A new approach to Corystospermales based on Triassic permineralized stems from Argentina

Anatomically preserved mature stems of late Middle Triassic corystosperms from the Paramillo Formation of Argentina are described and assigned to Cuneumxylon spallettii gen. nov. et sp. nov. The silicified specimens show features of the pith, primary xylem and successive rings of secondary xylem and phloem. The most striking characteristic is the anomalous secondary growth, represented by secondary xylem bounded by arcs of secondary phloem probably derived from successive repositioned cambia. Cuneumxylon has two kinds of unusual centrifugal secondary growth. The first shows unequal activity of different portions of the cambium on the circumference of the axes; the consequent restriction of cambial activity to certain restricted areas develops wedged stems, which often split. The second produces polyxylic stems following supernumerary cambial activity. As in extant plants growing in arid regions, the included phloem and the associated parenchyma may have had functional value avoiding desiccation of the outer tissues of the stem during droughts. Anatomical features of other Corystospermaceae were used to determine systematic affinities and to establish relationships among medullosans, corystosperms and cycads.     

Triassic plant fossils from Pollock Road,Southland, New Zealand

Pole. M. S., & Raine. J. I., 1994:03:28. Triassic plant fossils from Pollock Road, Southland. New Zealand. Alcheringa 18, 147–159. ISSN 0311-5518.

Sedimentary rocks of the Murihiku Supergroup considered to be latest Triassic (Rhaetian), crop out about 10 m below the Glenham Porphyry, near Glenham, New Zealand. They contain the vegetative macrofossils Marchantites sp. (Hepaticae), Pachydermophyllum praecordillerae (Frenguelli) Retallack and Pachydermophyllum benmorensis Anderson & Anderson (Peltaspermaceae), cf. Dicroidium dubium var. dubium Anderson & Anderson (Corystospermaceae), Desmiophyllum sp. cf. D. indicum Sahni (possible conifer), and Gingkophytopsis sp. (possible progymnosperm).

Reproductive material includes ovulate structures (Peltaspermum cournanei sp. nov.), pollenbearing structures (Antevsia sp.) and probably seeds of the Peltaspermaceae, ? Umkomasia (Corystospermaceae), and a possible progymnosperm microsporophyll.

The palynoflora is dominated by bisaccate gymnospermous pollen, mainly Alisporites spp., consistent with the presence of corystosperm macrofossils. Moderately common pollen of Cycadopites spp. may be of peltasperm origin.     

Elchaxylon,a new corystosperm based on permineralized stems from the Late Triassic of Argentina

Artabe, A.E. & Zamuner, A.B., March 2007. Elchaxylon, a new corystosperm based on permineralized stems from the Late Triassic of Argentina. Alcheringa 31, 85‐96. ISSN 0311-5518.

Anatomically preserved stems of Late Triassic corystosperms from the Río Blanco Formation of Argentina are described and assigned to Elchaxylon zavattieriae gen. et sp. nov. The polyxylic gymnospermous axes have two discontinuous cambial rings, which develop centrifugal secondary xylem and centripetal secondary xylem around the mesarch primary xylem bundles. The centrifugal undivided pycnoxylic secondary xylem incorporates uniseriate rays, uni- to biseriate radial pitting either alternate or opposite, and one or two simple pits in each cross-field. Atypical secondary growth is restricted to localized development of centripetal secondary xylem (inverted xylem). Elchaxylon could represent a basal member of the rhexoxyloid lineage. This basal clade is characterized by limited centripetal secondary xylem, lack of perimedullar bundles, and modest development of centrifugal xylem.

Analia E. Artabe [aeartabe@museo.fcnym.unlp.edu.ar] and Alba B. Zamuner [azamuer@museo.fcnym.unlp.edu.ar]. División Paleobotánica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque SN, 1900 La Plata, Argentina and Consejo Nacional de Investigaciones Científicas y Técnicas; received 20.5.2005, revised and accepted 14.9.2005.     

Rhexoxylon cortaderitaense (Menéndez) comb. nov., a species of permineralized stems newly assigned to the Corystospermaceae,from the Triassic of Argentina

Bodnar, J., June, 2008. Rhexoxylon cortaderitaense (Menéndez) comb. nov., a species of permineralized stems newly assigned to the Corystospermaceae, from the Triassic of Argentina. Alcheringa 32, 171–190. ISSN 0311-5518.

Anatomically preserved stems of Late Triassic corystosperms from the upper Cortaderita Formation in San Juan province, Argentina are described and assigned to Rhexoxylon cortaderitaense (Menéndez) comb. nov. These specimens were originally attributed to conifers, a group not closely related to corystosperms. The silicified axes preserve features of the cortex, pith, and secondary vascular system. Like all Rhexoxylon species, these axes have two discontinuous cambial rings that develop centrifugal secondary xylem (normal secondary xylem) and centripetal secondary xylem (inverted secondary xylem). Both types of xylem are separated by a band of parenchymatous tissue developed by a remnant cambium. The centrifugal xylem is divided by parenchymatous rays following differential cambial activity. The secondary xylem is pycnoxylic with uniseriate rays; tracheids have mainly biseriate and alternate bordered pits on radial and tangential walls, and one simple oval pit in cross-fields. The distinctive feature of R. cortaderitaense is the conspicuous amount of centripetal secondary vascular tissues not forming perimedullar bundles, this being interpreted as an apomorphic trait. This interpretation of the character modifies previous hypothesized phylogenetic relationships of corystosperm wood taxa.