Collinson, Manchester & Collinson, 2022

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 : 318-321

Genus Exbeckettia MANCHESTER et M.E. COLLINSON gen. nov.

T y p e. Exbeckettia mastixioides (E.REID et M.CHANDLER)

MANCHESTER et M.E.COLLINSON comb. nov.

P l a n t F o s s i l N a m e s R e g i s t r y N u m b e r.

PFN002684 (for new genus).

Exbeckettia mastixioides (E.REID et M.CHANDLER) MANCHESTER et M.E.COLLINSON comb. nov.

Text-fig. 6a–l , 7a–g

B a s i o n y m. Beckettia mastixioides E.REID et M.CHANDLER ( Reid and Chandler 1933: The London Clay flora, p. 456, pl. 25, figs 28–36).

H o l o t y p e. V.23002 (Natural History Museum,

London).

P l a n t F o s s i l N a m e s R e g i s t r y N u m b e r.

PFN002909 (for new combination).

N o m e n c l a t u r e. A new generic name: Exbeckettia MANCHESTER et M.E.COLLINSON is proposed because the name Beckettia E.REID et M.CHANDLER ( Reid and Chandler 1933) is a junior homonym of the moss genus Beckettia MÜLLER ( Müller 1898) .

E m e n d e d d i a g n o s i s. Diagnosis as presented by Reid and Chandler 1933 is hereby emended slightly to include two to four locules (compared to bilocular to rarely trilocular fruits in the original diagnosis).

R e m a r k s. Reid and Chandler (1933) established this genus and species for subglobular or broadly ovoid fruits with two to three locules, the locules U-shaped in cross section, each with an elongate dorsal germination valve (e.g., Text-figs 6e, h, i–k , 7a ). In addition, we noticed one tetralocular specimen that had previously been classified as Lanfrancia , which conforms to Exbeckettia in its anatomical composition ( Text-fig. 6l ). The fruits are 13–15 mm long and 11.5–13 mm wide. The single seed within each locule bears a ventral raphe as expected in Mastixiaceae . The endocarp surrounding each locule is smooth-surfaced with a relatively thin wall (0.8 to 1 mm thick on the ventral and lateral surfaces and 0.4 mm thick within the dorsal infolds). The bulk of the fruit stone appears to be mesocarp composed of parenchyma cells rather than fibres ( Text-fig. 7b, c ). A layer of horizontally oriented periclinal fibres (up to 4 cells thick) lines the locule except in the dorsal infold area ( Text-fig. 7b ). Similar fibres, also oriented horizontally, but in layers up to 10 cells thick, form the outer part of the endocarp wall (Textfig. 7d–g). Most of the endocarp is formed of longitudinally elongate, rice grain-shaped sclereids about 3–4 times longer than wide, which appear approximately circular in cross section, arranged in radial files, along which degradational splitting may occur ( Text-fig. 7a, f, g ).

A pair of canals, apparently marking the pathway of ovular bundles, runs longitudinally through the endocarp. The margins of the germination valve run tangential or parallel to the arms of the locule ( Text-figs 6e, i–l , 7a ), rather than intersecting directly with the locule, suggesting that they may have detached as pyrenes, rather than functioning in germination splitting, but as Reid and Chandler (1933) already discussed, it is possible that this is an artefact of preservation.

Reid and Chandler (1933) observed that the seed coat has two layers: an outer coat formed of many layers of polygonal cells 50 μm in diameter, as seen in impressions on the locule cast, and with a columnar arrangement, as seen in radial sections of the seed, and an inner coat two cells thick of small obliquely aligned cells that produce a finely striate surface.

The dorsal infolds of Exbeckettia are broad and the central area of each infold is occupied by mostly large isodiametric parenchyma cells or sclereids with smaller cells distributed peripherally (e.g., Text-fig. 7b, c ). Surrounding and infilling between the sclerenchymatous endocarps is a large area of parenchymatous tissue, composed of rounded isodiametric cells that include large cells interspersed among smaller cells ( Text-fig. 7c, e ). Reid and Chandler (1933) considered this tissue to be a part of the endocarp, however we treat this tissue as “mesocarp” because of its parenchymatous composition and striking similarity to the tissue surrounding the endocarp of the unilocular fossil, Diplopanax eydei STOCKEY, PIGG et LEPAGE ( Stockey et al. 1998). The parenchymatous tissue is not traversed by vascular bundles nor secretory canals. Despite excellent preservation, no resin cavities or canals are seen in Exbeckettia .

Reid and Chandler (1933) considered the relationship of this genus to Mastixia to be clear but emphasized that the fossil is distinguished by having more than one locule and by the mass of coarse parenchyma which mainly fills the angular spaces between the locules. The presence of an ovular bundle on either side of each locule, and of planes of separation running tangential to the lateral endocarp wall are additional similarities with Mastixia .

Mai (1993) adopted a broad concept of Beckettia , into which he sunk the London Clay genera Portnallia and Lanfrancia and added some Cretaceous species ( Knobloch and Mai 1986, Mai 1993). If his interpretation is followed then these would all belong now under the name Lanfrancia , due to the fact that Beckettia was a junior homonym. However, we do not agree with Mai’s (1993) interpretation that these species were congeneric. Lanfrancia is distinguished by more fibrous endocarps and apparently lacks the pair of ovular bundle channels expected to straddle each locule. Also, the tissue that forms between adjacent locules of Lanfrancia is not the same kind of parenchyma. In our current treatment we have also retained Portnallia as a distinct genus with justification provided below.

Figures

Text-fig. 6. Exbeckettia mastixioides (E.REID et M.CHANDLER) comb. nov. from the London Clay. a–f: Holotype, V. 23002, bilocular fruit. a–d: Surface renderings from micro-CT data. a: Apical view. b: Basal view. c: Lateral view with dorsal surface of locule facing forward. d: lateral view with interlocular septum facing forward. e, f: Digital sections from micro-CT data. e: transverse equatorial showing U-shaped locules and distinct dark endocarp (arrows). f: Median longitudinal intercepting both locules. g: lateral view of bilocular fruit with interlocular septum facing forward. V. 23013(3). h: Apical view of bilocular fruit with two exposed locule casts, V. 23013(1). i–l: Fruits in physical transverse section showing U-shaped locules and distinct locule lining. i: Bilocular fruit shown in (g). j: Bilocular fruit, V. 23007. k: Fruit with one locule larger than the other, V. 23006. l: Tetralocular fruit, V. 22993. Scale bar 5mm in (a–l), bar in (a) applies also to (b–d), bar in (e) applies also to (f), bar in (j) applies to (i–l).

Text-fig. 7. Exbeckettia mastixioides (E.REID et M.CHANDLER) comb. nov. Details of anatomy in transverse section on cut surfaces from bilocular fruit shown in Text-fig. 6i, V. 23013(3). a: Details of seed (S), locule (L), distinct endocarp planes of separation (arrows). b: Detail of endocarp adjacent to the locule, and surrounding mesocarp. Blue lines indicate thickness of sclerenchyma lining the locule. Note layer of horizontally oriented periclinal fibres a few cells thick, lining the locule (arrow). c: Enlargement showing parenchyma cells of the mesocarp decreasing in diameter toward the periphery. d: Enlargement showing fibres and sclereids of the endocarp. e: Sharp contact between endocarp and mesocarp. f, g: Detailed anatomy of endocarp including locule lining, and contact with mesocarp. Scale bars 2mm in (a), (b), 1mm in (c–g).