Cryogenian - Ediacaran to Cambrian Stratigraphy and ...

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Cryogenian - Ediacaran to Cambrian Stratigraphy and Paleontology of Guizhou, China. Morphological variability and types of preservation of Mobergella ...
Cryogenian - Ediacaran to Cambrian Stratigraphy and Paleontology of Guizhou, China

The 17 th Field Conference of the Cambrian Stage Subdivision Working Group, International Subcommission on Cambrian Stratigraphy and Celebration of the 30 th Anniversary of the Discovery of the Kaili Biota

Guizhou

2012

Edited by Yuanlong ZHAO, Maoyan ZHU, Jin PENG, Robert R. GAINES, Ronald L. PARSLEY

Journal of Guizhou University Natural Sciences

Cryogenian - Ediacaran to Cambrian Stratigraphy and Paleontology of Guizhou, China

Morphological variability and types of preservation of Mobergella radiolata a potential index species for the GSSP of Cambrian Stage 3 Yulia E. DEMIDENKO, Pavel Yu. PARKHAEV and Alexei Yu. ROZANOV Borissiak Paleontological Institute of the Russian Academy of Sciences

Cambrian Stage 3 of the newly developing strati-

boundary of the stage is difficult to substantiate on the

graphic scheme corresponds to the well - known Atdabanian Stage of the Siberian stage standard, i. e. the

basis of trilobites. Archaeocyath biostratigraphy works perfectly on the Siberian Platform and adjacent areas,

first trilobite bearing unit of the Cambrian system. In

but due to the limited dispersal of archaeocyaths at

various regions the first trilobites are different and ap-

the beginning of Atdabanian (Rozanov, 1992), it

pear at various stratigraphic levels, so the lower

does not provide the desired geographic span. Be-

157-

Journal of Guizhou University ( Natural Science) , Vol. 29 (Supplement I ) cause of that, SSF taxon Mobergella radiolata was

not strictly correspond to any morphological type, and

proposed as potential index species for the GSSP of

can be correlated with habitat peculiarities or simply

Cambrian Stage 3 (Rozanov et al. , 2011). The form

to the age of the animal. Significant variability is ob-

is easily recognizable and abundant at the level under

served in cross - sectional shape of the sclerite: we

the discussion. Due to its relatively wide geographic

found a series of intermediates between convex and

distribution, it has much better correlation potential

concave morphotypes, including combinated convexo

as compared with the first trilobites and locally occurring archaeocyaths.

- concave forms. The state of preservation also affects on the morphology of fossils, i. e. well - pre-

Studies on morphological variability and various

served sclerites bear prominent muscle ridges, where-

states of preservation of phosphate sclerites of Mober-

as the specimens with damaged surface can demon-

gella from the Lower Cambrian of the Siberian Plat-

strate pitted depressions on the places of muscle at-

form has recently resulted in establishment of several

tachments.

species - level taxa (Skovsted, 2003). The following

Thus, we suppose that only single species,

features were claimed as diagnostic: (1) number of

Mobergella radiolata , do exist in the Lower Cambrian

muscle scars on the interior surface of a sclerite; (2) thickness of the various portions of the sclerite; (3)

strata of the Siberian Platform, and the species M. hexactina and M. sibirica should be considered as its

sclerite profile in the cross - section, i. e. flat, con-

Jumor synonyms.

vex, concave, or a combination of these. However, the detailed study of extensive material

References

(hundreds of sclerites) from a number of localities in

Rozanov A Yu, 1992. Some problems concerning the Precam-

the middle and lower reaches of Lena River suggests

brian - Cambrian transition and the Cambrian faunal radi-

that none of these features can be used to distinguish species. All these morphological features occur in different combinations and vary from specimen to specimen, even in the same rock sample. It is impossible to separate the sclerites with different number of mus-

ation / / 1. Geol. Soc. London, 149: 593 - 598. Rozanov A Yu, Parkhaev P Yu, Demidenko Yu E, Skorlotova N A, 2001. Mobergella radiolata - a possible candidate for defining the base of Cambrian Series 2 and Stage 3 / / Bull. Museum Northern Arizona, 67: 304 - 306. Skovsted C, 2003.

Mobergellans (Problematica) from the

cle scars correspond with other features. The thick-

Cambrian of Greenland,

ness of sclerite margin is also very variable and does

Palaontol. Zeitschrift, 77 ( 2) : 429 - 443.

Siberia and Kazakhstan / /

Fig. 1. Various types of preservation and morphological variants of Mobergella radiolata from the base of Atdabanian Stage on Lena Hiver, Siberian Platform (scale bar - 300 mkm) .

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