A new marine woodground ichnotaxon from the Lower Cretaceous Mannville Group, Saskatchewan, Canada

Abstract. A new wood-boring ichnospecies is described from transgressive (lagoonal) deposits of the Lower Cretaceous Sparky Formation (Mannville Group) in west-central Saskatchewan, Canada. Apectoichnus lignummasticans new ichnospecies is a trace fossil that occurs in a thin coal bed and that was emplaced in an in situ xylic substratum (woodground). The ichnofossil is thin, elongate, unbranched, and straight to gently curved with a circular cross section and uniform diameter. Apectoichnus lignummasticans n. isp. is similar in many respects to modern borings in wood that are produced by marine isopods, e.g., Limnoria lignorum Rathke, 1799, for feeding and refugia. The recognition of Apectoichnus lignummasticans n. isp. in the rock record aligns with the modern observation that fossilized wood-boring assemblages should display higher ichnofossil diversities than commonly reported. Additionally, the stratigraphic occurrence of Apectoichnus lignummasticans n. isp. in association with other evidence of marine deposition reaffirms that certain wood boring morphologies (i.e., ichnotaxa) are useful as indicators of marine transgressions.


Geologic setting
Apectoichnus lignummasticans new ichnospecies occurs within the Lower Cretaceous (early Albian stage) Sparky Formation of the Mannville Group in the Western Canadian Sedimentary Basin. The Mannville Group was deposited during an overall transgression of the Boreal Sea. The Sparky Formation has bounding discontinuities that correspond to marine flooding surfaces, which separate it from the underlying General Petroleum and overlying Waseca formations (Morshedian et al., 2012;Fig. 1). The trace fossils were present in a wood clast in a coal bed within a cored well bore from west-central Saskatchewan. The coal bed in which Apectoichnus lignummasticans n. isp. was found was truncated during a marine incursion and is demarcated by a transgressive surface of erosion. Locally bioturbated, shallow-marine sandstone and mudstone units overlie the bored surface ( Fig. 1).

Materials and methods
Specimens.-The specimens were observed in a core from a wellbore near Bushy Lake, west-central Saskatchewan, Canada (Fig. 1). Two samples, UAI 0179 ( Fig. 2.1, 2.3) and 0180 (Figs. 2.2, 3), comprise a single piece of gregariously bored wood that is 98 x 67 x 15 mm at its maximum. The trace fossils occurred within the Lower Cretaceous Mannville Group (Sparky Formation) in association with a transgressive surface of marine erosion.
in analyzing various bioerosion features (e.g., Beuck et al., 2007Beuck et al., , 2008Schönberg and Shields, 2008;Tapanila, 2008). CT scanning is particularly suitable for substrata with empty borings due to the low attenuation of the tunnels relative to the substratum. This results in images with burrows that are easy to visualize and can be analyzed spatially across multiple dimensions. Although the specimens cannot be viewed directly in full relief, Micro-CT imaging allows observation of additional detail, trace morphology, and overall trace length.
Scanning was performed on a Skyscan 1172 Micro-CT. The X-ray source was set to 92 kV and 106 μA, and an Al+Cu filter was used to mitigate beam hardening. Data acquisition occurred at a resolution of 17 μm, with images taken at 0.5°i ncrements over a 180°rotation. Tomographic reconstruction was performed using © NRecon reconstruction software (Micro Photonics, Inc.), which includes procedures to reduce ring and beam-hardening artifacts, which are the most prominent noise features when CT scanning (Ketcham and Carlson, 2001). Reconstructed scans were rendered and analyzed using © CTvox Micro-CT volume rendering software (Blue Scientific) and a movie was made using the Flight Recorder functionality. The movie is available as Supplemental Data 1.
Repositories and institutional abbreviations.-Specimens examined in this study are housed in the Ichnology Research Group Trace Fossil Collection in the Department of Earth and Atmospheric Sciences, University of Alberta (UAI), Alberta, Canada. BM(NH) = The Natural History Museum, London.

Systematic ichnology
Borehole excavation in different substrata requires different behaviors and abilities (e.g., Dorgan, 2015). Thus, although ichnotaxa differ markedly from biological taxa, no paleontological, paleoecological, or taphonomic goal is achieved by lumping together traces excavated in lithic, osteic, and xylic substrata. This view was contested by Donovan and Ewin (2018), who considered Teredolites clavatus (xylic substratum) and Gastrochaenolites turbinatus Kelly and Bromley, 1984 (lithic substratum) to be synonyms because they are morphologically comparable. For the reasons discussed above, we consider substratum to be useful in taxonomic differentiation and advocate retaining both ichnotaxa (see also Zonneveld et al., 2015;Wisshak et al., 2019).

Ichnogenus Apectoichnus Donovan, 2018
Type ichnospecies.-Teredolites longissimus Kelly and Bromley, 1984. Emended diagnosis.-Elongate borings in xylic substrata and associated resins, nearly circular in cross section, with an approximately constant diameter. Borings are straight to sinuous or contorted and intertwined and predominantly occur parallel to the fibers of xylic substrata (modified from Donovan, 2018;Mayoral et al., 2020).
Remarks.- Kelly and Bromley (1984) proposed Teredolites longissimus as a new ichnospecies commonly associated with boring bivalves. This combination was recently challenged by Donovan (2018) who proposed the ichnogenus Apectoichnus to describe wood borings that lack the clavate morphology of Teredolites clavatus. Donovan (2018) retained the species-level taxonomy of Apectoichnus longissimus and thus, prior to the present work, Apectoichnus has been a monospecific ichnogenus. Subsequent contributions have validated the ichnotaxon Apectoichnus (e.g., Donovan and Ewin, 2018;Donovan and Portell, 2019;Wisshak et al., 2019). Mayoral et al. (2020) revised the diagnosis of Apectoichnus to include amber and other solid resins as possible host substrata. We further emend the genus-level diagnosis to recognize that the trace fossils are generally aligned wood fiber-parallel, which was previously considered diagnostic at the ichnospecies level (Mayoral et al., 2020).  Nicholson, 1873, Rhizocorallium Zenker, 1836, Teichichnus Seilacher, 1955, Diplocraterion Torell, 1870, Cylindrichnus Toots in Howard, 1966. TSE = transgressive surface of erosion.
Remarks.-Previous ichnospecific ichnotaxobases include length-to-width ratios > 5 and a tendency for the borings to  Mayoral et al., 2020). These characteristics are observed in the nonmolluscan borings examined herein and are therefore considered diagnostic at the ichnogenus level.
Occurrence Description.-The holotype and associated borings comprise an assemblage of Apectoichnus lignummasticans n. isp. emplaced in fossil wood. The gently curved holotype is displayed in the elevation view of sample UAI 0179 ( Fig. 2.1, 2.3). It has a preserved length of 22.8 mm, which is the longest of the assemblage, and a diameter of 0.9 mm (length-to-width ratio ∼25). An additional 20 borings were measured. Although the specimens cannot be viewed directly in full relief, Micro-CT imaging allows observation of additional detail, trace morphology, and overall trace length (Fig. 3). The preserved lengths vary considerably, averaging ∼ 12 mm. The assemblage displays a range in diameter from 0.4-1.2 mm (mean = 0.9 mm, N = 21). The length-to-width ratio ranges from 7-38 (mean = 15, N = 21); 17 of the ratios are between 10 and 25.
Etymology.-The species name is derived from the Latin lignum masticando ('wood chew').
Remarks.-Although size alone is not normally an ichnotaxonomic character, the small diameters of Apectoichnus lignummasticans n. isp., paired with large length to width ratios, make it readily discernible from Apectoichnus longissimus. The small diameters and the large length-to-width ratios of Apectoichnus lignummasticans n. isp.
are not ascribable to the teredinid bivalves that made Apectoichnus longissimus. The absence of radial bioglyphs that are sometimes associated with teredinid borings, and the lack of space available for calcareous boring linings or anterior caps further suggest a nonbivalve trace maker. The morphology of the structure is more consistent with mobile wood-boring marine arthropods (e.g., isopods, discussed below).

Discussion
Above we ascribe the occurrence of Apectoichnus lignummasticans n. isp. to marine and marginal-marine animals, owing to its similarity to borings made by the extant limnoiid isopod genus Limnoria Rathke, 1799 (Fig. 4). Nevertheless, it is worth emphasizing that Apectoichnus lignummasticans n. isp. is morphologically different from terrestrial wood-boring ichnofossils ( Table 1). The most common wood-boring species of Limnoria are L. lignorum Rathke, 1799, L. tripunctata Menzies, 1951, and L. quadripunctata Holthuis, 1949(e.g., Menzies and Turner, 1957Jones, 1963;Borges et al., 2014). Limnoria lignorum has a particularly widespread distribution due to its broad environmental tolerance (Borges et al., 2014). Environmental factors, e.g., temperature, salinity, and oxygen content, are directly related to the survival, distribution, and boring activity (i.e., egestion rate) of Limnoria; however, the most important constraint is the presence of an adequate food supply (i.e., wood) (Menzies, 1957). Mortality rates of Limnoria increase rapidly from 10-20°C, but boring activity is optimized in warmer water temperatures (20°C) and is significantly reduced below 10°C (Menzies, 1957;Eltringham, 1965). The limiting salinity for boring activity varies inversely with water temperature and ceases altogether in salinities < 10 ppt (Eltringham, 1961); the optimal salinity at 20°C ranges from 30-34 ppt (Borges et al., 2009). The amount of boring activity could also be directly related to dissolved oxygen content and is significantly reduced at levels < 3.0 ppm (Anderson and Reish, 1967). The presence of bored woodgrounds in the rock record is associated with transgressive coastal settings (Panos and Skacel, 1966;Bromley et al., 1984;Savrda, 1991;Shanley et al., 1992;Gingras et al., 2004). Indeed, the colonization of in situ woodgrounds by marginal marine and marine organisms requires a rise in relative sea level (e.g., Gingras et al., 2004). Importantly, Limnoria can survive for only ∼ 24 hours without water and thus commonly inhabits the intertidal zone (Menzies, 1957); populations are generally highest at low-tide levels where log-ground and woodground substrata are readily available. Longitudinal borings with oval cross sections (height:width = 1:3); interconnected by tangential tunnels, with radial tunnels of different sizes connecting to exterior; containing frass that is sometimes packed in backfill meniscae Ipites bobrowskianus Karpiński, 1962Genus Ips De Geer, 1775engraver beetle) Regularly branched borings with larger central tunnel and smaller radiating tunnels; aligned parallel to substratum Marine woodground ichnospecies Apectoichnus longissmus (Kelly and Bromley, 1984