Aarabia
Encyclopedia
Aarabia is a genus of extinct vascular plant
s found in central Morocco in outcrops of Early Devonian age (Emsian, around ). The leafless plant has a complex branching system with a main stem and at least three orders of side branches. In addition to these long branches, stems bore very short branches, which typically branched once into two curved sections. Spore
-forming organs or sporangia
were borne singly on reduced lateral branches in groups of at least three. The genus is thought to be related to the euphyllophytes – modern fern
s and seed plants.
in central Morocco
. They were 'adpressions' – a mixture of compressed and chemically altered remains of the original plant and impressions of its shape. Only central parts of the plant were found, the longest being 9 cm.
Stems (axes) were bare of leaves, spines or other protrusions, and were between 1.5 and 5 mm in diameter. Branching was 'pseudomonopodial', i.e. resulted in a 'main' stem and a lateral branch. There were up to four orders of branching; the branches were borne in two rows on opposite sides of the stem, producing a relatively two-dimensional structure. All orders of branches appear to have borne distinctive short branches which typically divided only once, the final sections having tips which curved away from each other and back towards the stem which bore them. Meyer-Berthaud and Gerrienne were unable to decide whether these short branches were generally capable of further growth or had ceased to grow and were adapted for some particular function.
Only poorly preserved specimens had sporangia. Up to three sporangia
were found on a "reduced lateral branch system". Individual sporangia were 1.8 to 4 mm long and 1 to 1.4 mm wide and probably hung downwards. No spore
s were found inside the sporangia, but large spores with an average diameter of 270 µm were found close to sporangia and may have been released from them. If so, Aarabia had megaspores and could be the earliest known plant with 'heterospory', i.e. two distinct sizes of spore.
in Central Morocco
. The specific epithet was chosen to reflect the short lateral branches.
species, Aarabia had short side branches in which the last sections curved back on themselves. On the other hand, Aarabia had relatively few sporangia in each fertile branching unit and they were single, not paired, whereas most trimerophytes had large clusters of sporangia carried in pairs which were often twisted around one another.
Meyer-Berthaud and Gerrienne added Aarabia to the data set which had been used by Kenrick and Crane in their analysis of the phylogeny of the polysporangiophyte
s, and generated a new phylogenetic tree. This placed Aarabia as the sister to the euphyllophytes. However, the analysis was regarded as "tentative", with 16 out of 33 chararacters which were used in constructing the tree being unknown. The diagram below is based on their results and the summary cladogram published by Crane et al.
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 found in central Morocco in outcrops of Early Devonian age (Emsian, around ). The leafless plant has a complex branching system with a main stem and at least three orders of side branches. In addition to these long branches, stems bore very short branches, which typically branched once into two curved sections. 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 singly on reduced lateral branches in groups of at least three. The genus is thought to be related to the euphyllophytes – modern fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s and seed plants.
Description
Fossils were found in an outcrop believed to be of Emsian age (around ) northwest of the town of AzrouAzrou
Azrou is a small Moroccan city 89 kilometers south of Fez, in the Meknès-Tafilalet region.-Geography:Azrou, which means rock in the indigenous Tamazight Berber language, is located in the heart of The Middle Atlas region of Morocco between Meknes, Fez, Ifrane, Midelt and Khenifra. The N13 road...
in central Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
. They were 'adpressions' – a mixture of compressed and chemically altered remains of the original plant and impressions of its shape. Only central parts of the plant were found, the longest being 9 cm.
Stems (axes) were bare of leaves, spines or other protrusions, and were between 1.5 and 5 mm in diameter. Branching was 'pseudomonopodial', i.e. resulted in a 'main' stem and a lateral branch. There were up to four orders of branching; the branches were borne in two rows on opposite sides of the stem, producing a relatively two-dimensional structure. All orders of branches appear to have borne distinctive short branches which typically divided only once, the final sections having tips which curved away from each other and back towards the stem which bore them. Meyer-Berthaud and Gerrienne were unable to decide whether these short branches were generally capable of further growth or had ceased to grow and were adapted for some particular function.
Only poorly preserved specimens had sporangia. Up to three 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 found on a "reduced lateral branch system". Individual sporangia were 1.8 to 4 mm long and 1 to 1.4 mm wide and probably hung downwards. No 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...
s were found inside the sporangia, but large spores with an average diameter of 270 µm were found close to sporangia and may have been released from them. If so, Aarabia had megaspores and could be the earliest known plant with 'heterospory', i.e. two distinct sizes of spore.
Taxonomy
Aarabia brevicaulis was named by Meyer-Berthaud and Gerrienne in 2001. The generic name is derived from the locality close to Jbel ben Aarab, near AzrouAzrou
Azrou is a small Moroccan city 89 kilometers south of Fez, in the Meknès-Tafilalet region.-Geography:Azrou, which means rock in the indigenous Tamazight Berber language, is located in the heart of The Middle Atlas region of Morocco between Meknes, Fez, Ifrane, Midelt and Khenifra. The N13 road...
in Central Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
. The specific epithet was chosen to reflect the short lateral branches.
Phylogeny
Aarabia shares features with basal euphyllophytes (Banks' trimerophytes). Two of these are dichotomous branching, which results in a clear difference between a 'main' stem and a lateral stem, and elongated sporangia, taller than wide, borne at the ends of fertile branch systems which are differentiated from other branches. Like some PsilophytonPsilophyton
Psilophyton is a genus of extinct vascular plants. Described in 1859, it was one of the first fossil plants to be found which was of Devonian age . Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China...
species, Aarabia had short side branches in which the last sections curved back on themselves. On the other hand, Aarabia had relatively few sporangia in each fertile branching unit and they were single, not paired, whereas most trimerophytes had large clusters of sporangia carried in pairs which were often twisted around one another.
Meyer-Berthaud and Gerrienne added Aarabia to the data set which had been used by Kenrick and Crane in their analysis of the phylogeny of the polysporangiophyte
Polysporangiophyte
Polysporangiophytes, also called polysporangiates or more formally Polysporangiophyta, are plants in which the spore-bearing generation has a structure of branching stems terminating in sporangia...
s, and generated a new phylogenetic tree. This placed Aarabia as the sister to the euphyllophytes. However, the analysis was regarded as "tentative", with 16 out of 33 chararacters which were used in constructing the tree being unknown. The diagram below is based on their results and the summary cladogram published by Crane et al.