A trace fossil made by a walking crayfish or crayfish-like arthropod from the Lower Jurassic Moenave Formation of southwestern Utah, USA

Arthropod repichnia lacking medial or paramedial impressions

Several arthropod locomotory (walking) ichnotaxa are readily distinguished from SGDS 1290 by (usually) lacking medial or paramedial impressions, but are worth comparing to SGDS 1290 to ascertain whether or not it might be a morphological variant of such ichnotaxa. These ichnotaxa are Acanthichnus (Hitchcock, 1858), Asaphoidichnus (Miller, 1880), Bifurculapes (Hitchcock, 1858), Coenobichnus (Walker, Holland & Gardiner, 2003), Copeza (Hitchcock, 1858), Danstairia congesta (Smith, 1909), Diplichnites (Dawson, 1873), Eisenachichnus (Kozur, 1981), Foersterichnus (Pirrie, Feldmann & Buatois, 2004), Hamipes (Hitchcock, 1858), Laterigradus (De Carvalho et al., 2016), Lithographus (Hitchcock, 1858), Hornburgichnium (Kozur, 1989), Maculichna (Anderson, 1975a), Merostomichnites (Packard, 1900), Mirandaichnium (Aceñolaza, 1978), Octopodichnus (Gilmore, 1927), Petalichnus (Miller, 1880), Pterichnus (Hitchcock, 1865), Tasmanadia (Chapman, 1929) and Umfolozia (Savage, 1971). Most of these ichnotaxa further differ from SGDS 1290 in the shapes and configurations of their tracks. The comparisons below specify track-making taxa only when one or more have been proposed for the ichnotaxon.

Acanthichnus tracks (Fig. 4A), attributed to a chelicerate such as a solifugid, are oppositely arranged, short, elongate impressions in two (or four, per Dalman & Lucas, 2015) parallel rows; tracks either are parallel to or angle slightly away from the trackway axis (Dalman & Lucas, 2015; Hitchcock, 1858). This morphology and organization are both unlike those of SGDS 1290.

Asaphoidichnus tracks (Fig. 4B), attributed to trilobites, are elongate to crescentic, possess 3–4 crescentic branches at one end, and are oriented oblique to the trackway axis (Miller, 1880). They are far more complex in structure than the tracks of SGDS 1290.

Bifurculapes (Fig. 4C), attributed to an insect, possibly a beetle (Getty, 2016), comprises adjacent pairs (rarely triplets) of slightly staggered, elongate, straight to crescentic tracks that lie parallel or slightly oblique to the trackway axis, unlike the tracks of SGDS 1290. Tracks in each pair sometimes converge toward one end in Bifurculapes. This ichnotaxon typically does not possess paramedial impressions, but a specimen described by Getty (2016: fig. 1) possess two such traces, albeit faintly, that lie closer to the tracks than to the trackway axis, as in SGDS 1290. These impressions are far less pronounced than their associated tracks, unlike those of SGDS 1290. Getty (2020) ascertained that Bifurculapes traces were made subaqueously and may have been made by a terrestrial insect that would have left different tracks subaerially.

Coenobichnus tracks (Fig. 4D), attributed to a hermit crab, are thick, crescentic to ellipsoidal, roughly parallel and closely appressed to the trackway axis, and asymmetrical, with the left tracks larger than the right tracks (Walker, Holland & Gardiner, 2003), all of which differentiate this ichnotaxon from SGDS 1290.

Copeza (possibly a variant and synonym of Lithographus (Lull, 1953; Rainforth, 2005; Fig. 4E)) consists of triplets of roughly oppositely arranged, linear, elongate tracks in which the anteriormost lies roughly perpendicular to the trackway axis while the posteriormost pairs lie parallel or oblique to the trackway axis (Lull, 1953). This rare ichnotaxon is thus unlike SGDS 1290.

Danstairia congesta (Fig. 4F) comprises circular to crescentic tracks in closely appressed sets of up to six that are oriented oblique to the trackway axis; tracks often overlap to form V-shaped structures (Walker, 1985), unlike in SGDS 1290.

Diplichnites (possibly including Acripes per Häntzschel (1975) and Hammersburg, Hasiotis & Robison (2018); also see below) tracks span a range of morphologies. D. aenigma (Fig. 4G), ostensibly the ichnospecies lectotype except that no specimen was designated as such (Stimson et al., 2018), typically comprises elongate, closely packed tracks in parallel rows on either side of the trackway axis; the tracks lie perpendicular to the trackway axis (Dawson, 1873). D. gouldi Type A (Fig. 4H) comprises parallel rows of closely spaced, oppositely arranged, simple, roughly circular to oblong to comma-shaped or irregular tracks with varying orientations to the trackway axis; D. gouldi Type B comprises closely spaced, elongate impressions oriented perpendicular, or nearly so, to the trackway axis, matching the general description of D. aenigma; D. gouldi Type C is similar to Type B, but the tracks are oriented oblique (~ 45°) to the trackway axis (Trewin & McNamara, 1994). D. cuithensis, attributed to large arthropleurid myriapods, is similar to both D. aenigma and D. gouldi Type B, but is very large and has widely spaced rows of tracks (Briggs, Rolfe & Brannan, 1979). D. binatus tracks often occur as closely appressed pairs of imprints (Webby, 1983). D. triassicus tracks are small and circular to ovoid rather than elongate, and frequently paired on either side of the trackway axis (Pollard, Selden & Watts, 2008); this ichnospecies has been alternately suggested to be a synonym of D. gouldi (Lucas et al., 2006b) or to pertain to Acripes (Machalski & Machalska, 1994; Pollard, 1985). D. metzi possesses a midline impression that is sometimes interrupted by connections between tracks in the closely appressed rows (Fillmore et al., 2017). The ichnogenus is in need of thorough review (Smith et al., 2003), but in all cases, the tracks are of different morphologies and arrangements than those of SGDS 1290.

Eisenachichnus tracks (Fig. 4I) are elongate, paired, and oblique to the trackway axis (rarely perpendicular), but the patterns of the pairs on either side of the trackway axis normally are asymmetrical: on one side, the paired tracks lie end to end, while on the other, they are more adjacent (Kozur, 1981). This morphology and arrangement are unlike those of SGDS 1290.

Foersterichnus (Fig. 4J), attributed to a crab, consists of widely spaced, paired rows of elongate tracks in sets of 3–4; rows are parallel to oblique to the trackway axis, and tracks in each set frequently overlap (Pirrie, Feldmann & Buatois, 2004). The wide spacing, clustering of tracks, and orientations of the tracks are unlike those of SGDS 1290.

Hamipes tracks (Fig. 4K) consist of closely spaced, paired, elongate to crescentic impressions oriented parallel to the trackway axis; the outer tracks are longer than their accompanying inner tracks, and the tracks are staggered or alternately arranged (Getty, 2018; Hitchcock, 1858). Track morphology readily differentiates Hamipes from SGDS 1290.

Laterigradus tracks (Fig. 4L), attributed to sideways-walking crabs, comprises asymmetrical trackways consisting of sets of up to four tracks (De Carvalho et al., 2016). Individual tracks vary widely in shape, ranging from elongate to tapered to comma-shaped to roughly circular. Track sets fall within a narrow trackway width and exhibit different stride lengths along the course of a trackway. While some individual track shapes resemble those of SGDS 1290, the overall arrangement and layout of the tracks is distinct.

Lithographus (including Permichnium sensu Minter & Braddy, 2009) tracks (Fig. 4M), which match those made by pterygote insects, especially extant cockroaches (Davis, Minter & Braddy, 2007), comprise trios (or pairs, in the case of the Permichnium variant) of elongate to comma-shaped, rather than circular or tapering, tracks that are arranged at different angles to one another, some of which are oriented perpendicular to the trackway axis, and others of which are oblique to the trackway axis (Guthörl, 1934; Hitchcock, 1858; Minter & Braddy, 2009). These track morphologies and arrangements are distinct from those of SGDS 1290. Hornburgichnium reportedly is similar to Permichnium, but has three tracks on either side of the midline instead of two, and at least one of each set is oriented parallel to the trackway axis (Kozur, 1989); it may also be a variant of Lithographus (Lucas et al., 2005b). Trackways of Lithographus can transition into trackways that Hitchcock (1858) called Hexapodichnus (Davis, Minter & Braddy, 2007; Minter, Braddy & Davis, 2007), so the latter may be considered a behavioral and/or substrate-consistency variant of the former, and also unlike SGDS 1290.

Maculichna (including Guandacolichnus and Paganzichnus of Pazos (2000) per Buatois & Mángano (2003)) tracks (Fig. 4N) comprise pairs (sometimes more) of small, circular to slightly elongate tracks arranged in closely appressed, slightly staggered rows. Pairings are oriented virtually parallel to the trackway axis (Anderson, 1975a); occasionally, short segments of linear, singular medial or closely spaced, paired paramedial imprints are also preserved that can be offset to one side of the trackway axis (Anderson, 1975a: figs. 8b, 8d and 8e). The pairing of Maculichna tracks differs from that of SGDS 1290. Aceñolaza & Buatois (1991, 1993) and Archer & Maples (1984) described Maculichna traces that exhibit the pairing of classic Maculichna from South Africa, but in which tracks are more ellipsoidal to shaped like slightly inflated isosceles triangles; the long axes of the triangles are oriented close to perpendicular to the trackway axis. Pazos (2000) recognized this morphology as the separate ichnotaxon Paganzichnus. This morphology is also unlike that of SGDS 1290.

The ichnospecies holotype of Merostomichnites narragansettensis (Fig. 4O) consists of parallel rows of roughly oppositely arranged circular to elongate to comma-shaped tracks, the long axes of which are perpendicular to the trackway axis (Packard, 1900). Merostomichnites beecheri tracks are circular and connected across the trackway axis by curvilinear, shallow, M-shaped imprints, creating a sort of segmented midline impression (Packard, 1900: fig. 4). These track and trace morphologies do not match those of SGDS 1290.

Mirandaichnium (Fig. 4P) consists of two rows of elongate, linear tracks that terminate laterally in small, circular impressions. Tracks are oriented perpendicular or oblique to the trackway axis, oppositely situated, and often grouped into series of eight (Aceñolaza & Buatois, 1993; Buatois et al., 1998), unlike those of SGDS 1290.

Octopodichnus (Fig. 4Q) ichnospecies, attributed to arachnids, have different morphologies. O. didactylus tracks are circular to crescentic to bifurcate or trifurcate oriented parallel to the trackway axis in alternating, arcuate sets of four (Sadler, 1993). O. minor tracks have a similar organization, but the tracks are more amorphous; O. raymondi tracks consist of clusters of four circular to crescentic marks arranged in checkmark-like patterns (Sadler, 1993). These track morphologies and distinctive arrangements are substantially unlike those of SGDS 1290.

Petalichnus (Fig. 4R), attributed to trilobites (Braddy & Almond, 1999), comprises sets of 2–3 elongate to crescentic tracks oriented perpendicular to the trackway axis (Miller, 1880). Anderson (1975b) and Braddy & Almond (1999) diagnosed Petalichnus tracks as sometimes bifurcate and occurring in series of 9–12; they further noted that the ichnotaxon needs review. Both track morphology and organization are unlike those of SGDS 1290.

Pterichnus tracks (Fig. 4S), attributed to isopods, frequently are segmented and are more linear and elongate than any in SGDS 1290. Tracks (or series of segments) are oriented oblique (Types 1 and 2 of Gaillard et al., 2005) or parallel (Types 3 and 4 of Gaillard et al., 2005) to the trackway axis, and approximately symmetrically arranged in two parallel rows (Gaillard et al., 2005; Hitchcock, 1865) that are somewhat closer together than are those of the morphologically similar Diplichnites. Types 3 and 4 of Gaillard et al. (2005) morphologically grade into Diplopodichnus (Uchman et al., 2011). Hammersburg, Hasiotis & Robison (2018) suggested that Pterichnus comprises undertracks of Lithographus and is thus a junior synonym of that ichnotaxon. In any case, Pterichnus tracks are readily distinguished from SGDS 1290.

Tasmanadia traces (Fig. 4T) consist of two rows of closely packed, elongate, linear tracks oriented generally perpendicular to the trackway axis; occasionally, tracks overlap at one end, creating narrow, V-shaped structures (Chapman, 1929; Glaessner, 1957). Morphologically, this ichnotaxon resembles Diplichnites gouldi Type B and Umfolozia (but lacks the organization of the latter), and differs from SGDS 1290 for the same reasons as those ichnotaxa.

Umfolozia (Fig. 4U), attributed to syncarid and peracarid crustaceans (Lima, Minter & Netto, 2017; Savage, 1971), consists of parallel rows of irregularly shaped to crescentic tracks oriented perpendicular or oblique to the trackway axis that follow a unique repeating pattern (Anderson, 1981; Savage, 1971) unlike anything discernible in SGDS 1290. Aceñolaza & Buatois (1993) noted morphological similarities between Mirandaichnium, Tasmanadia, and Umfolozia and postulated similar track makers.

In summary, SGDS 1290 is not a variant of any of these ichnotaxa.

Arthropod repichnia possessing one medial impression

Several other arthropod locomotory (walking) ichnotaxa are also readily distinguished from SGDS 1290 by possessing singular medial, rather than paired paramedial, impressions; again, comparison is warranted to ascertain whether or not SGDS 1290 is a morphological variant of such ichnotaxa. These ichnotaxa are Kouphichnium (Caster, 1938; Nopcsa, 1923), “Merostomichnites isp.” (Hanken & Stormer, 1975), Oniscoidichnus (Brady, 1947, 1949), Palmichnium (Richter, 1954), Paleohelcura (Gilmore, 1926), Protichnites (Owen, 1852), Robledoichnus (Kozur & Lemone, 1995), Shalemichnus (Kozur & Lemone, 1995), Stiallia (Smith, 1909) and Stiaria (Smith, 1909). As with traces lacking any medial impressions, these ichnotaxa further differ from SGDS 1290 in track morphology. As before, possible track makers are specified only when available.

Kouphichnium traces (Fig. 4V) are attributed to limulids and occur in a variety of configurations. Tracks in clear Kouphichnium walking traces that possess singular medial impressions (e.g., many K. lithographicum, but not, for example, K. minkinensis (King, Stimson & Lucas, 2019; q.v. Gaillard, 2011a; Shu et al., 2018)) typically occur in sets of up to five in rows oriented oblique to the medial impression and trackway axis; individual tracks range from circular and ellipsoidal to elongate, and can split into anywhere from 2 to 5 branches at their ends (Caster, 1938, 1944; King, Stimson & Lucas, 2019; Shu et al., 2018). Well-preserved Kouphichnium tracks are dissimilar to those of SGDS 1290.

Traces referred to as “Merostomichnites” (Fig. 4W) and attributed to the eurypterid Mixopterus by Hanken & Stormer (1975) consist of three elongate and crescentic tracks in oblique rows on either side of an intermittent medial impression; the tracks increase in size laterally, and some split into two or more branches on one end. In any of these cases, however, the tracks are substantially more complex than those of SGDS 1290.

Oniscoidichnus tracks (Fig. 4X) are elongate to crescentic, oriented perpendicular or oblique to the trackway axis, closely packed and closely appressed to the single midline impression (Brady, 1947; Davies, Sansom & Turner, 2006). In all these details, Oniscoidichnus traces differ markedly from SGDS 1290.

Ichnospecies of Palmichnium (Fig. 4Y), also attributed to eurypterids, vary in morphology. Generally, they comprise complex sets of tracks lying lateral to a medial impression that can be either continuous or discontinuous. Tracks range in shape from elongate to crescentic to ovoid to chevron shaped, and they generally parallel the trackway axis. The tracks occur in oblique rows in sets of up to four; in some traces, the lateralmost tracks are elongate and curved, while the more medial tracks are linear and oriented parallel to the trackway axis (Braddy & Milner, 1998; Minter & Braddy, 2009; Poschmann & Braddy, 2010; Richter, 1954). Tracks are both more numerous and differently shaped than those of SGDS 1290.

Paleohelcura (including Mesichnium per Braddy (1995) and Triavestigia per Kozur, Löffler & Sittig (1994); possibly a junior synonym of Stiaria; Fig. 4Z) traces, attributed to scorpions (Brady, 1947; Davis, Minter & Braddy, 2007), comprise small, circular tracks in sets of three in either rows, triangular arrangements, or checkmark-like patterns that lie external and oblique to the medial impression (Gilmore, 1926; Lagnaoui et al., 2015; Sadler, 1993). This distinctive layout is unlike that of SGDS 1290. De Peixoto et al. (2020) attributed traces lacking a medial impression and comprising closely appressed pairs or triplets of mostly elliptical tracks from the Upper Jurassic or Lower Cretaceous of Brazil to a new ichnospecies of Paleohelcura and attributed them to a pterygote insect track maker. Tracks in this ichnospecies are arranged in rows oriented oblique to the trackway axis, and track sets in this ichnospecies lie close to the midline. This morphology is also unlike that of SGDS 1290.

Protichnites traces (Fig. 4AA) comprise thick, often segmented medial impressions (sometimes absent except on trackway turns) flanked by oppositely arranged, subcircular to ellipsoidal to irregularly shaped tracks with varying orientations to the trackway axis (Burton-Kelly & Erickson, 2010; Collette, Gass & Hagadorn, 2012; Hagadorn & Seilacher, 2009). They differ substantially from the tracks of SGDS 1290.

Robledoichnus tracks (Fig. 4BB), attributed to flying insects, resemble tracks of Eisenachichnus but possess a discontinuous, faint medial trace consisting entirely of periodic, V-shaped marks flanked by asymmetrical pairs of tracks. On one side, the tracks are short, tapered, and oriented oblique to the trackway axis; on the other side, the tracks are longer and crescentic, oriented closer to perpendicular to the trackway axis (Kozur & Lemone, 1995). Lucas et al. (2005a) considered Robledoichnus a probable junior synonym of Paleohelcura or Stiaria, and the ichnotaxon differs from SGDS 1290 for similar reasons as those ichnotaxa, in addition to the asymmetry.

Shalemichnus traces (Fig. 4CC), for which only half a trackway is known, include a straight medial impression punctuated at intervals by V-shaped marks. This impression is flanked by sets of three tapered tracks in straight rows oriented perpendicular to the trackway axis; individual tracks have their long axes parallel to the trackway axis (Kozur & Lemone, 1995). Minter & Braddy (2009) considered Shalemichnus a junior synonym of Stiaria. The tracks of Shalemichnus bear some similarity to those of SGDS 1290, but the paramedial impressions of SGDS 1290 lack the V-shaped markings of the medial impression of Shalemichnus.

Stiallia traces (Fig. 4DD) consist of paired rows of long, linear impressions parallel or slightly oblique to the trackway axis and that frequently overlap. Stiallia pilosa lacks any medial or paramedial impressions, but Stiallia (Carrickia of Smith (1909)) berriana possesses a medial row of crescentic to chevron-shaped marks (Smith, 1909; Walker, 1985). Pollard (1995) suggested that Stiallia could be an arthropod swimming, rather than a walking, trace, though it also resembles traces made by bristletail insects walking in highly saturated mud (Getty et al., 2013: figs. 6F and 6G). Stiallia tracks are markedly unlike those of SGDS 1290.

Stiaria tracks (including some ichnospecies of Danstairia of Smith (1909); Fig. 4EE), attributed to scorpionids (Braddy, 2003; Lucas, Lerner & Voigt, 2013) and monuran insects (Genise, 2017; Kopaska-Merkel & Buta, 2013), are oppositely situated groups of 2–4 generally circular to tapered tracks in a linear to crescentic arrangement lying roughly perpendicular to the trackway axis (Walker, 1985). In some specimens and ichnospecies of Stiaria, the singular medial impression actually varies in position, meandering from medial to almost lateral to their tracks (Fillmore, Lucas & Simpson, 2012: fig. 26d; Walker, 1985: figs. 5b and 5c). In some Mississippian specimens from Pennsylvania, the medial impression is flanked by thin, discontinuous, but closely appressed paramedial imprints (Fillmore, Lucas & Simpson, 2012: figs. 26d–26g). Track arrangement alone differentiates Stiaria from SGDS 1290. Genise (2017) asserted that Stiaria should be considered a junior synonym of Siskemia (the latter has page priority over the former).

As with locomotory traces lacking medial impressions, SGDS 1290 is not a variant of any of these ichnotaxa.

Arthropod repichnia possessing three or more medial and paramedial impressions

Mitchellichnus (Fig. 4FF), attributed to archaeognathan insects (Getty et al., 2013), is distinguished from SGDS 1290 by possessing three medial impressions (Walker, 1985). Mitchellichnus tracks are complex, comprising two distinct types and arrangements. An inner set, lying close to the medial impressions, consists of apparently elongate tracks in sets of up to six that lie parallel to slightly oblique to the trackway axis; an outer set consists of larger, amorphous impressions (Walker, 1985). Tracks are thus more numerous in Mitchellichnus than in SGDS 1290, and the tracks differ in arrangement and morphology. Like Stiaria, Genise (2017) asserted that Mitchellichnus should be considered a junior synonym of Siskemia.

Keircalia (Fig. 4GG) is distinguished from SGDS 1290 by possessing four medial impressions (Smith, 1909; Walker, 1985). Keircalia tracks are crescentic to irregularly shaped, generally are oriented perpendicular to the trackway axis, and have no discernible arrangement (Walker, 1985). Both track morphology and organization are unlike those of SGDS 1290.

Arthropod repichnia possessing paired paramedial impressions

A few ichnotaxa, as well as some experimentally produced tracks of extant arthropods, resemble SGDS 1290 by possessing paired paramedial impressions in at least some specimens. Such ichnotaxa are Danstairia vagusa (Smith, 1909), Glaciichnium (Walter, 1985), Warvichnium (Walter, 1985), and Siskemia (Smith, 1909); similar extant traces include those made by notostracans (Trusheim, 1931) and crayfish (Fairchild & Hasiotis, 2011).

Danstairia vagusa (Fig. 4HH) possesses intermittent, thin, linear imprints that do not always parallel their accompanying trackways. Tracks are circular to triangular, generally have their long axes perpendicular to the trackway axis, and lack any coherent layout (Walker, 1985), unlike those of SGDS 1290. D. vagusa somewhat resembles Keircalia traces, but its tracks are spaced more widely apart.

Glaciichnium traces (Fig. 4II), which resemble traces made by isopods (Gibbard & Stuart, 1974; Lima, Minter & Netto, 2017; Uchman, Kazakauskas & Gaigalas, 2009; Uchman et al., 2011), comprise 1–3 elongate, linear tracks (“bars” that are divided into segments (Uchman, Kazakauskas & Gaigalas, 2009)) that lie oblique to the trackway axis and are staggered on either side of that axis, unlike the tracks of SGDS 1290; their linear, serial but discontinuous paramedial imprints are widely spaced, consistently abutting the medial ends of the tracks (Walter, 1985), farther apart than those of SGDS 1290. Lima et al. (2015) described the paramedial imprints in Brazilian specimens as comprising successive pairs of C-shaped imprints rather than strictly linear structures, further unlike SGDS 1290. Some Glaciichnium traces also possess a medial imprint as well (Uchman, Kazakauskas & Gaigalas, 2009; Walter, 1985).

Warvichnium traces (Fig. 4JJ) are complex, comprising pairs to multiple sets of linear, discontinuous medial and paramedial imprints flanked by varying numbers of linear to crescentic tracks in two or more sets: an inner set, close to the medial imprints, that are oriented slightly oblique to the trackway axis, and an outer set oriented closer to perpendicular to the trackway axis (Walter, 1985), quite unlike SGDS 1290. Getty (2020) noted similarities between Warvichnium and subaqueous Bifurculapes.

Among described arthropod repichnia, SGDS 1290 architecturally most closely resembles ichnospecies of Siskemia by possessing discreet, compact (not linear) tracks and track sets flanking paired paramedial impressions. Three ichnospecies of Siskemia are presently recognized (Walker, 1985):

• S. bipediculus (Fig. 4KK) comprises small, circular tracks in closely appressed pairs (occasionally trios) in rows oriented perpendicular or slightly oblique to the trackway axis; the pairs are spaced apart at approximately regular intervals and evenly distant from the uniformly straight and narrow paramedial impressions (Walker, 1985). The paramedial impressions lie close to the midline axis, well away from their adjacent tracks (the average ratio of the distance between a paramedial impression and its flanking track to the distance between the medial edges of the paramedial impressions is 1.34; see Supplemental Material).

• S. elegans (Fig. 4LL) has similarly shaped tracks in sets of up to four; the sets similarly lie well away from the likewise straight, narrow, and closely appressed paramedial impressions (the average ratio of the distance between a paramedial impression and its flanking track to the distance between the medial edges of the paramedial impressions is 1.75; see Supplemental Material). S. bipediculus and S. elegans differ primarily in the orientations of their track rows to the trackway axis and the continuities and thicknesses of their paramedial impressions (Walker, 1985), though these could be behavioral and/or substrate-driven variants.

• Siskemia latavia (Fig. 4MM; called “lata-via” by Smith (1909) and Walker (1985), but the ICZN does not permit hyphens in genus or species names) tracks comprise tapered or ovoid tracks arranged in roughly triangular sets of three. Most individual tracks have their long axes oriented oblique to the trackway axis; track sets have varying orientations to the trackway axis. Tracks in individual S. latavia sets usually are spaced well apart from each other; rarely do two individual tracks in a set contact one another. S. latavia tracks lack the regular spacing and arrangements of those of S. elegans and S. bipediculus, and occasional individual tracks lie close to the paramedial impressions, farther medially than in either of the other two Siskemia ichnospecies (the average ratio of the distance between a paramedial impression and its flanking track to the distance between the medial edges of the paramedial impressions is 0.80; see Supplemental Material). Each paramedial impression of S. latavia is slightly wider than those of the other two Siskemia ichnospecies (probably a function of the larger overall size of specimens attributed to this ichnospecies), but retain the close appression to the trackway midline and the uniform straightness.

The tracks of SGDS 1290 vary more in morphology than those of any known Siskemia ichnospecies, but grossly share their layout. Tracks in all three Siskemia ichnospecies have a staggered distribution, similar to, but less pronounced than, that of SGDS 1290. SGDS 1290 differs most markedly from any of the three Siskemia ichnospecies in the morphology and positions of the paramedial impressions: in SGDS 1290, the impressions vary in thickness along their lengths and undulate, in contrast to the thin, straight impressions of all three Siskemia ichnospecies. Additionally, the impressions in SGDS 1290 lie farther apart than those of the three Siskemia ichnospecies. In fact, all of the ichnospecies of Siskemia erected by Smith (1909), as well as both specimens later attributed to this ichnogenus (Getty et al., 2013; McNamara, 2014; Pollard, Steel & Undersrud, 1982) and Siskemia-like traces made by extant, archaeognathan insects (Getty et al., 2013), have such thin, linear, closely appressed paramedial impressions (sometimes offset toward one side of the trackway). The only time when archaeognathan traces approach the paramedial impression spacing of SGDS 1290 is when both abdominal styli (laterally) and gonostyli (medially) of the trace makers register impressions and imprints in wet mud, producing two sets of paramedial traces (Getty et al., 2013: figs. 6K and 6L), but even then the linear, lateralmost paramedial impressions still do not resemble the thick, undulating impressions of SGDS 1290. Simultaneously, in such wet mud, archaeognathan tracks are elongate and oriented oblique to the trackway axis, unlike those in SGDS 1290. In total, SGDS 1290 does not fit neatly into any known Siskemia ichnospecies and does not seem to be an archaeognathan insect trace.

Among traces made by extant arthropods, SGDS 1290 bears similarities to traces made by both notostracans and crayfish. Interpretive drawings of experimental traces made by notostracans figured by Trusheim (1931) depict elongate, crescentic, or tapered tracks oriented perpendicular to paramedial impressions; the tracks are arranged oppositely, unlike those of SGDS 1290. Additionally, the thin, linear paramedial impressions figured by Trusheim (1931) lie so far from the trackway axis that they often contact their accompanying tracks, a phenomenon that only occurs in SGDS 1290 near L4 and R4, where the lateral margins of its undulating paramedial impressions meander particularly far laterally. Tasch (1969) noted, though, that the drawings presented by Trusheim (1931) were misleading compared to his own experimentally generated notostracan traces. However, he described the morphologies of his notostracan tracks only as “minute en echelon stripes” (Tasch, 1969: 327), which does not adequately specify how they differed from those of Trusheim (1931); track details are impossible to discern in his lone photographic figure (Tasch, 1969: pl. 1.2). Gand et al. (2008) also conducted neoichnological experiments with notostracans, recovering locomotory traces that were less orderly than those illustrated by Trusheim (1931) (Gand et al., 2008: figs. 16.1, 16.2 and 17.1). Their extant notostracan tracks comprised multiple tracks with rather chaotic distributions lateral to their paramedial impressions, unlike the regular distribution seen in SGDS 1290. Gand et al. (2008) found their notostracan traces to fall within the “etho-morphotype” of Acripes, as exemplified by their novel ichnospecies A. multiformis from the Permian of France. (Linck (1943) and Pollard (1985) also referred Acripes (Merostomichnites of Linck (1943)) tracks to notostracans, but not based on neoichnological experiments.) A. multiformis traces, unlike classic Acripes (Matthew, 1910), possess paramedial imprints, albeit inconsistently. Hammersburg, Hasiotis & Robison (2018), Häntzschel (1975), Miller (1996) and Pemberton, MacEachern & Gingras (2007) all supported classic Acripes as a junior synonym of Diplichnites; the issue of synonymy is beyond the scope of this paper, but we note at least that the tracks in fossils that Gand et al. (2008) called A. multiformis differ from SGDS 1290 in the same ways that Diplichnites tracks do (see above). Lastly, Knecht et al. (2009: figs. 5 and 6) also illustrated traces made by extant notostracans (Fig. 4NN), which are “tidier” than those of Gand et al. (2008) and resemble those of classic Acripes and Diplichnites, albeit with discontinuous paramedial and curvilinear lateral (external) imprints. The tracks in these traces comprise irregular, ellipsoidal sets oriented oblique to the trackway axis, unlike those of SGDS 1290. In total, SGDS 1290 is unlikely to be a notostracan trace.

Fairchild & Hasiotis (2011) conducted neoichnological experiments with crayfish to examine their locomotory traces. These traces varied in morphology depending on substrate conditions (sediment grain size and saturation) and slope; in general, when clearest, they consist of sets of 1–4 circular, tapering, ellipsoidal, or elongate tracks, occasionally of different sizes, that are oriented parallel to the trackway axis and that lie lateral to a pair of undulating, variably thick paramedial impressions that lie closer to their flanking tracks than to each other (Figs. 3D and 4OO). Morphologically, the tracks and impressions match those of SGDS 1290, although the tracks made by extant crayfish often are larger than those of SGDS 1290 when produced in dry substrate (Fairchild & Hasiotis, 2011: fig. 9). Track sets in extant crayfish traces have complex arrangements: when comprised of multiple traces, they frequently are in rows oriented perpendicular to the trackway axis, but sometimes rows are oblique to the trackway axis. When fewer tracks are preserved, sets can appear to lie in single, parallel rows on either side of the paramedial impressions. Track positions can be opposite to staggered to alternating, also as in SGDS 1290. In both track and paramedial impression morphology, as well as in overall trace architecture, SGDS 1290 strongly resembles crayfish traces made in damp silt and clay (Fairchild & Hasiotis, 2011: figs. 2e and 2f), dry and saturated, very fine-grained sand (Fig. 3D; Fairchild & Hasiotis, 2011: figs. 4a, 4b, 4e and 4f), dry and damp, fine-grained sand (Fairchild & Hasiotis, 2011: figs. 5a–5d), and saturated medium sand (Fairchild & Hasiotis, 2011: figs. 6e and 6f). SGDS 1290 is preserved in, and was presumably registered in, a fine-grained sand, lithologically matching one set of experimental conditions in Fairchild & Hasiotis (2011). However, SGDS 1290 is not as detailed as many of the experimentally generated crayfish traces in comparable sediments. This could indicate one or more things: the fossil could be a slight overtrack (sensu Bertling et al., 2006) rather than a direct natural cast; the trace maker may have been partly buoyant; and/or trace-maker behavior and sediment consistency combined such that the lighter limbs did not impress as deeply as the heavier tail.