Inmontibusichnus charleshenryturneri, Knecht & Benner & Dunlop & Renczkowski, 2024

Knecht, Richard J., Benner, Jacob S., Dunlop, Jason A. & Renczkowski, Mark D., 2024, The largest Palaeozoic whip scorpion and the smallest (Arachnida: Uropygi: Thelyphonida); a new species and a new ichnospecies from the Carboniferous of New England, USA, Zoological Journal of the Linnean Society 200 (3), pp. 690-704 : 698-702

Inmontibusichnus charleshenryturneri isp.nov.

( Fig.7A, B )

Material: Holotype (part and counterpart) and only known specimen, MCZ:IP:198045(a, b) preserved as a full-body impression (trace fossil) on red shale.

Horizon and locality: The type specimen comes from the Late Carboniferous (Mid-Bashkirian) Wamsutta Formation of south-eastern Massachusetts, [ Fig. 3 , see Knecht et al. (2011) for background].

Collector: This specimen was collected by author R.J.K..

Etymology: This species is named in honour of the late African American zoologist Charles Henry Turner (1867–1923), best known for his work as an early pioneer in the field of social insect behaviour. Dr Turner earned a B.S. (1891) and a M.S. (1892) in Biology from the University of Cincinnati and was the first African American to earn a Ph.D. from the University of Chicago in 1907 (magna cum laude, Zoology). Dr Turner faced numerous obstacles due to racism, including restrictions and access to laboratories and research libraries, not being allowed to have students at the undergraduate or graduate level, limited academic employment opportunities, and low pay. Despite the many challenges, he managed to publish more than 70 papers including three in Science (the first African American to be published in this journal). He was also a leader and voice in the civil rights movement in St. Louis, Missouri, first publishing on the issue in 1897. He strongly believed that education was the key to eliminating racism and through his studies in comparative psychology and behaviour he identified two forms of racism: unconditioned response to the unfamiliar and learned or imitated behaviour. Despite a young death at the age of 56, exactly a century ago, Dr Charles Henry Turner had made many significant contributions in his lifetime, all while facing the hardships of racism and for this we name this new ichnospecies in his honour.

Diagnosis: Inmontibusichnus with an additional pair of robust, anteriorly-projecting appendage impressions that emerge from the anterior end of the larger, main impression.

Description: The specimen is on a ~1-cm thick slab of red, fine sandstone with thin shale drapes ( Fig. 7A, B ). The trace is preserved as a negative epirelief and a corresponding positive hyporelief and is bilaterally symmetrical overall and 15.6 mm in total length [including anteriormost impressions and telson; 9.59 mm in total length of prosoma and opisthosoma regions (excluding telson)]. It is divided into two main sections longitudinally that are equivalent to the tagmata of the arachnida body plan, i.e. cephalothorax (=prosoma) and opisthosoma ( Fig. 7A, B ). Most anteriorly, there is a set of anterior appendage imprints (~7.0 mm in total length), representing the pedipalps of the tracemaker, oriented at ~45° from the midline of the impression (~ 4.5 mm in length) before sharply turning towards the midline at a perpendicular angle (~ 2.5 mm in length). Between the anterior appendage impression is a thin horizontal impression, ~2.0 mm in length likely representing the coxae of the two pedipalps including endite impressions.

The cephalothorax region is 4.61 mm in length (from anteriormost pedipalp coxae impression to anteriormost impression of the opisthosoma). Along the length of this tagma, four narrow impressions diverge and taper distally, representing impressions of the coxae. The first coxa impression (~ 0.53 mm in length, ~ 0.35 mm at greatest width), which is the thinnest of all the coxae impressions, is only present on the left side of the trace and is directed anteriorly. The first coxa also has a significant portion of the leg impression preserved, including the trochanter, femur, and part of the patella ( Fig. 7 : I) in epirelief (MCZ:IP:198045a) but appears to be fully preserved in hyporelief on the counterpart (MCZ:IP:198045b). This is interpreted as the non-walking, antenniform leg of the thelyphonid tracemaker. Movement of this first leg is seen in Figure 7A, A 1 (MCZ:IP:198045b) leaving two impressions at time one (T 1) and time two (T 2). It is ~ 1.27 mm from the midline of the first coxa impression to the midline of the second coxae impressions. The second set of coxae impressions (~ 1.83 mm in length, ~ 0.49 mm at greatest width) are directed anterolaterally ( Fig. 7A, B : II). Part of the trochanter impressions is preserved on both sides and a possible fragment of the femur may have been imprinted on the left side. It is ~ 1.24 mm from the midline of the second coxae impressions to the midline of the third coxae impressions. The third set of coxae impressions (~ 2.15 mm in length, ~ 0.58 mm at greatest width) are directed anterolaterally ( Fig. 7A, B : III). Full trochanter and incomplete portions of femoral impressions are preserved on both sides. It is ~ 1.22 mm from the midline of the third set of coxae impressions to the midline of the fourth coxae impressions. The fourth set of coxae impressions (~ 2.66 mm in length, ~ 0.81 mm at greatest width) are directed posterolaterally ( Fig. 7A, B : IV). A partial trochanter impression is preserved on the left side. It is ~ 0.88 mm from the midline of the fourth set of coxae impressions to the anteriormost edge of the opisthosoma impression. Distal to the coxae impression are five short and narrow impressions interpreted to represent tarsal impressions of the tracemaker while in a natural standing posture ( Fig. 7A A 1 : tar).

The opisthosomal region ( Fig. 7A, B : op) is represented by an ovate impression, ~ 4.88 mm in length and ~ 2.59 mm in width at its greatest point (L/W ratio ~1.88). Abdominal sternites or segmentation can only be discerned in hyporelief on the counterpart (MCZ:IP:198045b). Close examination reveals nine distinct sternites. Discoloration near the posterior and scalloped edges are interpreted to be taphonomic artefacts caused by the body overprinting both plant material and the animal’s own deeply impressed tracks (see Fig. 8 ). A small, compact impression coming off the central posterior edge of the opisthosoma impression is interpreted as the impression of the pygidium ( Fig. 7A, B : py). Segmentation of the pygidium cannot be discerned in either part or counterpart of the specimen. Central and posterior to the likely pygidium impression is a long and narrow posteriorly oriented furrow ( Fig. 7A, B : te; ~ 3.45 mm in length, ~ 0.37 mm in width near the base and ~ 0.25 mm in width at the terminus), which represents the telson. Variation in thickness of the telson is interpreted to be a reflection of appendage movement and not actual differences in width, which is uniform in most thelyphonid telsons. There appears to be some possible segmentation in the telson impression but the sedimentary fabric surrounding the trace fossil makes this delicate character difficult to discern.

Associated trackway: A series of tracks that may constitute a trackway or a portion of one, are closely associated with the bodily impression on MCZ:IP:198045(a, b). The surface preserved is palimpsest, or at least affected by faint undertracks of amphibians, which is not uncommon in the fossiliferous Wamsutta Fm. units. The timing of vertebrate track production can be distinguished from that of the thelyphonid body impression and its associated tracks by the relative depth and definition of the two. Amphibian tracks are shallowly impressed and difficult to distinguish aside from digital pads ( Fig. 8 ), which indicates that they are either undertracks made on a surface above, or they were made when the surface had experienced some loss of water content. If the impressions of the two animals had been made simultaneously, it is likely that the amphibian producer would have made impressions at least as deep as the thelyphonid, which is presumed to have had low density and mass.

Modern thelyphonid walking legs bear paired, relatively large tarsal claws and a third smaller ‘spike’ that are together used for grasping. The two major claws diverge distally, and the distal portions of the claws and small spike beneath are the main points of contact with a substrate when walking ( Gallant and Hochberg 2017). This arrangement of tarsal claws produces a wedge-like impression, widening opposite the direction of travel (e.g. Schmerge et al. 2013).

Each individual track on MCZ:IP:198045(a, b) is wedge-like in form, and in some cases displays shallow bifid imprints at the terminus ( Fig. 8 ). The best-preserved tracks on the specimen of interest are 0.35 mm at the widest point and taper proximally, well within range of the distance between the distal portion of tarsal claws in modern thelyphonids as measured from photographic records ( Gallant and Hochberg 2017). Collectively, the tracks form a loose pathway moving toward the bodily impression. Directionality of tracks is indicated by the wedge-like morphology made by the paired tarsal claws pushing into the substrate opposite the direction of movement. Tracks on this slab have no clear pattern or track cycle and, therefore, may not qualify as a trackway (sensu Minter et al. 2007), but all individual tracks indicated are similar enough that they can be confidently related to the activities of one individual whose limbs repeatedly made contact with the substrate.

Studies of track-making in whip scorpions are rare. Schmerge et al. (2013) published one detailed study comparing trackways made by modern vinegaroons to examples of trackways made by modern and fossil arthropods. No fossil track or trackway, or burrow, definitively attributable to a whip scorpion has ever been recorded ( Hembree 2013, Schmerge et al. 2013). This paper represents the first trace fossils that can be directly attributed to a thelyphonid producer.

Discussion: Specimen MCZ:IP:198045(a, b) and associated trackway were likely made by an individual thelyphonid that was alive when the ichnofossils were produced. Possible behaviours that could explain full-body impressions of arthropods intergrading with trackways include end-of-life trackways and trails (Mortichnia; Seilacher 2007), moulting behaviour (Ecdysichnia; Vallon et al. 2015), and ‘resting’ (Cubichnia; Seilacher 1953).

Distinguishing trace fossils of living animals that incorporate a full-body impression from all possible taphonomic scenarios can be challenging. True Mortichnia are rare and most commonly preserved in subaqueous dysoxic to anoxic settings. Vallon et al. (2016) discouraged application of the category and found that some specimens classified as such actually record ecdysis ( Vallon et al. 2015). Many so-called ‘resting traces’ (Cubichnia of Seilacher 1953; ‘Volichnia’ of Walter 1978) or Full-body impressions (e.g. Knecht et al. 2011) made by a living arthropod may lack associated tracks or trackways entirely for a variety of reasons ( Martin 2006).

Criteria for establishing a full-body impression of a dead organism include:

1. Lying in an orientation other than upright, or in a ‘death pose’ (i.e. coiled, curled).

2. Entire impressions of limbs or other appendages that would have been held aloft in life, such as wings or antennae.

3. Limbs or other appendages folded beneath or over the body, disarticulated, or dislocated from the body.

Bodily impressions of organisms, especially those associated with trackways that do not exhibit the above features should not be treated as Mortichnia and also cannot be treated as ‘body moulds’ (sensu Minter et al. 2007). Indications that the causative organism was alive include:

1. An impression made solely by the ventral surfaces of the body and/or limbs.

2. Limb impressions that include only those portions of limbs or other appendages normally in contact with the substrate in life (i.e. tarsi rather than complete femora impressions).

3. An impression that shows movement of limbs or other appendages within tolerances (range of motion) that the animal would produce in life (limbs moving from joint axis; chelicerae probing substrate, etc.).

4. An impression that includes a trackway leading up to it or moving away, or both.

The bodily impression of a thelyphonid described here is no exception to these criteria, as it contains a number of features that indicate the causative organism was alive when it was made and no positive evidence to support its death. Multiple incomplete impressions of leg 1 representing a reasonable range of movement ( Fig. 7A, A 1 : T1, T2) and tarsal impressions that suggest a living and natural posture of the tracemaker ( Fig. 7B, B 1 : tar) are present. The lack of clear tracks or trackways leading away from the body impression may be due to poor preservation in that area, anterior to the body of the slab, where the surface is rough and lacks clarity.

The tracks and body impression of the thelyphonid maker were probably made in a transitional zone between subaqueous and subaerial environments as evidenced by the fine detail preserved in the trace fossils. The surface of interest, and most of the track-bearing surfaces in the Wamsutta Fm., are covered variably by a mm-scale layer of siltstone and shale that overlies very fine sandstone, which may represent waning flood deposition. After deposition, the surface dried enough to attain firmground status, yet remained plastic enough to faithfully reproduce the living causative organism’s body and its tracks (i.e. Davis and et al. 2007, Schmerge et al. 2013) in fine detail.

Figures

Figure 1. Image of an extant thelyphonid, Mastigoproctus giganteus giganteus Lucas, 1835.

Figure 3. Map indicating extent of Late Carboniferous sedimentary deposits in the region of interest (grey) with major cities indicated and comprising portions of south-eastern Massachusetts and eastern Rhode Island.Star at approximate location of recovered fossils thelyphonids. Basemap provided by ESRI; geological units merged from USGS state geologic map compilation.

Figure 7. The holotype of Inmontibusichnus charleshenryturneri igen.nov., isp. nov. (MCZ:IP:198045a,b).A thelyphonid full-body impression preserved as the mould (Fig.7A; concave epirelief) and cast (Fig. 7B; convex hyporelief) of an epichnial depression representing the ventral views of the tracemaker and their corresponding line drawings (Fig.7A1,Fig. 7B1). Image in Figure 7B, 7B1 has been flipped horizontally to match the same image orientation seen in Figure 7A,7A1. Keys to the anatomical abbreviations used can be found under the Material and Methods section.

Figure 8. Tracks associated with the body impression. Left: photo of slab whitened and greyscale inverted to accentuate tracks. Inset is magnified view of best series of wedge-like tracks, some with bifid extensions. Right: same field of view as left photograph with all tracks likely associated with body impression highlighted in red. Inset is sketch after Gallant and Hochberg (2017) SEM image of a thelyphonid tarsus (sans setae) highlighting two major claws and one minor claw (black scale bar in inset = 200 μm).