The only known specimen of
Regulus bulgaricus is a complete left ulna, 13.3 mm in length. It was collected on 20 September 1991 in a
ponor near Varshets, Bulgaria. It was first described by its collector, the Bulgarian paleornithologist Zlatozar Boev. Its species name,
bulgaricus, was given after the country in which the fossil was found. Boev diagnosed it as an extinct species of the genus
Regulus. The ulna is smaller than that of most passerines, and the shape of the articular surfaces identify it as a kinglet. It is distinguished from
R. regulus by a thicker base, a longer
olecranon, a larger
cotyla dorsalis, and smaller
quill knobs (papillae remigales caudales). Compared to
R. ignicapilla,
R. bulgaricus has a narrower proximal part of the
diaphysis, a shorter olecranon, and smaller
tuberculum retinaculi. Although Boev was unable to compare the fossil ulna with
R. goodfellowi, it can be excluded from this taxonomical comparison due to it sharing a superspecies with
R. regulus.
Regulus bulgaricus is the only fossil kinglet, and is possibly the ancestor of
R. ignicapillus. This was speculated based on the postglacial origin of a coniferous forest belt in the
Holarctic, which means that its avifauna is of a more recent origin. == References ==