The microscopic Cambrian fossil Saccorhytus was not a deuterostome, but a molt

Five years ago, based on findings from the Lower Cambrian Kuanchuanpu Formation, which is exposed in China’s Shaanxi Province, a small phosphate fossil was described: Saccorhytus coronarius. This microfossil, similar to a crushed scaly ball, attracted the attention of many scientists and popularizers of science, since, according to the authors of the description, the sacorite was a deuterostome (Deuterostomia) with pharyngeal gills and a mouth opening surrounded radially. folds In addition, the age of the surrounding deposits was estimated at 536–532 Ma, which corresponds to the beginning of the Cambrian period. All this allowed us to assume that the sacorite is very close to the beginning of the evolutionary branch of deuterostomes (to which we also belong). New findings of sacorite have made it possible to recognize several hollow tips, which were sensory organs. At the same time, no gill openings were found. Thus, the general set of external signs of sacorite corresponds to molting animals (Ecdysozoa), now represented by tardigrades, onychophores, priapulids and some other groups.

In my opinion, the best, in my opinion, popular presentation in Russian of the history of human origin – “I am looking for an ancestor” by the historian Nathan Eidelman – there was a funny picture in which Australopithecus, Pithecanthropus and other hominids are thrown out of pedestal where the true ancients and their ancestors, eoanthropus (human dawn). The latter, also known as the “Piltdown man” and who claims to be the most important link in human evolution, turned out to be a hoax from the beginning of the 20th century, which anthropologists and geologists managed to unravel only 40 years later.

Something similar happened recently with saccorite (Saccorhytus coronarius), whose description and interpretation as the oldest representative of deuterostomes, uniting chordates, hemichordates and echinoderms (some molecular biologists also classify xenoturbella as one of them), appeared five years ago in an authoritative scientific publication. Nature (see: Cambrian fossil Saccorhytus placed at the base of the evolutionary line of deuterostomes, “Elements”, 02/14/2017).

Of course, the bag was not a fake. In fact, it was found among other bone remains obtained by dissolving samples of phosphated limestone taken from the Lower Cambrian Kuanchuanpu Formation in southern Shaanxi. The age of this formation, compared to other Lower Cambrian deposits in southern China, for which radiometric dating has been obtained, is estimated between 536 and 525 Ma, and the sacorite comes from its older part. The Kuanchuanpu deposits are also known for other interesting finds, especially the oldest remains of molting worms, which in the modern fauna are represented by priapulids, kinorhynchus, loricifers, piloses and nematodes (see DJ Bottjer et al., 2020. Comparative Taphonomy and phylogenetic). sign of phosphatized Weng’an and Kuanchuanpu biotas).

The sacorite (more precisely, the hypothesis that it belonged to deuterostomes) had several problems. First, the authors of the description ignored the fact that the vast majority of similar-looking phosphate remains in this formation are fragments of moult skins or mineralized integuments from whole moult worms. Secondly, in order to find at least some similarity with deuterostomes in saccorite, it was compared with Cambrian Vetulicolia (Vetulicolia) and no less “problematic” Vetulocystidae (Vetulocystidae), whose belonging to deuterostomes is also more than doubtful. Vetulicolia, for example, had a shell covering the anterior part of the body, an articulated caudal region, and a mouth cone surrounded by tooth-like plates, as in anomalocaridids (radiodonts), the basal group of arthropods; paleontologists also “captured” them in the molting position, when the old outer skeleton splits about halfway so that its molted owner goes out into the wild (see A. Yu. Zhuravlev, 2014. Early history of Metazoa – a paleontologist’s view ).

In general, it is not possible to imagine how the gill apparatus worked in a creature with holes of various calibers scattered randomly over the entire surface of the body, either from the point of view of biology or from the point of view of physics. There are no such things in nature. It is not surprising that most Cambrian fauna specialists perceive the new candidate for “great ancestors” with great skepticism (see comments on the aforementioned Cambrian Fossil news Saccorhytus located at the base of the deuterostome evolutionary line).

In general, a more appropriate redescription of saccorite was immediately suggested. Reputation of the newspaper Nature -at least in paleontology-, alas, outstanding works are published there. But here’s what they actually stand out – whether it’s an in-depth study of truly interesting fossils or blatant nonsense – it doesn’t matter, as the citation index encourages both. Just remember the recent “hummingbird-sized dinosaur” that became a common lizard (L. Xing et al., 2020. Retraction note: Hummingbird-sized dinosaur from the Cretaceous period of Myanmar). In general, “it was never like that, and here it is again.”

In a new article of all the same Nature a group of scientists from scientific institutions in China, Europe, and the United States rethought the systematic membership of sacorite based on a comprehensive study of several hundred new specimens using a scanning electron microscope and a synchrotron X-ray microtomograph (Figs. 1 and 2 ).

rice  2. Saccorhytus coronarius

Most of the ellipsoid samples of saccorites (less than 1 mm in diameter) were found to be strongly compressed, with conical tubercles and broken protruding spines on the surface, with no remains of internal organs. An opening is distinguished on one of the sides of such an ellipsoid, which can be kept in an open and closed position. The opening is surrounded by thin radial folds of the integument. These folds are surrounded by 25-30 relatively large flattened hollow spines. Even further from the opening, on one side only there are several (1-5) particularly large spines. Rough radial folds of the integument and rows of small tubercles follow. In addition, on the surface there are several hollow cones of different sizes, which, if not broken, also end in spikes. Its location indicates the bilateral symmetry of the organism. Numerous small spines cover the rest of the sacorite surface.

The wall of the ellipsoid is two layers, usually with a gap between the layers. The presence of voids and the absence of a pronounced orientation of the apatite crystals in the wall indicate secondary post-mortem phosphating of the sacorite body.

Since the only hole in the sacorite was an opening, which may have been a mouth, and no other slits or pores were found on its surface, this organism could not be a deuterostome with gill slits or pouches. The preservation of the material, which is a flattened integument, indicates that the sacorite probably represents the moulting skin of a member of the Moulters clade (Ecdysozoa).

rice  3. Reconstruction of the appearance of the sacorite and its possible position among the metazoans

According to the authors of the paper under discussion, this organism was one of the ancient molting worms of the Cycloneuralia group, which includes the modern priapulids, loricifers and kinorhynchus. Along with worms and nematodes, these worms form a sister group to another branch of molting animals, which includes tardigrades, onychophores, and arthropods.

Source: Liu Yunhuan, Carlisle Emile, Zhang Huaqiao, Yang Ben, Steiner Michel, Shao Tiequan, Duan Baichuan, Marone Federica, Xiao Shuhai, and Philip CJ Donoghue. Saccorhytus is an early ecdysozoan and not the oldest deuterostome // Nature. 2022. DOI: 10.1038/s41586-022-05107-z.

Andrei Zhuravlev

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