Graptolites of the genus Appendispinograptus feature prominently in Late Ordovician biostratigraphy and studies of biodiversity. Well-documented, but enigmatic, secondary structures extend along the basal spines in mature specimens of three of the four common late Katian species of the genus: A. supernus, A. longispinus, and A. venustus. Using SEM imaging of isolated, three-dimensional specimens of A. supernus, we provide a detailed description of its early growth, as well as confirm that its secondary structures are composed of hollow tubes extending from the sicular and thecal apertures (‘parasiculae’ and ‘parathecae’, respectively). We also describe a collection of Canadian (northern Yukon) appendispinograptids that possess a large heart-shaped, sheet-like basal structure. Detailed comparison (width, thecal length and thecal inclination) with immature specimens from our collection, as well as collections from Russia, China, and Nevada, identifies these specimens as A. leptothecalis, a species not previously known to exhibit any secondary spine modification. The observation of spine modification in A. leptothecalis allows previously unclassified specimens from China, Siberia, and Nevada to be assigned to this species, clarifying our understanding of appendispinograptid biogeography and diversity. However, these structures are difficult to explain functionally. Unlike structures found in other Appendispinograptus species, those in A. leptothecalis decrease usable thecal space, shift drag proximally, and provide no obvious feeding advantage.