RESEARCH ARTICLE
Received 4 March 2024
Accepted 25 March 2024
Available online 23 October 2024
Keywords:
Craniiformean brachiopods, Gasconsia, Holorhynchus, Katian, East Baltic
ABSTRACT
In this study, the first trimerellide brachiopods from Estonia are described. They occur in the uppermost Katian (Upper Ordovician) shallow shelf environments and represent some of the largest Ordovician brachiopods. The trimerellides described are assigned to the genus Gasconsia Northrop, 1939, which disappears before the Hirnantian and reappears in the lowermost Wenlock after a long interregnum. In the Baltic Basin, the new data from Estonia expand the distribution area of Gasconsia from the Scandinavian facies belt in the west to the Estonian shallow shelf in the east. Despite poor preservation, the specimens studied revealed some structures in shell morphology important for species level identification. The stratigraphic position of Gasconsia is analyzed relative to the Katian-Hirnantian (Pirgu-Porkuni regional stages) boundary and the occurrence of the late Katian brachiopod Holorhynchus in Estonia. The improved data on the distribution of Gasconsia in time and space and a short discussion on brachiopod gigantism are presented.
Artiklis kirjeldatakse seltsi Trimerellida (klass Craniata) brahhiopoodide esmaleide, mis kuuluvad perekonda Gasconsia ja esindavad ии! ШК! Gasconsia gigantea. Need Vohilaiu saare paljandist leitud brahhiopoodid torkavad silma erakordselt suurte médtmetega (koja laius Кип! 120 mm), mis úletavad valdava osa ordoviitsiumi brahhiopoodide môôtmeid. С. gigantea esineb úlemordoviitsiumi Pirgu lademe Adila kihistu kôige úlemises osas, mis móningate paleontoloogiliste andmete pôhjal vôiks esindada ka úleminekulisi kihte ordoviitsiumi kôige Ülemisse Porkuni lademesse. Kirjeldatud ШК on sarnane Norras kirjeldatud liigiga С. worsleyi, mis esineb seal peaaegu samal stratigraafilisel tasemel kui Eesti ШК. Vôimalik, et С. gigantea nagu ka Pirgu lademe úlemise osa brahhiopoodid perekonnast Holorhynchus esindavad Pirgu lademe noorimat madalveelisele keskkonnale iseloomulikku kooslust. Samast Vohilaiu paljandist on párit mitu gastropood Maclurites neritoides leidu, mida Rootsi teadlased peavad rannaláhedase keskkonna indikaatoriks.
Perekond Gasconsia evolutsiooni üks епрага on esindajate puudumine alates ordoviitsiumi бриз toimunud massilise váljasuremise perioodist jaátumisega kaasnenud temperatuurilanguse ja keskkonnatingimuste muutuste tagajárjel. Gasconsia ilmus uuesti válja peale pikka perioodi siluri teisel poolel. Sellise levikuga taksoneid, mis taasilmuvad peale lühemat vói pikemat perioodi, nimetatakse Lazaruse taksoniteks.
Trimerelliidsete brahhiopoodide hulgas on mitmeid taksoneid, mis on môôtmete poolest erakordselt suured. lidsete eluvormide gigantism on oluline ka keskkonna mójutegurite (temperatuur, vee keemiline koostis, hapnikusisaldus jm) hindamisel.
Introduction
The trimerellides of the subphylum Craniiformea (Williams et al. 1996) form a separate evolutionary lineage of inarticulate brachiopods characterized by the early origination of articulation (Gorjansky and Popov 1986). In view of the stratigraphic range from the Sandbian (Upper Ordovician) to the Ludlow (Silurian), these brachiopods are known to inhabit warm water environments on both sides of the paleoequator (Torsvik and Cocks 2017; Chen et al. 2023). Trimerellides form a minor part in the Ordovician benthic faunas on different paleocontinents from Laurentia to Gondwana (Rong et al. 2017; Chen et al. 2023). The aragonitic shells of trimerellide brachiopods (Jaanusson 1966; Hanken and Harper 1985) are usually poorly preserved, which complicates the understanding of their morphology and functional mechanisms. Most investigations on trimerellides concentrate on systematic paleontology (see references in Popov and Holmer 2000; Chen et al. 2023).
A recent overview by Chen et al. (2023) first subdivided the Ordovician and Silurian trimerellide brachiopods into four groups (A-D) based on the shell outline, varying from elongate with a high ventral pseudointerarea (group A) to a transversely oval shell with a low ventral pseudointerarea (group D). A second subdivision of trimerellides by Chen et al. (2023) into three types (I-III) is based on the features of shell interiors, particularly the expression of platforms and vaults (weak, moderate, high). These types characterize the taxa, which could belong to different groups based on the shell outline. The species diversity of Katian (Ordovician) trimerellides in South China (Rong et al. 2017) and Australia (Webby and Percival 1983) during the Middle Ordovician is replaced in the Silurian by higher diversity in Laurentia and Baltica (Chen et al. 2023).
The first appearance of trimerellide brachiopods in Baltica is associated with the second wave of faunal immigration from Kazakhstan during the mid-Katian. Before that, trimerellides appeared in the Sandbian in Gondwana and Laurentia (Popov et al. 1997). Some Silurian trimerellides (Eodinobolus and Monomerella) and their Ordovician relatives (Eodinobolus) have been recorded in Estonia (Popov and Holmer 2000). The list of Baltic Silurian brachiopods (Rubel et al. 1984) comprises the species Trimerella cf. lindstroemi and Dinobolus cf. davidsoni without data on exact stratigraphic occurrences. These latter species are presumably represented by specimens (TUG 1091-62 and ELM 622:93:1, 622:93:2 from the Estonian geocollections database) from the Silurian of Gotland. À few specimens of Dinobolus (GIT 5061347, TUG 39-644 from the Estonian geocollections database) are known from the Jaani Regional Stage (RS; Wenlock) on the Saaremaa Island (western Estonia). The occurrence of trimerellides has also been mentioned in the Slite Group (mid-Wenlock) in Gotland (Sweden), forming reef-like structures there (Budd et al. 2011). The trimerellide brachiopod Gasconsia worsleyi Hanken and Harper, 1985 occurs in the Bonsnes Formation (Fm; upper Katian) in Norway (Owen et al. 1990), just below the Hirnantian strata. In Estonia, the new finds of Gasconsia occur on a similar stratigraphic level, in the uppermost Katian (Pirgu RS). The Estonian data expand the distribution area of trimerellides in the Baltic Basin from the Scandinavian facies belt in the west (Hanken and Harper 1985) to the Estonian shelf in the east. The trimerellides described in this paper are assigned to the genus Gasconsia based on many fundamental characters, including the shell shape, articulation structure, and a muscle system with an impressed, solid platform.
The aim of this study is to explain the taxonomic affiliation of the Ordovician trimerellides first identified in Estonia, show their relationship with the Norwegian species, and highlight their stratigraphic occurrence relative to the boundary between the Pirgu and Porkuni RSs (Katian and Hirnantian), and age relationship with the brachiopod Holorhynchus. The latter genus is a nominal taxon of the latest Katian brachiopod fauna (Brenchley and Cocks 1982; Brenchley et al. 1997, Rong and Harper 1988, Hints 1993; Paskevicius 2000; Shiino et al. 2014), which is widely distributed on different paleocontinents (Rong and Boucot 1998; Rong et al. 2004).
Material and notes on associated fossils
The five specimens of large trimerellide brachiopods described here were collected in 2022 by Ursula Toom and Bjorn Kroger from a small coastal outcrop in the eastern part of the Vohilaid Island (locality Vohilaid 8 by Stein 1937; Fig. 1). The small island of Vohilaid lies about 500 m east of the larger island of Hitumaa in northwestern Estonia. Small old quarries and coastal outcrops on Vohilaid, up to about 1 m in total thickness, expose Katian-Hirnantian boundary strata (the Pirgu-Porkuni transitional beds; Stein 1937). In the northernmost part of the island, the carbonate grainstone presumably belongs, at least partly, to the topmost Ordovician Porkuni RS, lower Hirnantian. The skeletal limestone in the easternmost locality (Stein 1937, locality 8 in fig. 129; Fig. 2) belongs to the Adila Fm (upper Katian). This formation comprises a rich association of various fossil groups, including corals, bryozoans, cephalopods, gastropods, and others, many of which are considered index fossils for the Pirgu RS (Kaljo et al. 1963; Männil 1990). In addition, the Adila Fm is striking for the presence of numerous species of corals and stromatoporoids, such as Proheliolites dubius, Sarcinula luhai, Catenipora parallela, and Clathrodictyon microundulatum, shared with the Norwegian pre-Hirnantian unit Sa, previously known as the Gastropod Limestone (Kaljo et al. 1963; Owen et al. 1990; Liang et al. 2013). In the Vohilaid section, Gasconsia is associated with Maclurites neritoides (specimens TUG 1723-1-7, identified by Mare Isakar), the index gastropod for the Adila Fm of the Pirgu RS (Isakar 1997). In Baltica, the warm-water Maclurites has been considered to belong to the shallow-water Benthic Assemblage 1-2 (Boucot 1975; Ebbestad et al. 2013), which could also be extrapolated to the youngest Katian deposits bearing Gasconsia.
The brachiopods described in this paper were collected from the upper part of the coastal outcrop section on the Vohilaid Island (Fig. 1). In this section, the Pirgu RS 1s represented by the strata commonly included in the Adila Fm (Fig. 2). However, Stein (1937) suggested the occurrence of transitional strata across the Katian and Hirnantian (Pirgu and Porkuni RSs) boundary.
All five specimens of Gasconsia studied here are represented by molds of convex dorsal valves of about the same size, showing some traces of partly preserved interior and exterior structures of shells. Only one ventral valve is partly preserved behind the dorsal valve. The measurements of shells and structures are, in some cases, approximate due to poor preservation.
Data on shelly fossils from the localities on Vohilaid are available in the Estonian geocollections database. The Estonian specimens of Gasconsia described here are housed at the Department of Geology, Tallinn University of Technology (institutional abbreviation GIT). The institutional abbreviations TUG and ELM mentioned above mark the specimens housed at the Museum of Geology of the University of Tartu and the Estonian Museum of Natural History, respectively.
On the taxonomy of Estonian trimerellides
The Estonian Upper Ordovician trimerellides described below are very large with a transversely oval outline, similar to the species of the genus Gasconsia (Northrop 1939; Hanken and Harper 1985; Mergl 1989; Watkins 2002). Given the shell outline, this genus belongs to group D (Chen et al. 2023), characterized by a relatively short pseudointerarea, and based on internal features, to type I (Chen et al. 2023). Specimens of type I have low platforms that are not vaulted and have feebly impressed or missing median ridges (Chen et al. 2023). The Estonian trimerellides, however, have a relatively high ventral pseudointerarea with an apical angle of about 120° (GIT 759-292b), and a two-partite, anteriorly trigonal platform that is vaulted with a strong wedge-like myophragm. The elements of the articulation mechanism could not be precisely observed in the studied specimens.
The updated description of Gasconsia, with its type species Gasconsia schucherti from the Silurian of Canada, and the description of G. worsleyi from the Upper Ordovician of Norway (Hanken and Harper 1985) represent a significant step towards a better understanding of trimerellide brachiopods. The similarity in the articulation and muscle systems of Gasconsia with the Ordovician genus Eodinobolus and the Silurian genus Dinobolus (Norford 1960) enabled the assignment of these three taxa to the subfamily Dinobolinae Li and Han, 1980. The Silurian species Dinorthis bohemicus (Barrande) differs essentially from Estonian trimerellides in dorsal interior features, such as the outline of anterior muscle scars, their subcircular ribbing, and furrow-shaped septa. The Estonian specimens differ from the species of Eodinobolus from Canada (E. magnifica, E. canadaensis, E. erectus; Norford and Steele 1969) in having a much larger size, a transversely oval shell outline, and specific details of dorsal interiors. The large size, transversely oval outline, dorsal socket, two-partite platform, central and anterior muscle scars, and external ornamentation of Estonian trimerellides support their identification within the genus Gasconsia.
Systematic paleontology
Subphylum CRANIIFORMEA Popov, Bassett, Holmer and Laurie, 1993
Class CRANIATA Williams et al., 1996
Order TRIMERELLIDA Gorjansky and Popov, 1986
Superfamily TRIMERELLOIDEA Davidson and King, 1872
Family TRIMERELLIDAE Davidson and King, 1872
Genus Gasconsia Northrop, 1939
Type species. Gasconsia schucherti Northrop, 1939, p. 161, pl. 12, figs 6-8; pl. 13, fig. 7; Hanken and Harper (1985), р. 246, text-figs 4B-G; the Gascons and Bouleaux Fms (Ludlow), Gaspé Peninsula, Quebec, Canada.
Diagnosis. Large to very large (up to 120 mm in width), semielliptical, gently to strongly biconvex shell; cardinal margin wide and cardinal extremities round. Ornament of concentric growth lines and some stronger ribs. Ventral valve with muscle field platform. Dorsal interiors with rounded dorsal sockets, pair of central muscle scars, and two larger anterior muscle scars with well-developed anterior-lateral directed ridges.
Species assigned and their occurrences (Fig. 3):
1. Gasconsia worsleyi Hanken and Harper 1985, p. 246, figs 2A, B, 3A-H, 4A; the Bonsnes Fm (upper Katian, Upper Ordovician), Ringerike district, Oslo region, southern Norway.
2. Gasconsia in Sweden is yet undescribed (unpublished data by courtesy of Lars Holmer are used here).
3. Gasconsia gigantea sp. nov. (this paper).
4. Gasconsia cf. worsleyi Popov et al., 1997, p. 591, fig. 4.29; upper part of the Oroj Fm (Climacograptus bicornis Biozone, upper Katian, Upper Ordovician), western side of the Shiderti River, southwest of Lake Eskeldykol, central Kazakhstan.
5. The species name Gasconsia transversa, applied for the brachiopods from the Silurian Much Wenlock Limestone (Silurian), England, was considered as a nomen nudum in a note by Davidson (1853; see Cocks 1978, p. 21), but was recognized as a formal specific name by Mergl (1989). As pointed out by Holmer et al. (2014), Gasconsia transversus described by Watkins (2002) from the late Silurian (Ludlow) of the Welsh Borderland seemingly lacks all evidence of an umbonal perforation and colleplax; most likely it is not conspecific with 7rifissura transversa. Thus, quotation marks are used for this species (see also the next below).
6. Gasconsia transversa (Salter); Mergl 1989, pl. 2, fig. 4; pl. 3, figs 1, 2; pl. 4, fig. 1; Kopanina Fm (Ludlow, Late Silurian), Bohemia, the Czech Republic.
7. Gasconsia schucherti Northrop, 1939, the type species of Gasconsia; see above.
8. Gasconsia sp.; Wenlock, Delphi, Indiana, USA (see Popov and Holmer 2000).
No fossil record of Gasconsia has been documented in the Hirnantian (latest Ordovician) and early Llandovery (Silurian). These brachiopods reappear in the USA (Laurentia) and England (Avalonia) in the Wenlock (Silurian) (Popov and Holmer 2000; Mergl 1989), and then in the Ludlow in Quebec, Canada (Laurentia) and Bohemia, the Czech Republic. The youngest Bohemian specimens occur in the Ludlow (Northrop 1939; Mergl 1989). It is assumed that Gasconsia may have become extinct in the interval of the Mid-Ludfordian extinction event (Kaljo and Martma 2006).
Gasconsia gigantea sp. nov. Figs 4-6
Derivation of the name. In Greek "vywavtwía" (giganteia), gigantea. The Estonian shells may grow up to nearly 120 mm in width and thus represent the largest known shell width of Ordovician brachiopods in the world.
Holotype. Mold of dorsal valve GIT 759-293 (Fig. SA1, A2), Vohilaid Island, NW Estonia, Adila Fm, Pirgu RS, uppermost Katian.
Diagnosis. Very large brachiopod with shell width up to 120 mm; outline transversely oval. Dorsal valve moderately convex, with thickness about half of valve length. Wedged depression marks a short median myophragm reaching up to middle of valve; laterally of that occur pair of large oval wing-shaped traces of anterior muscles and posteriorly pair of small, elongated teardrops central muscle scars (Figs 4 and SAI, A2).
Ventral pseudointerarea (Fig. 5B1, B2) about 0.33 as high as long, with apical angle of about 120°. Radial ornament of large ribs (wave-length of about 10 mm) and up to 9 fine ribs between them. Low, fine, tightly spaced rugae may occur randomly.
Description. Large dorsi-biconvex shell up to 120 mm wide, with transversely oval to semicircular outline, about half as long as wide; length/width ratio 0.55-0.60; maximum width of dorsal valve in posterior part, at about 0.2 of valve length from posterior edge; highest convexity at about midlength of dorsal valve, thickness about half of valve length. Cardinal extremities rounded, hinge line straight, shorter than shell width, anterior commissure rectimarginate. Ventral valve with apical angle of about 120°. Ornamentation with variable sized radial costellae and rugae-like concentric growth lines; distance between stronger ribs is about 10 mm, with up to 9 finer costellae between them; growth lines partially as low densely spaced rugae.
Dorsal valve (Fig. 4, Fig. SA1, A2, Fig. 6A1, A3, B1, B2) evenly convex, with slightly flattened lateral parts; thickness about half of valve length; highest point at mid-valve; length/ width ratio 0.55-0.60. On lateral profile, posterior curvature steeper than that of anterior part (Fig. 6A2). Cardinal angles rounded. Curved dorsal socket delineating posteriorly; oval platform (Fig. SA1, A2) about 30 mm in length and 24 mm in width, forming 0.25 of valve length and 0.16 of valve width. Dorsal bipartite platform extends to midline; two oblique nodes about 5 mm long in the middle of platform (Fig. 6B1, B2) mark deeply impressed central muscle scars. Anterior muscle scars (Fig. SAI, A2) represented as oval wing-like structures, open backwards, 15 mm in length and width, extending for about 0.2 and 0.3 of valve length and width. Wedged septa reaching up to mid-valve separates anterior scars. Scars covered by sub-radial pattern.
Ventral valve (Fig. 5B1b, B2) partly preserved behind dorsal valve GIT 759-292a, from where it has shifted about half of width to right side. Pseudointerarea about 0.3 as long as wide and about 0.4 of valve length, with flat surface; apex of pseudointerarea (apical angle) forming an angle of about 120%; apical homeodeltidium narrow, slightly shorter than pseudointerarea length, forming an angle of about 45° on top.
Comparison. The new species Gasconsia gigantea is most similar to the Norwegian species G. worsleyi Hanken and Harper, 1985. However, it differs in having a transversely oval or subsemicircular outline, with the length/width ratio of the dorsal valve about 0.8 instead of 0.6 in the Norwegian species; the dorsal valve of the Estonian species is half as high as long, whereas that of G. worsleyi is one quarter as high as long. The low ventral pseudointerarea in G. worsleyi is ca 0.1 as long as wide but 0.3 in the new species.
Estonian specimens of Gasconsia. Molds of dorsal valves, Vohilaid Island, locality 8 by Stein (1937), W Estonia (coordinates: 58.919734° N, 23.038809° E), Adila Fm, Pirgu- Porkuni transitional beds.
GIT 759-291 (Fig. 6A1-A3). Outline. Transversely oval, with length 61 mm, width over 100 mm, length/width ratio 0.60, thickness about 30 mm, thickness/length ratio 0.30. Highest convexity of valve at mid-valve; on lateral profile, curvature steeper in posterior than in anterior half. Hinge line shorter than maximum valve width; cardinal angles rounded.
Ornamentation. Some traces of ribbing and plication observed on surface of mold. A few concentric growth lines present close to margins. Interior features. Muscle scars occur laterally from trigonal myophragm, represented on mold as wedge-like depression. Two oval, open, backwards wing-like structures represent anterior muscle scars, occupying middle part of valve; reach anteriorly about 0.4 of valve length and form about 0.2 of valve width. Scars covered by sub-radial ridge-like pattern surrounded laterally by thickened border.
GIT 759-292a (Fig. 5B1a). Mold of an incomplete subcircular convex dorsal valve, with length about 41 mm, width about 70 mm, length/width ratio 0.58. Hinge line shorter than maximum width of valve. Cardinal angles rounded.
GIT 759-292b (Fig. 5B1b, B2). Part of ventral valve preserved behind dorsal valve GIT 759-292a. Single specimen with ventral pseudointerarea. Ventral valve pseudointerarea with apical angle of 120°, pseudodeltidium with angle of 45°.
GIT 759-293 (Fig. 5AI, A2). Holotype. Outline. Transversely oval dorsal valve with length 56 mm, width about 96 mm, length/width ratio about 0.58. Cardinal angles rounded; on lateral profile, curvature steeper in posterior than in anterior part. Radial ornament with intercalation of stronger and finer ribs. Interior features. Oval wing-like muscle scars separated by narrow wedge-like depression, marking median myophragm. Poorly preserved sub-radial pattern on muscle scars similar to that on specimen GIT 759-291.
GIT 759-294 (Fig. 6B1, B2). Dorsal valve, poorly preserved convex mold with length 66 mm, width 116 mm, length/ width ratio about 0.57, thickness about 30 mm, thickness/ length ratio about 0.45. Mold bears groove, marking myophragm in middle part of valve, reaching up to middle of valve. Muscle scars poorly impressed. Two small subparallel knobs marking central muscle scars occur posteriorly of septum close to hinge line. Ornament likely consists of a few ribs.
GIT 759-298 (Fig. 6C). Poorly preserved mold of dorsal valve. Surface damaged by boring organisms and erosion. Largest specimen in the collection, with transversely oval outline, valve width over 120 mm, length 68 mm, length/ width ratio about 0.6, thickness about 38 mm, length/ thickness ratio about 0.6; hinge line shorter than valve width.
Distribution of Gasconsia in Estonia
In the late Katian, Gasconsia has been known to occur in two regions: Baltica (Norway, Sweden, and Estonia) (Hanken and Harper 1985; Popov and Holmer 2000; this paper) and central Kazakhstan (Popov et al. 1997; Fig. 3).
In Estonia, the new species Gasconsia gigantea occurs in the upper Katian, as does Gasconsia in southern Norway (Hanken and Harper 1985). Gasconsia gigantea is found in the Adila Fm of the Pirgu RS, which comprises two chitinozoan zones: Conochitina rugata below and Belonechitina gamachiana above (Harris et al. 2004; Kaljo et al. 2017; Fig. 2). Exceptionally, the rock sample containing a specimen of Gasconsia (GIT 759-292) comprises the zonal chitinozoan Spinachitina taugourdeaui (identified by J. Nolvak) of the Porkuni RS. This may indicate a high stratigraphic position and the occurrence of transitional beds between the Pirgu and Porkuni stages, as suggested by Stein (1937). The occurrence of Porkuni zonal species S. taugourdeaui together with the Pirguan species Belonechitina gamachiana has been noted in some other sections (Aiamaa, Hints et al. 2005; Kaugatuma, Brenchley et al. 2003, Kaljo et al. 2008). In addition, the unusual mixed Pirguan ostracode fauna (Steusloffina cuneata, Daleiella rotundata, and others), together with species commonly found in the Porkuni RS (Apatochilina falacata and Gryphiswaldensia plicata; Meidla 1996), has also been identified in the uppermost part of the Adila Fm, distinguished as the Kabala Member (Mb; in Kaugatuma, mentioned as F1c-F2kb, Pôhjaka-Saare H-171, Virtsu-360 sections; Meidla 1996). The Kabala Mb has yielded the pentameride brachiopod Holorhynchus, known in Estonia only in a few sections on the Hitumaa Island (Lassi K-39, Hints 1993; Tamme K-38, Brenchley et al. 2003; Mánnamaa F-367, Nolvak 2008). Holorhynchus is a widely distributed brachiopod in the eastern and southern East Baltic (Brenchley et al. 1997; Paskevicius 1997). The occurrences of Holorhynchus and Gasconsia are valuable for understanding the latest Katian faunas and environments.
In Lithuania and Latvia, the Holorhynchus giganteus community in the Taucionys Fm characterizes the lagoonal facies of Benthic Assemblage 1 (Paskevitius 1997, 2000). The trimerellide brachiopod Gasconsia, based on the associated gastropod Maclurites, could also belong to shallowwater Benthic Assemblages 1-2 (Ebbestad et al. 2013).
In Norway, Holorhynchus and Gasconsia occur in different lithologies of the Oslo Region: the former in the Heroya Fm of the Fiker-Sandsvær district and the latter in the Bonsnes Fm of the Ringerike district (Owen et al. 1990). Both Gasconsia and Holorhynchus are found in environments ranging from peritidal flats to the inner shallow subtidal facies, which have patchy preservation in the geological record. The age of these brachiopods could be somewhat different, which needs to be clarified by future studies of time-related microfossils, especially those from the Kabala Fm with mixed faunas.
Gasconsia as a Lazarus taxon and gigantic brachiopod
In macroevolution, Lazarus taxa are assumed to be special taxa that disappear from the stratigraphic record for some time, particularly during a mass extinction, and re-appear after the event without any significant morphological changes. Lazarus taxa have been treated as extreme examples of a declining group (Rong et al. 2006). The trimerellide genus Gasconsia can be considered one of these taxa (Watkins 2002). In the stratigraphic range, it originates in the late Katian, just before the Late Ordovician Mass Extinction (LOME; Fig. 6). The order Trimerellida is recognized as a group of typical Lazarus taxa through the latest Ordovician and early Silurian (Chen et al. 2023). The disappearance of trimerellides, as well as the rarity of pentamerides and atrypides in the early-middle Hirnantian, not only in terms of richness and abundance but also in biodiversity, show the nature and effects of the LOME (Sheehan 2001; Rong et al. 2020). The dramatic temperature decrease (Finnegan et al. 2011) in tropical oceans during the LOME in the early- middle Hirnantian caused the disappearance of trimerellides and the establishment of the cold/cool water Hirnantia Fauna.
However, little is known about the reasons why the fossil record of Gasconsia has not been documented (Fig. 6) in the Llandovery when climate warmed up. It should be emphasized that the Lazarus taxa were usually represented by extremely rare species (Watkins 2002), and their population size drastically decreased during the crisis (Rong et al. 2006).
Another characteristic feature of Gasconsia is gigantism - described shells are up to 120 mm wide. The gigantism of ancient organisms in the geological record is an interesting topic (Vermeij 2016). Even a size of 10 mm can be considered gigantic, such as in the scolecodont of Devonian polychaetes (Eriksson et al. 2017); however, some Middle Ordovician endoceratid cephalopods reach up to 8-9 m in length (Vermeij 2016). Studies on gigantism in the geological record (e.g., Verberk and Adkinson 2013; Vermeij 2016) have highlighted the importance of this phenomenon in analyzing different ecosystem characteristics: temperature, planktic productivity, food quantity, and water chemistry, with special attention paid to the availability of oxygen.
From both global and regional perspectives, early Paleozoic brachiopods, particularly those from the Cambrian and Ordovician periods, are of medium and small sizes, based on data from China (Rong et al. 2017). Shells larger than 100 mm in width or length are rare. Large Ordovician brachiopods, such as Belubulla Percival, 1995 (Zhuzhaia, Xu and Li 2002), recorded in China, have a shell width of up to about 75 mm. However, the biggest known shells of Silurian stricklandiid brachiopods, Sinokulumbella, have a maximum shell width of about 100 mm (Rong et al. 2004). In the Baltic Ordovician, the largest brachiopod reaching 143 mm in width (Strophomenina) was recorded in the Porkuni RS (Röömusoks 2004) and supposedly belongs to the subfamily Strophomeninae. The Silurian trimerellide brachiopods of the genus Keteiodoros from Australia are of comparable size (width >145 mm; Strusz et al. 1998). Late Katian Gasconsia from Baltica and Kazakhstan might be linked to global climatic warming and increased nutrient supply (Brezinski and Kollar 2012). The temporal distribution of Gasconsia implies that its disappearance was caused by a dramatic temperature drop during the first phase of the LOME.
Conclusions
1. Based on distinctive characteristics, such as shell size, convexity, dorsal interiors, and a relatively high ventral pseudointerarea, the Ordovician trimerellides from Estonia are identified as a new species, Gasconsia gigantea. The new taxon is most similar to the Norwegian species Gasconsia worsleyi. Both occur in the upper Katian strata, just before the LOME.
2. Gasconsia gigantea sp. nov. occurs in biodetrital limestones, where it is associated with the gastropod Maclurites neritoides. In Sweden, the latter is considered a characteristic fossil of Benthic Assemblages 1-2.
3. The strata containing G. gigantea could be younger than those containing Holorhynchus in the Kabala Mb of the Adila Fm or more or less contemporaneous.
4. The trimerellide brachiopod Gasconsia represents a Lazarus taxon, whose descendants reappear in the Silurian. With a shell width of up to 120 mm, Gasconsia represents the largest Craniata and one of the largest brachiopods in Estonia, somewhat smaller than the large strophomenids in the Porkuni RS (lowermost Hirnantian).
Acknowledgments
The authors are grateful to the reviewers Lars Holmer and Yves Candela for their critical reading of the manuscript, which improved its quality. Many thanks go to the editor of the article, Kadri Pôdra, for useful remarks and corrections. We also thank Gennadi Baranov (Department of Geology, Tallinn University of Technology) for photographing the brachiopods. This study is a contribution to the IGCP project 735 "Rocks and the Rise of Ordovician Life" and was supported by the Estonian Research Council (PRG1701). The publication costs of this article were partially covered by the Estonian Academy of Sciences.
Corresponding author:
Linda Hints
Citation:
Hints, L. and Rong, J. 2024. Discovery of trimerellide brachiopod Gasconsia from the Ordovician of Estonia. Estonian Journal of Earth Sciences, 73(2), 124-133. https://doi.org/10.3176/earth.2024.12
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Abstract
Artiklis kirjeldatakse seltsi Trimerellida (klass Craniata) brahhiopoodide esmaleide, mis kuuluvad perekonda Gasconsia ja esindavad ии! ШК! Gasconsia gigantea. Need Vohilaiu saare paljandist leitud brahhiopoodid torkavad silma erakordselt suurte médtmetega (koja laius Кип! 120 mm), mis úletavad valdava osa ordoviitsiumi brahhiopoodide môôtmeid. С. gigantea esineb úlemordoviitsiumi Pirgu lademe Adila kihistu kôige úlemises osas, mis móningate paleontoloogiliste andmete pôhjal vôiks esindada ka úleminekulisi kihte ordoviitsiumi kôige Ülemisse Porkuni lademesse. Kirjeldatud ШК on sarnane Norras kirjeldatud liigiga С. worsleyi, mis esineb seal peaaegu samal stratigraafilisel tasemel kui Eesti ШК. Vôimalik, et С. gigantea nagu ka Pirgu lademe úlemise osa brahhiopoodid perekonnast Holorhynchus esindavad Pirgu lademe noorimat madalveelisele keskkonnale iseloomulikku kooslust. Samast Vohilaiu paljandist on párit mitu gastropood Maclurites neritoides leidu, mida Rootsi teadlased peavad rannaláhedase keskkonna indikaatoriks. Perekond Gasconsia evolutsiooni üks епрага on esindajate puudumine alates ordoviitsiumi бриз toimunud massilise váljasuremise perioodist jaátumisega kaasnenud temperatuurilanguse ja keskkonnatingimuste muutuste tagajárjel. Gasconsia ilmus uuesti válja peale pikka perioodi siluri teisel poolel. Sellise levikuga taksoneid, mis taasilmuvad peale lühemat vói pikemat perioodi, nimetatakse Lazaruse taksoniteks. Trimerelliidsete brahhiopoodide hulgas on mitmeid taksoneid, mis on môôtmete poolest erakordselt suured. lidsete eluvormide gigantism on oluline ka keskkonna mójutegurite (temperatuur, vee keemiline koostis, hapnikusisaldus jm) hindamisel.