Proteaceae

Carpenter, Raymond J. & Rozefelds, Andrew C., 2023, Leaf fossils show a 40 - million-year history for the Australian tropical rainforest genus Megahertzia (Proteaceae), Australian Systematic Botany 36 (4), pp. 312-321 : 319

Lobed-only leaves of Anglesea Proteaceae

Only two Proteaceae leaf types appear to be represented in the Anglesea Karo-mounted glass slide collection at MV, namely, M. paleoamplexicaulis and Banksieaefolia cuneata (R.S.Hill & Christophel) R.J.Carp., G.J.Jord. & R.S.Hill , and both these leaf species are only known from lobed leaves. In their nearest extant relatives (i.e. M. amplexicaulis and Musgraveinae), lobed leaves mostly occur in juvenile and understorey foliage, whereas adult and canopy leaves are simple ( Johnson and Briggs 1975; George and Hyland 1995; Hyland 1999 a, 1999 b; R. J. Carpenter, pers. obs.). Of further interest is that a third Anglesea lobed leaf-type was reported by Rowett and Christophel (1990), although without illustrations or other description, and not being identified. It is probable that this leaf type was Athertonia -like, as fossil cuticles extracted from Anglesea dispersed cuticle preparations are very similar to Athertonia diversifolia cuticle ( R. J. Carpenter, unpubl. data). As for the other two taxa, adult foliage in Athertonia is simple and juvenile foliage is lobed ( Johnson and Briggs 1975; Weston 1995).

It is possible that lobed-only leaves among these three taxa of Anglesea Proteaceae are an artefact of collection bias; Christophel (1994) remarked that most of the leaves recovered from Anglesea are architecturally indistinct, elliptical to ovate notophylls, and, therefore, adult leaves belonging to these proteaceous taxa may be present in the collections, but not recognised as such. However, arguing against this is the possibility that Christophel specifically searched for these leaves, because he would have been aware of the simple adult types of the nearest extant relatives, including Musgraveinae, inflorescence fossils of which also occur in the Anglesea sediments ( Christophel 1984). Also, because the routine procedure for curating Anglesea leaf fossils involved cuticle preparations ( Christophel et al. 1987), any simple leaves with proteaceous cuticular features should have been identified.

If correct, our inference of lobed-only leaves from three distinct tribes of subfamily Grevilleoideae is of ecological and evolutionary interest. Large, lobed leaves are a distinctive feature of numerous other tropical and subtropical Proteaceae , including Darlingia F.Muell. , Placospermum and Heliciopsis Sleumer ( Johnson and Briggs 1975) , but this leaf state is very unusual among co-occurring rainforest angiosperms. At the outer canopy level, unlobed and much smaller leaf forms predominate, likely serving the function of reducing heat loads ( Leigh et al. 2017). We propose that simple leaves evolved convergently in rainforest Proteaceae as post-Eocene environments became hotter and less equable with the northward drift of the Australian continent towards the Equator. In the much higher-latitude Eocene (and probably earlier) forests exclusively lobed-leaves sufficed whatever the size of the trees, and conferred a range of advantages, including efficient light capture (e.g. Givnish 1979).