''Urokodia'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of marine

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 ...

from the early

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

. The only known species is ''Urokodia aequalis'' from 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 China based on some 15 specimens.

Description

''Urokodia'' is an elongated arthropod measuring up to 3.5 cm. The dorsal

exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...

(

tergite A ''tergum'' (Latin for "the back"; : ''terga'', associated adjective tergal) is the Anatomical terms of location#Dorsal and ventral, dorsal ('upper') portion of an arthropod segment other than the head. The Anatomical terms of location#Anterior ...

) is composed of a cephalon (head) and a pygidium (tail shield) of subequal size. Both were initially thought to be nearly identical, possessing 4 pairs of large lateral spikes. However, subsequent studies revealed that the pygidium had a distinct triangular shape and a parallel row of tiny posterior spikes. Between the cephalon and pygidium was an elongated thorax of 14 or 15 segments. It possesses a pair of stalked eyes and stubby antennae below the front of its head. The remaining ventral region possesses subequal appendages with annulated bases and lamellar (lobe-like) outer branches, 3 pairs under both cephalon and pygidium and 1 pair under each thoracic segment. All but the pygidial appendages bore segmented, stenopodous (leg-like) inner branches.

Taxonomy

The ventral appendages of ''Urokodia'' had been unknown for a long time, and its dorsal features resemble Mollisoniidae (e.g. ''

Mollisonia ''Mollisonia'' is an extinct genus of Cambrian marine arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), m ...

'', '' Thelxiope'','' and Corcorania'') at first glance. This led to the previous classification of ''Urokodia'' as a member of the order

Mollisoniida Mollisoniida is an extinct order of chelicerates, living from the mid-Cambrian to Early Ordovician. The clade is united by the presence of various characteristics. These include an elongated dorsal exoskeleton, seven articulating tergites and ...

with other mollisoniids. Based on the discovery of ''Mollisonia'' appendages in 2019, the group is suggested to be stem-

chelicerates The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, tic ...

. However, a redescription published in July 2024 discovered the appendages of ''Urokodia'' differed markedly from those of mollisoniids or of any other chelicerates.

Phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analysis based on the new discovery instead places ''Urokodia'' as the most basal member of

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 ...

, an extinct group which includes

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s and their relatives.

References

Urokodia aequalis Arthropod from Maotianshan Shales
- URL retrieved September 11, 2010 Cambrian arthropods Maotianshan shales fossils Cambrian genus extinctions {{paleo-arthropod-stub