Department of geology northwest university

Professor Zhang Zhifei of our department instructs doctoral student Liu Yi to publish academic paper in National Science Review

Recently, Professor Zhang Zhifei, the research team of early life and environment innovation of Northwest University, directed by Liu Yi, a doctoral student, and Tim Topper, a young foreigner of the joint research group, and Dr skovsted, a natural history museum of Sweden, have carried out the latest observation and Research on the mollusk fossils found in Chengjiang biota through SEM and X-ray fluorescence analyzer. Based on the shell structure and internal anatomical characteristics, the team rejected the conclusion that the mollusks belong to the brachiopod crown group or the brachiopod sister group, and believed that the mollusks are not tentacle crown animals. In the phylogenetic tree, they should be placed in the crown wheel dry group, most likely close to the soft body animals. The results were recently published online in the national science review.

Triplicatella opimus

There are about 38 kinds of living animals on the earth. In the long geological history, trilobites, ancient cups, graptolites and many other kinds of animals have been extinct. Hyolitha is also a kind of extinct marine benthic invertebrate. It flourished from the Cambrian explosion 540 million years ago, and died out at the end of Permian. It is the main part of Cambrian evolutionary fauna.

Since the first report of Eichwald in 1840, the mollusks have been widely found and reported in Europe, North America, Australia, China and other places. The fossils of mollusks are generally preserved with cone tube, flap and appendage structure (the appendages are only found in mollusks at present). According to their morphology, they can be divided into two subgroups: hyolithids and orthothecids. In the Early Cambrian, they experienced rapid evolution and radiation events, with high diversity. But up to now, the research on its biological properties and systematics is still controversial.

In early studies, based on the similarity of shell morphology, the mollusks were placed under the phylum Mollusca for a long time, or independently into a phylum, the phylum mollusks.

In 2017, moysiuk et al. Found the feathered tentacle structure in the mollusca fossils of Burgess Shale, Canada, and interpreted it as a lopophore which has the same structure as the ciliary ring of the tentacle crown animals. In addition, the mollusks have two kinds of shells, namely, the operculum and the spinal canal. Therefore, the mollusks should belong to the lophophorates closely related to brachiopods. Then, some scholars found the structure of "fleshy stem" similar to brachiopod in the fossils of oncomelania, and further speculated that there was a close relationship between the mollusks and brachiopod, and even thought that the mollusks belonged to the sister group or crown group of brachiopod.

However, through careful observation and comparison of the shell structure, other scholars found that the soft tongue snail has a similar lamellar shell microstructure with the shell of mollusks in the same period, thus insisting on the relationship between the soft tongue snail and mollusks.

The latest achievements come from the early life and environment innovation research team of Northwest University. It is found that triplicatella (trilobites) in Chengjiang fossil Bank (the third stage of Cambrian) has fan-shaped tentacles. After a comparative study of a large number of fossils, the team found that the tentacle like feeding organ can stretch freely inside and outside the abdominal edge of the operculum. Based on the research on the muscles preserved in the three groove spirochetes, the team reconstructed the internal structure of the shell of the straight tube spirochetes except the digestive system for the first time, which enriched the internal anatomical information of the straight tube spirochetes. This result fills in the gap of oral anatomy of the fossil of oncomelania, and adds new evidence of key soft fossils to the systematic analysis of the fossil of Oncomelania in the past 200 years.

The scalloped tentacle feeding structure of triplicatella in Chengjiang fossil bank

In the comparative study of the functional morphology between the soft tongue snail and the tentacle crown animal (brachiopod and broomworm), the researchers found that the tentacle like structure of the soft tongue snail was different from the function of the lophophore of the tentacle crown animal (lophophorata), and was not homologous. The analysis is as follows:

(1) The tentacle like structure of the soft tongue snail does not distribute around the mouth, which does not meet the definition of lophophore;

(2) In function, the tentacle like feeding organ of Ligularia mollissima can stretch back and forth, instead of bending freely through the tentacles, beating the water to filter food;

(3) There are similarities between the tentacle like feeding structure and the shape of the tentacle crown in the larval stage of the brachiopod, but the tentacle crown in the larval stage of the brachiopod is mainly used for swimming rather than filtering.

Comparison of tentacles between the tentacles and the tentacle crown

In addition, the early life and environment innovation research team of Northwest University also observed the Yunnan sarcophagus in Chengjiang biota, and found that there was no sarcophagus like attachment structure in the beginning structure of spinal canal. The so-called "fleshy stem" is actually part of the residual fragments or scars of the vertebral canal shell. By comparing different lithofacies and fossil bank specimens, the researchers revealed the differences in the preservation of the end of the soft tongue spirulina, and revised the interpretation of the fossil "meat stem" of the straight tube Spirulina by scholars at home and abroad.

The so-called "fleshy stem" structure of the mollusk is actually part of the fragment or remnant of the sharp shell at the top of the vertebral shell

The above results are of great significance to the study of hyacinth. Specifically, these results:

(1) The internal anatomical structure of triplicatella, a fossil of Chengjiang fauna, was reported in detail for the first time, revealing the tentacle feeding structure of Oncomelania and filling the gap of oral anatomy of Oncomelania.

(2) Through the analysis of functional morphology, it is suggested that orthothecids feed on tentacle like sediments, indicating the evolutionary process of the mollusks from the non appendage to the appendage, so as to better adapt to the benthic life of the sea.

(3) Based on the shell structure and internal anatomical characteristics of the mollusks, it is considered that the mollusks do not belong to the tentacle crown animals, which denies the conclusion that the mollusks belong to the brachiopod crown group or the sister group of the brachiopod animals. It is suggested that the mollusks should be placed in the dry group of the brachiopod animals in the phylogenetic tree, which can be most closely related to the mollusks.

Paper information:

Are hyoliths Palaeozoic lophophorates?

https://doi.org/10.1093/nsr/nwz161

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