Saccomorpha papilio Hook, Radtke, Campbel et Golubic, 2024

Key feature (diagnosis): S. papilio is the name for the trace characterized by flat sporangial swellings widened into two opposite lateral lobes that resemble the shape of a butterfly with spread wings (Figs. 6a, b), diverging at angles between 45⁰ and 180⁰. Alternatively, the swelling may fuse to form a semicircular disk with several shallow lobes (Figs. 6c, d, e). The flat sporangial blade is itself anchored to the interior shell surface by numerous short (ca 2.5 μm long), thin (1 to 3 μm wide) tapered tunnels. On the opposite side, at the centre of the flat (the concave side) of the sporangial swelling is a spine (4–5 μm in diameter) extending toward the opposite shell surface (usually broken in cast preparations) Figs. 6b. Several (typically 4–7) tapered, monopodially branched tubular extensions originate between the swelling and the shell surface from a single centre, radiating in all directions (Figs. 6a, c). The main filaments, resin-replicated tunnels departing from the central body arch mildly counterclockwise for the first few hundred micrometres (Fig. 6c). They are initially 1–2 μm wide, and narrower with each subsequent ramification. These tubules form an extensive flat network, covering an area of 1–2 mm in diameter (Fig. 6a, c). The branching is monopodial and alternating, producing up to at least 7 orders of progressively thinner branchlets that adhere to the substrate surface, depart at an angle of about 70⁰, and run in remarkably straight lines. Filaments taper and become progressively finer as they ramify. The main filaments have a diameter of 1–2 μm at the base (Fig. 6d, e), while the finest branches are less than 0.1 μm wide (Fig. 6f). The extremely fine filaments originate at all ramification orders, appearing to have been created by tunnel-makers densely exploring the area. Very fine networks were observed to affect the substrate surface forming a small yet distinct etchfield around the termini of these finest grooves (Fig. 6f, Insert).