Additional observations on Zosterophyllum yunnanicum Hsü from the Lower Devonian of Yunnan, China

Investigation of unfigured specimens in the original collection of Zosterophyllum yunnanicum Hsü, 1966 from the Lower Devonian (upper Pragian to basal Emsian) Xujiachong Formation, Qujing District, Yunnan, China has provided further data on both sporangial and stem anatomy. We show that the sporangia dehisced into more or less equal valves through a mechanism that involved the development of large thick-walled cells. Furthermore the coalified xylem is composed of tracheids with G-type thickenings (predominantly annular secondary thickenings with small circular to irregular perforations in the intervening wall), confirming the presence of this form of vascular element in the genus Zosterophyllum. The species diagnosis is emended. Characterisation of dehiscence mechanisms in fossil sporangia is complicated by their different modes of preservation. A brief critical survey of the marginal features in bivalved sporangia in zosterophylls and other selected species is followed by a discussion on their putative functional significance. Preservation notwithstanding, we identify clear differences in mechanisms related to the nature of the underlying cellular structure of the dehiscence feature. The distinctive groove present in many species might represent a mechanism for regulating the timing of dehiscence in response to atmospheric conditions.