Taxonomy

Phylum: Lobopodia

Class: Xenusia

Order：Siberiida

Family: Siberiidae

Genus：Siberion

Specie: S. lenaicus

Description: 

Preservation of the holotype is far from being perfect and it is difficult even to decide,which side of the body it replicates.The belt of whitish (presumably calcium phosphate) matter along the body midline most probably represents the intestine and its location above the cuticle imprint suggests that it is seen from the inside of the body.In its posterior part, the cuticle annuli are represented by narrow impressions separated by wider flat areas, which suggests that there were regularly distributed transverse riblets on the body surface. Anteriorly, however, the picture is opposite-with flat areas separating these riblets in positive relief, although the phosphatic belt continues to run above them.Immediately behind the raptorial appendages, where the algal thallus covers the animal’s body, only one surface is discernible, which suggests that one is dealing with the external surface of the body.In

its anterior part, mostly the dorsum is represented but in the posterior part the venter contributed mainly to the relief.This makes problematic the meaning of the radially arranged probable oral structures and the series of minute oval structures bordering the intestine along its left side but apparently connected with cuticular annuli.It is proposed that the round objects in the head region represent mineralised bases of oral papillae located ventrally but seen from the dorsum across the cuticle. In case of similar but linearly arranged minute objects on the trunk, it is suggested that these are cuticular papillae arming the dorsal surface of the body.The location of the circle of possible oral papilli (or spines) significantly backwards in respect to the anterior end of the body indicates that the mouth was not strictly terminal but directed somewhat ventrally.There are about nine annuli per trunk appendage pair. The spacing of annuli is not strictly regular. Some of them disappear, others insert from the sides.There seems to be an indistinct zone of change from the dorsal dense annulation to a sparser annulation over the ventral belt.The appendages of anteriormost pair are much larger

than the appendages located behind.They are oriented almost transversely to the body axis, with their anterior margin only slightly behind the frontal end of the body.The annulation is not well demarkated by riblets or furrows and seems rather irregular. Near the anterior margin a series of elevations and depressions, more prominent than regular annuli, occur. Their tips are cut by the split rock surface but they are likely to be bases of spinose protrusions and are interpreted as such.The posterior margin of the head appendages is not preserved on either side of the body but on the left side of the holotype its base extends well behind. This makes them about three times as large as the following ‘trunk’ appendages.No appendage is completely preserved, but bases of

most of them are discernible on sides of the body. Tracing the body annulation helps in matching appendages from both sides of the body. There is a rather good evidence for 12 appendages of apparently uniform size.The appendage bases match well in pairs, up to the posteriormost 12th pair, but there is an appendage-like structure even further behind.Its size is similar to that of the nearby appendages but its axis strictly follows the axis of the body and, although this part of the body was exposed and carefully cleaned from the rock matrix with a needle, no sign of any additional structure that could serve as the second appendage of the same pair was encountered.

Age & Localities

Age：Early Cambrian, Botomian, Bergereniellus gurarii or early B. asiaticus trilobite Zone

Principal localities: ‘Algal lens’ on the right bank of the Lena River near the mouth of the Ulukhan·Tuoidakh Creek, Sinsk Formation