FINDS OF THE DUNKLEOSTEID PLACODERMS (PISCES, PLACODERMI) IN THE EUROPEAN PART OF RUSSIA (CENTRAL DEVONIAN FIELD)

The Dunkleosteidae Stensiö family includes large predatory Paleozoic armour fishes. Their remains are rare in the Devonian deposits of the European part of Russia (Central Devonian field). The finds of both Eastmanosteus Obruchev and Dunkleosteus Lehman genus representatives and indefinte dunkleosteids were detected in the area. currently there is a collection of some dunkleosteid remains at the Earth Sciences Museum of Moscow State University. The materials were collected in two localities in 2007-2008 years by the author. Plates of the head and trunk shields, preliminarily determined as Eastmanosteus aff. E. pustulosus (Eastman), were found in the sandy clay deposits of the Ardatovka Horizon (Givetian, Middle Devonian) in the Pavlovsk Quarry, Voronezh Region. An incomplete anterior ventro-lateral plate of Dunkleosteidae gen. et sp. indet., similar to the same plates of the Dunkleosteus-species, was discovered in the sandy deposits of the Zadonsk Horizon (Lower Famennian, Upper Devonian) in the Lime Factory Quarry not far from Livny Town, Orel Region. The brief description of the new findings supplementing the data on the Middle-Late Devonian dunkleosteids of the Central Devonian field is given.

Enigmatic encrusting fossils from the Upper Devonian of Russia: probable Rothpletzella microproblematica preserved in three dimensions

Intriguing microfossils encrusting certain Upper Devonian brachiopod shells from the Central Devonian Field, Russia, are reported for the first time. The fossils are pyritized, have fan-shaped morphologies and are formed by tightly-packed branches which divide dichotomously at different points in their development. The organisms preserved grew horizontally on shelly substrates. Comparisons with similar fossils known from the literature indicate that they do not represent any animal taxon commonly encrusting hard substrates. Instead, the morphology, organization and growth mode of these fossils are most similar to microfossils known under the name Rothpletzella, which so far have only been known from thin sections. Rothpletzella is a problematicum for which algal affinities have been proposed. The preserved branches of the fossils described here are too large for cyanobacterial cells. Their large size suggests their placement, along with other described Rothpletzella fossils, within the green algae order Bryopsidales. It is suggested that originally, these organisms possessed thalli encased within a thin, delicate calcified sheath. After burial the thalli underwent pyritization via sulphate reduction mediated by bacterial activity within low pH, dysoxic microenvironment, and their sheath dissolved. As three-dimensionally preserved, these algae provide a new, previously unrecognized, component within the Devonian encrusting communities.

Temporal dynamics of encrusting communities during the Late Devonian: a case study from the Central Devonian Field, Russia

In this study we focused on the dynamics of encrusting assemblages preserved on brachiopod hosts collected from upper Frasnian and lower Famennian deposits of the Central Devonian Field, Russia. Because the encrusted brachiopods come from deposits bracketing the Frasnian/Famennian (F/F) boundary, the results also shed some light on ecological differences in encrusting communities before and after the Frasnian–Famennian (F-F) event. To explore the diversity dynamics of encrusting assemblages, we analyzed more than 1300 brachiopod valves (substrates) from two localities. Taxon accumulation plots and shareholder quorum subsampling (SQS) routines indicated that a reasonably small sample of brachiopod host valves (n=50) is sufficient to capture the majority of the encrusting genera recorded at a given site. The richness of encrusters per substrate declined simultaneously with the number of encrusting taxa in the lower Famennian, accompanied by a decrease in epibiont abundance, with a comparable decrease in mean encrustation intensity (percentage of bioclasts encrusted by one or more epibionts). Epibiont abundance and occupancy roughly mirror each other. Strikingly, few ecological characteristics are correlated with substrate size, possibly reflecting random settlement of larvae. Evenness, which is negatively correlated with substrate size, shows greater within-stage variability among samples than between Frasnian and Famennian intervals and may indicate the instability of early Famennian biocenoses following the faunal turnover. The occurrence distribution of encrusters points to nonrandom associations and exclusions among several encrusting taxa. However, abundance and occupancy of microconchids remained relatively stable throughout the sampled time interval. The notable decline in abundance (~60%) and relatively minor decline in diversity (~30%) suggest jointly that encrusting communities experienced ecological collapse rather than a major mass extinction event. The differences between the upper Frasnian and lower Famennian encrusting assemblages may thus record a turnover associated with the F-F event.