Endressistemon E.M.FRIIS, P.R.CRANE, K.R.PEDERSEN, M.M.MENDES et J.KVAČEK, 2022

Friis, Else Marie, Crane, Peter R., Pedersen, Kaj Raunsgaard, Mendes, Mário Miguel & Kvaček, Jiří, 2022, The Early Cretaceous Mesofossil Flora Of Catefica, Portugal: Angiosperms, Fossil Imprint 78 (2), pp. 341-424 : 381

Genus Endressistemon E.M.FRIIS, P.R.CRANE, K.R.PEDERSEN, M.M.MENDES et J.KVAČEK gen. nov.

Type. Endressistemon cateficensis E.M.FRIIS, P.R.CRANE , K.R.PEDERSEN, M.M.MENDES et J.KVAČEK sp. nov.

Plant Fossil Names Registry Number.

PFN0002796 (for new genus).

Etymology. In honor of Peter K. Endress for his many contributions to understanding the flower and stamen morphology of angiosperms.

G e n e r i c diagnosis. Staminate structure consisting of a short, common stalk bearing two fully developed lateral stamens with a median axis-like structure between them. Stamens tetrasporangiate and dithecate with sessile, basifixed anthers attached directly to the common stalk. Thecae straight and parallel in each stamen, but the thecae of the two stamens diverging apically. Apex of the two stamens with prominent coriaceous projection that is also seen on the median axis-like structure. A bundle extends from the common base into the apical projection of the two stamens and the median axis-like structure. Pollen grains monocolpate, semitectate-reticulate.

C o m m e n t s o n t h e g e n u s. Interpreting the morphology and likely relationships of this androecial structure is not straightforward but based on the distinctive apical projection of the stamens, and the monocolpate pollen, we infer a systematic position among the grade of early diverging angiosperms lineages. Similar prominent apical projections are characteristic of the stamens of many extant Annonaceae (e.g., Endress and Hufford 1989, Van Heusden 1992), but stamens in Annonaceae are always free from each other. The trifid form of the staminal structure is more suggestive of the tripartite androecium of extant Chloranthus ( Endress 1987) and of the Late Cretaceous Chloranthistemon P.R.CRANE, E.M.FRIIS et K.R.PEDERSEN (Crane et al. 1989, Eklund et al. 1997). Both have two lateral and one median stamen typically borne on a common stalk and with the thecae positioned dorsally. However, Chloranthus and Chloranthistemon have a fully developed, tetrasporangiate median stamen and two bisporangiate lateral stamens, which is the reverse of the situation in Endressistemon in which the median axis-like structure appears to lack pollen sacs. The extinct chloranthoid Canrightiopsis also has an androecium consisting of three dorsally aligned stamens, but in Canrightiopsis the stamen bases are not united.

In the context of the androecia of these living and fossil chloranthoids it is possible that the median structure in Endressistemon is a reduced stamen or that the thecae have been torn away during fossilization. Alternatively, the median axis-like structure with its apical projection may be a bract, and the apical projections associated with the lateral stamens may also be derived from a bract fused to the stamens. This interpretation is supported by the tough, leaf-like nature of the apical projection, the axis-like form of the median structure, and the extension of three separate vascular bundles from the common base into the apical projections, which would be unusual in a typical angiosperm stamen. This interpretation would be consistent with the suggestion that the tough apical projection of the Hedyosmum stamen could be remains of a subtending bract fused with the stamen ( Endress and Doyle 2015, Doyle and Endress 2018).

Among extinct plants the Early Cretaceous Archaefructus G.SUN, DILCHER, SHAO L.ZHENG et Z.K.ZHOU also has two or three stamens borne together, but in Archaefructus , the anthers are not sessile and not borne on a common stalk and the stamen apices are less prominent ( Sun et al. 1998).