''Macivera'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of extinct

vascular plant Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignin, lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified Ti ...

s. Fossils were found in sediments in Bathust Island, Nunavut, Canada, from the upper Silurian ( Ludfordian, around ). The leafless stems (axes) branched dichotomously and were relatively thin, being between 0.7 and 1.0 mm wide.

Spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

-forming organs or

sporangia A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...

, which were elliptical, being longer than wide, were borne on the end regions of stems. ''Macivera'' is considered to be a

zosterophyll The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zostero ...

Description

The genus was first described from a small number of specimens found in sediments in Bathurst Island, Nunavut, Canada, which are considered to be of

Late Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

age (Ludfordian, around ). Stems (axes) were smooth, devoid of leaves, hairs or other protrusions, and were between 0.7 and 1.0 mm wide (around the lower limit of the size Boyce suggested as being compatible with the presence of

vascular tissue Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. T ...

), tapering down to 0.1 mm towards the apex of those stems which did not bear sporangia. The stems branched equally and dichotomously, with some suggestion of downwards branching at the base. The overall height of the plant is estimated to be some 8 cm.

-forming organs or

were borne towards the end of stems, apparently in clusters, angled slightly away from the upright. They were either borne directly on the stem or had short stalks, and do not appear to have been arranged into distinct rows. Sporangia were elliptical in shape, 1.3 to 2.0 mm high and 1.1 to 1.3 mm wide, so that they were longer than wide. As is typical of

zosterophylls The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosterop ...

, they split to release their spores via a linear opening at the opposite end to their attachment to the stem. Spores were found, around 50 μm in diameter, but may have been immature; they were of one size range (i.e. the plant was homosporous).

Taxonomy

The genus and species ''Macivera gracilis'' were named in 2002. The genus name is in memory of Elisabeth E. McIver, a

paleobotanist Paleobotany or palaeobotany, also known as paleophytology, is the branch of botany dealing with the recovery and identification of plant fossils from geological contexts, and their use for the biological reconstruction of past environments (pale ...

and collector of plant fossils from Bathurst Island. The specific epithet refers to the thinness of the stems.

Phylogeny

The position of the sporangia on the sides of stems and the way in which they split to release spores suggests a relationship with the

s. The distinctive features of ''Macivera'' are its small size, and the nature of its sporangia which are elongated rather than wider than high or kidney-shaped, and which have short or no stalks while not being borne in clear rows. The closest comparison was considered by Kotyk et al. to be with the genus '' Distichophytum'', but this has its sporangia in two more-or-less opposite rows. '' Hicklingia edwardii'' has similarly shaped and arranged sporangia, but they are considerably larger and stalked. Hao and Xue in 2013 listed the genus as a zosterophyll.

References

{{Taxonbar, from=Q6724155 Silurian life Prehistoric lycophyte genera Prehistoric plants of North America Zosterophylls