Siberiida

Siberiidae is a family of Cambrian lobopodians, sometimes called "jianshanopodians" or "giant lobopodians".

Morphology

Siberiids often reach large sizes, achieving as much as 22 cm, or even over 30 cm in length as seen in ''Jianshanopodia''.

Cephalon

The anterior of the cephalon is blunt, prominent appendages equipped with endites protrude laterally from it. In the case of ''Megadictyon'' the appendages terminate with sclerotized spines. The bottom of the head exposes an oral opening surrounded by an array of layers of plates and teeth which has been compared to the oral cones of radiodonts; the pharynx houses numerous pharyngeal teeth, and has been compared to that of priapulids. No eyes have been observed.

Limbs

Siberiids bear stout, unarthropodized, annulated limbs called lobopods. They bear gill-like papillae protruding dorsally from the series of annuli; These papillae were weakly sclerotized in ''Siberion'' and form branched feather-like structures in ''Jianshanopodia'' in order to greatly increase the surface area for respiration; the lobopods house internal fine canals which branch off into the papillae. Their trunk limbs also lack claws.

Trunk

Numerous uniform annulations ornament the broad trunk of siberiids, however ''Jianshanopodia'' exhibits primitive segmentation in its trunk region comprising at least 9 segments. The dorsal surface on ''Siberion'' is adorned by tw rows of small lateral tubercles corresponding to the annuli.

Caudal area

Siberiids exhibit a differentiated terminal caudal (tail) protrusion. In ''Jianshanopodia'' the caudal end forms a trilobed tail fan comprising the terminal protrusion and a pair of lobes positioned laterally to the median lobe, and pointing posteriorly; in front of the tail fan is a pair of smaller, distinct laterally oriented lobopods.

Internal anatomy

The digestive system is generally equipped with gut diverticulae allowing for more efficient digestion and metabolism; however ''Siberion'' may be an exception to that. Based on ''Jianshanopodia'', the respiratory papillae also house fine canals.

Other members of Siberiidae

''Parvibellus'' was initially described as a lower stem group arthropod; however, a recent study has suggested ''Parvibellus'' to be an early developmental stage of a siberiid based on the features such as a ventral mouth opening surrounded by circlets of tooth plates, and the lack of eyes and claws. It lacks the respiratory papillae generally possessed by siberiids, but that can be explained by its small size which allows it to respirate without any specialized structures.

There are also several more indeterminate siberiids from the Chengjiang Lagerstätte which have not been described yet. One of them bears anterior papillae which are not seen in other siberiids, terminal spines on the frontal appendages, and gut diverticulae.

Phylogeny

In the past siberiids have been considered a transitional group between ' Xenusia' and Radiodonta, but newer studies have recovered the phylogenetic relationships to be more complex and siberiids as basal total-group arthropods near ''Aysheaia'' which too bears differentiated raptorial frontal appendages.

Ecology

The respiratory papillae likely supported an active predatory lifestyle, similar to opabiniids, and radiodonts which too bear specialized respiratory structures, and raptorial appendages. This lifestyle is also supported by the gut diverticulae allowing for more efficient digestion and metabolism present in ''Jianshanopodia'' and ''Megadictyon''. Siberiids have been found in the Chengjiang Biota and the Sinsk Biota.

See also

*Amplectobeluidae
*"Hallucishaniids"
*Aysheaia
* Xenusion
* Hadranax

References

{{Taxonbar, from=Q93437384

Fossil taxa described in 2011
Lobopodia
Xenusia
Cambrian animals
Prehistoric arthropod families