Mastixioid Fruits (Cornales) From The Early Eocene London Clay Flora: Morphology, Anatomy And Nomenclatural Revision Author Manchester, Steven R. Author Collinson, Margaret E. text Fossil Imprint 2022 2022-08-26 78 1 310 328 http://dx.doi.org/10.37520/fi.2022.013 journal article 10.37520/fi.2022.013 2533-4069 7167911 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. Text-fig. 4. Mastixia cf. oregonensis (R.A.SCOTT) TIFFNEY et HAGGARD from the London Clay, originally included within the concept of M. cantiensis . a–c: V. 22960(1). a: Transverse fracture, showing c-shaped locule, dorsal infold, and sculptured endocarp, reflected light. b: Transverse digital section from micro-CT scan data. c: Surface view of ribbed endocarp extracted from micro-CT data. d: Transverse fracture, reflected light, V. 22955 (originally illustrated in pl. 25, fig. 4 of Reid and Chandler 1933 ). e, f: Transverse physical section, V. 22963(2) showing U-shaped locule and longitudinal dorsal infold. g–i reflected light. g: Detail from left of (d). h, i: Detail from right of (f). Scale bars 5 mm in (a–f), 1 mm in (g–i). 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.