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Laminar shell structure of Antigonambonites planus (Pander, 1830) (Brachiopoda, Billingsellida).

INTRODUCTION

The genus Antigonambonites Opik, 1934 was traditionally referred to the family Gonambonitidae (superfamily Clitambonitoidea), which is characterized by spondylium triplex, i.e. spondylium supported by three septae: one median and two lateral. However, the detailed study of its ventral muscle field revealed that in fact Antigonambonites lacks a true spondylium (Vinn & Rubel 2000). This became a reason for Popov et al. (2001) to refer Antigonambonites to the superfamily Polytoechioidea, family Polytoechiidae. Later their opinion was confirmed by the study of early ontogenetic stages of Antigonambonites Popov et al. (2007) and shared by Benedetto (2009). Finally Antigonambonites was formally assigned to Polytoechiidae in Topper et al. (2013).

The SEM study of the shell structure of clitambonitidines was first performed by Williams (1968), who mentioned that Vellamo, Eremotoechia and Antigonambonites were fibrous but he figured only Vellamo sp. Later he published a figure of the section through the shell of Antigonambonites planus (Pander, 1830) with a thin pseudopuncta noting that it was fibrous, but no regular fibrous mosaic is seen on this picture (Williams 1970, fig. 9). On the contrary, the section through the secondary layer of Vellamo shows standard stacking of fibres (Williams 1968, pl. 18, fig. 6). The fibrous secondary layer with a pseudopuncta has been figured for Estlandia marginata (Pahlen) (Vinn 2001). The stratiform laminar shell structure of Billingsella lindstromi (Linnarsson) from the suborder Billingsellidina of the order Billingsellida is well documented (Williams 1970; Williams & Cusack 2007). Thus in literature there are data on the shell structure of only five species of the entire order Billingsellida. Recent publications suggest that the shells of billingsellidines are laminar and the shells of clitambonitidines are fibrous, based on the data on the microstructure of these five species, one of which was not depicted and the photo of another is controversial. The insufficiency of data on shell fabric of this polyphyletic order comprising superfamilies of uncertain origin is obvious.

The present paper deals with the shell structure of A. planus, which is described here by the splints of one dorsal and two ventral valves from the upper Volkhovian (BII[gamma], Middle Ordovician) of the Leningrad Region, mouth of the Lynna River. The material studied is housed at the Borissiak Paleontological Institute of the Russian Academy of Sciences (PIN RAS), Moscow, coll. No. 4921.

DESCRIPTION OF THE A. PLANUS SHELL MICROSTRUCTURE

The shell fabric of A. planus is usually strongly recrystallized and the original microstructure is preserved rarely and in small areas. The primary layer has not been found but longitudinal rows of vertical fine crystals, possibly altered remains of original capillae, are observed on the umbonal region of one ventral valve (Fig. 1A). The rows are approximately 3 [micro]m high and 0.5-1 [micro]m thick.

The secondary layer of A. planus is typically cross-bladed laminar and pseudopunctate. The tabular laminae are best preserved on the rib crests (Fig. 1B, C). The thinnest laminae are about 250-500 nm thick, which possibly is their original thickness. The laminae are up to 20 [micro]m wide and composed of up to 14 laths. The lath thickness is 1-3 [micro]m, rarely 5-6 [micro]m (Fig. 1D-F). The surfaces of laminae show fine growth marks, which are 250-300 nm thick on the longitudinal sections of laminae and possibly correspond to daily deposition.

The umbonal thickening of the ventral valve is usually strongly recrystallized and only its superficial region has remained laminar, though usually these laminae are variously altered. Figure 1G shows a well-ordered set of laminae located closer to the outer surface of the ventral valve. Each lamina in this set is about 25 [micro]m wide but their superficial preservation is different. The surfaces of laminae that underwent early diagenetic stages are uneven and wrinkled but an area with original laths is preserved in one lamina (Fig. 1H). The thinnest laths are 1 [micro]m; thicker laths were possibly formed by fusion in the course of diagenesis. Very narrow (about 3 [micro]m) laminae (or very wide laths?) were found in the umbonal region of specimen 4921/661 but they show excellent cross-bladed mosaic (Fig. 2C).

Possibly the earliest diagenetic stages are shown in Fig. 1F, where the laths are partly slightly altered by the planes of growth. The possible initial stage of vertical fusion of two neighbouring laminae is displayed in Fig. 1I. In the following course of diagenesis the laminae became vertically and horizontally fused and lost their bladed structure; their surfaces turned variously wrinkled (Fig. 1K, L). These strongly thickened and wide diagenetic laminae compose a considerable part of the shell fabric.

The secondary layer may generally be subdivided into two main sublayers: the about 400 [micro]m thick outer sublayer with undular laminae of the ribs (Fig. 1N) and the about 130 [micro]m thick inner sublayer composed of planar but variously altered elements (Figs 1K, 2A, G). Anteriorly the inner sublayer is stratified into variously altered regions, from strongly thickened laminae to small isoform, irregularly shaped elements (Fig. 2G). It is likely that fig. 9 of Williams (1970) illustrates a similarly altered area of the A. planus secondary shell.

The shell wall near the spondylium is composed of three variously altered sublayers: the 55 [micro]m thick inner sublayer of cubical granules (this sublayer also separates lateral septae from the shell wall) (Fig. 2A), the 12-20 [micro]m thick solid interlayer of diagenetic calcite (Fig. 2B) and the about 400 [micro]m thick upper sublayer of cross-bladed laminae (Fig. 2C).

The diagenetic calcite is the next stage of laminae alteration. It is usually developed in the umbonal region and in the rib interspaces (Fig. 1N-P), while the ribs are composed mainly of original or diagenetic laminae (Fig. 1J).

A possible prismatic tertiary layer was found in two places on the inner surfaces of ventral valves. In the first case the remains of the tertiary layer are better preserved, 20-30 [micro]m thick, and composed of 9-14 [micro]m hight vertical columns, which are densely spaced and sharply bordered from the underlying laminae (Fig. 2D). The possibility that these columns are laminae prolongations as in the case of the tertiary prismatic layer of the fibrous shell is controversial. In the second case the columnar elements possibly became recrystallized and altered into 12 [micro]m high and 4 [micro]m thick vertical crystals (Fig. 2E, F). In both cases these areas of inner valve surface are smooth and lack tubercles; however, these vertical elements could also form in the course of diagenesis.

Other possible remains of the tertiary layer were found on the inner surface of the strongly recrystallized spondylium. Two spondylia studied by longitudinal splits are completely composed of diagenetic calcite (Fig. 1M).

The shell of A. planus is pseudopunctate but in most cases the taleolae were not observed and the pseudopunctae are composed only of deflected laminae (propunctae) (Fig. 2I-K). The cones are 30-50 [micro]m wide and irregularly and sometimes very closely (especially near the anterior margin, Fig. 2H) spaced. In few cases possible thin (5-10 [micro]m) taleolae were present (Fig. 2L). The length of the longest observed pseudopuncta is about 50 [micro]m.

CONCLUSIONS

It has been assumed that Polytoechioidea rather belongs to the suborder Billingsellidina than to Clitambonitidina (Topper et al. 2013). As A. planus is considered now to be polytoechioid, its laminar secondary shell is another evidence of close affinity between Polytoechiidae and laminar Billingsellidae. Unfortunately, it is the only polytoechioid species with documented shell microstructure as Williams (1968) did not picture the sections through the shell of Eremotoechia but only mentioned it was fibrous. Further investigation of the shell structure of brachiopods of the order Billingsellida probably will tribute to the clarification of their relationships.

Acknowledgements. This study was supported by the Arctic Program of the Presidium of Russian Academy of Sciences, project No. 44 'Paleontological Base for Reconstruction of the Early Paleozoic Paleogeography'. The referee O. Vinn and the anonymous referee are thanked for constructive reviews of the manuscript. The publication costs of this article were covered by the Estonian Academy of Sciences.

REFERENCES

Benedetto, J. L. 2009. Chaniella, a new lower Tremadocian (Ordovician) brachiopod from northwestern Argentina and its phylogenetic relationships within basal rhynchonelliforms. Palaontologische Zeitschrift, 83, 393-405.

Pander, C. H. 1830. Beitrage zur Geognosie des Russischen Reiches. St.-Petersburg, xx + 165 pp.

Popov, L. E., Vinn, O. & Nikitina, O. I. 2001. Brachiopods of the redefined family Tritoechiidae from the Ordovician of Kazakhstan and South Urals. Geobios, 32, 131-155.

Popov, L. E., Egerquist, E. & Holmer, L. E. 2007. Earliest ontogeny of Middle Ordovician rhynchonelliform brachiopods (Clitambonitoidea and Polytoechioidea): implications for brachiopod phylogeny. Lethaia,

40, 85-96.

Opik, A. 1934. Uber Klitamboniten. Publications of the Geological Institution of the University of Tartu, 39, 1-240.

Topper, T. P., Harper, D. A. T. & Brock, G. A. 2013. Ancestral billingsellides and the evolution and phylogenetic relationships of early rhynchonelliform brachiopods. Journal of Systematic Palaeontology, 11, 821-833.

Vinn, O. 2001. A new subspecies of the clitambonitidine brachiopod Estlandia catellatus from the Middle Ordovician of Osmussaar Island, Estonia. Proceedings of the Estonian Academy of Sciences, Geology, 50, 86-94.

Vinn, O. & Rubel, M. 2000. The spondylium and related structures in the Clitambonitidine brachiopods. Journal of Paleontology, 74, 439-443.

Williams, A. 1968. Evolution of the shell structure of articulate brachiopods. Special Papers in Palaeontology, 2, 1-55.

Williams, A. 1970. Origin of laminar-shelled articulate brachiopods. Lethaia, 3, 329-342.

Williams, A. & Cusack, M. 2007. Chemicostructural diversity of the brachiopod shell. In Treatise on Invertebrate Paleontology, Pt. H: Brachiopoda Revised. Vol. 6: Supplement (Selden, P. A., ed.), pp. 2396-2521. Boulder, Colorado.

Anna Madison

Borisyak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russia; Sunnyannmad@yahoo.com

Received 5 May 2017, accepted 7 June 2017, available online 16 October 2017

[c] 2017 Author. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/4.0).

Antigonambonites planus'e (Pander, 1830) (Brachiopoda, Billingsellida) laminaarne koja struktuur

Anna Madison

Antigonambonites planus'e koja struktuur on laminaarne, mitte fiibriline, nagu varem arvati. Laminaarne koja struktuur toetab Antigonambonites'e polutehhioidide hulka arvamist. Uuritud Ordoviitsiumi-vanuste kodade struktuuris on naha diageneesist tingitud muutusi. Kasijalgse laminaarne koja struktuur on paremini sailinud roonete kohal. Laminaarse koja struktuuri esinemine A. planus'el kinnitab polutehhioidide ja billingselliidide lahedast sugulust.

doi.org/10.3176/earth.2017.17
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Author:Madison, Anna
Publication:Estonian Journal of Earth Sciences
Article Type:Report
Date:Dec 1, 2017
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