Brigitte Meyer-Berthaud

Tyloses are protoplasmic swellings of parenchyma cells into the lumen of adjacent conducting cells. They develop as part of the heartwood formation process, or in response to embolism or pathogen infection. Here, we report the oldest fossil evidence to date of tylosis formation that occurs in permineralized wood of the (pro)gymnosperm Dameria hueberi from the Tournaisian (lower Mississippian, ca. 350 Ma) of Australia. Different developmental stages of tylosis formation are recognizable that range from small bubble-like protrusions to dense tyloses entirely filling the lumen of the conducting cells. The trigger for the development of tyloses in D. hueberi remains unknown. A survey of the fossil record of tyloses shows their occurrence in most groups of vascular plants since the Carboniferous. Future research in this field will screen even older (Devonian) fossils for evidence of tyloses and aim to understand the roles these structures have had in plant–pathogen interactions and plant hydraulic properties in the past.

The Famennian is a period of increased taxonomic diversity for the first representatives of the major plant groups, especially the spermatophytes (i.e. the seed plants) that, today, represent the most abundant and diversified plant group on Earth [ 1, 2]. Early spermatophytes evolved a unique ovule-centered reproductive syndrome that allowed them to germinate and grow rapidly in disturbed habitats and, secondarily, to colonize dry habitats unfavorable to the reproduction of their free-sporing predecessors [ 3, 4] (Fig. 1). In these ovulate organisms, the female gametophyte grows and produces gametes inside the indehiscent megasporangium (nucellus) that differentiates a pollen chamber retaining the pollen grains in its apical part. An integument encloses the nucellus. Like the Runcaria protovule that preceded them by 20 million years, Famennian ovules were all surrounded by a cupule, except perhaps Spermolithus and Warsteinia [ 5–7]. The latter, however, have been found isolated and may also have had a cupule remaining on the plant that produced them.

This paper discusses the main steps leading to the acquisition, during the Palaeozoic, of the seed habit. The earliest spermatophytes originated in the Late Devonian and produced small cupulate ovules characterized by an elaborated sporangial apex involved in pollination. The diversification of ovules during the Carboniferous led to a wider range of sizes and shapes, and an increasing importance of the integument. The earliest seeds containing embryos are reported in Carboniferous coniferophytes.

Archaeopterid trees were the main components of most Late Devonian forests. Their aerial axes characterised by a eustele with mesarch primary xylem strands, leaf traces departing radially from cauline bundles and secondary xylem tracheids with radial pits arranged in groups, are referred to the genusCallixylonZalessky. The nineteen species ofCallixylondescribed to date from North America, North Africa, Europe, Xinjiang and several parts of Russia range from the late Givetian to the Mississippian. In this paper, we describe a new species ofCallixylonfrom two specimens collected in the Famennian locality of Mader el Mrakib in eastern Anti-Atlas.Callixylon wendtiisp. nov. is characterised by the presence of sclerotic nests in the pith, a new character for the genus. Its wood shows narrow rays of variable height, with unevenly distributed ray tracheids. Small vascular traces crossing the wood within the two innermost growth rings are interpreted as evidence for short-lived leaves. This discovery adds to the diversity of the genusCallixylonin an area of northern Gondwana that may have been favourable to the establishment of a diverse community of archaeopterid trees.