Macivera
Encyclopedia
Macivera is a genus of extinct vascular plant
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
-forming organs or sporangia
, which were elliptical, being longer than wide, were borne on the end regions of stems. Macivera is considered to be a zosterophyll.
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
), 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.
Spore
-forming organs or sporangia
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, 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).
, 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.
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
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
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
-forming organs or sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
, which were elliptical, being longer than wide, were borne on the end regions of stems. Macivera is considered to be a zosterophyll.
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 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
Vascular tissue is a complex conducting 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. There are also two meristems associated with vascular tissue:...
), 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.
Spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
-forming organs or sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
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, 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 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 zosterophylls. 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 DistichophytumDistichophytum
Distichophytum is a genus of extinct vascular plants of the Late Silurian to Early Devonian , around . The genus has a tangled taxonomic history, also being known as Bucheria and Rebuchia ....
, but this has its sporangia in two more-or-less opposite rows. Hicklingia
Hicklingia
Hicklingia is a genus of extinct plants of the Middle Devonian . Compressed specimens were first described in 1923 from the Old Red Sandstone of Scotland...
edwardii has similarly shaped and arranged sporangia, but they are considerably larger and stalked.