Introduction
Cornulitid tubeworms are an order of encrusting tentaculitoids that are phylogenetically closely related and likely ancestors of free-living tentaculitids (Vinn and Mutvei, Reference Vinn and Mutvei2009; Vinn, Reference Vinn2010a; Vinn and Zatoń, Reference Vinn and Zatoń2012). The cornulitids have a global distribution and stratigraphic range from the Middle Ordovician to the late Carboniferous (Vinn, Reference Vinn2010a, Reference Vinnb). The biological affinities of cornulitids have long been debated (Herringshaw et al., Reference Herringshaw, Thomas and Smith2007), but with some certainty they belong to the Lophothrochozoa (Vinn and Zatoń, Reference Vinn and Zatoń2012). It is possible that cornulitids represent stem group phoronids (Taylor et al., Reference Taylor, Vinn and Wilson2010). Cornulitids are a paleoecologically important group of hard-substratum encrusters because they generally retain their original position on the substratum after fossilization (Taylor and Wilson, Reference Taylor and Wilson2003). Cornulitid tubeworms only inhabited normal marine environments, differing from their close relatives, the microconchids, which lived in waters of various salinities (Zatoń et al., Reference Zatoń, Vinn and Tomescu2012, Reference Zatoń, Wilson and Vinn2016; Shcherbakov et al., Reference Shcherbakov, Vinn and Zhuravlev2021). Fossils of cornulitid tubeworms are most common in shallow marine sediments. Cornulitids had a diverse ecology with several life modes that ranged from simple hard-substratum encrusters (Zatoń and Borszcz, Reference Zatoń and Borszcz2013; Zatoń et al., Reference Zatoń, Borszcz and Rakociński2017) to endobiotic symbionts of stromatoporoids and corals (Vinn, Reference Vinn2010a).
Cornulitids have rarely been studied from the Ordovician of Estonia, mostly because of their minor stratigraphical importance. The cornulitids from Estonia were first mentioned by Schmidt (Reference Schmidt1858) and Eichwald (Reference Eichwald1860). Recently, several species have been described from the Ordovician of Estonia (Vinn and Mõtus, Reference Vinn and Mõtus2012; Vinn, Reference Vinn2013; Vinn and Eyzenga, Reference Vinn and Eyzenga2021).
The aims of this paper are to: (1) systematically describe the fauna of small cornulitids and other tubicolous organisms from the Hirmuse Formation; and (2) discuss the phylogeny, diversity, ecology, and biogeography of the Ordovician cornulitids.
Geological background and locality
A shallow, warm, epicontinental sea covered what would become modern northern Estonia during the Late Ordovician. The Ordovician sequence of northern Estonia is relatively complete (i.e., all stages are present) and is represented mostly by carbonate rocks. There was a great climatic change in the Ordovician of Baltica when the paleocontinent drifted from the southern high latitudes to the tropical realm (Torsvik et al., Reference Torsvik, van der Voo, Preeden, MacNiocaill, Steinberger, Doubrovine, van Hinsbergen, Domeier, Gaina, Tohver, Meert, McCausland and Cocks2012). Carbonate sedimentation intensified during the warming of the climate (Nestor and Einasto, Reference Nestor, Einasto, Raukas and Teedumäe1997). The first evidence of a tropical climate, e.g., tabulate corals and stromatoporoids, appeared in the early Katian.
The large Vasalemma Quarry operated by Nordkalk is situated in Vasalemma village in northwestern Estonia (Fig. 1). The Katian limestones of the Vasalemma Formation and marls of the Hirmuse Formation are exposed in the quarry. They are excavated down to the ripple-marked upper surface of the Pääsküla Member of the underlying Kahula Formation (Hints and Miidel, Reference Hints and Miidel2008). The rocks of the Vasalemma Formation are represented in the quarry by a succession of bioclastic grainstones to 15 m thick. The grainstone layers contain numerous intercalated reef bodies. These reefs are composed of bryozoan framestone-bindstone, echinoderm bindstone, receptaculitid-bryozoan-microbial framestone, and tabulate bafflestone. The reef bodies can be > 50 m wide (Kröger et al., Reference Kröger, Hints and Lehnert2014). The marls and thin-bedded argillaceous limestones of the Hirmuse Formation overlie the Vasalemma Formation in the northeastern corner of the quarry. The Hirmuse Formation is characterized by a rich, normal, marine fauna including brachiopods, bryozoans, echinoderms, trilobites, and rugose corals (Hints and Meidla, Reference Hints, Meidla, Raukas and Teedumäe1997). The early Katian Guttenberg isotope carbon excursion (GICE) occurs in the middle and upper part of the Vasalemma Formation (Kröger et al., Reference Kröger, Hints and Lehnert2014).
Figure 1. Map of Estonia, with locality at Vasalemma indicated by a red dot. Dev. = Devonian; EST. = Estonia.
Materials and methods
The single sample AM-01-21 from the Vasalemma Partek Nordkalk Quarry, outcrop 19, containing ~5 kg of marl from the upper part of section, was washed with water, and 40 complete small tubes or fragments of larger tubes of cornulitids were manually picked from the washed residue. Specimens were later coated with platinum and photographed using a scanning electron microscope (SEM) at the Paleontological Institute of the Russian Academy of Sciences. Measurements were obtained from calibrated SEM images.
Repositories and institutional abbreviations
Types, figured, and other specimens examined during this study are deposited in the following institutions: Department of Geology, Tallinn University of Technology (GIT), and the Geological Collections of the Natural History Museum, University of Tartu (TUG).
Systematic paleontology
Class Tentaculitida Bouček, Reference Bouček1964
Order Cornulitida Bouček, Reference Bouček1964
Family Cornulitidae Fisher, Reference Fisher and Moore1962
Genus Conchicolites Nicholson, Reference Nicholson1872
Type species
Conchicolites gregarius Nicholson, Reference Nicholson1872.
Conchicolites kroegeri new species
Figure 2.1–2.4
Type specimens
Holotype, GIT 421-137, Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician; paratypes, GIT 421-135, 421-136, 421-138, 421-139, 421-140, and 421-134, same locality.
Figure 2. Cornulitid tubeworms from Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, Upper Ordovician: (1–4) Conchicolites kroegeri n. sp.: (1) holotype, GIT 421-137; (2) paratype, GIT 421-135; (3) paratype, GIT 421-138, with tube interior exposed; (4) paratype, GIT 421-139; (5) Conchicolites sp. indet. A, GIT 421-133; (6) Cornulites cf. C. sterlingensis Meek and Worthen, Reference Meek and Worthen1866, GIT 421-127; (7) Cornulites lindae n. sp., holotype, GIT 421-144.
Diagnosis
Small tube with very rapidly growing diameter and well-developed, densely spaced, regular, perpendicular ridges.
Occurrence
Only known from Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Description
Small tubes usually attached to the substrate only in their proximal part. The tubes are 0.6–1.7 mm long and to 0.6 mm wide at the aperture. Tube diameter expands rapidly in the initial growth stage. The angle of divergence is 20–30o. Free distal parts of the tubes sometimes tilt away from the substratum. The tube base is widened in the form of protruding perpendicular ridges making the horizontal contour of the tube base serrated. The perpendicular ridges are much stronger at the tube's contact with the substratum than on the top of the tube. There are 10 perpendicular ridges of 0.5 mm at the distal part of the tube. The perpendicular ridges are regular, densely spaced, well-developed, sharp, but rather low at the top of the tube and not always continuous along the exposed surface of the tube. The tube interior is smooth without any annulation.
Etymology
Named in honor of Björn Kröger for his detailed studies on the paleontology and sedimentology of the rocks exposed in the Vasalemma quarries.
Remarks
This new species is assigned to the genus Conchicolites because of its smooth tube interior. It closely resembles Conchicolites rossicus (Vinn and Madison, Reference Vinn and Madison2017) (Vinn and Madison, Reference Vinn and Madison2017, p. 238, fig. 2) by its regular, dense, well-developed, perpendicular ridges. This new species differs from Conchicolites rossicus in that its broad tubes expand much more rapidly in diameter than the tubes of Conchicolites rossicus. It also resembles Cornulitella minor Nicholson, Reference Nicholson1872 (Hall, Reference Hall1888, pl. 115, fig. 3) from the Upper Ordovician of North America with its regular, dense, well-developed, perpendicular ridges, but differs by its much broader tube. The broad tubes of Cornulites devonicus Pacht in Helmersen and Pacht, Reference Helmersen and Pacht1858 (Vinn et al., Reference Vinn, Musabelliu and Zatoń2019, p. 71, figs. 5A, B, 6A–H) resemble tubes of this new species, but they differ in having an annulated tube interior and external longitudinal striation.
Conchicolites sp. indet. A
Figure 2.5
Occurrence
Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Description
Fragmentarily preserved small tube (diameter to 0.4 mm). The tube slowly expands in diameter and is sparsely covered with prominent but thin, sheet-like, and high-perpendicular to subperpendicular ridges that tilt toward the tube aperture. The interspaces between the ridges are smooth and flat, rarely slightly concave. The ridges are ~10 um thick; distances between them vary from 50–180 μm.
Materials
GIT 421-133-2.
Remarks
Conchicolites sp. indet. A does not resemble any other Ordovician cornulitid. It differs from Conchicolites rossicus (see Vinn and Madison, Reference Vinn and Madison2017, p. 238, fig. 2) from the Katian of northwestern Russia, and Conchicolites kroegeri n. sp., by very long interspaces between the perpendicular ridges. Conchicolites sp. indet. A resembles Conchicolites sp. indet. 1 (Vinn and Wilson, Reference Vinn and Wilson2013, p. 365, fig. 13) from the Rhuddanian of Estonia in its peristome-like perpendicular ridges that are slightly tilted toward tube aperture, but it differs by its smaller size and unattached tube part. Conchicolites sp. indet. A resembles in its peristome-like perpendicular ridges the annulation of Cornulites? semiapertus Öpik, Reference Öpik1930 (Vinn, Reference Vinn2013, p. 109, figs. 3, 4) from the Darriwilian to Sandbian of Estonia, but it differs by its smaller size and smooth tube interior. We are not erecting a new species because we have studied too few specimens.
Genus Cornulites Schlotheim, Reference Schlotheim1820
Type species
Cornulites serpularius Schlotheim, Reference Schlotheim1820.
Cornulites cf. C. sterlingensis Meek and Worthen, Reference Meek and Worthen1866
Figure 2.6
- Reference Hall1888
Cornulites sterlingensis; Hall, pl. 115, fig. 7.
- Reference Vinn and Madison2017
Cornulites cf. sterlingensis; Vinn and Madison, p. 238, fig. 3.
Occurrence
Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Description
Small tube expanding moderately in diameter. The maximum diameter of the tube is ~0.5 mm. The tube bears a linear, deep attachment scar in its entire length. The tube is covered with strong, regular annulations with sharp crests. The interspaces between the annuli are deep and concave. There are six annuli/mm in the distal part of the tube. The tube is covered with strong longitudinal striae. There are approximately four striae per 0.1 mm.
Materials
GIT 421-127, 421-128, and 421-129.
Remarks
The studied specimens resemble Cornulites sterlingensis (see Hall, Reference Hall1888, pl. 115, fig. 7) from the Cincinnatian (Katian) of Ohio, but are much smaller. We consider our specimens juveniles and assign them tentatively to Cornulites sterlingensis. Our material also resembles Cornulites cf. C. sterlingensis from the Katian of northwestern Russia (Vinn and Madison, Reference Vinn and Madison2017), but our specimens are also somewhat smaller. Poorly preserved specimens of possible Cornulites cf. C. sterlingensis have also been described from the Takche Formation, Himalaya (Shabbar et al. Reference Shabbar, Saxena, Gupta, Singh and Goswami2022), but their state of preservation does not allow proper comparison with our specimens.
Cornulites lindae new species
Figures 2.7, 3.1–3.3
- Reference Vinn and Wilson2013
Cornulites sp. A; Vinn, p. 110, fig. 6.
Figure 3. (1–3) Cornulites lindae n. sp., Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, Upper Ordovician: (1) paratype, GIT 421-145, somewhat flattened; (2) paratype, GIT 421-151, somewhat flattened; (3) paratype, GIT 421-152; (4–7) Cornulites meidlai n. sp.: (4) holotype, TUG 1383-5, Rakvere, Keila Regional Stage, scale bar = 0.5 mm; (5–7) paratypes, Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician: (5) GIT 421-126; (6) GIT 421-155; (7) GIT 421-156.
Type specimens
Holotype, GIT 421-144, Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician; paratypes, GIT 421-145, 421-147, 421-151, and 421-152, same locality.
Diagnosis
Small, moderately expanding, almost straight tubes with delicate longitudinal striae, moderately developed annulation, and sharp, but not strong, annular crests.
Occurrence
Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Description
Small (to 1.9 mm long, 0.5 mm wide) nearly straight tube. The tube is circular in cross section and expands moderately in diameter. The angle of tube divergence is ~15o. Tubes are covered with more or less regular, moderately developed annulations. The annuli are stronger near the aperture, with sharp crests. The interspaces of the annuli are concave and V- to U-shaped in longitudinal section. The deepest part of the interspaces is usually located midway between two adjacent annular crests. There are six annuli per 0.5 mm near the tube aperture. The tube is covered with moderately developed regular longitudinal striae. The striae are stronger near the aperture. There are six striae per 0.1 mm at the tube aperture.
Etymology
Named in honor of Linda Hints for her studies on the paleontology and sedimentology of the rocks exposed in the Vasalemma quarries.
Remarks
Cornulites lindae n. sp. most closely resembles Cornulites sp. indet. A from the Sandbian of Estonia in its weak striae and the general shape of the tube, but it differs by its smaller size, slightly broader tube, and sharper annular crests. Despite some differences, we have synonymized Cornulites lindae n. sp. and Cornulites sp. indet. A (Vinn, Reference Vinn2013) because they likely represent different growth stages of the same species. This new species also somewhat resembles Cornulites serpularius (see Vinn and Wilson, Reference Vinn and Wilson2013, p. 360, figs. 3, 4) from the Wenlock of Estonia in its moderately expanding tube and relatively faint longitudinal striae. This new species differs from Cornulites serpularius in having sharper annular crests on the tube exterior and more regular annulation.
Cornulites meidlai new species
Figure 3.4–3.7
- Reference Vinn and Wilson2013
Cornulites sp. C; Vinn, p. 110, fig. 8.
Type specimens
Holotype, TUG 1383-5, Rakvere, northern Estonia, Keila Regional Stage, lower Katian, Upper Ordovician; paratypes, GIT 421-125, 421-126, 421-78, 421-155, and 421-156, Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Diagnosis
Tubes externally covered with strong reticulate ornament, with divergence angle ~12o and vesicular wall structure at the annular crests.
Occurrence
Vasalemma Partek Nordkalk Quarry, northern Estonia; previously reported by Vinn (Reference Vinn2013) from Rakvere (three complete shells), Rägavere (one complete shell), Keila to Rakvere stages, Katian, Upper Ordovician.
Description
Tube of moderate size covered with strong reticulate ornament. The mature tubes are 13–15 mm long and 3.0–4.0 mm wide at the aperture. Long free tube parts can occur. The tube fragments from Vasalemma with reticulate ornament are to 1.2 mm wide. The reticulate ornament is formed by equally strong and well-developed perpendicular growth lines and longitudinal striae. The annulation on the tube exterior is somewhat irregular and not always clearly visible. The tube interior is regularly annulated. The tube expands moderately in diameter. The divergence angle of the tube is ~12o. The tube wall is vesicular at the annular crests. The distance between annular crests is ~1.7 mm at the tube diameter of 4 mm. The basal edge is not widened. Emended from Vinn (Reference Vinn2013).
Etymology
Named in honor of Prof. Tõnu Meidla for his great contributions to the study of Estonian Ordovician fossils and facilitating paleontological research at the University of Tartu.
Remarks
The prominent longitudinal striae of the new species resemble those of Cornulites sterlingensis (see Hall, Reference Hall1888, pl. 115, fig. 7) from the Cincinnatian (Katian) of Ohio, but the former differs in the lack of well-developed external annulation and by stronger perpendicular growth lines. This new species also resembles Reticornulites reticulatus Lardeux, Jaouen, and Plusquellec, Reference Lardeux, Jaouen and Plusquellec2003 (Lardeux et al., Reference Lardeux, Jaouen and Plusquellec2003, p. 652–654, fig. 2) in its reticulate ornamentation with predominant transverse rings and fine longitudinal striae, but it differs by its regularly annulated interior.
Cornulites sp. indet. A
Figure 4.1, 4.2
Occurrence
Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Figure 4. (1, 2) Cornulites sp. indet. A, Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, Upper Ordovician: (1) GIT 421-142; (2) GIT 421-143; (3) Cornulites sp. indet. B, GIT 421-146, Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, Upper Ordovician; (4, 5) unidentified conical shell, GIT 421-148, Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, Upper Ordovician: (4) entire specimen; (5) setae-like structures on (4); (6) Lagenosypho sp. indet., GIT 421-131, Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, Upper Ordovician.
Description
Small tubes to 1.6 mm long and 0.35 mm wide. The tube is circular in cross section, slightly curved, with a long free part. There are no signs of attachment structures. The tube expands moderately in diameter. The angle of divergence is ~15o. The tube is covered with regular, well-developed annulation. The crests of the annuli are rather sharp and the interspaces are concave and V- to U-shaped in longitudinal section. There are approximately seven annuli per 0.5 mm in the distal part of the tube. The annuli are stronger near the aperture.
Materials
GIT 421-142, 421-143, 421-153, and 421-154.
Remarks
Cornulites sp. indet. A closely resembles Cornulites lindae n. sp. in its regular and moderately developed sharp annuli, but it differs in the absence of longitudinal striae. Cornulites sp. indet. A might belong to Cornulites lindae n. sp. if the lack of striae is an artifact of preservation.
Cornulites sp. indet. B
Figure 4.3
Occurrence
Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Description
Immature tube small (1.9 mm long, 0.4 mm wide), slightly meandering but generally straight, and externally covered with irregularly developed annuli. Some annuli are prominent on one side of the tube and weakly developed on the opposite side. There are approximately five annulations per 0.5 mm near the tube aperture. The tube expands moderately in diameter. The angle of divergence is ~15o. No attachment structures are visible. The mature tubes are large (11 mm long, 3.2 mm wide) and covered by faint longitudinal striae. There are approximately eight striae per 0.5 mm near tube aperture. The mature tubes are attached in their entire length and have a prominent widened base.
Materials
Immature GIT 421-146; mature GIT 421-124.
Remarks
The described specimens most closely resemble Cornulites? sp. indet. D (Vinn, Reference Vinn2013, p. 111, fig. 9) from the Vasalemma Formation in its somewhat irregular annulation and relatively narrow tube.
Unidentified conical shell
Figure 4.4, 4.5
Occurrence
Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Description
Small, straight, conical shell (1.2 mm long, 0.4 mm wide) that expands rapidly in diameter. The angle of divergence is ~19o. Inside the cone, on one side, is a slightly conical tube running throughout. The diameter of the external cone expands faster than the diameter of the internal conical tube. The diameter of the internal tube at the aperture of the external cone is 0.12 mm. The tilted ornamentation of the external tube is posteriorly directed and affiliated with the punctae. The punctae are ~10–15 μm in diameter (GIT 421-148) and well pronounced at one side of the cone, whereas on the other side, the punctae seem to be penetrated by conical appendages.
Materials
One almost complete shell (GIT 421-148).
Remarks
The appendages on its one side strongly resemble the phosphatized setae of lower Paleozoic brachiopods (Jin et al., Reference Jin, Zhan, Copper and Caldwell2007), but this does not prove a shared origin in terms of structure or secretion process. The structure of setae-like appendages in the conical shell is not known and thus cannot be compared with the structure of phosphatized setae in brachiopods.
Phylum Bryozoa Ehrenberg, Reference Ehrenberg1831
Order Cyclostomata Busk, Reference Busk and MacGillivray1852
Family Corynotrypidae Dzik, Reference Dzik1981
Lagenosypho Spandel, Reference Spandel1898
Occurrence
Vasalemma Partek Nordkalk Quarry, northern Estonia, Hirmuse Formation, lower Katian, Upper Ordovician.
Description
Conical zooecia with oval cross sections. The external walls of the zooecia are covered by fine growth lines to eight per 0.1 mm. The walls of the zooecia are very thin. The zooecia are 0.6 mm long and 0.45 mm wide at the widest side.
Materials
Single fragment (GIT 421-131).
Remarks
The described specimen has most in common with corynotrypids (see Dzik, Reference Dzik1981). It is tentatively assigned to Lagenosypho (personal communication, P.D. Taylor, 2022).
Discussion
Biological affinities of the unidentified conical shell
The general shape of this conical shell resembles those of cornulitids and tentaculitids. However, it is devoid of the perpendicular ornamentation characteristic of cornulitids. Moreover, there is a cylindrical structure that reaches through the tube from the proximal to the distal end of the shell. This cylindrical structure is likely not a substratum for the conical body around it, but a part of the organism because its diameter greatly increases from the narrow to the broad end of the shell. The inner tube is not located exactly in the center of conical shell but is closer to one side. The organism could have had differentiated ventral and dorsal sides. Such asymmetry shows that our problematic organism might be affiliated with bilateral animals. Among bilateral animals, nautiloids have conical shells with a siphuncle that is often located closer to one side of the shell wall. However, the inner tube of our organism is rather thick for a typical nautiloid siphuncle and there are no septae connected to the inner tube. Moreover, the punctate shell and setae-like structures are not known in cephalopods. The punctate shell structure and setae-like structures of this organism can be affiliated with lophophorates. The strange appearance of setae-like structures on only one side of the shell is likely an artifact of preservation. In modern brachiopods, setae are located at the anterior commissure of the shell. However, in the early Paleozoic linguliforms, orthides, and even tommotiids, the preserved setae (or punctae preserving the traces of setae) cover the whole shell surface. Therefore, the preserved conical structures are possibly homologues of the setae of lophophorates. One would expect in the case of a lophophorate with a conical shell to also see setae also around its aperture and not everywhere on one side of its exterior. Thus, it is possible that these setae-like structures are not homologues of brachiopod setae, making the possible lophophorate affinities of the organism uncertain.
Diversity and paleoenvironments
Vinn (Reference Vinn2013) concluded that cornulitids seem to be relatively rare in the Late Ordovician of Baltica, which contrasts with the situation in the Late Ordovician of North America where cornulitids seem to be more common (Hall, Reference Hall1888; Richards, Reference Richards1974; Morris and Felton, Reference Morris and Felton1993, Reference Morris and Felton2003). The numerous and diverse cornulitids from the Hirmuse Formation indicate that the diversity and abundance of cornulitids in the Ordovician of Baltica has been underestimated and it is likely the Baltic cornulitid fauna was not less diverse and abundant than the fauna of North America. Macroscopic cornulitid fossils of the Ordovician of Estonia were described by Öpik (Reference Öpik1930) and Vinn (Reference Vinn2013). However, no previous data exists on the diversity of small to microscopic fossil cornulitids. The large number of tiny cornulitids in the Hirmuse fauna demonstrates that many Ordovician cornulitids were very small and were not always present on large substrata, e.g., brachiopod shells, which were the most common substrata for cornulitids in the Ordovician. Instead, the Hirmuse cornulitids preferred small substrata often less than one millimeter in size. The cornulitids were collected from a clay, which represents a mud-bottom environment in the Hirmuse Formation. The muddy bottom fauna of the Hirmuse Formation might also differ from the encrusting fauna present on brachiopods in the limestones of the Vasalemma Formation. Considering the high diversity of the studied soft-bottom cornulitid fauna, it seems that clay-mud environments supported the highest cornulitid diversity in the Late Ordovician of Baltica. The muddy environment was also inhabited by numerous bryozoans (including erect corynotrypids), tiny brachiopods, and unknown organisms with conical shells. The biodiversity of tubicolous organisms in such muddy environments needs further study; such study could lead to reassessment of biodiversity curves for the Ordovician of Baltica.
Evolutionary notes
Some cornulitids, e.g., Cornulites sp. indet. A and Cornulites lindae n. sp. from the Vasalemma fauna, resemble free-living tentaculitids in their general shell shape and very regular, well-developed annulation. The free-living tentaculitids likely appeared sometime in the Ordovician (Vinn, Reference Vinn2010a). However, many supposed Ordovician tentaculitids could actually be tentaculitid-like cornulitids similar to some of our specimens. The free-living tentaculitids presumably evolved from cornulitid ancestors, so the discovery of intermediate forms among the Late Ordovician tentaculitoids is not surprising. These intermediate forms indicate that some tentaculitid characters, e.g., regular annulation and the almost straight shell, which were thought to be apomorphies of free-living tentaculitids, were actually inherited from ancestral cornulitids. The adaptation to encrust microscopic substrata led to the appearance of cornulitid tubes with long free parts that could have played a role in the evolution of typical free-living tentaculitid morphology. Cornulitids have a postlarval shell (Vinn and Mutvei, Reference Vinn and Mutvei2009), whereas in tentaculitids, the shell first appears in the larval stage as a long conical process (Farsan, Reference Farsan2005). The tentaculitids possibly evolved from cornulitids by changes in the life cycle, i.e., by the prolongation of the free-swimming stage with the larva metamorphosing into the juvenile while swimming and forming the shell in the water column as was proposed for other lophotrochozoans (Vinn and Mutvei, Reference Vinn and Mutvei2009).
Paleobiogeography of cornulitid fauna
The cornulitid fauna of the Late Ordovician of North America (see Hall, Reference Hall1888) somewhat resembles the cornulitid fauna of the Late Ordovician of Baltica (Vinn, Reference Vinn2013; present paper). Cornulites sterlingensis occurs both in the Late Ordovician of Laurentia (Hall, Reference Hall1888) and Baltica (Vinn and Madison, Reference Vinn and Madison2017; present paper). The species of Conchicolites described from the Late Ordovician of Baltica (Vinn and Madison, Reference Vinn and Madison2017; present paper) are also somewhat similar to Cornulitella minor (see Hall, Reference Hall1888, pl. 115, fig. 3). The cornulitid faunas of Gondwana are relatively well known. The Gondwanan cornulitids have been described from Morocco (Gutiérrez-Marco and Vinn, Reference Gutiérrez-Marco and Vinn2018) and Peru (Vinn and Gutiérrez-Marco, Reference Vinn and Gutiérrez-Marco2016); they also occur in Bohemia (Barrande, Reference Barrande1867), Spain (Verneuil and Barrande, Reference Verneuil and Barrande1855), France (Dreyfuss, Reference Dreyfuss1948), Sardinia (Spano, Reference Spano1974), and the Himalaya (Shabbar et al., Reference Shabbar, Saxena, Gupta, Singh and Goswami2022). Nevertheless, there are fewer common cornulitid species (i.e., Cornulites cf. C. sterlingensis) between Gondwana and Baltica. Moreover, small Conchicolites-like forms are unknown from Gondwana but such forms are known both from the Late Ordovician of Baltica (Conchicolites rossicus and Conchicolites kroegeri n. sp.) and Laurentia (Cornulitella minor) (see Hall, Reference Hall1888).
Acknowledgments
This paper is a contribution to the International Geoscience Programme (IGCP) Project 735, Rocks and the Rise of Ordovician Life (Rocks n'ROL). Financial support to OV was provided by the Institute of Ecology and Earth Sciences (University of Tartu) Research Grant and OV is also grateful to the Paleontological Society for a Sepkoski Grant. UT was funded by the Estonian Research Council, grant number PUTJD1106. We are grateful to C. Sendino and an anonymous reviewer for constructive comments on the manuscript.
Declaration of competing interests
The authors declare none.
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