Fossil pinnate Palm leaves from the Island Lagoon Flora, in the arid zone of South Australia.

Palms are an important part of the flora of the wet tropical and subtropical forests of eastern Australia, but are almost absent from the drier areas of the Australian interior, with only two species known from this area today, Livistona mariae from central Australia, and Livistona alfredii from the Pilbara region of Western Australia. Despite its large area, Australia is relatively species-poor in Palms compared with nearby landmasses, with only 54 species in 17 genera, compared to about 250 species on the island of New Guinea. 

The Palm flora of Australia contains a mixture of groups with different biogeographical regions, including Gondwanan groups, such as the Archontophoenicinae, Calamoideae, and Nypoideae, with fossil records in Australai which pre-date the Miocene, and Laurasian groups, such as Livistona spp., thought to have migrated from Southeast Asia since the Miocene, when monsoonal climates became prevalent across the region. Although of Gondwanan origin, the Archontophoenicinae are thought to have reached Australia from New Guinea in the Eocene, and subsequently dispersed from Australia to New Guinea in the Miocene. Beyond this, however, our understanding of the biogeographical origins of modern Australian Palms is severely limited by a paucity of fossils, particularly compared to the numerous fossil Palms of the Northern Hemisphere.

In a paper published in the journal Historical Biology on 25 September 2024, David Greenwood of the Department of Biology at Brandon University, and John Conran of rhe Environment Institute at the University of Adelaide, describe a new Palm species from fossil pinnate leaves from the Island Lagoon Flora for South Australia.

The Island Lagoon Flora is one of a number of ‘silcrete floras’ known the arid zone of South Australia, which produce a Plant fossils, which appear to have been species adapted to arid environments, with a smaller proportion of broad-leaved and Coniferous tree fossils. Age estimates for these floras have varied considerably since they were first recorded in the 1890s, with current estimates suggesting that different localities may reflect Eocene, Miocene, and Miocene-Pliocene assemblages. The Island Lagoon Flora is thought most likely to be of Miocene origin, probably contemporaneous with the Stuart Creek Silcrete Macroflora, though it is possible that it is older, possibly Eocene or Late Oligocene-Early Miocene.

The new Palm species is placed in the genus Phoenicites and given the specific name insula-lacuna, which is a Latin translation of 'Island Lagoon'. The species is described from two specimens, P14209 and P14467, both in the collection of the South Australian Museum. Both are incomplete portions of pinnate leaves, P14209 measuring 29.5 cm long and 27.7 cm wide, and P14467 measuring  23.9 cm long and 9.8 cm wide, with both showing at least 11 pinnae per side.

Phoenicites insula-lacuna. (A) Holotype P14209 showing whole specimen. (B) Paratype (P14467) with midvein at arrow. (C) Detail showing asymmetry of pinnae base (P14209). (D), (E) Detail of mid-pinnae showing midvein and secondary veins (P14209). (F) Rachis (P14209) showing patterned surface corresponding to ‘brown spots’ similar to those of extant Archontophoenix spp. (G) Detail of mid-pinnae with arrow showing midvein (P14467). John Conran in Greenwood & Conran (2024).

Greenwood and Conran note that there is little to differentiate the fossil genus Phoenicites from the living genus Archontophoenix, although they have chosen to use Phoenicites as the limited material available does not contain all of the diagnostic features for inclusion in the extant genus. This is a common situation in palaeontology, where all fossil species are morphospecies (species defined by their morphological appearance) rather than true biological species (which are defined by their ability to breed with other members of the species - something which fossils are incapable of doing).

(A)–(F) Extant Archontophoenix in the Adelaide Botanical Gardens and Waite Arboretum, University of Adelaide, or in habitat ((E) only). (A), (C) Archontophoenix alexandrae, whole leaf (A) and partial view of abaxial side (B) showing pinnae with prominent veins and pinnae rachis attachment. (B), (D) Archontophoenix cunninghamiana, partial view of abaxial side showing pinnae venation and rachis attachment, and (D) rachis showing brown spots that dry as ‘tuberculae’. (E) Archontophoenix purpurea and (F) Archontophoenix tuckerii showing pinnae venation and rachis attachment. John Conran and John Dowe in Greenwood & Conran (2024).

Modern members of the genus Archontophoenix are found in wet environments, such as freshwater swamps, rainforests, under monsoonal to seasonally dry climates. This is different from the drier climate generally recorded in the silcrete floras of South Australia. However, Greenwood and Conran note that one of the environments in which these Palms are found is rainforest gullies within (dry) tall Eucalypt forests, possibly providing a setting for the other more moisture-loving Plants found in these floras.

Map of Australia showing the Island Lagoon fossil locality, other South Australian Silcrete Flora sites, the arid zone (where the annual rainfall is less than 250 mm), the extant distribution of Archontophoenix (green circles) and the two extant species of Palm endemic in the arid zone (orange squares; Livistona alfredii in Western Australia and Livistona mariae in the Northern Territory).Abbreviations: NSW, New South Wales; NT, Northern Territory; Qld, Queensland; SA, South Australia; Tas, Tasmania; Vic, Victoria; WA, Western Australia. Greenwood & Conran (2024).

See also...