Solitary or weakly colonial herb with a single white tap root and a darkish brown vertical caudex, more or less branched apically into a crown covered by blackish brown marcescent leaf remains. Each caudex branch ending in a rosette at ground level with 5–8(10) leaves. Leaf rosettes 15–30 cm in diameter, with 0–5 hollow scapes usually 5–17(20) cm long and 4–5 mm thick. Scapes brownish or pinkish. The entire plant is glabrous.
Leaves 6–18 × 1.5–3 cm, with a winged petiole 1/3–1/4 of the entire leaf length. Blade oblanceolate to spathulate in outline, broadest in the distal 1/3, with 3–5 pairs of shallow to deep, acute, sharply dentate retrorse lobes (a runcinate leaf), end lobe usually triangular, subobtuse or acute, green with a broad pale or pink mid vein.
The primary inflorescence of Asteraceae is a head (capitulum) surrounded by an involucrum of one or more rows of phyllaries (involucral bracts). The flowers sit on a flat, concave or convex receptacle, sometimes with scales (the bracts of the single flowers). The flowers are epigynous with perianth at top of the gynoecium. The sepals are always transformed into a pappus, mostly by hairs or sometimes by scales. The 5 stamens are inserted in the corolla tube and the anthers form a ring through which the style grows and pushes the pollen outwards. Gynoecium of two fused carpels, 2 stigmas. The fruit is an achene with one seed.
Inflorescence a single apical head 2–3 cm broad. Involucrum of 2 rows of dark green phyllaries without appendages (horns). Outer phyllaries 12–18, 6–8 × 2.5–3.5 mm, about half as long as inner phyllaries, subappressed, narrowly triangular or oblong with a slightly extended obtuse apex. Inner phyllaries 10–20, 18–22 × 2.5–3,5 mm, appressed, narrowly triangular, the innermost ones with a narrow pale green hyaline margin. Receptacle flat, without scales. Flowers monosymmetric. Corollas ligulate (above the corolla tube all petals are fused in a long ligula facing outwards); ligulas >30, 15–20 mm, ending in (3)5 short, irregular teeth, yellow with a broad greenish lilac stripe on the outer surface.
The fruit is divided into a body, a narrowly cylindrical beak with a conical thickening at the base (the cone), and a pappus (transformed calyx) developing as a stalked, feathery umbrella on the fruit. Fruit body 4–5 × 0.8–1.1 mm, narrowly obovoid, dull straw coloured with shallow ribs, rugose below and with numerous and very pronounced teeth in the apical part, cone distinct, beak 6–8 mm, pappus rays 6–7 mm, white and obscurely dentate.
Asexual reproduction by seeds; very local vegetative reproduction by fragmentation of rhizome. Nearly all species of Taraxacum (and there are thousands, more than 1200 described from Europe alone) are agamospermous, i.e., with seed development independent of fertilization. Pollen may be well developed (yellow) or not (green or blackish green) but is assumed non-functional in nearly all species. Among the boreal and arctic species, sexual reproduction is suspected in only one species: the west arctic T. holmenianum Sahlin with a diploid chromosome number (2n = 16). The ploidy level of T. acromaurum is not known. Flowering and seed set is regular.
Fruits are easily spread by wind due to their umbrella-like pappus.
The runcinate leaves and the scabose heads with only ligulate flowers are unique for the genus Taraxacum in Svalbard. Taraxacum acromaurum is recognized from T. arcticum by its yellow flowers, by being much larger in all its parts, and by its strongly dentate, straw coloured fruits. Among the yellow-flowered species, it differs from both T. brachyceras and T. sect. Ruderalia in subappressed phyllaries and much more dentate fruits, from T. brachyceras in addition by no appendages (“horns”) on the phyllaries and much more deeply dissected leaves.
Moist heaths and meadows, and also on ruderal ground by huts. For some of its few collections are reported snowbeds with herbs and “urtemark” (meaning herb meadow). Probably indifferent as to soil reaction (pH).
Thermophilous. In Svalbard restricted to Bjørnøya where it is known from several stands in the southeast (Kvalrossbukta, Røedvika) and from Tunheim in the northeast.
The general range is restricted to Iceland, Jan Mayen where it is the most frequent species of Taraxacum, and Bjørnøya. The presence of this species only on quite isolated islands vouches for an efficient dispersal mechanism.
This is the only species of sect. Macrodonta in the arctic islands. The section is otherwise almost entirely restricted to Iceland and must be a late evolutionary development in the North Atlantic areas. On Bjørnøya, it has previously been reported as T. cymbifolium H.Lindb. ex Dahlst. but has been re-identified by D.O. Øvstedal (Bergen) as T. acromaurum, a revision we accept. The two names T. acromaurum and T. cymbifolium were previously considered synonyms but belong, according to Lundevall & Øllgaard (1999), to two different species, T. cymbifolium being restricted to Iceland and Scandinavia.
Lundevall, C.-F. & Øllgaard, H. 1999. The genus Taraxacum in the Nordic and Baltic countries: types of all specific, subspecific and varietal taxa, including type locations and sectional belonging. – Preslia 71: 43–171.