Ichnological terms follow those of Frey et al. (1984), Keighley and Pickerill (1994), and Retallack (2001). Stratigraphy and geological age are referred from Matsukawa et al. (2014), Sato and Yamada (2014), Sano (2015), and Yamada (2017).
Horizontal meniscate backfilled burrow
Fig. 4
Material: One specimen (TOYA-Fo. 7345) reposited in the collections of the Toyama Science Museum, Toyama City.
Preservation: Convex epirelief on a bedding surface within a weakly bioturbated (BI 1-2), fine-grained sandstone bed showing Bouma Tab or Tabd divisions.
Description: TOYA-Fo. 7345 is an unbranched, slightly sinuous horizontal burrow with meniscate backfill and lining structures (Figs. 4A), containing neither fecal pellets nor bioclasts. Any internal structures, such as normal- or inverse grading, are not recognized in the transverse-oblique fracture surface in contrast to horizontal and lateral views (Fig. 4B). Preserved maximum length and width are 9.6 cm and 2.9 cm, respectively. When a concave side of a meniscus (direction of burrowing; Keighley and Pickerill, 1994) is defined here as anterior, burrow width slightly decreases toward posterior. Except for a posterior broken portion, burrow height is almost constant (~1.7 cm). Surface of the burrow is smooth because of lacking transverse bellows-like ornaments, scalariform ridges, annulations and selective weathering of the infilled materials (Fig. 4C). Transverse view shows a semielliptical morphology whose height/width ratio is approximately 0.6.
The internal structure consists of texturally heterogeneous coarse-to-fine alternations exhibiting a dense-to-well spaced shuffled pattern, that is, asymmetrically arranged about the median line of the burrow and irregularly spaced (Fig. 4A). But intervals of the alternations have a faint tendency to be posteriorly spaced. The burrow infill also displays an imbricated or anteriorly dipping structure (dips ranging from 10° to 50°), when viewed in the lateral side (Fig. 4C-E).
Menisci composed of black mudstone are parenthesis-like, shallow arcuate or tortoise shell bracket-like, subrectangular in their horizontal shapes (Fig. 4A). Transverse width and thickness highly vary even in a single meniscus. Several menisci extend into the lining, but others pinch out not to extend fully across the burrow width. Moreover, the menisci often intersect or merge into each other.
Irregularly spaced, often pinched-out menisci result in an irregular succession of incompletely bounded non-compartmentalized packets and meniscate segments (Fig. 4A), which are composed of yellowish-brown, massive fine-grained sandstone showing an almost similar (or very slightly muddy) lithology to the host sandstone. The sandstone infill has inconstant longitudinal length owing to the irregularly and not parallelly arranged muddy menisci. Though thickness varies within a single sandstone packing likewise the meniscus, it tends to be rather shorter than the transverse width.
The lining is mostly weathered out, only well-preserved from middle to posterior portions of the right lateral side (Fig. 4A, C). This structure covers not only the burrow infill, but also very slightly extends horizontally into the host rock with nearly constant width (about 2 mm; Fig. 4A, C). The lining is a considerably thin, simple wall structure made up of black mudstone similar in lithology to the meniscus (Fig. 4A). Very faint longitudinal striations, which incline anteriorly at the same angles that of the infill, run on the lining surface (Fig. 4D). Interface between the lined burrow floor and the host sandstone is indistinct, rather irregular (Fig. 4C, E).
As a co-occurring trace fossil, a posteriorly inclined thin burrow containing structureless fine-grained sandstone intersects along the lining at a right anterior portion of the meniscate burrow (Fig. 4A, C).
Remarks: Ichnotaxonomy of the meniscate trace fossils are based on their wall characters and presence or absence of branching (D’Alessandro and Bromley, 1987). In the light of this criteria, TOYA-Fo. 7345 is difficult to be identified below ichnogenus levels because its preservation is too patchy to confirm whether originally branching or non-branching. However, though insufficient preservation, the Arimine specimen is partially comparable with the already known meniscate traces on the basis of its backfill and lining structures.
Adhesive meniscate burrows (AMB; Hasiotis and Dubiel, 1994), Ancorichnus Heinberg, 1974, Beaconites Vialov, 1962, Scoyenia White, 1929 and Taenidium Heer, 1887 are typical meniscate backfilled traces whose ichnotaxobases have been discussed in a large number of literature (e.g. Frey et al.,1984; D’Alessandro and Bromley, 1987; Keighley and Pickerill, 1994; Uchman, 1995; Retallack, 2001; Smith et al., 2008). Brief comparisons are attempted between the exemplified burrows and TOYA-Fo. 7345.
AMB are characterized by sinuous burrows consisted of a series of ellipsoidal packets filled with densely arranged, fine meniscus (Smith et al., 2008). Such features are never confirmed in the Arimine specimen.
Ancorichnus is a horizontal, weakly sinuous and cylindrical meniscate burrow with a wall structure (Frey et al., 1984; Keighley and Pickerill, 1994). In transverse section, the burrow shows a subelliptical morphology (Frey et al., 1984). Menisci do not always transversely extend to the wall but sometimes are more or less abruptly discontinued (Frey et al., 1984). These characteristics are also observed in TOYA-Fo. 7345. But the Arimine specimen dose not have smooth, structured mantle which is one of the diagnostic characters of the ichnogenus (Frey et al., 1984; Keighley and Pickerill, 1994). Endo- and hyporelief preservation of Ancorichnus (Frey et al., 1984) also differs from that of TOYA-Fo. 7345.
Beaconites resembles the Arimine specimen in respect to a horizontal, sinuous and walled meniscate burrow containing heterogeneous materials (Keighley and Pickerill, 1994). B. antarcticus Vialov, 1962 and B. coronus (Frey et al., 1984) have gently to moderately arcuate menisci (Keighley and Pickerill, 1994), likewise TOYA-Fo. 7345. However, strongly arcuate meniscate packets and unornamented linings diagnosed for this ichnogenus (Keighley and Pickerill, 1994) are not identical with the structures of the Arimine specimen.
Scoyenia occurs as a horizontal, gently curved burrow consisted of heterogeneous backfilled materials with a distinct longitudinal striated wall lining (Frey et al., 1984; D’Alessandro and Bromley, 1987). Such features can be also recognized in TOYA-Fo. 7345. But as pointed out by Frey et al. (1984), endo- and hyporelief preservation, conspicuous annulations and chevron-like menisci characterized Scoyenia are quite different from the Arimine specimen.
Sinuous, cylindrical Taenidium shears many diagnostic characters proposed in D’Alessandro and Bromley (1987), and Keighley and Pickerill (1994) with TOYA-Fo. 7345. For example, stacked, transversely short non-compartmentalized backfill (T. barretti (Bradshaw, 1981) ), epirelief of a horizontal burrow composed of packets considerably shorter than wide (T. satanassi (D’Alessandro and Bromley, 1987) ), and arcuate menisci (T. satanassi and T. serpentinum Heer,1887). Although not all of the morphologies in each ichnospecies correspond to the Arimine specimen. Specifically, an unwalled burrow containing deeply arcuate menisci (T. barretti), equally spaced and pelleted backfill (T. satanassi), annulations in the burrow boundary (T. satanassi and T. serpentinum), and well-spaced homogeneous menisci (T. serpentinum) are not recognized in TOYA-Fo. 7345.
In summary, the Arimine specimen has several but not all of the corresponding characteristics with the compared ichnogenera and ichnospecies. The anteriorly dipping non-compartmentalized backfill and the striated lining slightly horizontally extending into the host rock are in particular unique to TOYA-Fo. 7345.
Associated trace fossils: Sparsely occurring, unidentified horizontal-to-vertical burrows filled with dark sandy siltstone (except for the intersecting thin burrow).
Locality: Right bank of the middle stream of the Higashisakamori-dani Valley (36° 30′ 25.42′′ N, 137° 27′ 59.43′′ E; 1166 m in altitude), Arimine, southeastern Toyama Prefecture (Figs. 1, 2).
Stratum: Arimine Formation, upper part of the Kuzuryu Group.
Age: Oxfordian (early Late Jurassic).