''Retifacies abnormalis'' is an extinct

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

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 Early Cambrian sedimentary deposits in the Chiungchussu Formation or Heilinpu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized orga ...

of Yunnan, China. It is a member of the

Artiopoda Artiopoda is a clade 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 Trilobitomor ...

, and closely related to '' Pygmaclypeatus''.

Description

''Retifacies'' was relatively large sized, reaching a carapace length of , and a total length of , including the extended antennae and tailspine. The head shield was broad and short, and appears to have lacked eyes. The antennae had 17 segments, which telescoped into each other, with spines present on the underside of the antennae at the boundary between the segments. Also present on the head were four closely spaced pairs of appendages, the first three of which were uniramous (single branched), while the last was

biramous The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, ...

(two branched). The three uniramous appendages had six segments (podomeres), each ending with a terminal claw, with the endopod (lower leg-like branch) of the fourth biramous limb having a similar morphology. The trunk was divided up into 10 segments (tergites), all of approximately equal length and width, each of which were associated with pairs of biramous appendages. The endopods of these limbs had 5/6 podomeres with terminal claws, while the exopod (upper limb branch) comprised a semicircular lobe that bore over 12 paddle-shaped lamellae, the last of which bore

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

e (hair-like structures) at its edge. These limbs also bore exites, consisting of lamellae borne from the basal segment (basipod) of the appendage. The pygidium segment at the end of the body was proportionally large, and bore 5-6 pairs of biramous appendages. The segmented tailspine emerged from the underside of the pygidium. The carapace was covered in polygonal reticulated (net-like) ornamentation, which varied between specimens, corresponding to two morphotypes. Due to one morphotype only being found among the largest specimens the differences may be due to

ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the ovum, egg to adult. The term can also be used to refer to t ...

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 the Ancient Greek word (), meaning "the depths". ...

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

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'', 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'' and '' Sidneyia,'' 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 fos ...

in Canada, as part of

, a placement that has been confirmed in subsequent studies. Hou and Bergstrom suggested a close relationship to the genus '' Squamacula.'' However, after a redescription of both ''Retifacies'' and '' Pygmaclypeatus'' from the same deposit in 2022, the latter is considered to be ''Retifacies'' closest known relative, based on the fact that both taxa share a multi-segmented tailspine, among other morphological similarities, with a close relation to ''Squamacula'' being rejected. The clade containing ''Retifacies''+''Pygmaclypeatus'' was either classified in Trilobitomorpha, or in a more basal position within Artiopoda. Implied weights parsimony phylogeny after Berks et al. 2023.

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