Orthrozanclus

• The generic name Orthrozanclus is from other Greek. ὄρθρος - “dawn” (in the light of the phylogenetic classification of the animal as an early representative of its type) and ζάγκλον - “sickle” (in honor of its long sclerites).
• The species name reburrus is from lat. reburrus - “bristling” (indicates the appearance of the animal) ^[3] .
• The species name elongata is in honor of its elongated (compared to Orthrozanclus reburrus ) body ^[4] .

The first two fossil specimens of this genus were found by Charles Walcott , the discoverer of the Burgess shale deposits, but they were never described ^[3] . Another 9 copies were collected by the Royal Ontario Museum in the period from 1994 to 2000. All specimens were found in Walcot's quarry . Interest was first shown to him in 2006 by researchers from the University of Toronto J. Bernard Caron and Donald Jackson. In the scientific literature, it was first mentioned under the name scleritomorph C. There was no detailed description ^[5] . In 2007, Caron and Morris (a researcher from Cambridge) published a description of this fossil and gave it the name Orthrozanclus reburrus . In 2017, a second species related to the genus Orthrozanclus was described - Orthrozanclus elongata, which received its species epithet in honor of its elongated (compared to Orthrozanclus reburrus ) body. An instance of this species was discovered in the Maotian shales ^[4] .

Orthrozanclus had an elongated, approximately oval body, tapering at the back. The length of the specimens found lies in the range from 6 mm to 11.3 mm (including “needles”). The upper part of Orthrozanclus was convex, the bulge was bordered by a flat edge. The lower side of the animal was flat and unprotected, while the upper side carried several protective parts:

• a small shell covering the front of the body;
• three zones covered with sclerites that fit tightly to the body (one of these zones encircled it entirely);
• from 16 to 20 long needles on each side, bending upward.

Neither sclerites nor needles were mineralized and had round internal cavities in cross section. The shell was convex and had the shape of a triangle with rounded corners. In front, it had a protrusion, in the back it was raised, and a crest stretched along its longitudinal axis. Two types of rings could be distinguished on the sink: small rings indicating a gradual build-up of material along the edges, and large ones, possibly indicating the metamerism of Orthrozanclus . The function of the shell is still unclear ^[1] .

The anatomy of Orthrozanclus indicates that he was a benthic animal (living on the seabed). There is a possibility that he had a single muscular leg like modern gastropod mollusks ^[1] . Presumably, Orthrozanclus was not a predator, and the needles served him to protect him from enemies ^[6] .

The sclerites of Orthrozanclus in their structure resemble the sclerites of his contemporary of the Burgess schist, vivaxia ( Wiwaxia ). On the other hand, its anterior conch resembles in its structure fragments of the conch, also found in the Burgess sediments, whose owner was named Oikozetetes and was attributed to the chalkerid clade , most of which date back to the Early Cambrian. Similarities in shell structure have been demonstrated with other Early Cambrian fossils such as Ocruranus and Eohalobia . All these data made it possible to hypothesize the existence of the Halvaxiida clade, which, in addition to Orthrozanclus , includes all chalkeriids and vivaxia ^[1] . This hypothesis contradicts the hypothesis of Nicholas Butterfield, who believes that vivaxia is closer to annelids than to mollusks, while chalkeriids are closer to mollusks. Thus, the study of Orthrozanclus is important for elucidating the evolutionary history of mollusks or even all spiral mollusks ^{[7] [8] [9]} .

• Caron J.-B., Jackson DA Taphonomy of the Greater Phyllopod Bed Community, Burgess Shale (English) // Palaios : journal. - Society for sedimentary geology , 2006. - Vol. 21 , no. 5 . - P. 451-465 . - DOI : 10.2110 / palo.2003.P05-070R .
• Butterfield NJ Hooking some stem-group '' worms '': fossil lophotrochozoans in the Burgess Shale (Eng.) // Bioessays: journal. - 2006. - Vol. 28 , no. 12 . - P. 1161-1166 . - DOI : 10.1002 / bies.20507 . - PMID 17120226 .
• Conway Morris S., Caron JB. Halwaxiids and the Early Evolution of the Lophotrochozoans (Eng.) // Science: journal. - 2007 .-- 2 March ( vol. 315 , no. 5816 ). - P. 1255-1258 . - DOI : 10.1126 / science.1137187 . - . - PMID 17332408 .
• Markov A.V. A fossil animal was found close to the common ancestors of mollusks and annelids (neopr.) . Elements (March 6, 2007). Date of treatment December 19, 2013.
• Sigwart JD, Sutton MD Deep molluscan phylogeny: synthesis of palaeontological and neontological data (English) // Proceedings of the Royal Society B: journal. - 2007. - Vol. 274 , no. 1624 . - P. 2413-2419 . - DOI : 10.1098 / rspb.2007.07.07 . - PMID 17652065 .
• Butterfield NJ (2007). "Lophotrochozoan roots and stems" in the Palaeontological Association Annual Meeting . Budd, GE; Streng, M .; Daley, AC; Willman, S. Program with Abstracts : 26-27.  
• Butterfield NJ An Early Cambrian Radula (English) // Journal of Paleontology . - Paleontological Society 2008. - Vol. 82 , no. 3 . - P. 543-554 . - DOI : 10.1666 / 07-066.1 . (inaccessible link)
• Han Jian , Conway Morris Simon , Hoyal Cuthill Jennifer F. , Shu Degan. Sclerite-bearing annelids from the lower Cambrian of South China (Eng.) // Scientific Reports. - 2019 .-- 20 March ( vol. 9 , no. 1 ). - ISSN 2045-2322 . - DOI : 10.1038 / s41598-019-40841-x .

• John Timmer. New fossil unites three branches of life in the Cambrian (English) // Ars Technica. - 2007 .-- March 1.
• Fangchen, Zhao, Smith, Martin, Zongjun, Yin, Han, Zang, Maoyan, Zhu. High resolution images of Orthrozanclus elongata (en-us) // figshare. - 2017-11-14. - DOI : 10.6084 / m9.figshare.c.3841387.v1 .