''Aarabia'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of extinct

vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...

s found in central Morocco in outcrops of

Early Devonian The Early Devonian is the first of three epochs comprising the Devonian period, corresponding to the Lower Devonian series. It lasted from and began with the Lochkovian Stage , which was followed by the Pragian from and then by the Emsian, ...

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 A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cyc ...

were borne singly on reduced lateral branches in groups of at least three. The genus is thought to be related to the

euphyllophyte The euphyllophytes are a clade of plants within the tracheophytes (the vascular plants). The group may be treated as an unranked clade, a division Division or divider may refer to: Mathematics *Division (mathematics), the inverse of multipli ...

s – modern

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...

s and

seed plant A spermatophyte (; ), also known as phanerogam (taxon Phanerogamae) or phaenogam (taxon Phaenogamae), is any plant that produces seeds, hence the alternative name seed plant. Spermatophytes are a subset of the embryophytes or land plants. They inc ...

Description

Fossils were found in an outcrop believed to be of

Emsian The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 407.6 ± 2.6 million years ago to 393.3 ± 1.2 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after the ...

age (around ) northwest of the town of

Azrou Azrou (Berber: Aẓro, ⴰⵥⵔⵓ, Arabic: أزرو) (meaning “rock” or “stone”) is a Moroccan town 89 kilometres south of Fez in Ifrane Province of the Fès-Meknès region. Geography The market town of Azrou is located at a strategi ...

in central

Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria t ...

. 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

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 spores 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

in Central

. The specific epithet was chosen to reflect the short lateral branches.

Phylogeny

''Aarabia'' shares features with basal

s (Banks' trimerophytes). Two of these are pseudomonopodial 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 ''

Psilophyton ''Psilophyton'' is a genus (biology), genus of extinct vascular plants. Described in 1859, it was one of the first fossil plants to be found which was of Devonian age (about ). Specimens have been found in northern Maine, USA; Gaspé Bay, Queb ...

'' 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 Polysporangiophytes, also called polysporangiates or formally Polysporangiophyta, are plants in which the spore-bearing generation (sporophyte) has branching stems (axes) that bear sporangia. The name literally means 'many sporangia plant'. The cl ...

s, and generated a new phylogenetic tree. This placed ''Aarabia'' as the sister to the

s. 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.

References

{{Taxonbar, from=Q4661661 Early Devonian plants Emsian life Fossils of Morocco Prehistoric plant genera