''Xanthomyria'' is a genus of Cambrian

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 Ust-Mayan Formation of

Siberia Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...

. It contains one species, ''Xanthomyria spinosa ''.

Description

''Xanthomyria'' is known from two specimens, exhibiting great variation in size. Neither specimen preserves the head or legs, therefore its affinity is uncertain. However, various structures of the exoskeleton such as tuberculate and spinose tergites with a broad

rachis In biology, a rachis (from the [], "backbone, spine") is a main axis or "shaft". In zoology and microbiology In vertebrates, ''rachis'' can refer to the series of articulated vertebrae, which encase the spinal cord. In this case the ''rachi ...

and narrow

pleurae The pleurae (: pleura) are the two flattened closed sacs filled with pleural fluid, each ensheathing each lung and lining their surrounding tissues, locally appearing as two opposing layers of serous membrane separating the lungs from the media ...

rule out most Cambrian clades of arthropod, and unusually in this regard ''Xanthomyria'' is most similar to Carboniferous

archipolypoda Archipolypoda is an extinct group of millipedes known from fossils in Europe and North America and containing the earliest known land animals. The Archipolypoda was erected by Scudder (1882) but redefined in 2005 with the description of sever ...

ns such as ''

Euphoberia ''Euphoberia'' is an extinct genus of millipede from the Pennsylvanian epoch of the Late Carboniferous, measuring up to in length, that is small in Euphoberiidae which contains species with length about . Fossils have been found in Europe and ...

''. If this affinity is accurate, it would give millipedes an almost 100 million-year long

ghost lineage A ghost lineage is a hypothesized ancestor in a species lineage that has left no fossil evidence, but can still be inferred to exist or have existed because of gaps in the fossil record or genomic evidence. The process of determining a ghost line ...

from the late Cambrian to the late

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

. However, it is more likely that ''Xanthomyria'' instead belongs to a wholly extinct myriapod-like clade of arthropods along with forms like ''

Pseudoiulia ''Pseudoiulia'' is a genus of Cambrian arthropod known from the Chengjiang Biota of Yunnan, China, containing the single species ''P. cambriensis''. It is considered poorly known, but has been somewhat associated with other Chengjiang Biota fauna ...

'', due to true stem-group myriapods which appear very different having been found in the Cambrian, in the form of

Euthycarcinoidea Euthycarcinoidea are an enigmatic group of extinct, possibly amphibious arthropods that ranged from Cambrian to Triassic times. Fossils are known from Europe, North America, Argentina, Australia, and Antarctica. Description The euthycarcinoid bod ...

. The tail has a small flap-like posterior termination. The robust calcareous

tergites A ''tergum'' (Latin for "the back"; : ''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 'margin'. ...

also have very simple articulation surfaces. The trunk is roughly 5.2 cm long and had at least 34 segments (possibly over 60, if the smaller specimen is the tail end of a larger animal instead of being a juvenile), with a slight taper towards the back.

Etymology

''Xanthomyria'' derives from the Greek words ''xanthos'' meaning "yellow", and ''myrios'', meaning "countless" in reference to the yellowish colouration of the fossils alongside the inferred affinity with Myriapoda. The specific name ''spinosa'' derives from the long spines on each tergite.

References

{{Taxonbar, from=Q21368852 Enigmatic arthropod taxa Cambrian arthropods Paleozoic Russia Fossil taxa described in 2001 Fossils of Russia