Small, erect, annual herbs, to 30 cm tall. Leaves opposite, blade ovate to broadly elliptical, coarsely to finely serrate. Inflorescence a diffuse corymbose cyme. Phyllaries 5, distant, biseriate, subequal to equal, persistent, oblong-elliptical to obovate-elliptical, with shortly apiculate apices; receptacle flat, foveolate, glabrous, epaleaceous. Florets 7–14; corollas asymmetrical with the two inner lobes smaller, white, with distinct short constricted basal tube bearing glandular or eglandular hairs; throat short and broad-campanulate; lobes as long as wide or outer lobes of peripheral florets longer, short-papillose on inner surface and margins; lower part of filament glabrous; anther collars rather short; style base not enlarged, glabrous; arms rather short-clavate, densely short-papillose; papillae larger and less dense below clavate tip. Achenes prismatic, 5-ribbed, ribs setuliferous, narrowed and setuliferous above carpopodium; carpopodium inconspicuously, short-cylindrical; pappus 5 long, lanceolate squamellae, densely scabrid on margins, nearly smooth on outer surface, smooth on inner surface.

A little herb characterised by its asymmetrical corollas with two small inner lobes, flat and epaleaceous receptacle and presence of pappus.

Peru. Dept. Lima: prov. Huaral, Dist. Atavillos Bajo, NE of Pampas, road to Rupac, archaeological monument pre Inca, slopes with loamy clay soil, scrubland, −11.313333, −76.61333, 3033–3509 m a.s.l., 15 Apr 2018, (fl,fr), A. Cano, P. Gonzáles, E. Huamán, S. Riva & S. Rivera 22682 (holotype: USM-3070016!, isotypes: MO!, MOL!, US-3730645!, USM!).

Figure 1. 

Centenaria rupacquiana. A Plant B leaf (adaxial surface) C mature leaf (abaxial surface) D mature leaf (adaxial surface) E upper leaf F uppermost leaf bracteiform G capitula (upper view) H involucre I inner phyllaries J outer phyllaries K flower (frontal view) L flower (lateral view) M flower tube with glandular trichomes N flower (upper view) O style and stamens P, Q scale of pappus R achene without pappus. Photos by P. Gonzáles.

Slender and small herb 9–20(30) cm high, bearing 2–10 heads. Roots thin and delicate, 2–4 cm long. Stem unique, terete, 0.3–1(1.7) mm thick, often purplish tinged, rather densely pilose with several celled acuminate mostly spreading whitish or purplish hair up to 0.3–0.7(1) mm long, internodes 1–3, mostly (1)3–5(6) cm long. Leaves opposite, membranous, petioles short, ca. 1–1.5 mm long, pubescent like the stem, larger leaf blades 10–20 mm long, 7–12 mm wide, obtuse to acutish, at base cuneate, obtusely about 3-toothed on each side, triplinerved from near base and lightly reticulate veiny beneath, pilose on surface above and chiefly along veins beneath with hair like those of stem, thin herbaceous; upper leaves (subtending branches of inflorescence) mostly sessile, smaller, the uppermost bracteiform. Capitula homogama, discoid, mostly in groups of 2 or 10 at tips of stem and branches, 4–5 mm high, 2.5–4 mm thick, on densely pubescent pedicels 2–8(12) mm long, pedicel pilose and pilose-glandulose (globose-stippled glands); involucre 3–3.5(4) mm high, 2–2.3 mm wide, phyllaries 5, biseriate, imbricate, outermost phyllaries 3, membranous, elliptical to obovate-elliptical, obtuse, 3–3.5 mm long, 1–1.7 mm wide, 5-nerved, the phyllaries rather glabrate, apex tinted purple, slightly erose-ciliate, innermost phyllaries 2, elliptical to obovate, obtuse, 3.5–3.8 (4) long, 2–2.2 wide, 5-nerved, the phyllaries rather glabrate, apex erose-ciliate; receptacle flat foveolate; pales none. Flowers hermaphroditic, (7)9–12(14), corollas asymmetrical, the two inner lobes are smaller, white, funnelform, 2 (inner) to 3 (outer) mm long, outermost corollas 3–4 very asymmetrical, the tube 0.9–1.2 mm long, tube base expanded, the throat about 0.3–0.5 mm long, the longer teeth 1–1.5 mm long, triangular-lanceolate, the shorter about 0.2–0.3 mm long, triangular, apex acute, margin stout minute papillose, innermost corollas 7–8(10), the tube 0.7–1 mm long, tube base dilated, the throat about 0.2–0.5 mm long, the longer teeth 0.2–0.3 mm long, triangular-lanceolate, the shorter about 0.1–0.2 mm long, triangular, apex acute, margin stout, minute papillose, anthers oblong, apex obtuse, emarginate, subtruncate, exappendiculate, base obtuse, 0.2–0.3 mm long and 0.1–0.2 mm wide, filaments inconspicuous, ca. 0.1–0.2 mm, stylus crassiusculus, 1–1.3 mm, with 2 stigmatic arms 0.5–0.7 mm long, arms recurvate, clavate, densely papillose in the stigmatic region. Fruit an achene mature black, 1.5–2 mm long, 0.5–0.7 mm wide (above), prismatic, base attenuate, 5-ribbed, ribs setuliferous, narrowed and setuliferous above carpopodium; carpopodium inconspicuous, shortly cylindrical, less than 0.1 mm; pappus (only in the innermost flowers of the capitula, outermost flowers without pappus) of 5 lanceolate squamellae, densely scabrid on margins, nearly smooth on outer surface, smooth on inner surface, scales 1–1.2 mm long, 0.2–0.3 mm wide (lowermost), to 0.7 mm wide (middle part), united in the base, easily separable from the achene.

Known only from the type locality in Rupac, northeast from Lima Department.

Terrestrial plant growing on open area amongst shrubs, in the western Cordillera shrubland, between 3000–3500 m a.s.l. Co-occurring species include Paracalia jungioides (Hook. & Arn.) Cuatrec., Heliopsis buphthalmoides (Jacq.) Dunal, Dasyphyllum ferox (Wedd.) Cabrera and Vulpia megalura (Nutt.) Rydb. Flowering and fruiting between April and May.

The genus is dedicated to the centennial of the institutional foundation of the Natural History Museum of National University Mayor of San Marcos (1918–2018), for their hard work on the research, conservation, preservation and diffusion of the biodiversity of the country. All these actions are steadily increasing our knowledge of the flora and fauna of our native land. The specific epithet refers to Rupac, a small village with archaeological remains from the Atavillos culture, where the only two populations of this species are known from this place.

Centenaria rupacquiana is only known from the type collection and is therefore assessed as Data Deficient (DD) according to the UICN (2012, 2017) criteria. However, we recommend it should be considered critically endangered (CR), as it is only known from a single locality (Criterion B1a) with a continuing decline of its quality of habitat inferred from the intensive livestock in the area (Criterion B1b). Furthermore, is only known from two populations with an estimated number fewer than 250 mature individuals (Criterion C1a(ii)).

PERU. Lima: Huaral, Atavillos Bajo, near to village Pampas, road to archaeological monument Rupac, slopes with loamy clay soil, scrubland, −11.323055, −76.78138, 3033–3099 m a.s.l., 7 May 2018, (fl,fr), A. Cano, P. Gonzáles, E. Huamán & S. Riva 22721 (HUT!, HSP!, MO!, MOL!, SI!, US!, USM-307017!).

Centenaria belongs to the subtribe Piqueriinae of the Eupatorieae, being considered related to the genera Microspermum and Iltisia from Mexico and Central America and Ferreyrella of Peru, all having asymmetrical corollas with the two inner lobes smaller than the rest; however, Microspermum, Iltisia and Ferreyrella have no pappus or only a few capillary setae (Hind and Robinson 2007). Furthermore, Centenaria is evidently related to the genus Ellenbergia of Peru, which has a pappus of many segments but has symmetrical corollas.

The species described here is very similar to Ferreyrella; both have strikingly asymmetrical corollas, from which it differs primarily in having a flat epaleaceous receptacle (vs. slenderly conic paleaceous receptacle) and the presence of pappus (vs. lack of pappus). Although this distinction is usually important in the Eupatorieae, as it is in Compositae generally, it becomes merely a specific or even, in one case, a varietal character in Ageratum (Blake 1957). The flat receptacle and the pappus of lanceolate squamellae with densely scabrid margins in Centenaria are very suggestive of those of Ellenbergia, although the pappus is not so extremely united at the base into a cup-shaped piece, an additional significant feature not found in Ellenbergia.

Robinson et al. (2009) commented about a group formed by small, mostly rather ephemeral Eupatorieae, which usually have many-flowered heads. At this time, Guevaria, Ferreyrella, Ellenbergia and Centenaria would also fit in this group despite having fewer flowers.