Despite their very different lifestyles the Swifts (Apodidae) and Hummingbirds (Trochilidae) are thought to be closely related groups, placed within a single combined group, the Pan-Apodiformes, along with the Swift-like Tree Swifts (Hemiprocnidae). As a group the Pan-Apodiformes are thought to be most closely related to Nightjars, Potoos, Oilbirds and Frogmouths. Several Pan-Apodiform Birds have been described from the Eocene and Oligocene of Europe, but the American fossil record of the group is somewhat poorer. Despite this the group are thought to be of North American origin; the Hummingbirds are restricted to the Americas and the closest relatives of the group are at their most diverse in North America.
In a paper published in the Proceedings of the Royal Academy of Science: Series B, Biological Sciences on 1 May 2013, a team of scientists led by Daniel Ksepka of the Department of Marine, Earth and Atmospheric Sciences at North Carolina State University, describe a Pan-Apodiform Bird from the Early Eocene Fossil Butte Member of the Green River Formation of Lincoln County, Wyoming, which has been calculated to be 51 660 000 years old using Argon isotope dating.
The new Bird is placed in the genus Eocypselus, previously used to describe a single species from the Eocene of Denmark, and given the specific name rowei, in honour of John Rowe of The Field Museum in Chicago. Eocypselus rowei is an 8 cm bird preserved as part and counterpart on a divided slab. The specimen shows exceptional preservation, with a complete skeleton and intact feathering.
Eocypselus rowei, (a) main slab and (b) counterslab. Ksepka et al. (2013).
While the skeleton is almost complete, many of the bones split when the slab was broken open, so muscle insertions are not visible. The skull is poorly preserved, but has a beak of similar proportions to those of a modern Swift, but lacking the downturned tip.
Detail of the head of Eocypselus rowei. Ksepka et al. (2013).
The feathers around the dorsal part of the head, while not well preserved and hard to differentiate, retained preserved melanosomes (colour cells). These are densely packed rod-like eumelanosomes, which are typically associated with a glossy black colouring in modern birds, but occasionally are seen in iridescent colouring.
Scanning electron microscopy image of the part of the head of Eocypselus rowei, showing abundant three-dimensionally preserved eumelanosomes. Taken at the white spot in image (a). Ksepka et al. (2013).
Swifts and Hummingbirds share similar skeletal structures in their wings, with a short stout humerus and elongate carpometacarpus and phalanges, but achieve remarkably different wing shapes and highly specialized flying strategies through different muscle insertions and plumage. Eocypselus rowei has a humerus:ulna ratio within the range of modern Swifts, but a shorter overall wing-length than any Swift. The carpometacarpus and phalanges of Eocypselus rowei are shorter than those of Hummingbirds, but the primary wing-feathers are longer. This suggests a wing-shape less specialized than either the modern Swifts or Hummingbirds, and somewhat intermediate in form.
Comparison of wing structure in Eocypselus rowei (top), an extant Swift, Hirundapus caudacutus (middle) and an extant hummingbird, Archilochus colubris (bottom), with overall body outlines at right. Skeletal elements (right side) and spread wings (left side) from the same specimen were outlined and traced, with wings mirrored to create images for extant taxa. For the fossil taxon wing length was reconstructed from the leading primary, and dotted lines indicate the uncertain breadth of the wing. For comparison, wings were scaled to the same skeletal wing length and body outlines to the same head-to-tail length (excluding the long beak of the hummingbird). Ksepka et al. (2013).
Modern Swifts have shortened, reduced legs, poorly adopted to perching, while Hummingbirds have elongate hindlimbs and are confident perchers. Eocypselus rowei also has elongate hindlimbs, and appears well adapted to perching, suggesting that the Swifts lost this ability after they diverged from the Hummingbirds.
The left and right pes (feet) of Eocypselus rowei. Pedal digits are labelled I, II, III and IV. Ksepka et al. (2013).
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