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Calyptrochilum aurantiacum  is a plant species in the family Orchidaceae. Its bright orange and yellow flowers are a rarity among the Angraecinae. Its natural habitat is subtropical or tropical moist lowland forests, and it is threatened by habitat loss.

Description
Calyptrochilum aurantiacum is an epiphytic herb with a monopodial growth habit. It is erect and glabrous, up to 16 cm tall. Its stem is up to 15 cm long, 5-6 mm in diameter, and slightly flattened in the plane of the leaves; it loses its lower leaves with age and becomes woody. Roots up to 12 cm long and 2 mm in diameter extend from the basal part of the stem; they are silvery-green along their length due to velamen and are orange-green at the apex. The leaves, 2-4.5 by 0.6-1 cm, are alternately arranged, fleshy, oblong-lanceolate, and bilobed at the apices. They are channelled and thus v-shaped in cross-section, sheathing at the base, and grow 5-6 mm apart. The inflorescences are axillary (appearing from the leaf bases), racemose, and roughly capitate. They bear a dense cluster of 9-15 flowers and are shorter than the leaves, up to 18 mm long; the bracts are broadly triangular, like many vandaceous plants, and are 1.1-2 by 1.5 mm. The peduncles are 3-4 mm long and 1-1.5 mm in diameter, with a sheathing bract at the base and another at halfway.

The flowers are spirally arranged, non-resupinate, glabrous, and fleshy, with converging, keeled orange-red sepals and petals, as well as a yellow lip. The dorsal sepal is elliptic with a pointed tip, 3.8-4 by 1.5 mm; the lateral sepals are triangular, 4-4.3 by 2 mm. The petals are oblanceolate with a pointed tip, 3-5 by 1 mm. The lip is unlobed and unfringed, oblong with a pointed tip, 4.5-5 by 3 mm, with a fleshy callus on each side at the opening of the spur. The spur is slightly s-shaped and club-shaped, 1.5-2 by 1.5 mm. The column is short, 0.5-0.6 mm long; the rostellum is deeply notched and pincer-like, bearing two pollinia, which are round with 1 pore, attached by stipes to a hard crescent-shaped viscidium. The pedicels and ovary together are 2-2.5 mm long. The plant blooms and fruits from May to November during the rainy season. Its diploid number is n=34.

Taxonomy and naming
In 1986, the original describers of C. aurantiacum, P.J.Cribb & Laan, described the species as the sole member of the genus Ossiculum in honor of H. J. Beentje, who collected the type specimen along with other orchids in Cameroon. "Beentje", the Dutch for a small bone, was translated into its Latin equivalent, ossiculum.

Cribb and Laan noted that the genus closely resembled Calyptrochilum in terms of floral morphology. Both genera possessed an 's'-shaped clublike spur, and their rostella were similarly elongate and notched. O. aurantiacum bore a close resemblance to C. christyanum in particular, with both species possessing a yellow lip and similar spur shape. Cribb and Laan noted other similarities between the species and other genera known at the time as well. Diaphananthe, Chamaeangis, and Tridactyle all contained species with brownish-orange flowers, but none produced flowers as brightly pigmented as C. aurantiacum. The fleshy bipartite callus at the orifice of the spur was also speculated to be analogous to a lesser-developed version in some species of Tridactyle. Also, some species of the leafless genus Microcoelia had a rostellum which vaguely resembled that of C. aurantiacum, though in Microcoelia the viscidium is thin, as opposed to that of C. aurantiacum, which carried more substance.

Cribb and Laan also noted the unusually low chromosome number in Ossiculum and Calyptrochilum (2n = 34 and 2n = 36, respectively). The diploid number of the former was, at the time, the lowest recorded in the Aerangidinae, in which the two genera were originally placed based on a rather long rostellum length. Taking into account a 1983 paper by Arends and Laan that rostellum length was not as a reliable an indicator as chromosome count, the authors proposed that both Osiculum and Calyptrochilum be moved into Angraecinae, whose genera traditionally have low diploid numbers.

Later in 1986, Gasson and Cribb found the leaf structure of Ossiculum and Calyptrochilum to possess shared characteristics that did not belong to other genera in the Angraecinae. These features included "the same epidermal cell arrangements, spiral thickenings on some mesophyll cells, the presence of water storage cells, along with the absence of hypodermal and mesophyll sclereids and that of a palisade layer." These findings were confirmed in 2006 by Carlsward et al., but O. aurantiacum remained separate from Calyptrochilum because of its upright growing habit, slightly channelled leaves, nonresupinate flowers, pincer-like rostellum lobes, simple labellum with a two-lobed callus, and, most importantly, its distinctive golden flower color; the recognized Calyptrochilum species at the time had a pendent vegetative habit, flat leaves, linear rostellum lobes, tripartite lips lacking a callus, and mostly white to pale green flowers. Finally, in 2018, Stévart, M.Simo & Droissart combined the previous morphological and cytological data with genomic information, cementing the position of the former monotypic genus within Calyptrochilum.

Distribution and habitat
At the time of its initial publication, C. aurantiacum was known only from Cameroon.