Tridentea pachyrrhiza, previously known as Stapelia pachyrrhiza and commonly called the thick-rooted stapelia, is a clump-forming or mat-forming stem succulent that reaches 6 cm in height, 1,5 cm in thickness. From five to twenty-five four-angled stems may be grown. The variably shaped pale glaucous (blue-green) to grey-green and hairless stems, sometimes spotted red, have few large tubercular teeth.
Flowers grow in few-flowered inflorescences on fleshy stalks emerging from lower stem parts. A flower stalk may be up to 2,5 cm long. There are small pointed sepals at the base of the corolla. The bulging, flat-topped flower buds are pale reddish purple, the seams where the corolla lobes are joined dark, divulging something of the inside corolla colour to be shown later.
The five-pointed, star-shaped open flowers have short triangular to ovate lobes with acutely pointed tips and the centre flatly funnel-shaped, with just enough space for the corona to be sunken in the centre. The flower colour is black with yellow flecks or reddish black with yellow spots and a brown-red margin. In the photo the lobes are darker than the corolla centre. Lobe margins are fringed with long red hairs. The corolla reaches 7,5 cm in diameter.
Spreading, narrow, yellow corona lobes are visible in the flower centre. The outer corona lobes have three teeth each, the middle one bent with a groove at its tip and two broad lateral toothlets. The inner corona lobes are each divided into two horns.
The species distribution is restricted, found only in the west of the Richtersveld and southern, coastal Namibia.
The habitat is barren desert where extreme temperatures are common. The plants grow in rocky, sandy places where they are often nearly buried in the sand. They have thick, deep roots that anchor them in the loose sand of this windy terrain. Still, this plant is not considered to be threatened in its habitat early in the twenty first century (Williamson, 2010; White and Sloane, 1937; www.redlist.sanbi.org).