Remarks

Lissatrypide taxa occur abundantly in the Solvik Formation. All are smooth, but the shape and anterior margin vary widely. The internal configuration displays the clearest differences. The type species Lissatrypa atheroidea Twenhofel, Reference Twenhofel1914, which is absent in the Solvik Formation, has a unique, solid hinge pad with a cardinal pit covered by the cardinal process, and jugal processes that initially are directed ventrally and end in ring-like structures. Meifodia Williams, Reference Williams1951, has a short cardinal pit, and fairly thin, well-separated, and horizontal hinge plates. The crura are directed anteriorly initially. The genus Thulatrypa Huang et al., Reference Huang, Baarli, Zhan and Rong2016, has unsupported cardinal plates with elevated crural supports.

Type species

Eisaella uniplicata new species from the basal part of the Myren Member in the O/S boundary layers in the Oslo-Asker districts, Norway

Diagnosis

Medium-sized, smooth, biconvex, and thick-shelled valves with clear sulcus and fold. The umbo is small, adpressed, lacking a foramen and delthidial plates. Internally it lacks a pedicle collar, teeth are short, solid, and relatively slender, a low hinge pad rests on the valve floor, crura initially ventrally directed, and jugal plates create a ring-like feature. It has fewer than 10 whorls of the spiralia.

Occurrence

Found in the Oslo-Asker districts in Norway in uppermost Hirnantian to lower Aeronian strata.

Etymology

Named after Eisa, the goddess of old age in Norse mythology.

Remarks

Externally, Eisaella n. gen. is difficult to distinguish from juvenile specimens of Meifodia, with which it occurs. Compared with mature specimens, it is less transverse in outline and the sulcus originates farther anteriorly. However, external features are unstable and often adapt to specific environmental conditions. Internally, it differs strongly by having an elevated hinge pad and crura that initially are directed straight ventrally. Meifodia has hinge plates that are widely separated and fastened to the lateral shell wall and the crura are directed more laterally. Furthermore, Meifodia has jugal processes that are projected anteriorly with simple plates (Fig. 7), whereas Eisaella n. gen. has jugal plates that form a ring-like configuration (Fig. 6). Lissatrypa differs in lacking fold and sulcus and having a pedicle collar, apical or transapical foramen, and cardinal process. The two genera share hinge pads, fibrous crura initially directed ventrally before curving laterally, and jugal processes ending in plates with a ring-like structure. The configuration of the jugal processes was deemed the most stable character for taxonomic designation among lissatrypides by Nikiforova and Andreeva (Reference Nikiforova and Andreeva1961).

Copper (Reference Copper2004) speculated that Meifodia is the ancestor to Lissatrypa, with the oldest two species, L. atheroidea Twenhofel, Reference Twenhofel1914, from the Jupiter Formation, Anticosti Island and L. magna Rong and Yang, Reference Rong and Yang1981, from the Lojoping Formation in Hubei Province appearing in upper Aeronian strata. The new genus Eisaella is a more likely direct ancestor. The interior has most of the traits of a typical Lissatrypa, including the peculiar ring-like jugal plates.

Eisaella uniplicata new species
Figures 4.1–4.14, 5, 6

Figure 4.

(1–14) Eisaella uniplicata n. gen. n. sp. (1–5) Holotype, PMO 52.174, brachial, pedicle, anterior, posterior, and side view of valve; lower 12 m of Myren Member, Konglungø, Asker; (6, 7) PMO 20.885, posterior and side view of valve; lower part of Myren Member, Hovedøya, Oslo; (8) PMO 52.309d, anterior view, lowest part of Myren Member, Høyerholmen, Asker; (9) PMO 52.175 (sectioned), view of brachial valve, lower 12 m of Myren Member, Konglungø, Asker; (10) PMO 158.126, view of acid-treated brachial valve, ~55 m above base of Myren Member, Spirodden, Aske; (11) PMO 234.621, view of brachial valve, 20 m above the base of Myren Member, outcrop along E 18, Vakås, Asker; (12) PMO 158.158, mold of pedicle valve; (13) PMO 234.622, mold of brachial valve; (14) PMO 158.151, mold of pedicle valve; (12–14) from 21 m above base of Myren Member, along E18 Vakås, Asker. Eisaella minuta n. comb. (15–19) PMO 88.148, pedicle, brachial, side, anterior, and posterior view of valve, top of Myren Member, Malmøya, Oslo. Notice different scale for (15–19).

Figure 5.

Eisaella uniplicata n. gen. n. sp. Serial section of specimen PMO 52.175 lower 12 m of Myren Member, Konglungø, Asker. Eisaella minuta n. comb. Serial section of specimen PMO 51.968c, from Leangen Member, Leangen, Asker.

Figure 6.

Eisaella uniplicata n. gen. n. sp. Reconstruction of internal structures based on specimen PMO 52.175 (see Fig. 5). Eisaella minuta n. comb. Reconstruction of internal structures based on specimen PMO 51.968c (see Fig. 5).

Figure 7.

Comparison of reconstructed internal structures for Meifodia prima. Reconstruction of M. prima prima is based on PMO 52.309h from basal Myren Member, lowermost part of the profile at Spirodden, Asker. The middle figure is of M. cf. M. subundata, PMO 41.220b, from upper part of Myren Member, the south coast of Malmøya in Norway (modified from Copper, Reference Copper1995, fig. 8, p. 853). Meifodia prima ovalis is based on PMO 41.227 from Padda Member at Malmøykalven, Oslo.

Holotype

PMO 52.174 (Fig. 4.1–4.5), close to the O/S boundary at Konglungø, Asker.

Diagnosis

Medium-sized and evenly rounded shells with a well-defined pedicle sulcus and fold originating in the anterior part of the valves. Small and strongly incurved beak without a foramen. Strongly impressed, elongated, and triangular muscle field in pedicle valve. Short massive teeth and a low hinge pad with a long cardinal pit. Crura and jugal processes as for the genus, eight whorls of the spiralia.

Occurrence

Konglungø, Spirodden, Vakås along E18 and Høyerholmen, all Asker and Hovedøya, Oslo. From basal Myren Member up to 55 m above the base. Uppermost Hirnantian to lower Rhuddanian.

Description

Smooth, medium-sized, and dorsibiconvex valve with a small, strongly incurved beak. No foramen or delthidial plates. A broad and angular sulcus in the pedicle valve originates in the anterior part of the valve. Evenly rounded lateral sides with maximum width posterior, near the midline. Relatively high apical angle at 22° on average (Table 1).

Table 1.

Univariate summary statistics based on measurements of Eisaella uniplicata n. gen. n. sp. N = number of specimens measured, W = width, Lv = saggital pedicle length, U to I = umbo to start of fold, U to W = distance from umbo to the maximum width of the valve measured along the mid-length, T = maximum thickness, T ped = thickness of pedicle valve, T brach. = thickness of brachial valve, Angle = posterior angle of commissure, W F = maximum width of fold, D F = maximum depth of fold, St. dev. = standard deviation from the mean, and var.-covar. = variance-covariance matrix. Mean, maximum, and minimum are in mm.

Very thick interior posterior shell wall. Pedicle collar seems to be missing, but the preservation is poor. Short, strong, massive, teeth extended forwards with short lobes curving under projecting socket plates (Fig. 5). Strongly impressed pedicle muscle scars delineated by strong ridges deviating slightly and making a narrow, triangular muscle platform. Small, elongated, and strongly impressed adductor scars. Brachial muscle scars are also impressed and flaring out. Thick, horizontal, and strong hinge plates elevated on a cardinal pad with bulbous socket plates. Long cardinal pit without cardinal process. Median ridge in the posterior one-third to one-half of the valves. Crura extend up ventrally and curve laterally before they join the spiralia. Long jugal processes end in ring-like plates (Fig. 6). Eight whorls in the spiralia.

Etymology

Latin for single plication.

Remarks

Species same remarks as genus.

Holotype

LDM G 30/224 (Latvian National Museum of Natural History) Choldre borehole at 333.9 m, Latvia, Aeronian. Stored at the Museum of the Institute of Geology, Riga, Latvia.

Occurrence

Upper Myren Member, Malmøya, Oslo, Leangen Member, Leangen, Asker, uppermost Rhuddanian through Aeronian.

Description

Small, smooth valves, wider than long (length to width 85%, Table 2), with rounded margins and maximum width in the anterior part of the valve (maximum width to length, 65%). Posterior part is somewhat triangular (apical angle 135–140°), giving the shell a drop-like outline. Ventribiconvex to biconvex shell with a slight sulcus in the pedicle valve that varies, but starts in the anterior part. Small, strongly incurved beak overhanging the hinge line. Evenly rounded transverse cross-section and longitudinal profile rounded posterior that flattens out somewhat anteriorly.

Table 2

Measurements of Eisaella minuta n. comb. See Table 1 for explanation of abbreviations. PMO = Specimen numbers from the Paleontological collection of the Natural History Museum in Oslo, Norway.

Thick interior shell wall with a pedicle callist. Strongly impressed muscle scars in pedicle valve. Posterior part not captured in the sectioned specimen, but an eventual pedicle collar is unlikely since no foramen is visible. Teeth seem not to engage in the dental sockets. Low, thick, and very short brachial hinge pad with a broad v-shaped cardinal pit. Large, fibrous crural bases are gently arched ventrolaterally until they nearly touch the shell walls. Jugal processes originate close to the shell walls and are directed medially before they turn anteriorly a short distance. Secondary crystals prevent a clear view of the nature of the plates, but they are not as closely placed as seen in E. uniplicata n. gen. n. sp. Four whorls in the spiralia (Figs. 5, 6).

Remarks

Rybnikova (Reference Rybnikova1967) described this brachiopod from Rhuddanian and Aeronian strata as belonging to Glassia Davidson, Reference Davidson1881. She noted the species differed from later Glassia in the convexity of the shell and the sulcus in the pedicle valve. Copper (Reference Copper2004) referred Rybnikova's species to Lissatrypa. He described and sectioned L. cf. L. minuta from Telychian strata of the Hugley Shale near Wenlock Edge, Great Britain, and noted that the material probably was conspecific. However, the British material is lacking the sulcus in the pedicle valve. Furthermore, Rybnikova's drawings show a low pedestal as opposed to the high one in the British material. The British material seems to be true Lissatrypa, whereas the coeval Latvian and Norwegian material belong to Eisaella n. gen. The fibrous crura that initially are directed ventrally and then gracefully curve laterally are typical both of Lissatrypa and Eisaella n. gen. The Norwegian material is very scant, but similar to Lissatrypa minuta (Rybnikova, Reference Rybnikova1967). Lissatrypa minuta Kulkov, Reference Kulkov1967 from Wenlock strata in Gorno-Altai was excluded from the species by Copper (Reference Copper2004). It is close to Eisaella minuta n. comb. in outline and in having a sulcus in the pedicle valve. The figures are schematic, but a hinge pad is missing and the crura are different than the present material. It is excluded from Eisaella n. gen.

Baarli (Reference Baarli1987) collected and acid treated 10 kg samples from the Solvik Formation and counted the macro fossils for community analysis. Most of the 36 samples from Malmøya was donated by Dr. D. Worsley. ?Cyclospira Hall in Hall and Clarke, Reference Hall and Clarke1893, occurred commonly in the Eoplectodonta-Isorthis assemblage and is likely Eisaella minuta n. comb.

Type species

Hemithyris subundata McCoy, Reference M'Coy1851, SM A28851 from “Lower Llandovery,” Rhuddanian or early Aeronian, Mathrafal, near Meifod, Powys, Wales.

Remarks

Williams (Reference Williams1951) described two species of Meifodia and divided each into two subspecies. He proposed that the lineage underwent gradual evolution. Temple (Reference Temple1987) made a quantitative study of British specimens and found some of the same changes. However, this idea has not been thoroughly tested. Looking at the internal characteristics, Williams (Reference Williams1951) noted six traits that change gradually stratigraphically: (1) fusion of hinge plate acting as supports to the median ridge, (2) progressive strengthening of a transverse ridge bearing the pedicle muscle scars, (3) increasing definition of the pedicle chamber, (4) increasing massive development of the hinge plates and appearance of a septalium-like feature in later forms, (5) decreasing distinctness of brachial muscle scars, and (6) progressively more massive development of teeth. Meifodia occurs abundantly both as molds and whole specimens in the Norwegian sections near the base of the Solvik Formation in the western parts of the Oslo-Asker District and through the upper half of the Myren Member and the Padda Member in the eastern part of the district. The present study agrees with points 2, 3, and 5, related to changes in muscle scars and the definition of the pedicle chamber and point 6 as the teeth broaden. In spite of great variation in the Norwegian material, there is a general increase in the size of the cardinalia relative to the valve, which is in agreement with point 4 of Williams (Reference Williams1951) (Figs. 7, 8). This would accommodate the increase in width of the teeth. Point 1 of Williams (Reference Williams1951) is not so clear cut. The older of the two sectioned specimens has a slight lengthening of the inner socket plates anteriorly, which would correspond to the supporting plates mentioned by Williams (Reference Williams1951). Molds of older specimens show that some of these plates stay parallel (Fig. 8.1, 8.5), others converge towards the median ridge (Fig. 8.2, 8.3), but they do not meet in any of the six molds present, thus Williams's (Reference Williams1951) definition of plates not merging is correct. The illustrations by Temple (Reference Temple1987, pl. 14, figs. 1, 2, 7) show the same variation. The sectioned specimen from the upper range does not show a “septalium-like” feature, but it is present in three of the four molds (Fig. 8.10, 8.11), so the latter part of Williams’ (Reference Williams1951) point 4 is partially right. Cocks (Reference Cocks2019, pl. 37, fig. 2) also figured specimens from British material with this feature.

Figure 8.

(1–7) Meifodia prima prima. (1, 4) PMO 234.007, mold of conjoined valves, posterior views with pedicle valve on top and bottom position, respectively, 20 m above base; (2, 5) PMO 234.609, mold of conjoined valve, posterior views with pedicle valve on top and bottom position, respectively, 8–10 m above the base; (3) PMO 234.006, mold of brachial valve, 20 m above the base; (6) PMO 158.145, mold of pedicle valve 21 m above base; all molds from base of Myren Member, Solvik Formation, outcrop along Motorway E18 parallel to Nye Vakås Vei, Asker; (7) PMO 41.220g, pedicle view, part of the collection investigated by Copper (Reference Copper1995) from the southern coast of Malmøya, Myren Member. (8–12) Meifodia prima ovalis, uppermost Myren Member, Ulvøya bad, Ulvøya, Oslo; all from Solvik Formation; (8, 9) PMO 234.614, molds of conjoined valve, (8) brachial view, (9) pedicle view; (10) PMO 234.612, brachial view of mold; (11, 12) PMO 234.613, brachial (11) and pedicle (12) views of mold. (8–12).

The hinge pad with the cardinal pit changes into a high median septum or pedestal between the hinge plates (Figs. 7–9) before it fuses with a lower and thinner ridge, as seen in thin sections from the lower and higher ranges in Norway. Williams’ (Reference Williams1951) descriptions do not include this feature and it is not evident in figures from Temple (Reference Temple1987) or Cocks (Reference Cocks2019) from Britain. Instead, M. prima ovalis shows a short transverse bar in front of the cardinalia—something not seen in Norwegian material. Both must have acted as supports for the cardinalia and they may be local variations of such support. Molds from both low and high stratigraphic levels in Norway show that the pedestal within the cardinalia is a variable feature from very high to non-present. There may be a trend since it is present, although not very high, in two specimens out of six in the lower range and high in five out of six in the upper range.

Figure 9.

Meifodia prima prima. Serial section of specimen PMO 52.309h from basal Myren Member, lowermost part of the profile at Spirodden, Asker. Meifodia prima ovalis. Serial section of specimen PMO 41.227 from Padda Member at Malmøykalven, Oslo.

Copper (Reference Copper1995) made thin-sections of specimen PMO 41.220b from Malmøya in Norway (Fig. 7). This comes from a collection of 10 valves collected by the Norwegian paleontologist Kiær from the southern side of Malmøya. They are marked as level 6b, which corresponds to the upper half of the Myren Member (upper Rhuddanian to lower Aeronian). The southern side exhibits an anticline with the Padda Member of Aeronian age on the two sides and the upper half of the Myren Member with the lowest strata of middle Rhuddanian age in the middle. Most material comes from the point on the southern beach with these lowest strata. Hence, it is most likely that these are upper Rhuddanian in position.

Williams (Reference Williams1951) did not describe the jugal processes. There is a marked difference between the stratigraphically older specimens with fairly thick jugal processes directed straight anteriorly and younger specimens with slender processes that nearly touch (Fig. 7). The thin section of Copper (Reference Copper1995, fig. 8) shows jugal processes in an intermediate position, so this might be an additional trend. Otherwise, the thin section differs markedly from sections made by the present author (Figs. 7, 9) in that there are no signs of a “pedestal.” The height and presence of the “pedestal” increase upwards stratigraphically, so low or none is a primitive feature. Furthermore, there are no supporting plates lateral to the hinge plates, but the thin-sections that potentially could reveal this point are not published. Acid treatment of one of the other specimens in Copper's collection (Fig. 8.7) shows pedicle scars that conform to those of Meifodia subundata forma typica of Williams (Reference Williams1951). The valve was partially filled by secondary calcite, making it hard to observe the features of the cardinalia, but a fairly wide cardinal pit and a “pedestal” are present, together with converging support plates. Copper (Reference Copper1995) assigned his specimen to Meifodia cf. M. subundata based on the differences found in the thin section. This study suggests that the thin section made by Copper (Reference Copper1995) is an outlier among specimens with transitional features that fall between the two end members. The collection is here included in Meifodia prima prima.

Williams (Reference Williams1951) did not distinguish stratigraphically older and younger material by external characteristics. Differences are difficult to discern visually (Fig. 10). To test actual differences, specimens from the basal layers of the Solvik Formation in Asker (upper Hirnantian–lower Rhuddanian) were measured and compared to material from upper Myren and Padda members at Malmøya (Aeronian). The Meifodia specimens from the collection with the one specimen sectioned by Copper (Reference Copper1995) and others from the same level, upper Rhuddanian, were also measured. The specimens are from the collections of the Paleontological Museum in Oslo, including additional specimens of Aeronian age collected by the author. The museum material is quite abundant, but many lack accurate stratigraphic information so those without information were omitted from this study. The result was 32 specimens from the basal Myren Member (uppermost Hirnantian–lower Rhuddanian strata) and 20 specimens from the uppermost Myren and Padda members (Aeronian strata), together with the specimens from Copper's (Reference Copper1995) study (upper Rhuddanian). Ten different features were measured (see Fig. 3, Tables 3, 4). Principal component analysis (PCA) was conducted to visualize differences between the populations and to supplement basic statistical summaries for the three populations. To mitigate for variations in body size, components 2 and 3 were utilized.

Figure 10.

(1–9) Meifodia prima prima. (1–5) PMO 52.379, pedicle, brachial, posterior, side, and anterior views, lower part of Myren Member, Spirebukta, Asker; (6, 7) PMO 52.133, pedicle and anterior views, basal Myren Member, Konglungø, Asker; (8, 9) PMO 52.055a, brachial and pedicle views, basal Myren Member, Høyerholmen, Asker. (10–17) Meifodia prima ovalis. (10–12) PMO 21.009, pedicle, brachial, and anterior views uppermost Myren Member, Malmøya, Oslo; (13–17) PMO 41.240a, posterior, brachial, pedicle, side, and anterior views, Padda Member, Malmøya, Oslo. All Solvik Formation.

Table 3.

Univariate summary statistics based on measurements of Meifodia prima prima. See Table 1 for explanation of abbreviations.

Table 4.

Univariate summary statistics based on measurements of Meifodia prima ovalis. See Table 1 for explanation of abbreviations.

The result mimics what was clear from internal molds and thin sections—wide variation within individual populations (Fig. 11.1). The lower and upper populations plot in different areas, but these areas overlap. The most important feature separating them is the angle of the posterior margin (Fig. 11.2). This concurs with larger beaks that jut out farther and larger cardinalia in the stratigraphically younger population. Two other features found to be of importance are the distance from the posterior end to maximum width and the distance from the posterior end to the start of the fold. This means the shells from higher levels tend to be rounder in outline and the fold tends to start farther posteriorly. The small collection used by Copper (Reference Copper1995) plots very much in the middle of these two areas (Fig. 11.1).

Figure 11.

(1) Principle component analysis (PCA) of ten measurements compiled from three populations of Meifodia prima using components 2 and 3 and showing convex hull. Specimens taken from the lower half of Myren Member (uppermost Hirnantian and basal Rhuddanian) are shown as “x,” specimens used by Copper (Reference Copper1995) and other shells from the same horizon in the lower upper half of the Myren Member (upper Rhuddanian) are shown as black squares, and specimens from the upper Myren Member and the Padda Member (Aeronian) are shown as open circles. (2) Eigenvalues from the measurements used in Figure 12.1. (3). Bivariate plot of the apical angle towards maximum width with linear trend and R² value line for all the data points and linear trends for the stratigraphically lower population and the population occurring towards the top of the Solvik Formation. (4). The same bivariate plots and trends for length from umbo to inflex of sulcus towards maximum width.

Bivariate plots (Fig. 11.3, 11.4) correlating the size of the valves represented by maximum width and other measurements confirm the large variability in morphology. The distance from umbo to the inflex of the sulcus increases with size, as expected, although the increase is smaller in the later populations (Fig. 11.4). The exception is the angle of the posterior margin (Fig. 11.3). The angle is nearly independent of size of the shells (R² = 0.0727, Fig. 11.4), but the lower populations occurring in strata around the O/S boundary plot lower (i.e., the angle is consistently lower) than the combined middle and upper populations from Malmøya from the upper 60 m of the 190 m thick Solvik Formation.

As pointed out by Williams (Reference Williams1951) and Temple (Reference Temple1987) and shown in this study, there is a gradual change in the prevalence of certain internal features between the stratigraphically oldest and youngest specimens. This study adds converging jugal processes and changes in external shape that occur with time with a beak that becomes more prominent, shells that tend to be rounder and have a sulcus that starts farther anterior with time. The PCA supports a pattern of gradual phyletic evolution where no final separation of the two phena has occurred. Individuals from the older and younger population may be distinctly different from each other, but they fall along a continuum with extreme specimens, which makes them difficult to separate.

Temple (Reference Temple1987) stated that the type material is poorly constrained in age and locality and consists of just three specimens. He regarded M. subundata as a nomen dubium. Instead, he recognized M. prima prima and M. prima ovalis; two of the subspecies determined by Williams (Reference Williams1951). Cocks (Reference Cocks2019) elevated M. ovalis to species level. A species is an arbitrary segment of an evolving lineage in time (Vaux et al., Reference Vaux, Trewick and Morgan-Richards2016). That is a fitting definition and an elevation to species for M. ovalis may be defended in that light. However, there are still large overlaps in features and the evolving traits could probably still be reversed. To keep them as subspecies, as recommended by Temple (Reference Temple1987), is more prudent. The Solvik Formation where Meifodia occurs throughout was deposited over ca. 5.6 Ma and most species have a shorter duration than that (Patzkowsky and Holland, Reference Patzkowsky and Holland1997). Exceptions are species with a large geographic distribution (e.g., Meifodia prima), which tend to have greater longevity than endemic species (Heim and Peters, Reference Heim and Peters2011) since traits can be reintroduced from elsewhere. In conclusion, this is an example of gradual phyletic evolution or a biased random walk where a wide range of variation in traits is expected as seen in the coeval genus Stricklandia (Williams, Reference Williams1951; Baarli, Reference Baarli1986). Practically, a handful of specimens has to be used for definition of subspecies.

Holotype

SM (Sedgwick Museum) A30114, a brachial valve from strata of Rhuddanian age at Esgair-fwyog, northeast of Llandovery, Dyfed, Wales.

Occurrence

Høyerholmen, Ostøya, Bærum, Konglungø, Spirebukta, Spirodden, and Vakås in Asker, near the base of the Solvik Formation. Sjursøya and Malmøya in Oslo. All from strata of uppermost Hirnantian to middle Rhuddanian age.

Description

Medium-sized, strongly dorsibiconvex, smooth shells, broader than long (average, length 77% of width; Table 3). Wide variation in width to length, but generally, the proportions increase with size. Broad hingeline with small, adpressed beak and small (most often hidden or apical) foramen. Low posterior angle becomes lower with increasing width. Maximum width well in front of the midline (Table 3). A characteristically strong and often angular sulcus in the pedicle valve varies and is proportionally deeper in small valves. Sulcus initiates at about mid-length and may inflex abruptly. Corresponding fold in the brachial valves starts in the posterior third and is sometimes outlined by a small groove on each side.

Very thick posterior shell wall (Fig. 9). Narrow, deep muscle platform has strongly impressed adductor scars and a faintly impressed transverse ridge (Fig. 8.4, 8.6). Less well-impressed diductor scars splay out at ~45° angle. Short and medium thick teeth. Well-impressed and large muscle scars in the brachial valve. A very short cardinal pit and relatively small, short, and horizontally directed hinge plates. A characteristic median pedestal (median septum) separates them, descends to the floor at the front of the hinge plates, and goes on as a low ridge to about one-third of the shell length (Figs. 7, 9). Short, brachial continuations of the inner socket plates, or septal support, may be parallel or less commonly angled towards the median septa. Thin crura splay out ventrolaterally. Far-separated jugal processes stay high ventrally and splay out into thin plates. Eight to nine whorls in the spiralia (Fig. 9).

Remarks

The pedicle muscle marks, as seen in Figure 8.6, conform to those described by Williams (Reference Williams1951, SM A 30118), although they are more strongly imprinted than those seen in most other valves. Williams described the subspecies prima as biconvex. The degree of convexity in the Norwegian material varies, but the brachial valve is consistently the deeper valve.

Holotype

SM A301214, one-quarter mile southwest of Llwyn-yr-iâr, north-east of Llandovery, lower Aeronian.

Occurrence

Malmøya, Malmøykalven, and Ulvøya bad, all in Oslo in uppermost Myren and the lower parts of Padda members, basal to middle Aeronian.

Description

Smooth, transverse (average length 76% of the width), dorsibiconvex valves with a somewhat rounded outline (maximum width close to mid-length). Broad hingeline; adpressed and fairly prominent beak creating an average posterior angle of 20° (Table 4). Strong, deep sulcus with rounded flanks originates at about mid-length and occupies half of the valve width. Corresponding fold in the brachial valve originates in the anterior third. Fold may have small plica. All these characters vary, as seen in Figure 10.

Thick shell posteriorly. Muscle scars sit on a platform with small, deeply impressed adductor scars, flanked by large, broad, and flabellate diductor scars. Strongly impressed muscle scars with high, narrow ridge in the pedicle chamber, divided by a median indentation (Fig. 8.12). Fairly thick, short, and massive teeth (Fig. 9). Medium-well impressed and oval muscle scars in the brachial valve. Very short, broad cardinal pit. Bottom of the pit stays elevated as a pedestal or median ridge and increases in height towards the front of the cardinalia, continuing in a short and low median ridge (Figs. 7, 9). Fairly delicate, strongly splayed out, and horizontal hinge plates. Fine crura directed laterally with thin, long jugal processes ending in horizontal plates that nearly touch in the middle of the valve. Eight whorls are visible.

Remarks

The internal characters, as seen in molds, correspond to what was described for the species in Williams (Reference Williams1951). As found by him, the brachial muscle scar is faintly impressed. The brachial valve relative to the pedicle valve is shallower than what is found in older populations on average (Table 4).

Genus Thulatrypa Huang et al., Reference Huang, Baarli, Zhan and Rong2016

Type species

Thulatrypa gregaria Huang et al., Reference Huang, Baarli, Zhan and Rong2016, PMO 158.116, uppermost Leangen Member of the Solvik Formation, Bleikerveien.

Remarks

The type species of Thulatrypa is very abundant in the Aeronian strata of the Oslo-Asker districts. Specimens with unsupported cardinal plates jutting into the valve displaying peculiar crural supports (see Fig. 12), similar to the type species Thulatrypa gregaria, are common throughout the Solvik Formation. However, the shape, size of the shells, and the number of spiralia vary widely.

Holotype

PMO 158.116, uppermost Leangen Member of the Solvik Formation, Bleikerveien, Asker.

Figure 12.

Cross sections of the posterior parts of the brachial valves from the type species Thulatrypa gregaria, T. vikenensis n. sp., T. huangi n. sp., and ?T. new species showing the cardinal plate with crural supports and crus. Note how the fibers in the cardinal plate curve up into the crural supports. The cross-sections are not drawn to scale.

Figure 13.

(1–4) Thulatrypa gregaria. (1, 2) PMO 21.084, pedicle and side views, from Padda Member, Malmøya, Oslo; (3, 4) PMO 52.727h, side and pedicle views, uppermost Spirodden Member, Vettre Brygge. (5–22) Thulatrypa huangi n. sp. (5–9) PMO 158.291g, holotype, brachial, pedicle, side, anterior, and posterior views; (10–13) PMO 158.107r1, brachial, pedicle, side and posterior views; (5–13) 52–57 m above base of Leangen Member, Skytterveien; (14–18), NIGP 174600, posterior, brachial, pedicle, side and anterior view; (19) NIGP 174601, anterior view; (14–19) upper part of Leangen Member, Skytterveien; (20) PMO 105.872, mold of pedicle valve, 10 m above base of Leangen Member, Skytterveien; (21) PMO 234.625, mold of brachial valve, 41 m above base of Leangen Member, Leangbukta; (22) PMO 158.119, mold of pedicle valve, 16 m above base of Leangen Member, Skytterveien; (23–28) ?Thulatrypa n. sp. (23–26) PMO 234.635, posterior, brachial, ventral, and side views, 4 m above base of Leangen Member, Skytterveien; (27, 28) PMO 158.222, brachial and pedicle views, 20 m above base of Leangen Member, Leangbukta; (3–28) all Asker.

Occurrence

Malmøya and Malmøykalven in Oslo, Skytterveien, Leangen, Vettrebukta, and Vettre Brygge in Asker, Løkkeåsen in Sandvika, uppermost Rhuddanian through Aeronian.

Remarks

Thulatrypa gregaria is one of the most common brachiopods in the Aeronian, especially in the Asker area. It was well described by Huang et al. (Reference Huang, Baarli, Zhan and Rong2016); however, its exact stratigraphic range was not established in that article. Specimens in a collection from uppermost Rhuddanian strata at Vettrebukta (Fig. 13.3, 13.4) are generally smaller and more biconvex compared to the large, strongly dorsibiconvex specimens found farther up in the section (Fig. 13.1, 13.2). There is an increase in convexity of the valve with increasing size in T. gregaria (Huang et al., Reference Huang, Baarli, Zhan and Rong2016, fig. 12B). Internal features seen in thin sections of the early specimens concur with T. gregaria (Fig. 14). PCA plots using the same ten external measurements used for Meifodia (Fig. 3) show that the two populations (using convex hulls) plot in different areas, but intersect slightly (Fig. 15). The main differences between the populations are in width, umbonal angle, and the distance from umbo to inflex of the sulcus, with differences in brachial depth as a secondary component. Much of this is due to ontogenetic change., but a larger intersection in PCA was expected because the populations overlap in size. The early specimens are likely phenotypes of the later T. gregaria. Thus, the range of that species is extended down into the uppermost Rhuddanian.

Figure 14.

Thulatrypa gregaria. Serial section of specimen PMO 52.727 from the top of Spirodden Member at Vettre Brygge, Asker (stratigraphically low Thulatrypa) compared with selected cross-sections of the holotype specimen, PMO 158.116 (redrawn from Huang et al., Reference Huang, Baarli, Zhan and Rong2016, fig. 6).

Figure 15.

PCA of 10 measurements on individual specimens (see fig. 3) of Thulatrypa gregaria from uppermost Rhuddanian (denoted by black, filled circles in area of plot designated “Lower”) and middle Aeronian (crosses in area of plot designated “Upper”).

Holotype

PMO158.291 g from 52–57 m above the base of Leangen Member, Solvik Formation, (mid-Aeronian), Skytterveien, Asker. A whole shell (Fig. 13.5–13.9).

Diagnosis

Smooth, biconvex, and medium size, with maximum thickness in the posterior part of the valve. Small, adpressed beak with apical foramen and clear growth lines anteriorly. Pedicle valve with wide, gentle sulcus and lightly incised triangular muscle scars. Unsupported cardinal plate with medium-strong crural support and thin crural bases with fibrous crura. Four to six whorls in the cardinalia.

Occurrence

Malmøya, Oslo. Vettre, Leangbukta, and Skytterveien, Asker. Occurring throughout Aeronian.

Description

Medium-sized, smooth, and evenly biconvex with rounded margins and a wide, gentle sulcus in the pedicle valve. Slightly wider than long valves (length 87% of width on average; Table 5) and maximum thickness in the posterior part of the valve giving it an almond-shaped, sidewise cross-section. Clear growth lines over most of the valves, especially anteriorly. Very small and strongly incurved beak with small foramen. Delthidial plates not visible.

Table 5.

Univariate summary statistics based on measurements of Thulatrypa huangi n. sp. See Table 1 for explanation of abbreviations.

Pedicle valve with lightly incised, triangular muscle field, small adductor scars and long, narrow diductor scars. Strong, solid, broad, and short teeth (Fig. 16). Very thick pedicle shell wall posteriorly, probably with a pedicle callist. No clear cardinal process or cardinal pit. Cardinal plate with moderately thick socket plates, concave hinge plates, and fairly thin crural-base supports. Supports start far posterior and continue onto the unsupported cardinal plate. Ventrolaterally directed, delicate, and fibrous crura (Fig. 17). Jugal processes not observed, although a hook-like structure seen in the cross-section at 2.9 mm (Fig. 16) may represent part of one. Four wide whorls in the medio-anteriorly directed spiralia.

Figure 16.

Thulatrypa huangi n. sp. Serial section of specimen PMO 158.107s1 from the top of Leangen Member at Skytterveien, Asker. Thulatrypa vikenensis n. sp. Serial section of specimen PMO 209.199 from 20 m above the base of the Myren Member, outcrop along E18 motorway, Vakås, Asker.

Figure 17.

Reconstruction of internal structures based on specimen PMO 158.107s1 for Thulatrypa huangi n. sp. (see above), for Thulatrypa vikenensis n. sp. based on specimen PMO 209.199 (see above), and ?Thulatrypa n. sp. based on specimen PMO 153.873 (see below).

Etymology

Named in honor of Dr. Bing Huang from Nanjing Institute of Geology and Palaeontology who loaned me specimens from his collection.

Remarks

This species of Thulatrypa is close to T. vikenensis n. sp. that occurs from the base of the Solvik Formation and overlaps slightly in age near the base of Aeronian. It differs in that T. huangi n. sp. has an almond-shaped cross-section where maximum thickness is in the posterior part as compared with a flatter valve that tapers steeply near the anterior margin. Thulatrypa vikenensis n. sp. has an anacline area and foramen that is mesothyroid, while the beak is strongly incurved with apical foramen in T. huangi n. sp. It differs from T. gregaria in size, being biconvex, having a fairly narrow triangular muscle scar in the pedicle valve, and an apical foramen.

Holotype

PMO 234.041 from 8–10 m above the base of the Myren Member, Solvik Formation, Vakås, along E18, Asker. A conjoined shell.

Diagnosis

Medium-sized, rounded, equally biconvex to slightly ventribiconvex and relatively flat Thulatrypa. Length close to width and the cross-sections, both longitudinally and transversely, in the two valves are evenly convex. Anterior margin rectimarginate with well-developed growth lines anteriorly. Beak very small and orthocline to anacline with mesothyridid foramen. Thick, prismatic shell. Narrow pedicle collar, no cardinal process present on a cardinal plate supported by bulbous socket plates.

Occurrence

Vakås along E18, Spirodden, Skytterveien, all Asker and Malmøya, Oslo, lowermost Rhuddanian to lower Aeronian.

Description

Medium-sized, smooth, low biconvex, with wide hingeline (Fig. 18). Maximum width is around mid-length and the margin is evenly rounded. Average length to width is 93% and thickness to width 54% (Table 6). Anterior margin is rectimarginate to slightly uniplicate. Beak very small, orthocline to anacline, with mesothyridid foramen and tiny triangular delthidial plates. Thick, prismatic shell. Growth lines spaced two to three mm apart anteriorly. The muscle field in the pedicle valve is wide, triangular, and fairly well impressed. Adductor scars are not visible on the specimen. A pedicle callist with a narrow pedicle collar is seen in the one available mold (Fig. 18.15). Brachial muscle scars are weakly impressed on the side of a short median ridge. The teeth are fairly large, simple, and directed anteromedially. There is no cardinal process, but a cardinal plate with well-developed crural support (Figs. 16, 17) is present. The crural bases are fairly large and rounded. The socket plates are large and the cardinal plate juts unsupported out into the valve. The crura continue into the pedicle part of the valve. Jugum is not known and parts of the spiralia are lying sidewise in the brachial valve. There are 3–4 whorls present and the distance between them indicates that there was maximum space for 6–7 whorls.

Figure 18.

(1–16) Thulatrypa vikenensis n. sp. (1–5) PMO 234.041, holotype, pedicle, brachial, side, posterior, and anterior views, 8–10 m above base of Myren Member, Vakås, along E18; (6, 7) PMO 158.873 brachial and side views, 19 m. above the base of the Leangen Member, Skytterveien; (8–10) PMO 209.199 (sectioned) pedicle, brachial, and anterior views, 20 m above the base of the Myren Member; (11–14) PMO 234.619, posterior, brachial, pedicle, and side views, 8–10 m above the base of the Myren Member; (15, 16) PMO 234.610 mold, brachial and pedicle views, 8–10 m above the base of the Myren Member; (8–16) along motorway E18 Vakås, Asker. (17–24) ?Becscia pentagona n. sp. (17–20) PMO 87.387a, holotype, pedicle, brachial, side, and posterior views, lower part of Myren Member at the west side of Ostøya, Bærum; (21–24) PMO 88.163, brachial, posterior, anterior, and pedicle views, lower part of Myren member, Spirodden, Asker. (25–28) ?Glassia sp., PMO 87.601, brachial, pedicle, side, and posterior views, middle to top of the Myren Member, Malmøya, Oslo.

Table 6.

Univariate summary statistics based on measurements of Thulatrypa vikenensis n. sp. See Table 1 for explanation of abbreviations.

Etymology

From the major county (Viken) it which it occurs.

Remarks

The present material occurs from the base of Rhuddanian into Aeronian. Externally it conforms to the shape of Lissatrypa that first appeared in Aeronian strata, although the beak is not much incurved. It also has the mesothyridid foramen and pedicle collar of Lissatrypa, while the cardinal plate with crural supports aligns it with Thulatrypa. Eisaella n. gen. from basal Rhuddanian is close to Meifodia in shape and a likely ancestor of Lissatrypa. Since Meifodia, Eisaella n. gen., and Thulatrypa appear together in O/S boundary layers, this must have been a time of intense evolution of the lissatrypides on the Baltic continent. The divergence of these genera was yet not finished and a possible common ancestor may be found in older Hirnantian strata. Another unstable species is ?L. hunanensis (Rong et al., Reference Rong, Xu and Yang1974), from lower Aeronian in China, which was moved from Thulatrypa to Lissatrypa (Huang et al., Reference Huang, Harper, Zhou, Zhan, Wang, Tang, Ma, Wang, Chen, Zhang and Luan2019), but shares characteristics with both genera.

Thulatrypa orientalis occurs in beds of middle Rhuddanian age of South China, and Huang et al. (Reference Huang, Baarli, Zhan and Rong2016) postulated that Thulatrypa migrated from South China to Baltica where T. gregaria occurs from upper Rhuddanian strata. Thulatrypa vikenensis n. sp. occurs in layers of uppermost Hirnantian age into early Rhuddanian, so this postulation might be revised. The species may have followed the southern gyre from Baltica to China. However, all other atrypide genera described from the Solvik Formation seem to originate in the east and follow the currents westward (Baarli, Reference Baarli2021a, Reference Baarlib), raising the possibility that the genus remains to be found in older strata in China.

?Thulatrypa new species
Figures 12, 13.23–13.28, 17, 19

Figure 19.

?Thulatrypa n. sp. Serial section of specimen PMO 153.873, from near the top of the Leangen Member, Skytterveien, Asker.

Occurrence

The species occurs at Leangbukta and Skytterveien, Asker throughout Aeronian strata.

Description

Very small, slightly biconvex with a strongly projecting beak, anacline interarea with long, narrow delthidial plates. Cardinal angle between 30–35° (Table 7), with anterior part of the valve evenly rounded and rectimarginate margin. Pedicle callist and medium-strong, short teeth curving below the cardinal plate. Bulbous socket plate and concave hinge plates. The cardinal plate curves gently ventrally. Small crural supports border thick crural bases that continue onto a cardinal plate jutting unsupported into the valve. Crural bases extend vertically into the pedicle valve initially before they curve laterally. There are five whorls in the spiralia.

Table 7.

Measurements of ?Thulatrypa n. sp. See Table 1 for explanation of abbreviations. PMO = Specimen numbers from the Paleontological collection of the Natural History Museum in Oslo, Norway.

Remarks

Only three poorly preserved specimens are known of this species. For that reason, it has not been formally defined. The material does not fit comfortably within Thulatrypa. It is a very small species with an outline and shape far different than any other Thulatrypa. It is provisionally aligned with Thulatrypa because it exhibits the typical cardinal plate that juts into the valve with the peculiar crural base supports (Figs. 12, 17). However, the crura initially project ventrally as in Lissatrypa, not laterally as in other species of Thulatrypa. This might be assigned to a new genus if sufficient material becomes available.

Type species

Becscia scissura Copper, Reference Copper1995 from the lower parts of the Becscie Formation, uppermost Hirnantian into Rhuddanian, west side of Fox Point, Anticosti, Canada.

Remarks

Genera occurring in strata of transitional Hirnantian and Rhuddanian age are Becscia Copper, Reference Copper1995, and Idiospira Cooper, Reference Cooper1956, belonging to the subfamilies Septatrypinae Kozlowski, Reference Kozlowski1929, and Idiospirinae Copper, Reference Copper, Williams, Brunton and Carlson2002, respectively. What genus and family the material below represents depends on the nature of the jugal processes, which are obscure in the material described below.

Holotype

PMO 87.387a (Fig. 18.17–18.20), from the basal Myren Member at the west side of Ostøya, Asker.

Diagnosis

Very small, ventribiconvex, smooth shells with maximum width in the posterior half. Adpressed and pronounced beak. Outline pentagonal with the two more or less-pronounced folds starting far posterior. Sulcus between the folds deepens abruptly near the anterior margin. Cardinal pit present between slightly divided hinge plates.

Occurrence

Ostøya, Høyerholmen, Asker; all from basal Solvik Formation.

Description

Small, smooth shell with two characteristic pedicle folds starting far posterior ending in a sulcus (Fig. 18.24). Fold in the brachial valve starts at mid-length. Deeply ventribiconvex shells, longer than wide (length 104% of width; Table 8) with a slightly pentagonal outline. Pronounced beak (apical angle 115°), strongly curved, and adpressed.

Table 8.

Univariate summary statistics based on measurements of ?Becscia pentagona n. sp. See Table 1 for explanation of abbreviations.

Pedicle valve with thin shell wall. Large dental cavities and teeth that curve below the socket plates (Fig. 20). Low and relatively robust hinge plates supported by socket plates. Initially, horizontal and nearly touching, but farther anterior diverge and curve up into the pedicle valve. Valve filled with secondary crystals making the thin structures of the crura and jugal processes impossible to discern. Clearly seen dorso-medially directed spiralia with four whorls.

Figure 20.

?Becscia pentagona n. sp. Serial section of specimen PMO 87.387b, basal Myren Member, Ostøya, Asker.

Figure 21.

?Becscia pentagona n. sp. Reconstruction of internal structures based on specimen PMO 87.387b (see above).

Etymology

From Latin pentagonum, five-angled or pentagonal.

Remarks

Externally, the material is close to Becscia with the typical lateral and diverging corrugations, although an apical foramen was not confirmed. Internally, it differs in that the inner socket ridges are close, but not fused into a cardinal plate as is found in Becscia. Idiospira does not have the typical corrugations, but may have a sulcus anteriorly with a crenulation that gives a w-shaped brachial fold, a feature not seen in this material. It is also biconvex to dorsibiconvex, not ventribiconvex. Internally, Idiospira has separate hinge plates like the Norwegian material. The present material is assigned to Becscia and the subfamily Septatrypinae based on the close external configuration, but a more confident taxonomic assignment depends on discovery of material with visible jugal processes.

Copper and Jin (Reference Copper and Jin2014) ascribed the tiny, ribbed Atrypina gamachiana Twenhofel, Reference Twenhofel1928, to Becscia, while Modzalevskaya (Reference Modzalevskaya, Nekhorosheva and Sobolevskaya2018) described B. scissura Copper, Reference Copper1995, with strong secondary folds from Rhuddanian aged strata in Taimyr. No ribs or secondary folds are seen in the present material.

Type species

Glassia obovate Sowerby, Reference Sowerby1839, from Lower Ludlow Beds, Mathon Lodge, Malvern Hills, Herefordshire, Great Britain.

Remarks

Only one well-preserved specimen of this taxon (PMO 87.601) is present. It has the typical outline of Glassia. However, Glassia typically occurs in younger strata from upper Llandovery and into Devonian, so the assignation to genus is questionable. Species of Glassia are mentioned as occurring fairly commonly at Malmøya in samples collected by Dr. D. Worsley and described by Baarli (Reference Baarli1987) in Aeronian strata.

Remarks

Representatives from this order are not common in the Solvik Formation. Sheehan (Reference Sheehan1977) collected Hindella kiaeri Sheehan, Reference Sheehan1977, from a layer 3–4 m above the base of the Solvik Formation at one locality at Bekkestua, in Bærum. The species also occurs in older beds of the Langåra Formation. Just a couple of pedicle valves that probably represent H. kiaeri have been found at localities investigated in this contribution. They occur at 3 m and 8 m above the base of the Solvik Formation at Konglungø and Vakås, respectively, in Asker (Fig. 22.21, 22.22). A pedicle valve belonging to Cryptothyrella Cooper, Reference Cooper1942, occurs in Aeonian strata from the top of the Leangen Member at Jongsåsveien in Sandvika (Fig. 22.23). Otherwise, there is only one common athyridide, Shelvothyris Cocks, Reference Cocks2019, which will be discussed below.

Figure 22.

(1–20) Shelvothyris bivittata n. sp. (1–5) PMO 88.723 (sectioned), brachial, pedicle, side, posterior, and anterior views, uppermost Myren Member, Malmøya; (6–9) PMO 11.811, anterior, pedicle, brachial, and side views (note that the beak is partially eroded), Solvik Formation, Sandvika. (10, 11) PMO 153.944, brachial and posterior views, middle of Leangen Member. (12, 13) PMO 153.998, brachial and pedicle mold of whole, flattened valve, lower part of Leangen Member; (14) PMO 153.871, mold of brachial valve and (15) PMO 158.900, mold of pedicle valve; (14, 15) top of Leangen Member; (16) PMO 234.015, molds of pedicle and brachial (holotype) valve, 11 m above base of Leangen Member; (17) PMO 234.626, molds of brachial and pedicle valve, upper part of Leangen Member; (18) PMO 234.627, longitudinal cross-section of whole valve and mold of posterior part, lower part of Leangen Member; (19) PMO 158.900, top of Leangen Member; (20) PMO 153.997a, lower part of Leangen Member; (10–20) all from Skytterveien, Asker. (21, 22) Hindella kiaeri (21) PMO 234.010, pedicle valve from 8 m. above the base of the Myren Member, Vakås, along E18, Asker; (22) PMO 158.223, mold of pedicle valve, three meters above base of Myren Member, Konglungø, Asker. (23) Cryptothyrella sp., PMO 234.620, top of Leangen Member, Jongsåsveien, Sandvika.

Type species

Terebratula furcata Sowerby, Reference Sowerby1839. Lectotype reselected by Cocks (Reference Cocks2019), GSM Geol. Soc. Coll. 6909a, from original material collected by Sowerby from Bog Quartzite, Shropshire, Great Britain.

Remarks

Copper and Jin (Reference Copper and Jin2017) listed the type species of this genus, Terebratula furcata Sowerby, Reference Sowerby1839, as Cryptothyrella. Cocks (Reference Cocks2019) questioned this because they defined Cryptothyrella as a medium to large genus with a narrow medial groove in the pedicle apex. Moreover, the dental plates in this material fail to diverge strongly medio-ventrally, as is typical in Cryptothyrella, but is more like those typical of Hindella. However, the dental plates in Hindella are long, whereas Shelvothyris has short ones. The genus is, therefore, considered valid.

Holotype

PMO 234.015, brachial valve from 11 m above the base of the Leangen Member (lower Aeronian), Skytterveien, Asker.

Diagnosis

Shelvothyris with a very long brachial septum covering 50–85% of the length of the valve. Strap-like, moderately impressed muscle scars in pedicle valve diverge just slightly anteriorly. Relatively slender teeth.

Occurrence

Skytterveien, Asker, Sandvika, Malmøya, Oslo. Occurring through Aeronian strata.

Description

Small, biconvex to dorsibiconvex smooth shell with drop-shaped outline, evenly rounded with maximum width in anterior part (umbo maximum width to length 58%; Table 9). Rectimarginate to slightly plicate anterior margin is (Fig. 22.5, 22.6). Strong and incurved pedicle beak with small, triangular, pedicle delthidial plates flanking relatively large apical foramen. Brachial valve with a weaker incurved beak.

Table 9.

Measurements of Shelvothyris bivittata n. sp. See Table 1 for explanation of abbreviations. PMO = Specimen numbers from the Paleontological collection of the Natural History Museum in Oslo, Norway.

Pedicle shell valve thickened posteriorly with shorth, dental lamellae that curve slightly anteriorly (Fig. 23). Relatively slender teeth, extending anteriorly as they curve below the hinge plates. Well-impressed, y-shaped muscle scar with a thin medium groove is somewhat variable in outline. Most diverge modestly at ~20°, but they may splay up to 50°. Two trunks of vascular channels continue anteriorly on each side of the muscle scars. Relatively thin hinge plates posteriorly supported by a median septalium. Farther anteriorly, where the septalium support is absent, they thicken before the crura splay off in a medial position. Septalium comprising 50–85% of the length of the valve, becomes shallower anteriorly. Weakly impressed muscle scars in the brachial valve. The spiralia are not preserved whole.

Figure 23.

Shelvothyris bivittata n. sp. Serial sections of specimen PMO 88.723, uppermost Myren Member, Malmøya.

Etymology

Latin for the double ribbon-like muscle scars in the pedicle valve.

Remarks

Cocks (Reference Cocks2019) described two species from England and Wales, Shelvothyris furcata and Shelvothyris sp. The present material differs from both primarily in the very long and initially deep median septa. The latter species is not well described, although it differs both from S. furcata and S. bivittata n. sp. in having very short dental plates. The type species has short, stumpy teeth and well-impressed muscle scars. The teeth in S. bivittata n. sp. are relatively slender and curl below the hinge plates for a considerable distance anteriorly, and the muscle scars are not so deep. The apical foramen seems larger than in the type material. Shelvothyris furcata was not sectioned by Cocks (Reference Cocks2019). He could, therefore, not assign the species with certainty to a family, but chose the subfamily Whitfieldellinae Alvarez, Rong, and Boucot, Reference Alvarez, Rong and Boucot1998. Whitfieldella Hall and Clarke, Reference Hall and Clarke1893, is a later genus with a complex jugum. Copper and Jin (Reference Copper and Jin2017) simplified the taxonomy and included genera with a simple jugum in the subfamily Hindellinae, which is followed here. Shelvothyris is very common in the high-energy, middle part of the Solvik Formation, but whole specimens are rare. The whole shell that was sectioned for this study had spiralia that were fragmentary. However, the configuration of the cardinalia and the placement of crura (Fig. 23) seem to fall close to Hindella and Koigia Modzalevskaya, Reference Modzalevskaya1985. The earliest complex jugum is known from late Aeronian. Confident assignment of the genus is still not certain, but it likely belongs to Hindellinae.

Remarks

To complete the treatment of the atrypides in the Solvik Formation, three very rare species of the family are added herein. Two shells of Atrypina (Atrypina) Copper, Reference Copper, Williams, Brunton and Carlson2002, of the subfamily Atrypinae McEwan, Reference McEwan1939, occur in the Leangen Member at Leangen and Skytterveien (Fig. 24.1–24.5). The subfamily Clintonellinae Poulsen, Reference Poulsen1943, is represented by two genera, ?Anabaria Lopushinskaya, Reference Lopushinskaya1965, with two valves at the base and 43 m above the base of the Leangen Member at Vettrebukta and Skytterveien, respectively (Fig. 24.6–24.10). Clintonella aprinis Verneuil, Reference Verneuil1845, appears as molds in one single shale horizon, 32.5 m above the base of the Leangen Member, Leangbukta (Fig. 24.11–24.13), together with other rare brachiopods and graptolite fragments.

Figure 24.

Atrypina (Atrypina). (1–5) PMO 51.968t, brachial, pedicle, posterior, anterior, and side views, from Leangen Member, Leangen, Asker. ?Anabaria sp. (6–10) PMO 157.985, posterior (note dental lamellae), anterior, brachial, pedicle, and side views of valve, 0–5 m above the base of the Leangen Member, Vettrebukta. (11–13) Clintonella aprinis from 32 m above the base of the Leangen Member, Leangbukta. (11) PMO 105.238, mold of pedicle valve; (12) PMO 157.989, mold of brachial valve; (13) PMO 157.990 mold of brachial valve.