Orthogonum arbor Hook, Radtke, Campell et Golubic, 2024

Key feature (diagnosis): O. tenue consists of narrow, very straight, rarely branched tubular traces, with irregularly wound, crowded terminals (Fig. 14 a-a). Lateral branches are short with attenuated ends. The main tubules are 5.5 to 10 (6.9 ± 1.35, n = 32) μm wide. Comment: This trace needed to be described as separate Orthogonum ichnospecies as it forms trace assemblages that are consistently narrow, and distinct from O. lineare. The main trunk diameter of O. tenue corresponds to the diameter of the secondary branches of O. tubulare Radtke 1991. The trace was observed in the shell of the pteropod Limacina.