Mastixia parva E.REID et M.CHANDLER, 1933

Manchester, Steven R. & Collinson, Margaret E., 2022, Mastixioid Fruits (Cornales) From The Early Eocene London Clay Flora: Morphology, Anatomy And Nomenclatural Revision, Fossil Imprint 78 (1), pp. 310-328 : 315-316

Mastixia parva E.REID et M.CHANDLER, 1933

Text-fig. 3a–i

H o l o t y p e. V.22972 (Natural History Museum,

London).

R e m a r k s. This is one of the oldest occurrences of the extant genus, Mastixia . The holotype has degraded from pyrite decay since it was initially imaged by Reid and Chandler (cf. Text-fig. 3a, b ), but micro-CT scanning still reveals the internal structure ( Text-fig. 3c–g ). The unilocular, single-seeded fruits have endocarps 7–10 mm long, with a relatively smooth surface, and a few shallow longitudinal furrows. Reid and Chandler (1933) observed a circular scar at the apex marking the limit of an accrescent calyx. They described the mesocarp as 0.6 to 0.98 mm thick, formed of polygonal or rounded cells, typically ca 30 µm in diameter. The endocarp opens at germination by a large longitudinal dorsal valve with a median longitudinal groove on its external surface ( Text-fig. 3h ) corresponding with a deep longitudinal dorsal infold projecting into the locule as a rounded ridge, forming a c-shaped locule as viewed in transverse section ( Text-fig. 3d–g, i ). The wall is about 0.8 mm thick, formed of a layer of transversely aligned elongate cells and parenchyma. The locule lining is composed of transversely aligned fibres. As in extant Mastixia , the separation zones of the germination valve run tangential to the limbs of the locule.

Additional examples that appear likely to represent the same or a very similar species were called simply “ Mastixia sp. ” by Manchester (1994) from the middle Eocene Clarno Nut Beds locality of Oregon. Those specimens clearly show the pair of ovular bundle canals running parallel to the germination valve margins ( Manchester 1994: pl. 10, fig. 2) – a characteristic feature of this extant genus. Other species of Mastixia came to prominence later in the European Tertiary, including Mastixia rattazzii MARTINETTO in the Oligocene and M. amygdaliformis (SCHLOTH.) KIRCHH. ( Hably and Erdei 2013), M. lusatica MAI , and M. thomsonii MAI ( Mai 1970) in the Miocene.

Figures

Text-fig. 3. Mastixia parva E.REID et M.CHANDLER. a–g: Holotype, V. 22972. a: Ventral view (original illustration from pl. 25, fig. 13 of Reid and Chandler 1933), reflected light. b–g: from micro-CT data. b: Dorsal view of specimen in (a) now suffering from encrustation due to pyrite decay; isosurface rendering. c: Translucent volume rendering, dorsal view showing two limbs of the locule and longitudinal groove. d–g: Digital transverse sections at various positions showing c-shaped locule, longitudinal dorsal infold, endocarp wall, and degradational cracks. h, i: V. 22983(1). h: Dorsal view showing longitudinal infold. i: Physical transverse section showing c-shaped locule and longitudinal dorsal infold. Scale bars 5mm in (a–h), applies also to (b–g), 2mm in (i).