Palaeopascichnus linearis ( Fedonkin, 1976 )

Martin, Jean-Paul Saint, Charbonnier, Sylvain, Martin, Simona Saint, Cazes, Lilian & André, Jean-Pierre, 2025, New records of Palaeopaschichnus Palij, 1976 from the Ediacaran of Romania, Geodiversitas 47 (1), pp. 1-16 : 7-8

Palaeopascichnus linearis ( Fedonkin, 1976)

( Figs 6 ; 7 )

SYNONYMY. — See details in Kolesnikov et al. (2018a) and Kolesnikov & Desiatkin (2022). The original name – Margaritichnus linearis Fedonkin, 1976 – was first proposed for a supposed trace fossil (chains of faecal pellets). The species was later revised with the diagnosis emended to correspond to a body fossil (see Kolesnikov et al. 2018 a and Kolesnikov & Desiatkin 2022).

EMENDED DIAGNOSIS. — Test agglutinated, elongated, curved or rectilinear, occasionally branching, consists of a single series of globular or ellipsoidal chambers 1-15 mm in width. The series occasionally diverge dichotomously. Chambers are relatively consistent in size within a series or gradually increase in width before diverging, but the length-to width ratio of the chambers is relatively constant along the series. The wall thickness does not exceed 1 mm. Number of chambers in a series ranges between 3 and 5 and 30-40.

TYPE AGE. — Late Ediacaran.

EXAMINED SPECIMENS. — Romania • 1 specimen (in situ specimens and cast); Casimcea ; MNHN. F.A92246 • Rahman ; 1 specimen (in situ specimen).

DESCRIPTION

Casimcea specimens

The specimen from the S1 surface is preserved as positive epirelief visible on a roughly rectangular area (c. 30 × 70 cm) exhibiting some slightly marked ripples ( Fig. 6A, B ). It consists of meandering segments with parallel edges, of fairly constant width (8 mm in average). The segments are composed of a series of generally consecutive apparently incurved-arcuate elements, about 4 mm thick, with low positive epirelief ( Fig. 6C ). They were considered as chambers by Kolesnikov et al. (2018b). Elements are separated by a shallow incurved depression (c. 1 mm wide). The parallel edges of the segments are often visible and marked by a fine negative epirelief groove (0.5 mm wide; Fig. 6C ). One imprint differs by the lack of the successive elements between the two parallel lines ( Fig. 6D ). The photogrammetry of the mold reveals the bumpy appearance of the surface and the arrangement of the fossil ( Fig. 6B, C ). The lateral delimitation of segments seems to correspond to a tubular structure enclosing the arcuate elements ( Fig. 6B ). In several cases, a divergence of the segments occurs ( Fig. 6C ). Just before the branching, the terminal element is wider than the preceding elements and those of the two divergent segments, up to 1 cm. In two cases, a segment can cross another but overlapping ( Fig. 6C ). This superposition may result from the crushing of two distinct parts of the initial body, not located in the same plane.

The largest surface S2 exhibits discreet meandering imprint with parallel lines and segmented elements ( Fig. 7A ). Another imprint, perpendicular to the previous one, with two parallel lines without distinguishable constitutive elements ( Fig. 7B ) is here also present as for the S1 surface ( Fig. 6B, D ).

Rahman specimen

The specimen is clearly visible only over a length of about 15 cm. It is probably only part of a larger body fossil. It corresponds to a slightly sinuous imprint with parallel edges and constant width (c. 1 cm). It is composed of millimeter thick arcuate elements in slightly marked positive relief ( Fig. 7C, D ). These characteristics are similar to those on Casimcea S2 surface.

DISCUSSION

According to recent studies, the Romanian specimens have palaeopascichnid characteristics: 1) bedding plane-parallel modular fossils consisting of simple or more complex series of closely spaced millimetric circular, sausage- or kidney-shaped units ( Jensen et al. 2018); 2) macroscopic modular chain-like fossils ( Kolesnikov 2019), serially arranged, millimetre-scale allantoid chambers ( Hawco et al. 2019); and 3) serially or cluster-like arranged, millimetre- to centimetre-scale globular or allantoid chambers ( Kolesnikov & Desiatkin 2022). Morphometric and taxonomic analyses characterized and discriminated different species of Palaeopascichnus or morphospecies ( Hawco et al. 2019; Hawco 2020; Kolesnikov et al. 2018b; Kolesnikov 2019; Desiatkin et al. 2021; Kolesnikov & Desiatkin 2022). Based on these works, the Romanian specimens are here assigned to Palaeopascichnus linearis ( Fedonkin, 1976) because of their relative consistency in the size of arcuate elements (considered as chambers) throughout the series, which is not seen in the type species. Palaeopascichnus linearis shares common characters with the type species, such as the possible branching of the series, as observed in the Romanian specimens.

Figures

FIG. 6. — Specimens from Casimcea surface S1: A, View in situ of the Palaeopascichnus Palij, 1976 specimen; B, photogrammetric representation of the cast showing the poorly marked ridges at the surface (black arrow), the network of arcuate meandering elements and the “empty” meandering parallel imprints (white arrow); C, detailed view showing the well-marked outline of segments,the clear superposition of segments (a), the subdivision of segments with elements increasing in size just before the subdivision (b); D, detail of S1 specimen (close up of B, white arrow) with arcuate elements and imprint with parallel lines of constant width (c. 1 cm), without internal structures. Photos: J. P. Saint Martin.

FIG. 7. — A, Specimens from Casimcea surface S2 with a meandering succession of slightly arcuate element (black arrow) poorly expressed and imprint with parallel lines of constant width (c. 1 cm), without internal structures (white arrow) similar to Fig. 6D; B, same picture as A with specimens highlighted;C, D, speci- men from Rhaman; C, meandering succession of arcuate elements (arrow) on S2 surface; D, detail of C. Photos: J. P. Saint Martin.