Connaroxylon dimorphum Baas, Manchester, Wheeler & Srivastava, 2022

Baas, Pieter, Manchester, Steven, Wheeler, Elisabeth A. & Srivastava, Rashmi, 2022, Validation of the names linked to the oldest fossil Connaraceae wood (Connaroxylon, Connaroxylon dimorphum), Phytotaxa 558 (2), pp. 249-250 : 249-250

Connaroxylon dimorphum Baas, Manchester, Wheeler & Srivastava , sp. nov.

Description:— Growth rings indistinct or faint and then marked by radially flattened fibers. Vessels diffuse, 20-25% solitary, remainder in radial multiples of 2–3(–6 or rarely more than 10; longest multiples in wood adjacent to pith), 14–30 per sq.mm, circular to oval when solitary, flattened at the place of contact when in radial multiples; tangential diameter 62 (29–115; SD 17) µm, radial diameter 79 (43–116; SD 23) µm. Perforations simple. Vessel elements 387 (174–743; SD159) µm long. Intervessel pits alternate, 8–10 µm, bordered with slit-like or round apertures; vessel—ray pits mostly with strongly reduced borders and circular to elongate. Thin-walled tyloses occasionally present. Fibers of two types: 1) thin-walled and in parenchyma-like bands of varying widths alternating with 2) thick-walled in zones that vary from narrow to wide. Most fibers non-septate, but some septate. Fiber wall pitting not observed, probably minutely bordered to simple, typical of libriform fibers. Radial fiber diameter 20 (14–25; SD 4) µm. The parenchyma-like fiber bands are much more common in the juvenile wood than in the more mature wood towards the periphery of the sample. Axial parenchyma scanty paratracheal, extremely sparse, in strands of 5–6 cells, cells 16–30 µm in radial diameter. Rays almost exclusively uniseriate, but a few rays with short biseriate portions, 11–23 per mm, heterocellular, composed mainly of square or upright cells with a few procumbent cells present in biseriate central portions, or upright, square and weakly procumbent cells mixed throughout the rays. Height 2–20 cell or 539 (146–942; SD 241) µm. Crystals. Prismatic crystals present in chambered upright ray cells and in chambered fibers. Herringbone patterned rays and tubules. In radial sections some of the rays show a herringbone pattern with cells converging towards narrow tubules with dark contents. Pith. The pith is wide, rounded to pentagonal with central large-celled parenchyma. The peripheral pith is small-celled, in some regions giving an impression of internal phloem, separated by some small-celled parenchyma layers from the protoxylem vessels.

Type (holotype):— Birbal Sahni Institute of Palaeosciences, Lucknow Museum 40975 .

Illustration:— Illustrations of the holotype have been published as figs. 1–9 in Baas et al. (2017: pp.127, 128).

Connaroxylon dimorphum is the oldest known fossil wood of the Connaraceae , and one of the oldest fossils of the order Oxalidales , to which the family belongs. Because its identity/botanical affinity is robust, and the fossil is well dated at 65 +/- 1 Ma, we encourage future students of the phylogeny of Oxalidales and related angiosperm orders to use this record to calibrate molecular clock analyses.