Orthogonum arbor Hook, Radtke, Campell et Golubic, 2024

Key feature (diagnosis): O. arbor is a solitary, relatively short, tubular form with a notable resemblance to a tree (Fig. 13a). Like a tree, it consists of a "trunk'' which terminates with a dense "canopy" of branches. The trunk begins its growth at the point of entry into the substrate and curves to continue to grow, mostly parallel to the shell surface, as a straight or gently arched tunnel for 100 to 600 μm. It is circular in cross section with a diameter of ca 10 μm, gradually increasing distally to 12 μm. It may sometimes ramify (Fig. 13a) or change direction if encountering obstacles (Fig. 13b). It mostly remains unbranched and mildly curved (Fig. 13c). The "canopy" results from a burst of ramifications at the end of the straight "trunk," with an average branch point distance of 30 μm. The branching pattern is planar, sympodial, expanding up to 5 orders, with a circumference of about 150 μm (Fig. 13a). The planar growth of the tree trace may be dictated by the thinness of the shell. Departures of this pattern were noticed in thicker shells where the taxon develops a threedimensional canopy (Fig. 13b). Branches are up to 130 μm long, sometimes arched, and always shorter than the main trunk. Their diameter is narrower at the branch point, and gradually widens distally, up to 50 μm, assuming a clavate shape. Older branches also enlarge laterally, becoming flat and oval in cross section. As the widened branches fill most of the space between, they crowd and become mutually compressed and rectangular in cross section. This trace is typically an integral member of the deep sea ichnocoenosis (Fig. 13d).