Macivera

Did You Know?

• The genus name honors paleobotanist Elisabeth E. McIver.
• The specific epithet gracilis refers to the thinness of the stems.
• Macivera was first described from a small number of specimens.

Description

The stems of Macivera were smooth, lacking leaves or hairs, and ranged from 0.7 to 1.0 mm in width, tapering to 0.1 mm near the apex of non-sporangiate branches. Branching was equal and dichotomous, with some evidence of downward branching at the base. The plant is estimated to have reached about 8 cm in height. Sporangia were borne near the stem tips, often in clusters and angled slightly outward; they were attached directly or on short stalks and not arranged in distinct rows. Each sporangium was elliptical, 1.3 to 2.0 mm high and 1.1 to 1.3 mm wide, thus longer than wide. As typical for zosterophylls, they opened by a linear slit at the distal end. Spores about 50 μm in diameter were found, possibly immature, and all of one size, indicating homospory.

Phylogeny

The lateral position of the sporangia and their method of dehiscence align Macivera with the zosterophylls. Its small size and elongated sporangia, which are borne without clear rows and have short or absent stalks, distinguish it from related genera. Kotyk et al. noted closest resemblance to Distichophytum, though that genus has sporangia in two opposite rows. Hicklingia edwardii shares similar sporangial shape and arrangement but has larger, stalked sporangia. Hao and Xue (2013) listed Macivera as a zosterophyll.