Hibbertia jayhornii K.R.Thiele sp. nov.

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WA: Mount Lesueur National Park: 3.1 km E along Brumby Track from Cockleshell Gully Road then c. 150 m by foot SE to breakaway edge, 20 Sept. 2020, K.R. Thiele 5662 (holo: PERTH 09261893; iso: AD, MEL)

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Low shrubs to 0.2(–0.5) m high, abundantly multi-stemmed at base and resprouting after fire; young branchlets moderately pubescent with short sub-stellate hairs sometimes overtopped by longer, simple hairs, and with tufts of short hairs in the leaf axils. Leaves spreading, scattered, linear to narrowly oblong, (5–)8–12(–15) mm long, 0.7–1(–1.3) mm wide, the margins prominently recurved to a usually distinctly sunken midrib, the abaxial surface mostly obscured; adaxial surface prominently tuberculate, with sparse, long, slender, sinuate hairs arising from the tubercles; abaxial surface densely papillate where visible along either side of the midrib, which is smooth and glabrous or very sparsely tuberculate-hairy as in the adaxial surface; apex subacute due to the shortly excurrent midrib. Flowers sessile, terminating short-shoots or main stems; flower-subtending bracts 7–11, broadly ovate to narrowly triangular, chartaceous, 1.5–5 mm long, glabrous to sparsely pilose especially along the centre-line, minutely fimbriolate, the lowermost grading to the leaves. Sepals broadly ovate, 5–6(–7.5) mm long; midribs not prominent; outer sepals broadly acute, glabrous, minutely fimbriolate; inner sepals similar in size to the outer but broader, often obtuse and with scattered, minute sub-stellate hairs. Petals 5, yellow, broadly obovate, 10–12 mm long, emarginate. Stamens 10, all on one side of the gynoecium and curving over it like a hand of bananas; filaments 0.8–1.2 mm long, fused at the base into a robust claw; anthers rectangular, 1.8–2.5 mm long, dehiscing by introrse, longitudinal slits. Staminodes 2(3) either side of the stamens. Carpels 2; ovaries compressed-globular, densely pubescent; styles inserted excentrically on the carpel apex, parallel and curved beneath the stamens, 1.5–2 mm long. Ovules 2 per carpel. Fruiting carpels and seeds not seen.

Selected specimens examined: Green Head-Coorow Road (4569687); Mount Peron (6508561); Cervantes (3459144); Coomallo Creek (3021432, 5685834); Mount Lesueur (3021440); Carnamah-Eneabba Road (6312020); York (7980221)

Beverley (5419417, 7949103, 8839891); Wandering (8815232); Dongolocking Nature Reserve (4149092, 4569598, 8828091, 9261753); Tarin Rock (3097749); Calingiri (3069729, 9247165, 9261818); Piawaning (3069575, 3069680)

Bolgart (3069133); Wongan Hills (3069486, 4482360). For full specimen details, see the following batch search of the ALA for this set of specimens: https://biocache.ala.org.au/occurrences/search?q=qid:1668086934122#tab_mapView

Diagnostic features. Hibbertia jayhornii can be distinguished from all other species in the H. aurea-H. crassifolia group by the combination of obtuse, strongly tuberculate leaves with long, pilose, non-hooked hairs arising from the tubercles, the (usually) weak midrib with the lamina margins loosely recurved to each other, and the papillate abaxial lamina.

Phenology. Flowers from July to October with a peak in September.

Distribution & habitat. Hibbertia jayhornii as delimited here occurs in four rather disjunct areas in south-western Western Australia (Fig. 9). A northern population extends from Tathra National Park to the Nambung River, with most collections from around Lesueur National Park. Two populations are from around the Wongan Hills–Calingiri and the York–Wandering areas respectively. A southern populations occurs in Dongolocking Nature Reserve and at Tarin Rock.

Conservation status. Widely distributed, including in a number of national parks and nature reserves, and not considered to be under threat.

Etymology. Named in honour of Jay Horn (Florida Gulf Coast University). Jay's PhD thesis on the phylogenetics and structural botany of Dilleniaceae and Hibbertia significantly increased our understanding of Hibbertia and its evolution and taxonomy. Jay collected H. jayhornii and other members of H. crassifolia (as it was understood at that time), and clearly recognised the species named here after him as distinct.

Notes. As well as occurring in several geographically disjunct areas, Hibbertia jayhornii is morphologically somewhat variable and may in future need to be segregated further into two or more species. Its morphological variation appears to be at least partially geographically structured. Plants from the Mount Lesueur–Nambung River area (which includes the type) are very low, strongly resprouting shrubs to 0.15 m high, usually with many short, slender, erect, often little-branched stems from a well-developed rootstock, and have rather dark green, narrow and generally very pilose leaves, and floral bracts 1.5–3 mm long. In other populations the plants are generally more erect, to 0.3(–0.5) m high, are less strongly (or possibly not) resprouting, have somewhat shorter, broader leaves that are greyish-green when dried and may or may not be strongly pilose, and floral bracts 3–5 mm long. However, the geographic patterning is complex: collections from near Calingiri are more similar to plants from Mount Lesueur plants than they are to the geographically close Wongan Hills plants; conversely, a collection from Tathra National Park (J.W. Horn 4010) is more similar to the southern populations than it is to the geographically close Mount Lesueur plants. The forms intergrade to some extent. For these reasons, a broad circumscription is employed here pending further field work perhaps combined with molecular sampling of the populations and morphological variants. If the species is further subdivided, the name H. jayhornii will apply to the distinctive populations in the Mount Lesueur area.