Sarcandraxylon sanjosense, Pipo & Iglesias & Bodnar, 2020

Pipo, M. Laura, Iglesias, Ari & Bodnar, Josefina, 2020, A new vesselless angiosperm stem with a cambial variant from the Upper Cretaceous of Antarctica, Acta Palaeontologica Polonica 65 (2), pp. 261-272 : 263-265

Sarcandraxylon sanjosense sp. nov.

PFNR: PFN001017.

Etymology: From the San José pass (James Ross Island), a nearest locality where the fossil was found.

Holotype: IAA-Pb 621, a stem 3 mm long and 5 mm wide in a rock sample mounted on SEM stub; six cellulose-acetate peels mounted in glass slides (three in transverse section; and three in tangential section). Type locality: San Jose Pass (equivalent to CF cite from Iglesias 2016), northern James Ross Island, Antarctic Peninsula (63°54’14” S; 57°54’40” W; Fig. 1 ).

Type horizon: Early–middle Campanian (~83–77 Ma), Late Cretaceous, Upper Beta Member, Santa Marta Formation, Marambio Group, James Ross Basin.

Diagnosis.—Secondary vascular cylinder with interfascicular rays and axial elements in segments. Homocellular and heterogenous interfascicular rays consisting of square and procumbent parenchyma cells. The secondary xylem segments contain tracheids and uniseriate homocellular and homogeneous rays with elongated upright cells. Growth rings are indistinct. Secondary phloem with fibers arranged in caps. Bark composed by dead cells of secondary phloem and cork cells which are accumulated in a multilayered rhytidome.

Description.—The slender stem is 5 mm in diameter, circular in transverse section ( Fig. 2A ). Although its periphery is not completely preserved, the whole shape remains almost intact. The central circular pith is 1.87 mm in diameter. It is composed of large isodiametric parenchyma cells (40– 70 μm; mean 50.36 μm). The primary vascular system is formed by one cycle of collateral vascular bundles, only 19 could be counted due to preservation (although may reach almost 30 in the whole), with endarch protoxylem (cell sizes 2.5–5 μm), and metaxylem cells size 5–7.5 μm (mean diameter 5.92 μm) ( Fig. 2B ). The secondary vascular cylinder is radially dissected, due to a cambial variant derived from one vascular cambium with differential activity, in which the interfascicular cambium only produces ray parenchyma cells resulting in axial vascular elements in segments (AVES pattern). The secondary xylem lacks vessels, and is composed by tracheids (cellular diameters range 12.5–25 μm; mean diameter 17.9 μm) and uniseriate homocellular and homogenous rays. These fascicular rays are two cells high and composed of elongated upright parenchyma cells ( Figs. 2C, E , 3A, C, D ). Growth rings and axial parenchyma are absent. The radial thickness of xylem segments is up to 202.5 μm. The interfascicular rays are broad (67 μm), multiseriate (5–6, up to 8 cells wide), homocellular, and heterogenous; consisting of square and procumbent parenchyma cells (5– 10 μm; mean 7.33 μm) ( Fig. 3B ). The secondary phloem is arranged in half-moon shaped bundles and wide interfascicular rays ( Fig. 2D ). Phloem cells are poorly preserved; only some collapsed cells were identified, leaving a large central gap from the center to the base of bundles. Caps of phloem fibers are present externally in the bundles, with a mean size of 26.25 μm per 68.75 μm. The fibers are hard to measure because they are small and poorly preserved. The bark is composed by dead cells of secondary phloem, and cork cells which are accumulated in a multilayered rhytidome (mean bark growth 50 μm). It has a sinuous shape, with external shallow valleys in the interfascicular areas of phloem, and pronounced ribs over the phloem fiber caps ( Fig. 2D ).

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 1. A. Location of the new fossil locality in Antarctic Peninsula, James Ross Island (gray). B. Map of James Ross Island at the northern Antarctic Peninsula. C. Map with the Santa Marta Formation outcrops (area shaded in gray) at the northern James Ross Island, showing location of the new fossil locality (asterisk). Modified from Olivero 2012 and Carvalho et al. 2013.

Fig. 2. Light micrographs of the stem of chloranthacean angiosperm Sarcandraxylon sanjosense gen. et sp. nov. (IAA-Pb 621), San José Pass, Antarctica, early–middle Campanian. A. Transverse section of the complete stem with pith, secondary xylem, phloem, and bark. B. Protoxylem (black arrow) and metaxylem (white arrow) in a vascular bundle. C. Secondary xylem in transverse section, pith in the left (white arrow), two interfascicular rays and fascicular secondary xylem (fx) intercalated. White arrow pointing uniseriate fascicular ray. D. Detail of bark and secondary phloem. Note phloem with AVES pattern as well. Phloem cells not preserved (p) and phloem fibers cap (black double head arrow) separated by interfascicular rays (ir). Bark (white double head arrow) with a continued thickened layer, forming shallow ribs. E. Tangential section of the stem, arrows pointing fascicular uniseriate rays; at both sides interfascicular rays (ir). Scale bars: 50 μm; except A, 1 mm.

Fig. 3. SEM images of the secondary xylem of the chloranthacean angiosperm Sarcandraxylon sanjosense gen. et sp. nov. (IAA-Pb 621), San José Pass, Antarctica, early–middle Campanian. A. Transverse section of the stem, showing one vascular bundle in the middle, pith toward the right, white arrows pointing uniseriate fascicular rays. B. Parenchymatic cells of an interfascicular ray in transverse section showing the main shape and piritization of the cell walls. C. Secondary xylem in longitudinal tangential section, showing fascicular xylem (fx) and interfascicular rays (ir). D. Longitudinal section of fascicular xylem; D1, showing tracheids and a two cells tall uniseriate ray (white arrow) with elongate upright cells; D2, detail showing cell walls partially replaced with framboidal pyrite (white arrow). Scale bars: A, D, 100 μm; B, 40 μm; C, 200 μm; E, 50 μm.