Branta dickeyi Miller 1924

Branta dickeyi Miller, 1924

Material. USNM PAL 641972, r carpometacarpus: proximal end with damage to trochlea carpalis, collected July 2002 (Fig. 1F).

Description. A large anserine carpometacarpus with a long extensor process of the alular metacarpal that is perpendicular to the long axis of the bone and narrows distinctly towards the tip. In contrast, the extensor process is shorter in Chen and Anser; and shorter and wider at the tip in Coscoroba and Cygnus. The fossil agrees well in morphology with the largest comparative skeletons of Branta canadensis but exceeds them in size (at least if skeletons of captive-reared individuals are excluded from comparisons). It is distinctly larger than other North American species of Anserinae. The extensor process resembles large individuals of B. canadensis in being long and robust. Rough-surfaced exostoses are present at the tip of the extensor process; these tend to be present in large males of B. canadensis but can also occur in other Anseriformes.

Measurements. In the fossil, the maximum depth from the carpal trochlea through the extensor process is 28.6 mm. This exceeds the range for 12 modern carpometacarpi of B. canadensis measured by Emslie (1995; range 21.8-25.8) and nine measured by me (range 19.8–25.3, including two from South Dakota, within the original range of the largest subspecies of Canada Goose, B. canadensis maxima Delacour). It closely matches the measurements recorded by Emslie (1995) for two Irvingtonian fossil carpometacarpi of the fossil species Branta dickeyi, one from Florida and one from Oregon, both of which measured 28.7 mm.

Remarks. The large fossil goose B. dickeyi has the osteological characteristics of Branta but is roughly the size of a Tundra Swan (C. columbianus). It has previously been reported from three disparate localities in North America: the Rancholabrean McKittrick tar seeps of California (Miller 1924), a Blancan locality in Malheur County, Oregon (Miller 1944), and an early Irvingtonian locality in Florida (Leisey Shell Pit, Emslie 1995). There are few complete bones among these fossils, and not many bones from each locality. I have ascribed the large but fragmentary carpometacarpus of Branta from Cumberland Bone Cave to the species, although this does require the assumption that incomplete remains from widely separated localities represent a single species.

In the Oregon and Florida sites, B. dickeyi co-occurs with smaller Branta fossils that match B. canadensis in size. This supports the view that the fossil species represents an extinct phyletic lineage rather than a chronospecies of its modern relative. However, if we consider the modern species B. canadensis as a potential modern analog, interpreting the fossils becomes more complex. B. canadensis is migratory within North America and exhibits considerable geographic variation in body size across its broad range (Aldrich 1946; Delacour 1951). Populations that breed at high latitudes in northern Canada and Alaska tend to migrate farther than, and to be smaller in body size than, those that breed at mid-latitudes (Mowbray et al. 2002). (This is independent of the human-mediated expansion in the distribution of large-bodied, resident Canada Geese that began in the 1960s due to captive propagation and release (Ankey 1996; Mowbray et al. 2002.)) Such a pattern of size variation and migration could theoretically cause individuals of the same species, but quite different body sizes, to be present in the same fossil site.

The largest modern subspecies of Canada Goose, B. canadensis maxima Delacour, bred in the Great Plains well west of Cumberland Bone Cave and was thought to be extinct when it was described (Delacour 1951). A small population of these birds was discovered in 1962 and taken into captive propagation. Releases of the captive-reared birds into the wild after habituation to human-modified habitats enabled them to expand in geographic distribution, become resident year-round in many regions, and increase exponentially in population size (Ankey 1996; Mowbray et al. 2002). Some captive-reared individuals of this subspecies in the USNM collection approach B. dickeyi in skeletal size, although it is unclear whether birds with no history of captivity attain the same body size. In ascribing the Cumberland Bone Cave fossil to B. dickeyi, I have followed the lead of prior authors and have left unresolved the question of whether the fossils of this species represent larger-bodied populations that are ancestral to modern B. canadensis.