Archaeoniscus () is a genus of prehistoric isopods that first appeared during the Bajocian stage of the Middle Jurassic. It is a widespread genus with a paleogeographic distribution encompassing the continental margin environments of the central Atlantic Ocean and the western Tethys Ocean. Fossils of Archaeoniscus suggest that this genus lived in diverse aquatic habitats, including the marine, paralic, and freshwater environments. While earlier descriptions suggested that it may have had an ectoparasitic association with fishes, some researchers argue that at least two species, A. aranguthyorum and A. coreaensis, lived a benthic free-living lifestyle based on morphological characteristics that are either unsuitable for or unrelated to parasitic behavior.
Six species have been named: two species are known from the Jurassic period rocks in Europe, and the other four are known from the Cretaceous period rocks in Europe, North America and East Asia. Of these species, the taxonomic position of the only Late Cretaceous species, A. texanus, is uncertain, for some researchers claim that this species should be neglected because of the significant morphological differences in comparison to the other species within the genus. Some specimens of the most completely known species, A. aranguthyorum, suggest that the female individuals of this species possibly had brood pouches, indicating that the genus may have had sexual dimorphism. An alternative interpretation suggests that the brood pouches may actually be the impressions of the hindgut and anus. Additionally, two unnamed indeterminate species are known from the Early Cretaceous rocks in Mexico and Egypt.
Discovery
Reverend Peter Bellinger Brodie first discovered the crustacean fossils from the Purbeck Group in the Vale of Wardour, England. He asked Henri Milne-Edwards to examine some specimens and formally describe them. The specimens sent to Milne-Edwards measured in length, though other descriptions indicate their lengths were about . Brodie claimed that he also found larger specimens that are about the size of small trilobites. Milne-Edwards identified that the specimens belong to an order Isopoda, and suggested that it may belong within the range of a family Cymothoidae, possibly an intermediate species between Serolis and Cymothoa. In 1843, Milne-Edwards proposed a genus name Archaeoniscus and a species name A. brodiei in honor of the discoverer Rev. P. B. Brodie. From the same lithostratigraphic group, Charles Willcox collected various insect fossils and sent them to John O. Westwood. Upon his examination, Westwood found several isopod specimens which varied in size, being either equal to or larger than A. brodiei. Westwood considered them to be a new species of Archaeoniscus, so he designated these specimens under a new species name A. edwardsii in honor of Milne-Edwards. While A. brodiei is discovered from the Late Jurassic (Oxfordian-Tithonian) strata in England, France and Germany, A. edwardsii is only discovered from the Early Cretaceous (Berriasian) strata in England.
In 1918, Wilhelm Haack erected a monotypic family Archaeoniscidae and claimed that it is distinct from the suborder Oniscidea based on the large pleotelson, the shapes of antennulae, antennae and uropods. In 1992, Robert W. Wieder and Rodney M. Feldmann examined various isopod fossils from Austin Chalk in Texas, United States. Of these fossils, Wieder and Feldmann considered some specimens that are only known from posterior exuviae to belong to Archaeoniscus based on the raised axial node on the pleotelson which they claim to be distinct from almost every other known isopod species. Although they tentatively referred these specimens under a new species name, A. texanus, they acknowledged that the taxonomic placement of these specimens is difficult; still, they considered this placement prudent enough and better than erecting a new genus name and establishing the taxonomic affinities for the specimens. The stratigraphic age of Austin Chalk is during the Late Cretaceous (Coniacian-Santonian), so A. texanus is the youngest known species of this genus.