Aberlemnia
Encyclopedia
Aberlemnia is a genus of extinct vascular plant
s of the Early Devonian (around ), which consisted of leafless stems with terminal spore
-forming organs (sporangia
). Fossils found in Scotland were initially described as Cooksonia
caledonica. A later review, which included new and more complete fossils from Brazil, showed that the specimens did not fit the circumscription
of the genus Cooksonia; accordingly a new genus Aberlemnia was proposed.
quarry, Scotland. Other fossils now assigned to Aberlemnia caledonica have been found in Wales, Brazil and possibly Bolivia. Plants consisted of smooth leafless stems (axes) up to 1.4 mm wide, decreasing in width at each branching. Specimens branched up to five times, at angles between 25 and 55°, mainly dichotomously, although those from Brazil had some trichotomies. Spore-forming organs or sporangia
were borne at the ends of the stems. Individual sporangia varied in shape. Smaller ones were more or less circular in outline, larger ones were kidney-shaped (reniform), up to 2 mm high and 3 mm wide. The difference in shape is interpreted as being due to growth and maturation. To release their spores, the sporangia split into two valves along the border opposite to the stem on which they were attached (i.e. distally).
caledonica by Edwards in 1970. According to a review of the genus Cooksonia by Gonez and Gerrienne, the sporangium of the type species (C. pertoni) is formed by a widening of the end of a stem. At maturity the sporangium is topped by a flattish disk (an operculum) and releases its spores when this breaks up. The sporangia of C. caledonica are quite different. No existing genus was considered to cover the precise morphology of this plant, so that a new genus Aberlemnia was put forward. The name is based on the location where the first fossils were found, Aberlemno
in Scotland.
study in which they included the two best-characterized species of Cooksonia, C. paranensis and C. pertonii, together with Aberlemnia caledonica (then still called C. caledonica) produced the following cladogram:
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 of the Early Devonian (around ), which consisted of leafless stems with terminal 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 (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...
). Fossils found in Scotland were initially described as Cooksonia
Cooksonia
Cooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian ; the group continues to be an important component of the flora until the Early Devonian, a total time span of...
caledonica. A later review, which included new and more complete fossils from Brazil, showed that the specimens did not fit the circumscription
Circumscription (taxonomy)
In taxonomy, circumscription is the definition of the limits of a taxonomic group of organisms. One goal of taxonomy is to achieve a stable circumscription for every taxonomic group. Achieving stability can be simple or difficult....
of the genus Cooksonia; accordingly a new genus Aberlemnia was proposed.
Description
Fossils from which the genus was first described were found in the AberlemnoAberlemno
Aberlemno is a parish and small village in the Scottish council area of Angus. It is noted for three large carved Pictish stones dating from the 7th and 8th centuries AD ; the stones can be viewed at any time in spring-autumn, but are covered by wooden boxes in the winter to prevent frost damage...
quarry, Scotland. Other fossils now assigned to Aberlemnia caledonica have been found in Wales, Brazil and possibly Bolivia. Plants consisted of smooth leafless stems (axes) up to 1.4 mm wide, decreasing in width at each branching. Specimens branched up to five times, at angles between 25 and 55°, mainly dichotomously, although those from Brazil had some trichotomies. Spore-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 at the ends of the stems. Individual sporangia varied in shape. Smaller ones were more or less circular in outline, larger ones were kidney-shaped (reniform), up to 2 mm high and 3 mm wide. The difference in shape is interpreted as being due to growth and maturation. To release their spores, the sporangia split into two valves along the border opposite to the stem on which they were attached (i.e. distally).
Taxonomy
Specimens were first attributed to CooksoniaCooksonia
Cooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian ; the group continues to be an important component of the flora until the Early Devonian, a total time span of...
caledonica by Edwards in 1970. According to a review of the genus Cooksonia by Gonez and Gerrienne, the sporangium of the type species (C. pertoni) is formed by a widening of the end of a stem. At maturity the sporangium is topped by a flattish disk (an operculum) and releases its spores when this breaks up. The sporangia of C. caledonica are quite different. No existing genus was considered to cover the precise morphology of this plant, so that a new genus Aberlemnia was put forward. The name is based on the location where the first fossils were found, Aberlemno
Aberlemno
Aberlemno is a parish and small village in the Scottish council area of Angus. It is noted for three large carved Pictish stones dating from the 7th and 8th centuries AD ; the stones can be viewed at any time in spring-autumn, but are covered by wooden boxes in the winter to prevent frost damage...
in Scotland.
Phylogeny
Gonez and Gerrienne consider that Cooksonia is the most basal of the stem group of the lycophytes, whereas their genus Aberlemnia diverged later and is more derived. Consistent with this position, the genus has a combination of inherited features or plesiomorphies, such as dichotomous branching and terminal sporangia, with more advanced features, such as bivalved sporangia, which are characteristic of the lycophytes. A cladisticCladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
study in which they included the two best-characterized species of Cooksonia, C. paranensis and C. pertonii, together with Aberlemnia caledonica (then still called C. caledonica) produced the following cladogram: