The Hibbertia aurea-H. crassifolia species group currently comprises the two species H. aurea Steud. and H. crassifolia (Turcz.) Benth., both of which are endemic in the south-west of Western Australia. The boundary between these two species has been unclear for many years, and each is morphologically diverse. In this paper the H. aurea-H. crassifolia species group is revised and nine species recognised, substantially clarifying species boundaries within it.

Members of the Hibbertia aurea-H. crassifolia species group all have small, linear, ericoid leaves (with the leaf margins usually closely revolute to the midrib or to each other), sessile flowers subtended by several to numerous quite prominent bracts, stamens on one side of two densely pubescent, 2-ovulate carpels, with the anthers parallel to each other and curved over the short styles like a hand of bananas, and staminodes lateral to the stamens. The only other Western Australian species that shares these features is H. robur K.R.Thiele; based on morphology, this species would be expected to be part of this group, but a preliminary unpublished molecular phylogeny being developed as part of a Flora of Australia project indicates that it may not be (T. Hammer pers. comm.). A comprehensive and well-resolved phylogeny of the genus is needed to ascertain relationships more precisely. This paper deals only with H. aurea, H. crassifolia, and their segregate new species.

Hibbertia aurea was described by Steudel (1845) based on Preiss 2152b. There are eight duplicates of Preiss 2152, seven of which were annotated b by Steudel and comprise the type series for H. aurea. The eighth duplicate was annotated a and is the type of H. pallida Steud.; it is subtly though clearly different from the other seven. The specimens show the key differences described by Steudel for these two species, with the b series having 6–8 prominent bracts subtending the calyx and the a specimen having fewer, smaller bracts. Steudel also described H. aurea as having golden-yellow petals, while his choice of epithet indicated that he regarded that the petals of H. pallida were paler yellow. The specimens differ also in habit, with the a specimens comprising well-branched stems and the b specimens having several unbranched stems from a basal rootstock (although the HBG specimen of the a series also has unbranched stems).

Bentham (1863) regarded Steudel's differences between H. aurea and H. pallida to be insufficient for their recognition as species, and listed the latter as a synonym of the former, a move followed by all subsequent authors. He also erected H. aurea var. obtusa Benth., into which he placed another Steudel name, Pleurandra glaucophylla Steud. There is confusion over the typification of P. glaucophylla, likely beginning with an error by Steudel and compounded by an error by Bentham. Pleurandra glaucophylla is the basionym of Hibbertia glaucophylla (Steud.) K.R.Thiele, previously known as Hibbertia rupicola (S.Moore) C.A.Gardner, with H. aurea var. obtusa as a synonym (see Thiele & Hammer 2022).

A second variety, H. aurea var. brevifolia Diels, was erected by Diels & Pritzel (1904). The type specimen was presumably destroyed in the bombing of the Berlin Herbarium in 1943. While the protologue is inadequate to fully determine its identity, the locality (south of the Moore River near Mogumber) places it within the distribution of Steudel's H. pallida and beyond that of H. aurea.

Hibbertia crassifolia was described, as Pleurandra crassifolia Turcz., by Turczaninow (1849), based on a Drummond type (Nova Hollandia, Drummond IV., 120), and transferred to Hibbertia by Bentham (1863). A variant with distinctly pilose leaves was phrase-named as H. sp. Mount Lesueur (M. Hislop 174) at the Western Australian Herbarium in 2002. Thiele (2013) returned the phrase-name to synonymy under H. crassifolia, commenting that it falls well within the variation found throughout the range of H. crassifolia, but leaving open the possibility that future work may allow the segregation of taxa within the species sens. lat. This paper does just that, and clarifies the placement of H. sp. Mount Lesueur.

Bentham (1863), and later Wheeler (2004), gave the differences between H. aurea and H. crassifolia as leaf indumentum and apex shape (H. aurea being nearly glabrous with acute leaves c. 12 mm long, and H. crassifolia having obtuse, hoary leaves 4.5–7 mm long) and sepal indumentum (H. aurea having shining, glabrous or minutely stellate-hairy sepals and H. crassifolia having pubescent sepals). However, at the commencement of this study, many specimens at PERTH placed in either species shared some features supposedly characteristic of the other, and the boundaries between them were unclear.

Extensive fieldwork and close study of the material at PERTH has shown that the H. aurea-H.crassifolia species group comprises nine distinct species differing mainly (and consistently) in leaf morphology and the number and nature of the bracts subtending the flowers. The new species described here are H. consuta K.R.Thiele, H. coracula K.R.Thiele, H. ericoides K.R.Thiele, H. jayhornii K.R.Thiele, H. lesueurensis K.R.Thiele, and H. williamsiorum K.R.Thiele. Hibbertia aurea and H. crassifolia are both recircumscribed, and H. pallida is reinstated as distinct from H. aurea.