The modern Avian Order Apodiformes includes the Swifts, Apodidae, Treeswifts, Hemiprocnidae, and Hummingbirds, Trochilidae. The morphological distinctness of this group of highly specialised fliers has made determining how they are related to other Bird groups difficult, although modern phylogenetic techniques have revealed that they are the sister group to the Owlet-Nightjars, Aegotheliformes. Although a number of fossil Apodiformes are known, with the oldest coming from the Early Eocene London Clay of southern England, the early history of the group is not well understood, as, in common with most Birds, the skeletons of Apodiformes tend to be small and delicate, not lending themselves to preservation, so that most fossils assigned are of isolated, and often fragmentary, bones.
The earliest known members of the group are Primapus lacki, which is known from a nearly-complete humerus from the London Clay at Bognor Regis in West Sussex, and the distal portion of a second humerus from the Isle of Sheppy in Kent, and Eocypselus vincenti, which was described from associated wing and pectoral girdle bones from Walton-on-the-Naze in Essex, with partial skeletons assigned to the same species described from the early Eocene Fur Formation in Denmark. A second species of Eocypselus, Eocypselus rowei, has been described from the Early Eocene Green River Formation of Wyoming.
The classification of these early Apodiform Birds remains unclear. Primapus lacki has been assigned to the Aegialornithidae, an extinct group which also includes four species from the Middle Eocene-Early Oligocene Quercy fissure fillings of France, and which may be stem group Apodiformes or more closely related to Swifts than other members of the group. Eocypselus vincenti is sometimes considered to be an early member of the Hemiprocnidae, but more usually to an Apodiform stem group, the Eocypselidae.
In a paper published in the journal Ibis on 22 March 2024, Gerald Mayr of the Ornithological Section of the Senckenberg Research Institute and Natural History Museum Frankfurt, and Andrew Kitchener of the Department of Natural Sciences at National Museums Scotland, and the School of Geosciences at the University of Edinburgh, describe multiple new skeletons of Apodiform Birds from the Early Eocene London Clay at Walton-on-the-Naze in Essex, from the collection of the late Michael Daniels, an Edinburgh resident and amateur palaeontologist who passed away in 2021, leaving his collection to National Museums Scotland. From this material they describe three new species of Eocypselus, as well as new specimens of Eocypselus vincenti and Primapus lacki, which add significantly to our understanding of these Birds.
The first new species described is Eocypselus geminus, where 'geminus' means 'twin' in reference to the fact that this species closely resembles Eocypselus vincenti. It is described from a partial disarticulated skeleton, specimen NMS.Z.2021.40.116, which comprises a partial rostrum and mandible, the axis, a furcula (wishbone) broken into three pieces, the right scapula, the right coracoid in a piece of matrix, a partial sternum, a partial left humerus, the left radius, the proximal portion of the right radius, the right phalanx proximalis digiti majoris, both carpometacarpi, fragments of the pelvis, a right tarsometatarsus lacking its proximal end, a few pedal phalanges and the anungual phalanx. A second specimen is also referred to this species, NMS.Z.2021.40.117, which comprises the left coracoid, the omal extremity of the right coracoid, the left humerus, the proximal portion of the right humerus, the left ulna, and the proximal portion of the right ulna. A mediolaterally narrow premaxilla found associated with this specimen belongs to a different, non-Apodiform species.
While Eocypselus geminus resembles Eocypselus vincenti closely, the two species can be distinguished on the basis that the processus procoracoideus of the coracoid projects less strongly towards the processus acrocoracoideus in Eocypselus geminus than in Eocypselus vincenti, the furcula of Eocypselus geminus has wider omal sections of the scapi clavicularum than that of Eocypselus vincenti, and the humerus of Eocypselus geminus, which has a stouter shaft and a dorsoventrally wider distal end than that of Eocypselus vincenti.
The second new species described is named Eocypselus paulomajor, where 'paulomajor' is intended to mean 'somewhat larger' in Latin, in reference to the size of the species, which is larger than Eocypselus vincenti. This species is described from a single specimen, NMS.Z.2021.40.118, which comprises a partial furcula, a left coracoid, and a left ulna.
The third new species described is named Eocypselus grandissimus, where 'grandissimus' means 'the largest' in reference to the fact that this specimen is larger than all other members of the genus. It is described from a single specimen, NMS.Z.2021.40.119, which comprises a few vertebrae, both coracoids, both scapulae, the cranial portion of the sternum, the distal end of the right humerus, the proximal end of the left humerus, a partial left carpometacarpus, the distal end of the left femur, and one pedal phalanx. A distal left humerus associated with the specimen belongs to a non-Apodiform species.
A number of further specimens are referred to Eocypselus, but not to any particular species. All are of similar size to Eocypselus geminus and Eocypselus vincenti, but cannot be confidently be assigned to either species, and may potentially represent additional, undescribed, species. although without further, more complete, specimens to refer to, it would be impossible to be certain of this.
A new specimen of Primapus lacki is also described. This specimen, NMS.Z.2021.40.133, comprises the furcula, the right coracoid, the left and right humeri lacking distal ends, the left radius, a partial right carpometacarpus, and a single pedal phalanx.
Mayr and Kitchiner consider the Aegialornithidae and Eocypselidae to be stem-group Apodiform Birds, forming sister taxa to the crown group Apodiformes. Primapus lacki represents the earliest representative of the Aegialornithidae, a group which persisted at least 19 million years, into the Early Oligocene. In contrast, the Eocypselidae are known only from Early Eocene deposits, and appear to have been a short-lived group. The crown group Apodiformes are unknown in the Early Eocene, with the earliest known example being Scaniacypselus szarskii, thought to be an early Swift, from the Middle Eocene Messel Shale of Germany.
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