True Arthropods in Stem Groups" (Variation in appendages in early Cambrian bradoriids reveals a wide range of body plans in stem-euarthropods) research paper. Researchers have conducted a new morphological study of the fossils of high muscle insects in the World Natural Heritage "Chengjiang Biota" and obtained new and important understandings. Zhai Dayou associate researcher is the first author of the paper, researcher Liu Yu and researcher Hou Xianguang are the corresponding authors.
Fossils of tall muscle worms in the Cambrian Chengjiang biota. Left: Duo's small Kunming insect; Middle: Chen's Kunyang insect; right: Indian insect. Modified from Zhai et al., 2019, Communications Biology. Yunnan Provincial Key Laboratory of Paleontology Research Courtesy of Yunnan Province
Arthropods are the most diverse animal phylum on Earth since the Ezoogens. During the long epoch of geological history, most of the arthropods that once lived on Earth were extinct due to brutal competition for survival, but the rich fossil record in the strata still bears witness to the colorful past of these historic species.
The high muscle worm (taxonomic Order Bradoriida) is a group of arthropods that lived from the Cambrian to the Early Ordovician, and they were particularly prevalent in the early Cambrian strata of southwestern China. In most strata, the hypermyolars have only preserved their shells, while the soft bodies are decayed, and only in a few specifically buried fossil reservoirs such as the Chengjiang biota, there are appendage structures preserved. Because the shell is similar to that of mesozoites, hypermyosomes have long been considered a class of mesosomes. Researcher Hou Xianguang of Yunnan University et al. studied the fossils of tall muscle worms that preserve soft bodies, and proposed that they do not belong to mesozoans, which greatly impacted the academic understanding of the taxonomic position of tall muscle worms and also affected the understanding of the evolutionary history of mesosomes, an important crustacean taxon. However, due to the small size of the individual and the hard shell, most of the appendage structures of the high muscle worm are still difficult to study and unknown. The application of high-precision X-ray computed tomography (micro-CT) provides an opportunity to unravel this mystery.
Using GE Phoenix Nanotom M X-ray scanner and Xradia 520 Versa X-ray microscope, the researchers scanned and reconstructed the morphological information of three high-muscle insect specimens produced in chengjiang biota in Yunnan Province with a body length of only a few millimeters and fine structures of appendages preserved at the micron scale-
The results showed that there were huge differences in the appendage structure of these three types of high muscleworms. Among them, the most common high muscle worm in Kunming, Dow's Little Kunming Worm, has 12 pairs of appendages, including 1 pair of single-branched antennae, 4 pairs of double-branched appendages with two rows of inner lobes, 5 pairs of double-branched appendages with one row of inner lobes, and 2 pairs of single-branched tail appendages with specific morphology and different shapes, and its appendage composition is 1+4+5+1+1. The slightly smaller and less common Chen's Kunyang insect has 13 pairs of appendages, including 1 pair of single-branched antennae, 3 pairs of different double-branched head appendages, 8 pairs of double-branched appendages with short outer limbs and developed inner lobes, and a pair of short, thick, single-branched tail appendages with underdeveloped inner lobes, and its appendage composition is 1+1+1+1+8+1.
In addition, the researchers also found a specimen of indian insects (undetermined species) that had not previously been reported in the Chengjiang biota, which had 12 pairs of appendages, including a pair of single-branched antennae and 11 pairs of posterior appendages with well-developed inner lobes and their upper bristles, with an appendage composition of 1+11. Among the above three species of high muscle insects, the appendage arrangement pattern of Chen's Kunyang insect is similar to that of the jaw, which is a highly evolved type; the appendage arrangement pattern of Indian insect is similar to that of leaf foot, odd shrimp and large appendage arthropods, etc., which is a very primitive type; the appendage arrangement pattern of Duo's small Kunming insect is very unique, and it is difficult to find similar types in the arthropod family.
Hypermyosomes partially appendages. Top left: Duo's little Kunming insect; top right: Indian insect; bottom: Chen's Kunyang insect. Modified from Zhai et al., 2019, Communications Biology. Yunnan Provincial Key Laboratory of Paleontology Research Courtesy of Yunnan Province
The results of the study showed that the difference in appendage patterns between different hypermyosomes was far beyond the scope of the classification of hypermyosomes as an order level, and had reached the subphylum level. This suggests that the taxonomic work of Cambrian arthropods based on mesococcal morphology is grossly biased by the failure to comprehensively cover soft body morphological information and greatly underestimates the species diversity of early arthropods, particularly bivalve-shelled arthropods. Research also shows from another aspect that early arthropods have independently evolved a variety of body forms through evolutionary radiation to adapt to and occupy different ecological environments.
The research was funded by the National Natural Science Foundation of China, the Yunnan Provincial Science and Technology Department Fund, the Yunnan Paleontological Innovation Team Project and other scientific research projects.