''Retifacies'' (''Retifacies abnormalis'') is an extinct

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

, that lived in the lower Cambrian (about 518 million years ago). Its fossil remains have been found in the

Maotianshan Shales The Maotianshan Shales are a series of Cambrian, Early Cambrian deposits in the Chiungchussu Formation, famous for their ''Lagerstätte, Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. ...

of Yunnan, China. It is a member of the

Artiopoda The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilo ...

Description

This large animal could reach 55 centimeters in length, even though most of the specimens found were smaller. The cephalic shield was short, and was followed by a dozen

tergites A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'mar ...

that overlapped each other. The rear part of the body consisted of a large shield and a long segmented tail. All the tergites had a surface made up of irregular polygons, unknown among other Maotianshan arthropods. Ventrally there were probably no eyes placed on peduncles, and immediately alongside there were two long antennae with short bristles. Like many primitive arthropods, this animal also had the classic biramous appendages, eighteen pairs in number. Three of these were positioned under the head, ten corresponded to the thoracic wipes and five were present in the caudal shield. The appendages consisted of a branch with ambulatory function (endopodium) with a thorny internal margin, and a branch - gill (exopodium) formed by a fin-shaped structure, consisting of twenty bristles.

Classification

The vaguely similar appearance to that of a trilobite originally led scholars to think that ''Retifacies'' was a possible relative of '' Helmetia'' or ''

Naraoia ''Naraoia'' is a genus of small to average size (about 2-4½ cm long) marine arthropods within the family Naraoiidae, that lived from the early Cambrian to the late Silurian period. The species are characterized by a large alimentary system and s ...

'' , two other primitive arthropods related to trilobites (Delle Cave and Simonetta 1991, Hou and Bergstrom 1997). Subsequently, another study on showed how ''Retifacies'' was related to other basal arthropods, as ''

Emeraldella ''Emeraldella'' is a genus of arthropod known from the Middle Cambrian of North America. The type species ''E. brocki'' was described in 1912 from the Burgess Shale. 21 specimens of ''Emeraldella'' are known from the Greater Phyllopod bed, where ...

'' and ''

Sidneyia ''Sidneyia'' is an extinct arthropod known from fossils found from the Early Cambrian-age Maotianshan Shales to the Mid Cambrian Burgess Shale formation of British Columbia. 144 specimens of ''Sidneyia'' are known from the Greater Phyllopod bed, ...

'' , from the

Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest foss ...

in Canada, as part of

, a placement that has been confirmed in subsequent studies. After a redescription of both taxa. '' Pygmaclypeatus'' from the same deposit is considered to be a sister taxon, based on the fact that both taxa share a multi-segmented tailspine.

Lifestyle

Due to a lack of information about the gut morphology or contents, the reconstruction of the ecology of ''Retifacies'' is speculative. The authors of the 2022 redescription suggested that ''Retifacies'' was a

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

organism (living on the seafloor) that scavenged on the remains of soft-bodied organisms or organic matter.

References

Sources

* Hou, Chen & Lu, 1989. Early Cambrian new arthropods from Chengjiang, Yunnan. Acta Palaeontologica Sinica, 28, 42–57. * Chen & Zhou, 1997. Biology of the Chengjiang fauna. Bulletin of the Natural Museum of Natural Science, 10, 11 – 106. * Hou & Bergstrom, 1997. Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. Fossils and strata, 45, 116 pp. * Edgecombe & Ramskold, 1999b. Relationships of Cambrian Arachnata and the systematic position of Trilobita. Journal of Paleontology, 73, 263 – 287. * Hou, Bergstrom, Wang, Feng & Chen, 1999. The Chengjiang fauna. Exceptionally well-preserved animals from 530 million years ago. 170 pp. Yunnan Science and Technology Press, Kunming, Yunnan Province, China. {{Taxonbar, from=Q3933748 Artiopoda Cambrian genus extinctions