Four incompletely preserved caudal vertebrae lacking the neural arches of two fossil sirenian individuals of Halitherium schinzii (Oligocene) from the Rhine area in Germany and northern Belgium reveal osteological alterations. The caudal vertebrae possess a transverse process with growth retardation. This asymmetry indicates that the affected transverse processes are less developed than their counterparts and, consequently, deviate from the norm. Computed tomography (CT) scans reveal osteosclerotic patterns, a morphological feature that characterizes sea cows and supports the nonpathological state of the vertebrae. Additionally, no indications of vertebral fractures or any other occurrences due to external factors are present. This is the oldest documentation of such an anomaly in any sirenian and is interpreted here as hypoplasia, the underdevelopment of an organ or parts of it that might cause a functional deficiency. Anat Rec, 2011. © 2011 Wiley-Liss, Inc.