Split Slab (part: GRI #441a; counterpart: GRI #441b)
Invertebrate Trackway
This small slab (GRI# 441) of Coconino Sandstone (Permian, AZ) bears an invertebrate trackway, the footprints of an arthropod crawling across the sand. The trackway consists of alternating series (groups) of three, small, mostly circular imprints with a narrow medial impression. This means the producer was hexapodous (walked on three pairs of legs), walked on the tips of pointed legs, and most likely dragged a terminal filament (narrow tail) to produce the medial impression.
The currently accepted name of this trackway is Paleohelcura tridactyla Gilmore, 1926 (=P. dunbari Brady, 1961), but Cedric L. Clendenon, Biology Ph.D., has made a strong case for using Stiaria intermedia (Smith, 1909) as a senior synonym, which is used for similar trackways.
Paleohelcura is conventionally interpreted as the trackway of a scorpion made on dry sand, but this interpretation is problematic because scorpions are octopodous (walk on 4 pairs of legs), produce wider medial impressions if they do drag their metasoma (usually held above them), and make much larger imprints in dry sand.
Furthermore, predictive modeling (based on neoichnological experiments of various arthropods in dry, damp, wet, and underwater conditions) indicates this trackway (and all Paleohelcura trackways in the Coconino Sandstone) probably formed on wet sand or underwater (Clendenon and Brand, 2024).
Other potential producers include giant archaeognathans (bristletails) and small eurypterids, but as there are no body fossils in the Coconino Sandstone, the actual producer remains unknown.
441a detail: the concave epichnia side of the trackway (scale in cm)
Specimens bought by GRI from the personal collection of Joe R. Young, Jr. Based on the lithological characteristics, it is most likely that these specimens were collected near Ash Fork, Arizona.
Reference:
Clendenon, C.L. and Brand, L.R., 2024. Quantitative analysis of experimental trackways of scorpions, tarantulas, and crayfish. Ichnos, 30(3), pp.235-266. https://doi.org/10.1080/10420940.2024.2322937
441b detail: the convex hypichnia side of the trackway (scale in cm)