The solitary rugose coral Keyserlingophyllum is characterised by its large size and the development of lonsdaleoid dissepiments. Detailed investigations into its ontogeny and intraspecific variation are limited, primarily due to the difficulty in obtaining a sufficient number of well-preserved specimens that include neanic stages. In Baoshan, Southwest Yunnan, six exceptionally preserved specimens were retrieved from the Mississippian Shihuadong Formation. These specimens provide an adequate sample of a fossil population and were serially thin-sectioned to facilitate a thorough analysis of morphological variation. Examination of septal insertion patterns across all six specimens revealed significant variations in the length, thickness, and arrangement of the septa, especially in the counter or counter-lateral area, both across ontogenetic stages and between individuals. The notable elongation or irregular thickening of septa may be linked to adaptations for maintaining stability on a soft substrate in a shallow marine setting. Based on these findings, a revised diagnosis of Keyserlingophyllum is proposed, accompanied by a systematic reassessment of all species previously classified under Keyserlingophyllum and Humboldtia in China. Applying the updated diagnosis to Keyserlingophyllum, a total of 13 species are confirmed as belonging to this genus there. (c) 2026 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights are reserved, including those for text and data mining, AI training, and similar technologies.