Coragyps atratus (Bechstein, 1793)

James, Helen F., 2020, The Irvingtonian Avifauna of Cumberland Bone Cave, Maryland, Zootaxa 4772 (1), pp. 111-131 : 118-119

Coragyps atratus

USNM PAL 641970 , r femur : proximal end and most of the shaft, collected October 8, 1999 ( Fig. 1D ) .

Description. The bone displays adult features including a distinct crista and fossa trochanteris, articular facet for the antitrochanter, and femoral head with the neck and the fovea ligamentum capitis developed, but it is clearly immature in that the exposed shaft is densely covered with short, fine striations, the fossa trochanteris is spongy, and the head retains dense, fine pores. It agrees remarkably in size, form and fine morphological details with Coragyps atratus , including the shaft curvature, proportions of the proximal end including the thick neck, the sharp and moderately elevated form of the crista trochanteris, and the proximal placement of the pneumatic foramen on the cranial surface. Larger than Cathartes aura and with the femur shaft distinctly bowed rather than straight as in that species. An immature individual of H. leucocephalus with similar bone surface textures (USNM BIRDS 611757 f) has a much longer femur (length from trochanter to the prominent nutrient foramen on the caudal aspect of the shaft, 57.7 mm in H. leucocephalus vs. 37.0 in the fossil) but is less advanced towards adult morphology (e.g., femoral head much less produced, crista trochanteris not distinct, no fossa trochanteris or fovea ligamentum capitis, articular facet for the antitrochanter bulbous). Likewise, the fossil is much smaller than the extant Aquila chrysaetos , and the adult features that the fossil has attained differ in form from the two eagles (crista trochanteris less elevated proximally above the proximal articular surface than in H. leucocephalus , more elevated and more distinct than in A. chrysaetos ). Amplibuteo concordatus Emslie and Czaplewski 1999 , an extinct eagle known from Blancan and early Irvingtonian sites in Florida, was not directly compared but is unlikely to be relevant due to its eagle-like rather than Coragyps -like osteology.

Measurements. Howard (1968) summarized measurements from a large series of Coragyps occidentalis ( Miller 1909b) bones from the Rancho La Brea tar pits, of which only the least transverse breadth of the shaft (8.9 mm) can be observed in the Cumberland Bone Cave femur. For this measurement, Howard reported a mean of 9.1 mm in modern C. atratus and a minimum of 9.0 mm in 65 femora of C. occidentalis .

Remarks. The late Pleistocene (Rancholabrean) species C. occidentalis , is known from multiple localities in southwestern North America and is believed to have become extinct along with the Pleistocene megafauna ( Brasso & Emslie 2006). Bones of this species are larger on average than in modern C. atratus ( Howard 1962, 1968; Frailey 1972). Because the fossil femur has not attained adult surface textures, we cannot exclude the possibility that its full adult size would be above the range of modern C. atratus and within the range of the larger late Pleistocene species. It is notable that C. atratus has not moved into most of the southwestern range of C. occidentalis in the time since the extinction of the latter, roughly ten thousand years ago. This suggests that C. occidentalis occupied arid habitats that are not suitable for C. atratus .

The immature bone of C. atratus provides evidence that the species was breeding in or near the cave. We can rule out the possibility that the bird had flown from elsewhere to reach Cumberland Bone Cave by analogy with a growth series of California Condor (G. californianus) skeletons. The developmental stage of the fossil most closely matches that of a condor that died at 136 days old (USNM BIRDS 658188), in which the femur is likewise at or close to full adult size and form, has a finely striated shaft, dense pores on the femoral head, and the fossa trochanteris still spongy. The fossil femur is more advanced developmentally than a bird that died at 80 days old (USNM BIRDS 658191), which has not attained adult size and has the articular ends not fully formed or ossified. In two older condor chicks that died ten or fifteen days after the usual age of fledging (AMNH 32153, 184 days, and USNM BIRDS 658221, 187 days), the surface of the femoral shaft has attained the smooth surface texture and rugose topography seen in adult birds, although both femora retain some porous structure on the proximal articular surface.

The G. californianus skeleton with the femur at a similar stage of development to the fossil has a relatively underdeveloped forelimb, with the major wing bones still spongy and growing at the articular surfaces and the carpals and metacarpals unfused, and has the sternum still entirely porous and spongy (see USNM BIRDS 658188). It is no surprise, then, that California Condor chicks typically fledge at an older age (about 30 to 40 days older) than that of the individual that best matched the fossil ( Snyder & Snyder 2000). Assuming the developmental pattern is similar in Coragyps , the fossil vulture from Cumberland Bone Cave would not yet have been able to fly.

Figures

FIGURE 1. Fossils from Cumberland Bone Cave compared with modern taxa.A) fossil left humerus of Meleagris sp., USNM PAL 641969, caudal aspect; B) left humerus of Meleagris gallopavo, USNM BIRDS 556334 f, caudal aspect; C) fossil left humerus of Meleagris sp., USNM PAL 641067, caudal aspect; D) fossil right femur of Coragyps atratus, USNM PAL 641790, cranial aspect; E) right femur of C. atratus, USNM BIRDS 647499 m, cranial aspect; F) fossil proximal right carpometacarpus of fossil Branta dickeyi, USNM PAL 641972, ventral aspect; G) proximal right carpometacarpus of B. canadensis, USNM BIRDS 488182 m, ventral aspect; H) left tarsometatarsus of Falcipennis canadensis, USNM BIRDS 557527 f, plantar aspect; I) fossil left tarsometatarsus of Bonasa umbellus, USNM PAL 641979, plantar aspect; J) left tarsometatarsus of B. umbellus, USNM BIRDS 600360 f imm., plantar aspect; K) fossil left tarsometatarsus of Ectopistes migratorius, USNM PAL 769090, plantar aspect; L) tarsometatarsus of E. migratorius, USNM BIRDS 292904 (captive), plantar aspect; M) fossil left femur of Megascops guildayi, USNM PAL 769089, caudal aspect; N) left femur of Megascops asio, USNM 623630 f, caudal aspect; O) fossil right ulna of M. guildayi, USNM PAL 641984, ventral aspect; P) right ulna of M. asio, USNM BIRDS 623630 f, ventral aspect; Q) right tarsometatarsus of Perisoreus canadensis, USNM BIRDS 639077 f, dorsal aspect; R) fossil right tarsometatarsus of a species of jay (Corvidae, aff. Perisoreus/Cyanocitta), CM 24274, dorsal aspect; S) fossil right tarsometatarsus of a species of jay (Corvidae, aff. Cyanocitta/Perisoreus), USNM PAL 641989, dorsal aspect; T) right tarsometatarsus of Cyanocitta cyanea, USNM BIRDS 499504 f, dorsal aspect. Scale bar 1, for images A–G, = 2 cm. Scale bar 2, for images H–N, = 1 cm.