Spicules and microspheres in the heliolitid tabulates from the Silurian strata of Gorny Altai, Siberia
Introduction
Calcareous spicules (sclerites) widely occur in the soft body of modern octocorals (Bayer, 1956). The first report on possible presence of similar structures in Paleozoic chaetetids was published by Gray (1980). He described pseudomorphs of spicular structures in Carboniferous Chaetetes from the Tinant limestones, North Wales (Great Britain). Fossilized soft bodies of polyps in the Paleozoic tabulate coral colonies of Favosites were identified in thin sections by Copper (1985) and they were three-dimensionally preserved in the specimens from the Llandovery Jupiter Formation on Anticosti Island, Canada (Cooper, 1985; Copper and Plusquellec, 1993; Chatterton et al., 2008). Meanwhile Kaźmierczak (1989, 1996) interpreted calcite and pyrite structures within the skeleton of the alveolitid tabulate Squameoalveolites fornicatus (Schlüter) from the Emsian and Eifelian deposits in Poland as pseudomorphs of sclerites (spicules), though the idea later was questioned by Wood et al. (1990).
A discovery of diagenetically unmodified within-corallum skeletal elements of polyps in transverse and longitudinal thin sections of Silurian Heliolites from the Ludlow Upper Douro Formation on the Somerset Island (Canadian Arctic Archipelago) was described by Dixon (1996). Later, these authors described spicules and accompanying elements of the calices of Favosites and Heliolites from the Jupiter Formation (Llandovery), Cape Phillips Formation (Wenlock), and Upper Douro Formation (Ludlow) (Chatterton et al., 2008).
Three types of spicules were identified (Chatterton et al., 2008): (1) point spicules attached to the bases of tentacles; (2) collaret spicules from the upper (collar) part of the polyp; and (3) capstan spicules covering all the polyp body and tentacles. All these three types of spicules were found in the Canadian favositid coralla, but only point spicules were identified in the heliolitid coralla. The shape of tabulate corals spicules resembles spicules of the Silurian probable octocoral Atractosella (Bengston, 1981), as well as spicules of some sponges (Sethmann et al., 2006). Preservation of such internal structural elements of polyps required special environmental conditions. Apparently, the coralla were buried while their polyps were still alive.
(Dixon (2010) also described the fossilized soft bodies of Heliolites from the Barlow Inlet Formation on the Devon Island, the Canadian Arctic Archipelago (based on transverse and longitudinal thin sections). The microspheres represent hollow spheroidal bodies covering dead polyps. These resemble calcispheres widely known from Paleozoic limestones (Wilson, 1975; Dixon, 2010) although in shape and size they resemble also oocytes of modern coral polyps (Shlesinger and Loya, 1991), but such interpretation was found unlikely by Dixon (2010).
Section snippets
Geological setting
Silurian strata are widespread in the Gorny Altai (Fig. 1). They are mostly represented by a rhythmic, often repeated, alternation of terrigenous and carbonate rocks (Sennikov et al., 2008). In sections with dominant terrigenous sedimentation, limestone layers and lenses are frequently found. Bioherms, solitary reefs and reef massifs, along with the adjacent series of terrigenous-carbonate-biogenic (often clastic) formations are common in the studied sections of the Altai .Throughout the
Materials and methods
The heliolitid coralla were collected from the “Generalka-slope” section in 2016 and 2017. Beside tabulate and rugose corals, stromatoporoids, crinoids, brachiopods, trilobites, ostracods, gastropods, and some ichnofossils were found in limestone lenses of the section. Longitudinal thin sections revealed a detritus of calcareous green algae. The tabulate corals assemblage from the “Generalka-slope” section consists of Squameolites junggarensis Lin et Wang, 1981, Propora sp., Heliolites
Systematic paleontology
Class Anthozoa Ehrenberg
Order Heliolitida Frech
Suborder Heliolitina Frech
Superfamily Proporicae Sokolov
Family Plasmoporidae Sardeson
Genus Squameolites Bondarenko, 1963
Type species: Squameolites squamiger Bondarenko, 1963, p. 50, from the Ludlow of Kazakhstan.
Squameolites junggarensis Lin et Wang, 1981
(Fig. 3, Fig. 4)
Holotype: Squameolites junggarensis Lin et Wang, 1981, p. 68, pl. 34, fig. 1.
Diagnosis: Corallites 1.6–2.4 mm (mean 2.0 mm) in diameter, surrounded by tubules with 0.2–0.6 mm in
Morphology of spicules
Spicules are preserved only in the Altai heliolitids from units 5, 6 and 7 of the “Generalka-slope” section. Good preservation of the material enabled observations in both transverse and longitudinal thin sections (Fig. 3). Point spicules are clearly visible in three transverse and seven longitudinal thin sections.
The spicules in the corallites can take up to 2/3 of the area of the tabulae in calices. The size of point spicules in the studied sections is 0.4–0.7 mm along the long axis
Their
Small rounded calcite “bodies” (microspheres)
Besides spicules, small rounded calcite “bodies” (microspheres) were discovered in thin sections of the Silurian Altai tabulate corals (Fig. 5a–f). Such “bodies” are located in different places across the coral colony, including its surface. In some thin sections, clusters of these calcite bodies are located inside the sediment very close to the edge of the colony where a new layer of the coral colony and corallites begin to grow up (Fig. 5f).
A similar picture of the distribution of
Depositional environments
The Silurian Altai deposits, in which the heliolitid Spicules have been identified, represent the backreef basin environments strongly influenced by contribution of fine terrigenous material from the land. The calcareous green algae occurring in the Generalka-slope section of the Kuimov Formation indicate a depth of the sea bottom within the euphotic zone, not more than 150 m, probably between 30 m and 80 m. There are no tempestite or other storm-induced structures. The absence of traces of
Acknowledgments
The study was supported by the Russian Foundation for Basic Research. Authors thank Prof. Jerzy Dzik for reviewing the original text of the paper, which helped to introduce some substantial clarifications to Discussion section of the manuscript, make it more compact and significantly improve English. The authors are grateful to Dr. Kun Liang for all the advice that improved the article and expanded the discussion of important questions.
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