Biostratigraphy of the Ludlow chitinozoans from East Baltic drill cores; pp. 170–184

Abstract

The distribution of chitinozoans in the East Baltic Ludlow sequences was studied in the Ohesaare, Ventspils, Pavilosta, and Dubovskoye drill core sections. The Angochitina elongata, Eisenackitina lagenomorpha, and Eisenackitina barrandei biozones were described and correlated with the regional stratigraphical units, as well as with conodont and vertebrate biozones in the Ohesaare core.

At what stratigraphical level is the mid Ludfordian (Ludlow, Silurian) positive carbon isotope excursion in the type Ludlow area, Shropshire, England?

“…Even in the Ohesaare core, where the hiatus is of greater duration and the excursion is not recorded, strata both above and below the hiatus are still within the same chitinozoan biozone (Eisenackitina lagenomorpha Biozone), although this is much reduced in thickness by comparison with the other East Baltic cores studied by Nestor (2009). From a conodont biostratigraphical viewpoint, the level of the stratigraphical gap in the Ohesaare core is interesting: it occurs between the Paadla and Kuressaare regional stages (Jeppsson et al 1994, fig.…”

“…For example, Pterochitina perivelata is part of Paris's (in Kříž et al 1986) Přídolí assemblage, but has recently been found by Nestor (2009) throughout much of the Ludfordian of the Pavilosta core, Latvia. Similarly, Ancyrochitina pedavis, stated to be 'restricted to the uppermost Ludlow' by Paris (in Kříž et al 1986), occurs in both the E. lagenomorpha and E. barrandei biozones of the Pavilosta core (Nestor 2009). …”

“…Although used successfully as a biozonal index in southern Sweden (Grahn 1996), in the East Baltic region E. philipi is rare, prompting Nestor (2009) to erect an E. lagenomorpha Biozone for the lower Ludfordian. The FAD of confidently identified E. philipi in the East Baltic region is at the base of, or very low in the E. lagenomorpha Biozone (Nestor 2009), suggesting that the bases of the E. philipi and E. lagenomorpha biozones are at an approximately synchronous level. It appears that the LAD of E. philipi is at a lower stratigraphical level than that of E. lagenomorpha: this is certainly the case on Gotland (Laufeld 1974) and in the East Baltic (Nestor 2009).…”

“…According to Nestor (2009, fig. 4) this level is within the lower part of the Eisenackitina barrandei Biozone.…”

“…The isotope excursion occurs at a level (upper Mituva Formation) at which chitinozoan diversity declined markedly and in the overlying Ventspils Formation chitinozoans are 'scarce'. Nestor (2009) related the diversity decline to the Lau Event and refers to 'intense extinction of chitinozoans'. A major extinction at this stratigraphical level is not apparent from the data from the other cores (Ohesaare, Pavilosta and Dubovskoye) studied by Nestor, however, nor from her summary range chart ( fig.…”