Glaciichnium Walter, 1985

Uchman, Alfred, Gaździcki, Andrzej & Błażejowski, Błażej, 2018, Arthropod trace fossils from Eocene cold climate continental strata of King George Island, West Antarctica, Acta Palaeontologica Polonica 63 (2), pp. 383-396 : 387-392

Ichnogenus Glaciichnium Walter, 1985

Type ichnospecies: Glaciichnium liebegastensis Walter, 1985 , Liebegast in Germany, Pleistocene .

Emended diagnosis. —Horizontal trace fossil composed of median trail and lateral, long, straight or slightly curved tracks. The tracks can be composed of up to four segments.

Remarks. —The original diagnosis by Walter (1985: 104) reads: “Relativ geringe Fährtenbreiten bis zu 5 mm. An die in Bewegungsrichtung liegenden C-Eindrücke schliessen sich jeweils vorn B und A, die schräg zur Bewegunsrichtung liegen, in nach hinten gestaffelter Anordnung an”. This can be translated as: Relatively small trackways up to 5 mm wide. In the direction movement, C-footprints always terminate before footprints B and A, and adjoin toward the direction of movement, with diverse arrangement from the rear side”. Glaciichnium australis isp. nov.

Figs. 5A–D, 6A–D, 7A–D, 8A.

Etymology: From Latin australis , southern; in reference to Southern Hemisphere.

Type material: Holotype: ZPAL Tf.8/2007.1, a slab of very fine-grained silty sandstone ( Figs. 6A , 8A ); paratype: ZPAL Tf.8/2007.2.

Type horizon: Mount Wawel Formation , Eocene .

Type locality: N of the Dragon Glacier , Admiralty Bay area, King George Island, Antarctica .

Diagnosis.— Glaciichnium having unilobated or bilobated median trail and lateral tracks inclined at an angle of 65–85° in respect to the median trail.

Description. —The trace fossil was observed on parting, upper or lower surfaces that resulted from cracking the rock along laminae. It is preserved as trackways and their undertracks. The trace fossil is formed by the median trail and lateral tracks (in fact preserved as undertracks). The whole structure is 10 mm wide. It displays different morphologies with transitions between them. The median trail is a smooth, semicircular furrow when epichnial, and a ridge when hypichnial, 2–3 mm wide, with a transition to a bilobate furrow ( Figs. 6D , 7A–C ) or ridge (Figs. 5A, C, D, 7D), with a low median ridge or furrow ( Fig. 6A, C ). The furrow can be bounded by low levees ( Fig. 7A ). The course of the median trail is irregularly winding, or with longer, gently bent segments separated by more abrupt turns of 15–25° every 10–15 cm. Very rarely single loops are present ( Fig. 7D ). The lateral tracks are straight or slightly curved linear furrows or ridges, 7–9 mm long, less than 1 mm wide, except for the inner terminal part, which can show a triangular widening up to 1.5 mm at the base and is unifid (terminology after Trewin 1994) at the tip. Some of the tracks are segmented into two or three depressions or ridges ( Fig. 6A ). They start from the edge of the median trail or at a distance of 2 mm away. They are perpendicular to the median trail or slightly oblique under the angle of 65–85°. The lateral tracks are 2–4 mm apart. They are distributed symmetrically on both sides of the median trail or only on one side. In some specimens the lateral tracks are represented only by single depressions, which disappear along the course of the trace fossil. In this case, only the central furrow or ridge may be present ( Figs. 6D , 7C ). It can be bilobate with a transition to a unilobate, less distinct furrow

Fig. 7B ) or a ridge without lateral tracks. Locally, the trace fossil is covered by thin, flattish blanket of sediment (Fig. 5B), up to 10 mm long and elevated over the lower bedding surface, fitting the width of the entire structure.

Remarks. —In general, the emended ichnogeneric diagnosis matches partly the diagnosis of Protichnites Owen, 1852 by Keighley and Pickerill (1998: 95), which reads: “Trackways that, in epirelief, consist of any number of simple, or short striae imprints in similar series on either side of a central, intermittent or permanent, groove or grooves.” The main difference is that the imprints in Glaciichnium are long when completely preserved. The same concerns the diagnosis of Stiaria Smith, 1909 typified by S. quadripedia Smith, 1909 ( Fig. 8F, G ), which reads: “A trackway consisting of a single groove between two rows of imprints which occur in groups of two to four. These groups are set transversely or obliquely to the midline of the trackway” ( Walker 1985: 290). However, the imprints in S. quadripedia are short. Also S. intermedia Smith, 1909 ( Figs. 8H , 9 ) shows similar features. Moreover, its tracks are locally asymmetric in respect to the midline. Siskemia Smith, 1909 , typified by S. elegans , 1909 ( Fig. 8E ) displays bilobate or trilobate central ridge, short and curved imprints ( Walker 1985). However, the median trail of Glaciichnium australis is also locally bilobate. It is an open problem if these ichnogenera should be synonymised on the basis of the common two elements: central groove(s) and lateral tracks. Genise (2017: 498) suggested that Stiaria, Siskemia , and Mitchellichnus should be synonymised. The ichnotaxonomic relation of Glaciichnium to these ichnogenera is a separate problem. However, in this stage of knowledge, when the intraspecific variability of these ichnogenera is still poorly known, the possible synonymization would be premature. Instead, they can be grouped in the new ichnofamily Protichnidae.

The median trail and oblique, long or slightly curved tracks are the feature of Glaciichnium , which till now was a monospecific ichnogenus represented only by Glaciichnium liebegastensis Walter, 1985 . However, the latter is narrower

H

by half and its lateral tracks are inclined at a smaller angle with respect to the median trail (30–40° vs. 65–85° in G. australis ; Fig. 8B, C ). The bilobate median trail of G. australis , without lateral tracks ( Figs. 6C , 7C ), resembles the trace fossil Diplopodichnus Brady, 1947 ( Uchman et al. 2011), but this morphotype of G. australis is only an incomplete morphological variant of this ichnotaxon, probably preserved as an undertrack. Unilobate central trails, preserved as epichnial furrows or hypichnial ridges ( Figs. 6D , 7C, D ), probably undertracks, are very similar to the trace fossil Helminthoidichnites Fitch, 1850 (e.g., Uchman et al. 2009). Nevertheless, the transitions from the unilobate to the bilobate trail and to the bilobate trail with lateral tracks show that they are only preservational variants of G. australis .

Most specimens of Glaciichnium liebegastensis , including the holotype ( Fig. 8B ), display well expressed lateral tracks composed of two or three segments, and the median trail bounded by discontinuous, longitudinal, inwardly concave arcs. However, in some specimens, the median trail is continuous ( Walter 1985: pl. 2: 5, 7; Uchman et al. 2009: fig. 5C). These specimens of G. liebegastensis are the most similar to G. australis , which shows the continuous median trail. The continuous median trail occurs in all preservational variants of G. australis , while some preservational variants of G. liebegastensis do not display the median trail. The differences somewhat challenge the ichnogeneric affinity of G. australis , but solution of the problem requires extensive studies of the ichnofamily Protichnidae, which are beyond the scope of this paper.

In general, Glaciichnium australis is very similar to the Recent trace of the caddisfly larva Philopotamus montanus ( Donovan, 1813) and other caddisfly larvae of the family Leptoceridae , with agglutinated house tubes observed in shallow puddles on a dirt road along the stream in the Lejowa Valley, Tatra Mountains, southern Poland ( Figs. 8D , 10 ). The larvae walked on the very soft muddy bottom of a puddle using three pairs of segmented legs. P. montanus lives in colder waters in submontane areas of this region. Its size is similar but the lateral tracks are inclined at an angle 30–40° with respect to the median trail. However, similar traces can be produced by apterygote insects ( Getty et al. 2013).

The median trail is a trace of the body dragged on the muddy bottom, while the lateral tracks are footprints made during locomotion. The bilobate segments of the furrow indicate some double appendages along the body, such as the uropods of isopods. The levees resulted from accumulation of sediments pushed out from the furrow by ploughing the body. The local blanket of sediment covering the trace fossil consists of remnants of laminae on which the trace was imprinted. The local asymmetry of the trace fossils can be caused by currents tilting the tracemaker, which responded by placing greater pressure of appendages from one side than from the other side of its body. The variations in preservation can be caused also by subtle changes in sediment consistency. The segments showing transitions from bilobated to unilobated furrow without lateral tracks can be interpreted as a lateral transition from more to less cohesive substrate (compare Uchman and Pervesler 2006).

Glaciichnium australis can be interpreted as a locomotion and possibly a feeding trace (pascichnion). Possibly, the animal fed on microbe-rich detritus at the surface. The tracemaker of Glaciichnium australis is unknown, but isopods (by similarity to Asellus aquaticus , tracemaker of G. liebegastensis , and to Recent trace of Armadillidium vulgare ; see Uchman et al. 2011) and trichopteran insects (by direct observations of traces of Philopotamus montanus and others) are so far the most reliable candidates.

Stratigraphic and geographic range. —Middle–late Eocene, King George Island, Antarctica.

Figures

Fig. 6. Arthropod trace fossil Glaciichnium australis isp. nov., middle–late Eocene, Mount Wawel Formation, King George Island, Antarctica, slab ZPAL Tf.8/2007.1. A. The holotype (redrawn in Fig. 8A), epichnion. The narrow diagonal furrows are damage scratches. B. Very irregular form running between the lower plant stems. C. Hypichnial forms preserved mostly as the double central trail resembling the trace fossil Diplopodichnus. D. Epichnial, unilobated median trails resembling the trace fossil Helminthoidichnites. Also delicate median bilobated trails are present. → Fig. 5. Arthropod trace fossil Glaciichnium australis isp. nov., resting trace, and lower plant stems on lower bedding surface, middle–late Eocene, Mount Wawel Formation, King George Island, Antarctica, slab ZPAL Tf.8/2007.8. A. General view, the long G. australis running from the base to the top shows different preservational variants. The knobs are plant stems. The resting trace (rt) in the lower part. B. Fragment of the long G. australis with thin blankets of underlying laminae covering the trace fossil. C. Fragment of the long G. australis crossed by another preserved mostly as the median trail, several cross sections of the lower plant stem. D. The resting trace (rt), several cross sections of the lower plant stem, and G. australis preserved mostly as the median trail. E. Resting trace, drawing (E1) and photograph of close view (E2).

Fig. 8. Comparison of drawings of holotype Glaciichnium australis sp. nov. (A) to Glaciichnium liebegastensis (B holotype, C), trackway of a Recent caddisfly larvae Philopotamus montanus (D), lectotype of Siskemia elegans (E), Stiaria quadripedia (F neotype, G), and lectotype of Stiaria intermedia (H). B, C from Walter (1985: fig. 4A and B, respectively); E from Walker (1985: fig. 2a, part); F from Pollard and Walker (1984: pl. 2: 2); G from Walker (1985: fig. 5a); H from Walker (1985: fig. 5c).

Fig. 7. Arthropod trace fossil Glaciichnium australis isp. nov. preserved mostly as unilobated or bilobated median trails, middle–late Eocene, Mount Wawel Formation, King George Island, Antarctica, slab ZPAL Tf.8/2007.25. A. Unilobate median trail passes into irregular trackway, produced probably in low cohesive substrate. B. A transition between unilobate and bilobate median trail. C. Unilobate and bilobate median trails as epichnial furrows. D. Hypichnial ridges, which are unilobated median trails resembling the trace fossil Helminthoidichnites.

Fig. 9. The lectotype of Stiaria intermedia Smith, 1909 (see also Fig. 8H), Epichnion, Old Red Sandstone, Lower Devonian, Dunure (GSE 14075), Scotland, UK. Published with permission of the British Geological Survey in Edinburgh.

Fig. 10. Recent traces (A–C) of the caddisfly larvae Philopotamus montanus (Donovan, 1813) (D), Lejowa Valley, Tatra Mountains, southern Poland.