''Yunnanocephalus'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of ptychopariid

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

. It lived during the late

Atdabanian Cambrian Stage 3 is the still unnamed third stage of the Cambrian. It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4, although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approx ...

and

Botomian Cambrian Stage 4 is the still unnamed fourth stage of the Cambrian and the upper stage of Cambrian Series 2. It follows Cambrian Stage 3 and lies below the Wuliuan. The lower boundary has not been formally defined by the International Commission on ...

stages, in what are currently Antarctica, Australia and China. It was a "moderately common" member of the

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

. ''Yunnanocephalus'' is the only genus currently assigned to the Yunnanocephalidae family.

Description

''Yunnanocephalus'' is a small (about ) trilobite with an inverted elongated egg-shaped outline. Its headshield (or cephalon) is ovate and twice as wide as long, slightly wider than and not confluent with the articulate middle part of the

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...

(or

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

). The raised central section (or

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior to ...

) is weakly defined, without clear furrows except for one defining the occipital ring, tapering forward, with the front straight and confluent with the eye ridges. The occipital ring is about as wide as the border and the axial rings of the thorax. The distance between the glabella and the frontal border is slightly wider that the border. The visual surface of the eye is rather small (about 20% of the length of the cephalon), with the front about halflength of the cephalon. The natural rupture line (or suture) anterior of the visual surface is parallel to the midline or converges slightly forward and from the back of the eye diverges to the middle of the widely rounded back edges of the cephalon. The thorax has 14 segments, with short triangular spines, the 3rd and 4th slightly wider than the frontal segments, and a small tailshield (or

pygidium The pygidium (plural pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compo ...

) consisting almost fully of the axis and post axial boss, with two vaguely defined axial rings.

Ontogeny

During the

ontogenetic 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 egg to adult. The term can also be used to refer to the st ...

development of ''Y. yunnanensis'' between a late larval phase ( meraspis degree 9, 2½mm) and the adult (or

holaspis ''Holaspis'' is a genus of equatorial African lizards in the family Lacertidae. These lizards are capable of gliding flight for distances of 30 meters (98 feet). Etymology The New Latin word "''Holaspis'' " is derived from the Greek words " as ...

) the facial suture changes from

proparian Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

gonatoparian Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

. Although this development has also been observed in '' Duyunaspis duyunensis'', it is really uncommon. Usually, proparian holaspid trilobites developed from opisthoparian meraspids due to a mechanism called

heterochrony In evolutionary developmental biology, heterochrony is any genetically controlled difference in the timing, rate, or duration of a developmental process in an organism compared to its ancestors or other organisms. This leads to changes in the ...

Distribution

* ''Y. longioccipitalis'' is known from the Lower Cambrian of Antarctica (Atdabanian,

Shackleton Limestone The Shackleton Limestone is a Cambrian limestone formation of the Byrd Group of Antarctica. The age of the formation is established to be Cambrian Stage 3, dated at ranging from 520 to 516 Ma. This period correlates with the End-Botomian mass ext ...

, 81.9° S, 158.7° E). * ''Y. macromelos'' has been found in the Lower Cambrian of Australia (Botomian, Mernmerna Formation, Angorichina station, east side of The Bunkers, 1 km S Balcoracana Creek, South-Australia, 31.2° S, 138.9° E). * ''Y. yunnanensis'' occurs in the Lower Cambrian of China (Atdabanian, Yuanshan or Qiongzhusi Formation, Chengjiang biota, near Haikou, Anning, Jinning, Jingmacun, Kunming, Yunnan, 24.8° N, 102.6° E; and Maotianshan Bed 4, 24.0° N, 102.0° E). ''cited on''

References

External links

Order Pitychopariida at trilobite.info
{{Taxonbar, from=Q5974962 Ellipsocephaloidea Ptychopariida genera Cambrian trilobites Trilobites of Antarctica Cambrian trilobites of Australia Cambrian trilobites of Asia Fossils of China Fossil taxa described in 1936 Cambrian genus extinctions