Institutional abbreviations. INGUJ—Institute of Geological Sciences, Jagiellonian University, Kraków, Poland; MB.W.—Palaeontological Collection of the Museum of Natural History in Berlin; SMF—Palaeontological Collection of the Senckenberg Institute, Frankfurt am Main, Germany.
Ichnogenus Oravaichnium Plička and Uhrová, 1990
2020 Oblongichnus—Bel Haouz et al.: 4, figs. 4–8
Type ichnospecies. Oravaichnium hrabei Plička and Uhrová, 1990.
Diagnosis (revised after Uchman et al. 2011). Straight, winding or meandering unbranched burrow with subrectangular or carinate cross section, homogeneous fill, and smooth or faintly chevroned margin.
Remarks. Originally diagnosed as ‘cylindrical trace’, a revised diagnosis was provided by Uchman et al. (2011) to stress the ‘box-like (non-carinate) cross section’. While this diagnosis remains appropriate for the type ichnospecies of Oravaichnium, this ichnogenus comprises a wide range of morphotypes. Based on the variability of Oravaichnium as analysed from the Muschelkalk, we revise the ichnogeneric diagnosis to include congeneric forms with a carinate cross section.
Oblongichnus solodukhoi Bel Haouz et al., 2020, the type ichnospecies of Oblongichnus Bel Haouz et al., 2020, is herein regarded as a junior subjective synonym of Oravaichnium hrabei (see below) and consequently Oblongichnus as junior synonym of Oravaichnium. Oravaichnium may grade into Protovirgularia by developing or preserving a faint chevron-like ornamentation, but differs from it by the characteristic cross section and an ornamentation that may occur on both, the ventral (lower) and dorsal (upper) burrow part.
Oravaichnium hrabei Plička and Uhrová, 1990
Figures 4, 5 and 6
Lectotype and paralectotypes. The original material of O. hrabei from the Eocene of Slovakia is housed in the Moravec Depository of the Brno Museum, Czech Republic, and consists of a large slab with numerous burrows (catalogue number Ge26017, not Ge2601 as mistakenly stated in Uchman et al. 2011). Plička and Uhrová (1990) as well as Uchman et al. (2011) figured that slab and collectively treated all burrows on it as ‘holotype’, instead of selecting a single specimen, which would be a requirement according to the rules of the International Code of Zoological Nomenclature (ICZN 1999). Consequently, the best preserved and developed burrow on that slab is herein defined as the lectotype of O. hrabei, all other specimens being paralectotypes (Fig. 4).
Other material. Two catalogued slabs from the Middle Triassic limestone (Muschelkalk) of Poland, which contain several specimens (INGUJ214P/SO6, Gr6), numerous specimens observed in the field, not only in Poland but also in Thuringia (Germany) and France.
Diagnosis (after Uchman et al. 2011). In hypichnial view, a smooth, continuous ridge, subquadrate in cross section, irregularly meandering, winding or looping.
Description. The herein designated lectotype (Fig. 4, arrows) is preserved as hypichnial ridge at the base of a fine-grained sandstone slab. It is ca. 490 mm long, has an almost constant width of 2–3 mm and a height of ca. 3 mm. The burrow is irregularly winding (but not crossing over), with a relatively long straight portion, and curves rarely reaching an angle of 90°. Its cross section is subrectangular, non-carinate, with slightly rounded edges. Other burrows of that slab are almost straight, slightly curved to irregularly meandering, 60–220 mm long, 2–3 mm wide and ca. 3 mm high. Faint striae are present on the side walls (margins) of the burrows and run perpendicular to the burrow axes. These striae are best pronounced in the bends of the burrows.
The Middle Triassic specimens are preserved as exichnial and hypichnial full relief (Fig. 6). The burrows have a straight or arcuate, winding, or irregularly meandering course, and are horizontal to oblique, rarely vertical to the bedding. The continuous ridges or endichnial burrows have a subangular cross section, which is commonly slightly rounded. In rare cases, ridges are composed of continuous portions with slightly different course, resembling short probes of the burrows (Fig. 6d). The ridges are 4–11 mm (commonly 4–5 mm) wide, generally smooth, and their fill is micritic similar to the host rock.
Remarks. Oravaichnium hrabei from the Polish, German, and French Muschelkalk comprises exichnial and full-relief hypichnial burrows, which were formed post-deposition (Stachacz and Matysik 2020). It is relatively rare in the Lower Muschelkalk of Poland, compared with the abundant O. carinatum isp. nov. Nevertheless, transitional forms of these two ichnospecies occur (Table 1). Specimens composed of probe-like portions with diverging courses originated from a stepwise movement of the producer, a supposed bivalve. O. hrabei from the Triassic is distinctly larger than specimens originally described from the Eocene flysch (e.g., Plička and Uhrová 1990; Uchman et al. 2011).
O. hrabei was originally described from the Neogene of the Carpathians, where it occurs in turbiditic beds and is interpreted as predepositional trace formed in a deep marine environment (Uchman et al. 2011). Although, at first it was assumed that ‘… this trace fossil is very rare and has so far been found only in the Eocene flysch deposits of the Carpathians’ (Uchman et al. 2011), material from the Muschelkalk indicates its widespread distribution once this common ichnotaxon is properly recognised.
Subquadrate in cross section, partly chevroned specimens of O. hrabei resemble arthrophycids, i.e., Arthrophycus linearis and different ichnospecies of Phycodes, both attributed to polychaetes worms (cf. Seilacher 2000: figs. 2, 14). Especially, Arthrophycus brongniartii formed by arthropods (Rindsberg and Martin 2003) shows cross section and sculpture similar to O. hrabei. Nevertheless, A. brongniartii is a burrow with complex morphology (cf. Rindsberg and Martin 2003: fig. 3), while O. hrabei is smooth, or faintly chevroned. The V-shaped chevron of O. hrabei corresponds to small fusiform (in lateral cross section) bivalve shells, and subquadrate cross section is most likely a result of burrowing with slightly parted valves. Moreover, numerous bivalves associated with O. hrabei support small bivalves as the producer of this trace fossil.
Oblongichnus solodukhoi Bel Haouz et al., 2020 is very similar to O. hrabei in having the same morphology, variable orientation, and course, but mainly differs from it by having ascribed a thick lining or burrow wall. Close inspection of the figured type specimens reveals that their ‘lining’ consists of the same sandy substrate as the burrow fill and most likely results from a lateral shift of the producer. This adjustment feature resembles an incipient spreite rather than the lining of the burrow wall. ‘The thickness and kind of lining are variable’ (Bel Haouz et al. 2020: 99), and the holotype only shows a narrow interspace between burrow fill and host rock, suggesting a very thin lining, if any. For these reasons, O. solodukhoi is regarded as a junior subjective synonym of O. hrabei.
Occurrence in the Muschelkalk. Lower Gogolin Beds in Żyglin and Płaza; Upper Gogolin Beds in Strzelce Opolskie, Górażdże; Górażdże Beds in Górażdże; Terebratula Beds in Górażdże; Upper Muschelkalk in Thuringia, Germany, and France.
Oravaichnium carinatum isp. nov.
Figure 7, 8, 9, 10, 11 and 12
Etymology. From Latin carinatus, meaning ‘keeled, having a keel-shaped ridge’.
Type locality. Żyglin near Tarnowskie Góry, active quarry, southern Poland.
Type stratum. Lower Gogolin Beds, Lower Muschelkalk.
Paratypes. One catalogued slab containing eight specimens (INGUJ214P/Zy1) and 17 extracted burrows (INGUJ214P/Mil1 to Mil17) from the Lower Gogolin Beds at Żyglin and Milowice, southern Poland.
Other material. Several slabs containing a few tens of specimens from the Lower Gogolin Beds at Żyglin, Moczydło, Milowice and Moczydło, the Upper Gogolin Beds at Górażdże, and the Lower Terebratula Beds at Strzelce Opolskie and Górażdże, Górażdże Beds in Strzelce Opolskie and Górażdże, southern Poland. Moreover, numerous specimens were observed in the field, both in Poland and in Germany.
Diagnosis. Horizontal to oblique, straight or arcuate, wall-like burrow with a downwardly or upwardly carinate, amygdaloid or pear-shaped cross section, essentially tapering in one or both directions, homogeneous fill, and smooth or faintly chevroned margin.
Description. Hypichnial, wedge-shaped or arcuate, convex ridges or endichnial walls with smooth or weakly ornamented margins, generally tapering towards one or both terminations. The burrows are filled with the same massive sediment (micrite) as the host rock, or with slightly darker sediment (Figs. 7, 8, 10). It can display an indistinct backfill with scattered, ellipsoidal faecal pellets of 0.2–0.6 mm (commonly 0.3 mm) in diameter and 0.2–0.6 mm in length, which are filled with micrite lighter in colour than the burrow fill (Fig. 11a, c, f, g, i, j, o, s). The burrows have labile cross-sectional morphologies even through individual specimens (Fig. 11), which range from a relatively wide and oval shape, to pear or elongated amygdaloid shape, to slim burrows with a sharp keel and burrow terminations plunging into the bed in some cases. Most specimens thin upwards; fewer thin towards their base. Tapering upward and downward have both been observed within the same burrow (Figs. 11a, d, 12).
Burrow width, measured in the same cross sections at 1/3 and 2/3 of burrow height (Fig. 2, Table 1), is 3.8–6.0 mm (w1, Fig. 2) and 2.5–5.5 mm (w2), respectively; the w1/w2 ratio varies between 0.73 and 0.93 (average 0.84, Table 1). The pear-shaped cross section is the most common morphological variant and appears in various combinations (Fig. 11), whereas some ridges are relatively high and form a vertical or oblique wall (Fig. 11l, p). Most of the 422 measured burrows have a width/height ratio between 0.3 and 1.7 (Fig. 13). The ridges are generally smooth, rarely corrugated or irregularly serrated (Fig. 10a), although faint ornamentation formed by longitudinal narrow grooves or chevron-like patterns may occur (Figs. 7c, d, 9a–f, 14a–c).
Burrow fill is generally the same as the host rock, except for the specimens from the Terebratula Beds where it is commonly darker or composed of coarser material than the host sediment (Stachacz and Matysik 2020). Some burrows have the shell of the bivalves? Myoconcha bicostata or Pleuromya rugosa at their terminations, although their relationship to the burrow (e.g., accidental or producing it) remains questionable (Fig. 15). The ridges run parallel or obliquely to the bedding, in some cases having a rotation around their axis up to 90°. The original upper surface of the ridges may display a slight furrow (Fig. 9f), which may be a secondary feature due to reburrowing. The maximum width of the ridges is 2–29 mm (3–7 mm wide in more than 90% of 422 measured specimens) and their height is 2–20 mm (3–12 mm in ca. 90% of 422 measured specimens), with a width/height ratio ranging from 0.26 to 2.25 (Fig. 13, Table 1). The observed burrow fragments are 50–100 mm long.
Remarks. Occasionally visible backfill with sediment pads indicates active fill, and chevron-like patterns (Figs. 9a–f, 14a–c) correspond to features known from the ichnogenus Protovirgularia, which in many cases is produced by bivalves (e.g., Seilacher and Seilacher 1994). O. carinatum differs from O. hrabei by its carinate cross section (Table 1). Although the general shape of the burrow varies, upwardly tapered, mostly pear-shaped variants dominate (Fig. 11). In comparison, O. hrabei is subrectangular in cross section, with a width/height ratio of about 1. The burrow orientation of O. carinatum in the sediment is different, usually oblique to the bedding, instead of horizontal as in the type material of O. hrabei (cf. Plička and Uhrová 1990; Uchman et al. 2011). Nevertheless, Oblongichnus solodukhoi Bel Haouz et al., 2020, which is herein regarded as a synonym of O. hrabei, is also variably oriented and even includes steeply inclined burrows similarly to O. carinatum isp. nov.
Overall morphology and the shape of the cross section of O. carinatum with its upward-decreasing width resembles Protovirgularia triangularis (Macsotay, 1967) and its more deeply sculpted possible senior synonym P. pennata (von Eichwald, 1860), which, however, has its keel at the base and is composed of densely arranged striae instead of sporadic ornamentation (Uchman 1998). Beside smooth burrows of O. carinatum, faint striae to more pronounced chevron-like patterns may occur, indicating transitional forms between O. carinatum, P. triangularis and P. pennata. O. carinatum shows a wide spectrum of morphological variation with a transition field to O. hrabei and Planolites beverleyensis (cf. Plička and Uhrová 1990; Knaust 2007; Uchman et al. 2011).
Occurrence. The Gogolin Beds in Żyglin, Wojkowice, Mikołów, Libiąż-Moczydło, Żelatowa, Płaza, Wygiełzów, Jaworek, Centawa, Pogorzyce, Strzelce Opolskie and the Terebratula Beds in Górażdże and Strzelce Opolskie. O. carinatum occurs in great abundance in the Lower Muschelkalk of Poland and Germany, where this trace fossil forms a characteristic ichnofabric (Stachacz and Matysik 2020).
Ichnogenus Lockeia James, 1879
Lockeia siliquaria James, 1879
Material. Two catalogued slabs containing several specimens (INGUJ214P/SO4, Gr10), additional specimens observed in the field.
Description. Hypichnial convex semirelief forming small, distinct, asymmetrical, mound-shaped short ridges. The shape of the mounds resembles that of small, almond-shaped bivalve shells, and is steeper on one side than the other. The mounds are, 5–7 mm wide, 7–15 mm long and up to 10 mm high. Some specimens occur at the termination of Oravaichnium carinatum isp. nov.
Remarks. Stout forms of Lockeia have been ascribed to L. amygdaloides (Seilacher, 1953), and narrow forms to L. siliquaria James, 1879. However, Seilacher and Seilacher (1994) and Schlirf et al. (2001) regarded L. amygdaloides as morphological variants of L. siliquaria, because they occur in the same beds together with transitional forms between these two ichnospecies. L. siliquaria is the resting trace produced by bivalves in various environments, mostly shallow-marine settings (Seilacher and Seilacher 1994).
Occurrence. Gogolin Beds in Żyglin, Żelatowa, and Terebratula Beds in Strzelce Opolskie.
Lockeia cf. siliquaria
Figures 17a, 18a, b
Material. Four catalogued slabs with several specimens (INGUJ214P/Ze10 to Ze14), numerous specimens observed in the field.
Description. Hypichnial convex semirelief forming elongate, obtuse-triangular mounds or asymmetrical ridges, 20–50 mm long, 10–18 mm wide and up to 20 mm high. The shape of the mounds resembles that of triangular bivalve shells, and one side is much steeper than the other. Some specimens plunge into the bed or form long, continuous ridges.
Remarks. The specimens described here as Lockeia cf. siliquaria are much larger and more elongate than the small form of L. siliquaria as described above. The shape of this resting trace corresponds to the shape of the bivalve genus Hoernesia, which occurs in great abundance within the same interval.
Occurrence. Terebratula Beds in Górażdże and Strzelce Opolskie.
Ichnogenus Protovirgularia M‘Coy, 1850
Protovirgularia dichotoma M‘Coy, 1850
Material. Two catalogued slabs containing at least 14 specimens (INGUJ214P/Ze10, 11), more specimens were observed in the field.
Description. Hypichnial convex semirelief or full relief, curved, meandering or winding ribbon or crest, up to 40 mm long, composed of continuous series of U- or V-shaped pads that form a chevron-like pattern. The burrow is subquadrate or carinate in cross section. The distinct chevron-like pattern locally disappears, in which case only irregular wrinkles are visible or the surface is smooth (Fig. 17b). The pads are 3–15 mm wide and their width in individual burrows is constant.
Remarks. Some specimens show smooth portions (Fig. 17b) typical of Oravaichnium hrabei and represent intermediate forms between both ichnospecies. Other specimens show features typical of both Lockeia siliquaria and P. dichotoma (Fig. 17a) and represent transitional forms between these ichnospecies. Claus (1965) reported dichotomous trails from the Upper Muschelkalk of Thuringia, Germany, and tentatively attributed them to Protovirgularia. Based on their dichotomous morphology, this trace fossil can be included in P. dichotoma (see Han and Pickerill 1994). P. dichotoma also occurs as burrows and trails in the Lower Muschelkalk of Berlin-Rüdersdorf (Fig. 17c–f) and Thuringia (Knaust 2007), as well as in the sandy facies at the basin margin in the Trier area (Knaust and Hagdorn 2020). Smooth burrows from the Polish Muschelkalk have usually been assigned to Planolites or Palaeophycus by earlier authors (e.g., Szulc 2000: fig. 22B, C; Kowal-Linka and Bodzioch 2010: fig. 3B; Chrząstek 2013: pl. 3). Because a chevron-like pattern is diagnostic for Protovirgularia (e.g., Uchman 1998), completely smooth specimens (without ornamentation) are herein assigned to Oravaichnium carinatum or O. hrabei. Although Protovirgularia is commonly interpreted as bivalve trace based on experimental ichnology, it can also be produced by polychaetes as assumed in the older literature and proven by preserved remains of their producers in the Upper Muschelkalk of Germany (Knaust 2021b).
Occurrence. Upper Gogolin Beds, Żelatowa quarry; Lower Muschelkalk, Berlin-Rüdersdorf; Upper Muschelkalk, Thuringia.
Ichnogenus Ptychoplasma Fenton and Fenton, 1937
Ptychoplasma cf. excelsum Fenton and Fenton, 1937
Figure 18c, d
Material. Two catalogued specimens (INGUJ214P/Zy8, Ze8), several specimens observed in the field.
Description. Hypichnial convex full-relief or semirelief, straight or winding discontinuous ridges composed of fusiform to amygdaloid mounds, partly connected in their upper part (usually within the bed) and then forming a serrated wall (Fig. 18c). The mounds are conical or indistinctly carinate in cross section, smooth, 12–30 mm long, 5–15 mm wide, 10–25 mm high and separated by 10–20 mm.
Remarks. Specimens described here as Ptychoplasma cf. excelsum range in mound size and shape, and although mostly conformable with P. excelsum Fenton and Fenton, 1937, other elements known from P. conica Pieńkowski and Uchman (2009) and P. vagans (Książkiewicz, 1977) occur as well. High lability of morphology, even within the same burrow, is common.
Occurrence. Gogolin Beds in Żyglin, Żelatowa and Płaza.