Cymastrobus irvingii Evreïnoff, Meyer-Berthaud, Decombeix, Lebrun, Steemans, Tafforeau, 2017

Evreïnoff, Mathilde, Meyer-Berthaud, Brigitte, Decombeix, Anne-Laure, Lebrun, Renaud, Steemans, Philippe & Tafforeau, Paul, 2017, A new Late Devonian isoetalean lycopsid from New South Wales, Australia: Cymastrobus irvingii gen. et sp. nov., Palaeontologia Electronica 19 (8), pp. 1-16 : 3-6

Cymastrobus irvingii Evreïnoff, Meyer-Berthaud, Decombeix, Lebrun, Steemans, Tafforeau sp. nov.

Figures 1-5

Diagnosis. Large bisporangiate cone that may exceed 8 cm long and 5 cm wide. Cone axis narrow, about 10% the width of the cone, containing a ring of primary xylem showing a corrugated outline with an almost continuous band of exarch protoxylem; sporophyll traces departing from the bays of the primary xylem cylinder. Sporophyll-sporangium units arranged helically, about 8-10 per gyre. Sporophylls comprised of a long, narrow pedicel widening distally but without alations, and a delicate distal lamina oriented perpendicularly to the pedicel; pedicels showing an abaxial keel and a distal heel, the latter forming hexagonal shields protecting the sporangia externally. Megasporangia and microsporangia in distinct parts of the cones; megasporangia proximal, enclosing a large number of megaspores. Casts of megaspore central body up to 500 µm in diameter, showing numerous small circular pores arranged in several rows around the trilete mark, smooth elsewhere. Casts of microspore central body less than 100 µm in diameter, showing one small pore between the rays of the trilete mark.

Holotype. Specimen NMVP 161998 , Museum Victoria, Melbourne, Victoria, Australia.

Type locality. Barraba, New England, New South Wales, Australia; left bank of the Manilla River, upstream from the Connors Creek crossing, Mandowa Mudstone.

Age. Famennian.

Derivation of name. In honor of Mr. John Irving who discovered the Barraba plant locality and collected the cone.

Description

General features. The bisporangiate cone is 7.5 cm long and 5.8 x 2 cm wide ( Figures 1.1 , 2.1 ). It is incomplete, the proximal and distal extremities being missing. It has been laterally compressed through its whole length and distorted distally ( Figure 2.2 ). A split in the distalmost part reveals the cone axis ( Figure 1.1 ). Sporophylls are helically arranged. On the compressed face, however, they may appear organized in vertical rows ( Figure 2.4, 2.8 ). The megasporophylls occupy the basal third of the cone, a zone about 2.8 cm long ( Figure 2.1, 2.2, 2.5, 2.6, 2.7 ). Apart from their spore content, the mega- and microsporophylls are similar in shape and size.

Cone axis. The cone axis measures about 6 mm in its widest dimension. It does not show any conspicuous tapering from one end to the other ( Figure 2.1, 2.2 ). In transverse section, the stele which is broken in the distal part is 1.8 x 2.6 mm wide ( Figures 1.2 , 2.3 ). It shows a 250 µm thick ring of primary xylem surrounding a central zone where a group of large thin-walled cells remains preserved ( Figure 1.3, 1.5 ). These pith cells measure 40-55 µm in diameter. The outer edge of the xylem ring is irregularly undulated. Protoxylem tracheids form an almost continuous band of 9-15 µm wide cells at the periphery ( Figure 1.3-5 ). Metaxylem tracheids increase from 20 to 48 µm in diameter inwards. Outside the ring of primary xylem and separated from it by a 40 µm thick zone of small thin-walled cells, are pockets of poorly preserved elements that may have corresponded to the primary phloem ( Figure 1.4, 1.5 ). Beyond, a narrow band of 16-30 µm wide cells represents the inner cortex.

Vascular traces to the sporophylls are numerous and produced helically ( Figure 2.3 ). They depart from the grooves of the corrugated xylem cylinder ( Figure 1.3-5 ). At the level of emission, their section tends to be radially elongated ( Figure 1.5 ). They measure about 60 µm tangentially and 80 µm radially, and are comprised of about 20-24 tracheids. Vascular traces increase in size as they cross the cortex. On the outer edge of the inner cortex their xylem strand is 85-90 µm wide and has a circular outline. It is bordered on the abaxial side by a 40-50 µm wide crescent-shaped sheath of thin-walled cells that do not exceed 13 µm in diameter ( Figure 1.4 ). The traces follow a steep course upwards in the rest of the cortex until they reach the base of the sporophylls ( Figure 2.2 ). Their diameter in the outer cortex is about 140 µm ( Figure 2.3 ).

Sporophyll-sporangium units. The sporophyll-sporangium units are tightly packed ( Figure 2.1 ). Their number per gyre is estimated to range between eight and 10. They are slightly raised in the distalmost part of the cone, diverge from the axis at approximately 90° lower down, and bend downward progressively in the lower half, with an angle of up to 115 ° in the megasporangiate part ( Figure 2.1, 2.2 ).

Sporophylls comprise a 19-22 mm long pedicel and a poorly preserved distal lamina consisting of dissected threads of tissues oriented perpendicularly to the pedicel ( Figures 3.2-5, 4.1 -7). Pedicels are narrow ( Figures 2.5 -6, 4.3) but expand both laterally and abaxially in the second half of their length ( Figures 2.6 -8, 3.3-4, 4.3). They are 200 µm wide proximally and reach a maximal width of 5.5 mm distally ( Figures 2.8 , 3.4-5 ). In profile view, the pedicels are about 500 µm high when they diverge from the cone axis. At about 3 mm from their insertion point, they differentiate a keel on the abaxial side ( Figure 3.2 ). The keel increases in size distally until it fuses with a heel reaching a height of 4 mm ( Figures 3.2 , 4.4 -7). In front view, the distal parts of the pedicels form diamond-shaped shields partly hiding the sporangia ( Figure 2.4 ). A single vascular strand runs through the whole length of the pedicels. In the lamina, synchrotron images show it as a vertical structure of low density that stands in central position among the surrounding threads of laminar tissue ( Figures 2.1 , 3.5 , 4.1-5 ).

The sporangia measure 16-18 mm long, 3-5 mm high, and 4-6 mm wide ( Figures 2.2 , 3.4-5, 4.4 -7). They are slightly shorter than the pedicels to which they are attached over their whole length by a narrow pad of tissue ( Figure 3.4-5 ). Their width exceeds that of the pedicels in their proximal half and becomes comparable distally, except at the tip where the sporangia become narrower ( Figures 2.5 -8, 3.4-5, 4.1-2). Sporangium wall is thin (80-90 µm). All the sporangia have retained their spore contents. The mega- and microsporangia both contain several hundred spores ( Figures 2.2, 2.5 -7, 4.1-2, 4.6, 4.7).

Spores. The internal casts of megaspores collected inside the megasporangia have a spherical outline and measure 420-490 µm in diameter ( Figure 5.1 ). They do not show any ornamentation except on the proximal face where a faint trilete mark characterized by 100 µm long rays is surrounded by a triangular area with concave sides showing more than a hundred pores ( Figure 5.2 ). Individual lobes of this trilobed pored zone are about 140 µm long and 85 µm wide. The pores are regularly arranged in three to four rows on each side of the rays. They are circular to oval and 2-3 µm deep. Their diameter ranges from 4 µm close to the rays, up to 9 µm outside.

The internal casts of microspores collected inside the microsporangia have a spherical outline and measure 60-65 µm in diameter. They show a small trilete mark with 4 µm long rays. Three 2.5 µm wide pores are visible between the rays, close to the proximal pole ( Figure 5.3 ). Some microspores have retained fragments of an outer envelope showing densely arranged minute pores on the inner side.

Figures

FIGURE 1. Cymastrobus irvingii gen. et sp. nov. NMVP 161998. 1, General view of the cone; cone axis at arrow. 2, Distal part of cone axis in transverse section. 3, Detail showing the stele (St) and five sporophyll traces at arrows. 4, Detail showing the wavy outline of the xylem ring, presumed location of the phloem (Ph), inner cortical cells (IC) and three sporophyll traces at arrows. 5, Detail showing the emission of a sporophyll trace from a groove of the primary xylem ring.

FIGURE 2. Cymastrobus irvingii gen. et sp. nov. NMVP 161998. Virtual sections, X-Ray synchrotron microtomography. 1, Cone in tangential section. 2, Cone in radial section; note the proximal position of the megasporangia. 3, Cone axis in transverse section. 4, Outer portion of the cone in tangential section showing four sporophyll-sporangium units in longitudinal row. 5-8, Inwards to outwards series of tangential sections through the cone showing the progressive changes in size of the sporophyll pedicels (P) and the sporangia (S).

FIGURE 3. Cymastrobus irvingii gen. et sp. nov. NMVP 161998. Virtual sections, X-Ray synchrotron microtomography. 1-5, Proximal-distal series of longitudinal sections through a sporophyll-sporangium unit; note the heel (H) in 1, keel (K) in 2 and 3, microsporangium (mS) and longitudinal pad of tissue (P) in 4 and 5, vascular strand (VS) in 5.

FIGURE 4. Cymastrobus irvingii gen. et sp. nov. NMVP 161998. Virtual reconstructions and volume rendering visualization of chosen anatomical units within the X-Ray synchrotron microtomography scan. 1-2, General view of the cone showing two reconstructed sporophyll-sporangium units, the proximal one producing megaspores, the distal one microspores. 3, Reconstructed sporophyll showing the enlarging pedicel and dissected lamina. 4-5, Two reconstructed sporophyll-microsporangium units in profile view. 6-7, Two reconstructed sporophyll-megasporangium units in profile view. VS: vascular strand.

FIGURE 5. Cymastrobus irvingii gen. et sp. nov. NMVP 161998. 1, Cast of a megaspore central body showing numerous small circular pores arranged in several rows around the trilete mark. 2, Detail of previous view. 3, Casts of microspore central bodies; the largest one shows three pores between the rays of the trilete mark (arrows).