Lochkovian and Pragian strata from selected sections in the Spanish Central Pyrenees have provided one of the best Conodont sequences in the world permitting the development and global correlation of hi-resolution bioand chronostratigraphic scales. This scale is better for the Lochkovian, especially for the middle Lochkovian, but Pragian records in combination with records from the Barrandian area have enabled the proposal of an alternative Pragian Conodont biozonation for European sequences. Other fossils are not as abundant and have not received much attention, except the Ostracods from the Gerri de la Sal sections and some Fish remains from the Compte-I section.
Lingulate Brachiopod remains are present in many Conodont samples from Lochkovian and Pragian rocks but commonly they are in low numbers, poorly preserved and with spotty records that difficult a thorough study on this group. However, the section Compte-I presents an excellent record that enables the first systematic study of this group in the Pyrenees and its precise stratigraphical location within a detailed time framework constructed by the accompanying Conodont taxa.
In a paper published in the journal Bulletin of Geosciences on 16 May 2020, Michal Mergl of the Centre of Biology and Geosciences at the University of West Bohemia, and José Valenzuela-Ríos of the Department of Botany and Geology at the University of Valencia, present a systematic study of Lingulate Brachiopods in the Pyrenean section Compte-I, and place them into the fine biostratigraphic Conodont scale for future correlations.
The current knowledge of Lower Devonian micromorphic Lingulate Brachiopods outside Bohemia and Australia is poor and therefore not adequate for correlation and evolutionary suggestion, and any new stratigraphical exactly placed data are of high importance.
The Compte section is located near the former national road N-260 between the localities of La Pobla de Segur The Compte section is located near the former national road N-260 between the localities of La Pobla de Segur in Spanish Pyrenees. All the fossils come from the Lochkovian and lower Pragian strata of the section Compte-I. This section spans the Lochkovian to Emsian, but the betterstudied part is the Lochkovian and early Pragian. Previous studies have described in detail this part of the section and presented the first detailed bioand chronostratigraphic interpretations, and identified the three-fold Lochkovian subdivision (lower, middle and upper) and several zones within these parts. Moreover, the Lochkovian/Pragian interval was narrowed for the first time in the Pyrenean sections. Subsequently, José Valenzuela Ríos and Jau-Chyn Liao presented the first general study on thin section and microfacies to characterise the lithofacies present in the Compte-I section in 2012. A more recent study established a Conodont-based detailed correlation between the Spanish Central Pyrenees and the Prague Synform for middle and upper Lochkovian strata, including the Lochkovian/Pragian transition in the Compte-I section.
The Organophosphatic Brachiopod described by Mergl and Valenzuela-Ríos were obtained as the side product of the research primarily focused on Conodonts. In total, 140 limestone samples were collected and dissolved in diluted formic acid (6–8%) in order to search for conodonts. The insoluble residue was decanted and sieved of 1.5 mm permitted size separation of larger fragments. The finer residue (smaller than 1.5 mm) was dried and Brachiopod shells were hand-picked under the binocular lens.
The first specimen described are assigned to the genus, Barrandeoglossa (due to the fragmentary nature of the specimens, none are assigned to species level). This is a small fragment of dorsal valve which shows convex smooth larval shell. That is posteriorly bordered by a thickened posterior shell margin forming distinct convex strip tapering toward the apex. Other details are unknown.
Although the material is very poor, the thickened posterior margin is a distinct feature of the genus. The fragment may belong to Barrandeoglossa perneri, a common species in the Kotýs Limestone of the Lochkovian age in the Barrandian area of the Czech Republic, but because data about the ventral valve are unknown, an open taxonomical position is preferred.
The next specimens are assigned to the genus Acrosaccus, as Acrosaccus sp. A. These are a hundred dorsal valves and their fragments. Shell moderately thick-walled, 6 mm wide in the largest known fragment. The dorsal valves range from elongate elliptical to gently transversally oval outlines, with some valves nearly circular. Anterior and lateral margins are evenly rounded, the posterior margin is slightly less rounded. Valve convexity is low both in transverse and longitudinal profiles, but the brephic valve is prominently convex and elevated above the adjacent surface of mature shell. The apex is situated in posterior ¼ of the valve. The larval shell subcircular, evenly arched and smooth, 350–400 μm wide. It is bounded by an elevated periphery from the post-larval shell. The brephic shell is gently transverse, about 1.5 mm broad, with transversally oval outline and evenly rounded periphery. Its surface is covered by weak growth fila more distinct around the periphery. Generally smoother surface distinguishes the brephic shell from the mature shell surface. Whole brephic shell with the larval shell attached to its posterior is gently inclined and posterodorsally faced.
The ornament of the dorsal valve consists of entire or imperfect concentric rugellae. New imperfect rugellae originate lateral to the apex, at about one-fourth of shell length. Some rugellae are short, and rapidly change into low, rope-like fila on surface of the interspace. Entire rugellae rapidly increase in size with shell growth, with new rugella always somewhat higher than the preceding one. Interspaces vary in width, having a flat bottom or bearing low, rope-like concentric fila. Rugellae are lamellose, resting almost perpendicularly to shell surface, with weakly thickened bases and gently thicken crests. Rugellae between the apex and the posterior margin are lower but the interspaces have coarser growth fila. Surface of interspaces and slopes of rugellae are covered by oblique lines. Microornament consists of densely crowded uniformly sized subcircular pits of 3 μm diameter in regular honeycomb-like arrangement.
Interior of the dorsal valve bears large, obliquely elongate and clearly impressed central muscle scars inclined 50° each another and separated each another by a low median ridge. Visceral area is elongate rhomboidal in outline, 30% as wide as the valve and anteriorly extends to midvalve. Its posterior part is divided by a thin myophragm. Anterior and lateral margins bear distinct imprints of radially arranged terminal canals of the vascular system. Some of canals extend from gently divergent narrow canals of vascula media, which extend from anterior corners of central muscle scars. Small concave epithelian cell moulds cover the visceral field and adjacent parts of he interior. Ventral valve is unknown.
The fragments are similar to some Acrosaccus-like discinids with prominent concentric rugellae. However, lack of information about the ventral valve makes generic affiliation of Spanish specimens tentative. Several similar discinids of the Acrosaccus shape of the Lower Devonian age are known from the Barrandian area of the Czech Republic but none of them has surface of interspaces and slopes of rugellae covered by oblique fila as exhibits the Acrosaccus sp. from Pyrenees. Likewise, none of fragmental ventral valves from our sample from Compte-I section show this distinct microornament. Among the European species, Acrosaccus sp. from the Suchomasty Limestone (Emsian) is most similar, but lacks the above mentioned oblique fila and its rugellae are much robust and much closely spaced. A similarly shaped Acrosaccus vertex was described from the Chýnice Limestone (upper Emsian), but the lamellose rugellae of Acrosaccus sp. are distinctly higher and more closely crowded. Acrosaccus sp. from the Acanthopyge Limestone (Eifelian) of the Barrandian area has similar lamellose rugellae in the dorsal valve, but its rugellae are lower than rugellae of Acrosaccus sp. Similar species ?Acrosaccus sp. has been described from several limestone members of the Lower Devonian age of New South Wales, Australia, but none of illustrated specimens has as high rugellae as Acrosaccus sp. from Pyrenees. Unfortunately, the fine details is Australian shells are unknown.
The ventral valve of Acrosaccus sp. is unknown from our samples, while the dorsal valves are numerous and fairly well preserved. This disproportion may be explained by disintegration of thinner and much fragile ventral valve, likely having low conical shape. Similar disproportion in number of dorsal and ventral valves has frequently been observed in samples in the Barrandian area. Similar variations of the shell outline observed in Acrosaccus sp. are known in other Lower Palaeozoic Discinoids (e.g. Orbiculoidea sp.). It is likely related to biotope of a particular specimen. It is worth a mention that the outline of brephic shell is constantly circular despite differently outlined postlarval shells. Pyrenean specimens surely belong to a new species but shortage of important piece of regarding the ventral valve led Mergl and Valenzuela-Ríos to prefer an open taxonomic position.
This species is the commonest Lingulate Brachiopod in samples of the Compte-I section.
The next group of specimens are also referred to the genus Acrosaccus, as Acrosaccus sp. B. This material comprises ten fragmentary ventral and dorsal valves.
The available shells are imperfect and therefore the species is poorly known. Fragments indicate that shell is thin-walled and subcircular to slightly elongate oval in outline. Dorsal valve has prominent, moderate convex subcircular brephic shell which is strongly tilted posterodorsally above a steep posterior slope of valve. Size of the brephic valve is around 500 μm. The mature shell bears fine thread-like concentric growth fila.
The ventral valve is low conical, with distinct brephic valve which slightly overhang the earliest part of the pedicle track. Pedicle track is narrowly triangular, with posteriorly moderate expanding listrial plates and narrow evenly broad inner listrial plate. Length of the pedicle track is about 1.5 mm. It is posteriorly closed by mature shell bearing the same external ornament as adjacent posterolateral parts of the valve. The mature ventral valve is covered by concentric growth fila gradually increasing in a size toward shell periphery. The early lines encircling the apex are thread like and very fine, but fila from 2 mm sized shell are much higher, convex, with broad bases separated by unevenly sized interspaces.
Attribution of ventral valves to dorsal valves is somewhat tentative, but it is based on similar type of ornament and biconvex shells profile similar to some species of the genus. Acrosaccus sp. A differs by distinct type of microornament and by coarse rugellae. This species is present in samples CP-I/34, CP-I/40A, CP-I/46B, CP-I/51B, CP-I/56, CP-IW/52C of the Compte-I section.
The next specimens are referred to the genus Chynithele as Chynithele aff. fritschi. This material comprises two dorsal valves.
The dorsal valve is subcircular, about 2 mm wide, gently concave, thick-walled, rectimarginate. The apex is situated at posterior one-fifth of the valve. The larval shell and brephic shell are not clearly differentiated. The brephic shell is weakly convex, about 280 μm wide, encircled by very fine entire concentric rugellae of the early mature shell.
Ornamentation consists of mostly entire rugellae, with rare inserted imperfect rugellae which rapidly attain the same size of nearby entire rugellae. Sizes of rugellae progressively increase with shell growth. Rugellae are gently inclined forward. The crest of each rugella is broader than the base of rugella and form a short shelf that slightly overhang the anterior slope of the rugella. Interspaces have flat or shallow concave bottom and are slightly wider than crests of adjacent rugellae. Microornament consists of small circular pits of 3 μm diameter closely spaced. Pits covers entire surface of the mature shell.
Both specimens show distinct features of the genus: a weakly concave dorsal valve, generally entire, prominent and gradually larger rugellae with overhanging crests, and a microornament of fine circular pits. The genus Chynithele is represented by several successive species (Chynithele fritschi, Chynithele ventricona, Chynithele intermedia, Chynithele amoena) in the Barrandian area. These are known from the Emsian to Eifelian. Apart from the stratigraphically earliest species, others species differs from the Spanish specimen by generally lower much robust rugellae and much transverse outline of dorsal valves. The species Chynithele fritschi from the lower Emsian (Zlíchov Formation) was erected on poorly preserved dorsal valve showing only internal characters. This species comes from the upper part of the Zlíchov Formation (lower Emsian) and its relationship to approximately coeval Chynithele ventricona requires a detail revision. Undescribed species of Chynithele that differs from the Emsian and Eifelian species of the Barrandian area is also known from the Kotýs Limestone (Lochkovian) of the Barrandian: this to date unnamed species has similar shaped concentric rugellae as the Spanish species.
The next specimen, one fragment of ventral valve, is assigned to the genus Lochkothele, as Lochkothele cf. intermedia.
The apical part of the ventral valve belongs, judging from growth lines, to subcircular, gently elongate thin walled shell. Larval shell is slightly transversely oval, about 500 μm wide, with poorly defined border. The pedicle track is deeply incised, 0.8 mm long, evenly wide along its whole length. The track is posteriorly terminated by reversely V-shaped incision. The floor of the pedicle track is undifferentiated, evenly concave and covered by growth lines. Exterior of mature ventral valve is covered by growth fila accentuated in regular intervals into low rugellae. The first rugella appears 0.7 mm posteriorly from the apex and other rugellae are next arranged in regular distances 0.2 mm apart. Microornament is unknown in the specimen from Compte-I section.
The earliest Lochkothele is known from the Wenlock (Homerian). Several, usually rare occurrences of Lochkothele come from the Lochkovian, the Emsian and the Eifelian. All known species of the genus have distinct semitubular undifferentiated pedicle tracks, weakly ornamented ventral valves with closely packed concentric growth fila, and thin and low thread-like rugellae. The single available fragment may belong to Lochkothele intermedia, the common species of Lochkovian age in the Barrandian area.
The next specimen, a fragment of dorsal valve, is assigned to the genus Schizotreta, as Schizotreta sp. A.
Single available dorsal valve is elongate elliptical with the marginal apex and belongs to individual approximately 3 mm long judging from shaping of rugellae. The shell wall is moderately thin. The valve is weakly convex in transversely profile and gently depressed in the median sector. The larval shell is distinct, smooth, subcircular, 300 μm wide, convex and gently inclined towards the posterior margin. The postlarval shell is covered by rod-like concentric rugellae with new ones originating in a posterolateral sector by intercalation. Both concentric and new rugellae are uniformly sized, arranged in regular intervals 120–150 μm apart along the axis of valve. Rugellae are gently inclined outwards, with anterior slope shortly undercut. Broad and flat floor of interspaces bears growth fila, which are distinct posterolaterally and suppressed anteromedially.
The valve is similar to dorsal valves of Schizotreta vaneki from the Chýnice Limestone of the Barrandian area. However, this species is significantly younger (upper Emsian) and the ventral valve is unknown in the Compte-I locality. Moreover, the Spanish specimen differs by more elongate outline which has weak taxonomic significance and may be exaggerated by a tectonic deformation. Therefore Mergl and Valenzuela-Ríos prefer an open taxonomical position.
The next group of specimens, thirty-one fragments of dorsal valves, are also assigned to the genus Schizotreta, as Schizotreta sp. B.
The dorsal valve is elongate oval, 2 mm wide, with maximum width at posterior one-third. Anterior margin is semicircular, sides are less rounded and the posterior margin is only gently curved, with nearly straight part posterior to the brephic valve. The shell convexity is very low and decreases anteriorly. The apex is situated to one-fourth to one fifth of the valve, separated from the posterior margin by narrow band of steeply sloping shell bearing some rugellae. Larval and brephic shells are poorly known due preservation. The mature shell is covered by distinct but low, rope-like concentric rugellae, which are uniformly sized. Size of particular rugella gently changes with its location on valve; posterolateral part of rugella is higher and more distinct than its anteromedian partitions. Interspaces are broader than rugellae, with flat floor and surface covered by uneven growth fila. Interior and ventral valve are unknown.
The valve differs from Schizotreta sp. A from the Pyrenees and from Schizotreta vaneki from the Chýnice Limestone (upper Emsian) of the Barrandian area by much densely crowded concentric rugellae.
The next specimen, an incomplete dorsal valve, is referred to the genus Praeoehlertella, as ?Praeoehlertella sp.
The dorsal valve is poorly known, but from the development of growth lines may be deduced that shell is very broadly oval to almost circular, with evenly curved margins including the posterior margin. The apex is situated at posterior one-third, with low posterior slope bearing gently coarser ornament than developed on the rest of the valve. The brephic shell is about 300 μm wide, gently vaulted, smooth and slightly tilted posterodorsally. The surface of post-larval shell bears densely crowded, low and broad concentric growth fila separated by narrow slit-like interspaces. One distinct largely extending growth lamella is present. Microornament consists of minute circular pits (about 1 μm in diameter) densely covering entire surface excluding the brephic shell.
Mergl and Valenzuela-Ríos's material is poor and therefore is referred to Praeohlertella with uncertainty. Outline, convexity, location of the apex on the dorsal valve, and type of ornamentation are close to Praeohlertella lukesi from the Suchomasty Limestone (upper Emsian) of the Barrandian area.
The final group of specimens, 44 dorsal and 11 ventral valves, are referred to the Acrotretid genus Havlicekion as Havlicekion cf. holynensis.
The shell of these specimens is fairly large for the genus with the ventral valve 1.5 mm tall having a broadly unisulcate commissure. The dorsal valve is subcircular, with evenly rounded posterior margin and weak and broad sulcus. Larval shell has two distinct nodes. The larval shell is 200 μm wide, slightly overhanging the posterior edge of post-larval shell. Dorsal pseudointerarea is anacline, with straight anterior edge and very shallow widely triangular median groove. Median septum is high, thin, having steeply sloping anterior edge towards a weakly defined marginal brim. It occupies 80% of valve length. The surmounting plate is inclined at about 45° toward shell floor, the lower rod is at about 35°. The surmounting plate anterior tip is at the midvalve, the tip of the lower rod at two-third of the valve. The crest of surmounting plate is rounded, slightly flattened near its anterior end. The end is blunt. The proximal part of the lower rod is less distinct, but its distal part is distinct but less robust than the surmounting plate. Anterior end of the lower rod deviates to right or left side from the plane of symmetry. Cardinal muscle scars are large, elongate oval, 30% as long as the valve.
The ventral valve is tall cone, with clearly flattened and fairly broad catacline ventral pseudointerarea. The larval shell is asymmetrically conical with slightly extended part near the foramen. The larval shell is pierced by large circular ventrally directed foramen just above steep posterior slope. Lateral and anterior slopes of the valve are gently convex in lateral profile having gradually changing slope from moderate near beak to steeper in later growth stages.
Ornament of the dorsal valve consists of low concentric rugellae of uniform size separated by narrow interspaces. The concentric ornament is more distinct on dorsal valve. The ventral valve is weakly rugellate having rugellae crossed by radial striae. The width of rugellae is 8 to 10 μm. This combination forms a drapery-like relief, which obscure the rugellate ornament. Larval shell is covered by uniformly sized flat-bottomed circular pits of 5 μm diameter. Pits are regularly arranged, closely spaced and only rarely intersecting.
The specimens are very similar to Havlicekion holynensis from the Kotýs Limestone of the Lochkovian age of the Barrandian area. There is only one difference on dorsal median septum. Distance between the surmounting plate and lower rod is smaller in Havlicekion holynensis and a blade-like part of septum between them is distinctly shorter than those in Havlicekion cf. holynensis from Spain. The surmounting plate on Havlicekion holynensis may be weakly concave in lateral profile and is triangular in a cross-section. but this is straight and rod-like in Havlicekion cf. holynensis . Current knowledge of phenotypic plasticity of the shell interior of Biernatiids is not satisfactory for evaluation of the taxonomical value of these minor differences. Therefore Mergl and Valenzuela-Ríos are aware using observed minor differences between specimens for definition of separate species.
Our current state of knowledge of the Silurian and Devonian organophosphatic micromorphic Brachiopods extracted from carbonates is rather weak. Many issues of their palaeogeography, palaeoecology, evolutionary history and some aspects of their shell morphology remain to be solved. This deficiency contrasts with extent of information about morphology and evolutionary history of the group based on samples from early Cambrian to latest Ordovician; mostly extensive or summarizing works have been published in last decades.
This difference is not caused by sampling methods, because organophosphatic microbrachiopods can be easily extracted by weak acid solution from limestone of any age. However, unlike Cambrian and Ordovician rocks, abundance, diversity and disparity of organophosphatic Microbrachiopods are markedly lower in the Silurian and Devonian carbonates. Therefore, due to their rarity and unattractiveness, few works are focused on Devonian Microbrachiopods. To date, the reliable data referred to them came from two distant areas: New South Wales, Australia and the Barrandian area of the Czech Republic. Scattered data about the ubiquitous Biernatid Opsiconidion also comes from other areas with Devonian rocks (Arctic Canada, Novaya Zemlja, Mongolia, Podolia, Ukraine, Carnic Alps). Research on these Devonian Microbrachiopods has often been focused on shell morphology and ultrastructure of larval shell.
Regarding current knowledge of Devonian Microbrachiopods, the new Pyrenean record of Microbrachiopods from lower Lochkovian to early Pragian represent important contribution to elucidate the Lower Devonian Microbrachiopod history. Three taxa ascribed as Acrosaccus sp. A, Schizotreta sp. B and Havlicekion cf. holynensis range across the Lochkovian and reach the stratigraphic level very close to the Lochkovian–Pragian boundary. Barrandeoglossa sp., Schizotreta sp. A, ?Praeoehlertella sp., and Lochkothele cf. intermedia are restricted to middle Lochkovian in the section Compte-I , and only Acrosaccus sp. B crosses the Lochkovian–Pragian boundary. The Pyrenean records of Chynithele aff. fritschi begin close to the base of the Pragian in Compte-I section. Although fragmentation and rarity of shells hamper accurate taxonomic evaluation, a general composition of lingulate fauna indicates uniformity of Microbrachiopods in Early to Middle Devonian tropical marine fauna.
The Microbrachiopod assemblage from Spanish Central Pyrenees is similar to ones known from the Lochkovian and the Pragian of the Barrandian area, the Czech Republic. Dominance of small Discinoids, a Glossellid Barrandeoglossa and a Biernatiid Havlicekion cf. holynensis indicates a moderately shallow environment, similar to depositional environment of the Kotýs Limestone (Lochkov Formation, Lochkov) in the Barrandian area. This agrees with previous palaeoenvironmental interpretations. Absence of a Lingulate Paterula and presumable absence of Biernatiid Opsiconidion (although cannot be excluded, that small fragments of Biernatiids from Pyrenees belong to this genus) indicate a limited influx of deeper and/or hypoxic environment in Pyrenees. Large thick-shelled Lingulates are totally absent in our samples from Pyrenees. In general, the Spanish Pyrenees assemblage is remarkably similar to the ones from Lower Devonian, Barrandian area and to ones reported from New South Wales, Australia. The Spanish Pyrenean fauna likely represents one of so far rare examples of the 'standard' Microbrachiopod association of shallow to moderately deep environment of Lower Devonian tropical climatic zone.
All three Devonian occurrences of Microbrachiopods (New South Wales, Central Bohemia, and newly the Spanish Pyrenees) are typical by dominance of very small sized ('dwarf') shells. All areas were located within tropical climatic zone with intensive carbonate production on reefs and carbonate platforms in the Early Devonian. Small size of organophosphatic Brachiopods in these areas greatly contrasts with many times larger shells of the organophosphatic Brachiopods that populated siliciclastic shelves, mostly but not exclusively present in high-latitude sites during the Early Devonian. More or less well known medium- to large-sized Discinoids are known from Argentina, Bolivia, Uruguay, Paraguay and Brazil, Antarctica, Falkland Islands, South Africa, and Libya and Algeria. Similarly large Discinoids are known from siliciclastics of the Rhenish Massif, Germany and New York, U.S.A.. Some represents the largest organophosphatic Brachiopods so far known with shell size exceeding 60 mm width. Indeed, their shell size indicates that phosphorus was easily accessible in surrounding water.
Small size of organophosphatic Brachiopods from limestone was likely consequence of nutrient scarcity, especially phosphate depleted waters at ancient reefs. On the Recent reef complexes nutrient-poor (oligotrophic) waters dominate. That is in striking contrast with enormous biotic diversity at the Recent reefs. The similar circumstances could be suggested for the Devonian reefs. The dissolved inorganic phosphorus in form of phosphate is particularly important for reef organisms in the Recent reefs. Reef waters have about only 20% concentration of phosphate in comparison with concentration of dissolved phosphate in deep waters. In areas with upwelling waters the input of phosphate may be significantly higher. Run-off of nutrient rich water from land is another significant input of phosphate into nearby sea.
Run-off of phosphate from land to shallow siliciclastic shelves was likely the significant trigger for production of large shelled organophosphatic Brachiopods in the Devonian. This could be the case of large-sized Discinoid populations on high latitude cratonic basins on Gondwana or low latitude shelves with significant input of siliciclastics. Recent large Discinoid Brachiopods occur in shallow littoral zone near upwelling sites (west coast of Africa and west coast of South America) while their tropical allies are small sized, rare and often occupying a cryptic habitat. Similarly, the analogous depletion of phosphate in waters in the Devonian reef contributed miniaturization and retreat of organophosphatic brachiopods from this environment in mid-Palaeozoic.
The Microbrachiopod assemblage from Spanish Central Pyrenees is similar to ones known from the Lochkovian and the Pragian of the Barrandian area, the Czech Republic. Hence the Spanish Pyrenean fauna likely represents one of so far rare examples of the 'standard' Microbrachiopod association of shallow to moderately deep environment of Lower Devonian tropical climatic zone. Small size of organophosphatic Brachiopods from limestone was likely consequence of nutrient scarcity, especially phosphate depleted waters around the ancient reefs. This also contributed to retreat of organophosphatic Brachiopods from this environment during the mid-Palaeozoic.
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