New specimens of Scaniacypselus szarskii, a stem group representative of the Apodidae (true swifts) from the middle Eocene Messel oilshale in Germany, are described. These fossils show that Scaniacypselus differs from crown group Apodidae in a number of distinct and previously unrecognized osteological features. Notably, the sternum of Scaniacypselus is shorter than that of extant true swifts, the ulna proportionally longer, the carpometacarpus shorter, and the internal index process on the proximal phalanx of the major wing digit less developed. In details of its humerus morphology, Scaniacypselus is distinguished from all extant apodiform birds. Scaniacypselus further has a much shorter tarsometatarsus than most crown group Apodidae. It is hypothesized that short legs are plesiomorphic for Apodidae as a total group (stem group and crown group representatives), but that crown group Apodidae primitively have an elongated tarsometatarsus as an adaptation for clinging to vertical surfaces. The less specialized wing and pectoral girdle morphology suggests that Scaniacypselus was probably not as aerial as extant Apodidae. The differences in foot morphology indicate that it was more arboreal than its living relatives and had different breeding and roosting habits. Crown group Apodiformes probably diverged well after the middle Eocene, and the derived nesting behavior may have contributed to their evolutionary success.