Yarravia
Encyclopedia
Yarravia is a genus of extinct vascular plant
s mainly known from fossil
s found in Victoria, Australia
. Originally the rocks in which they were found were considered to be late Silurian
in age; more recently they have been found to be Early Devonian (Pragian, around ). Specimens consist only of incomplete leafless stems, some of which bore groups of spore
-forming organs or sporangia
which were fused, at least at the base.
in which they were found, now called the Wilson Creek Shale, was for many years regarded as of late Silurian
age. The dating was based on graptolite
fossils which were identified as Monograptus
uncinatus, which is of early Ludlow
age (around ). These were subsequently described as a new species, M. thomasi, which proved to be of Early Devonian
age (Pragian, around ). Rocks of Silurian age do occur in the region and the fossil plant Baragwanathia
has been found from both late Silurian and the Early Devonian strata. However, Yarravia was found only in the Devonian shales. Tims and Chambers say that the material described by Lang and Cookson was from near Yea
, some distance from the location originally given.
Fossils collected near Rebreuve
, Pas-de-Calais Department, northern France, were named Yarravia minor by Danzé-Corsin in 1956. The strata in which they were found were initially considered Pragian, but are now thought to be Emsian (from around ).
The sporangia were borne in a group at the ends of stems. The specimens are flattened, so that the original shape is not entirely clear, but up to five or six upright elongated sporangia appear to have been radially arranged on a base formed by a widening of the stem; there may have been a central space. At the base the sporangia are fused together, but their tips may be free. The entire structure can be described as a 'synangium'. Blackened material inside the sporangia was assumed to have been spores.
Two species were described from the Australian material. In Y. oblonga, the synangium (the group of fused sporangia) was longer than wide, being about 7 mm long by 1.25 mm wide. The tips of the sporangia were free and appear to consist of tissue not containing spores. The synangium of Y. subsphaerica was significantly larger and as long as wide, being about 1 cm in length and width. The tips of the sporangia were less prominent.
, of similar age, which is known from both Australia and China, had a similar radial cluster of sporangia at the tips of stems; the two genera are distinctive in this way compared to plants from other regions. It has been suggested that Yarravia may simply be Hedeia in a different state of preservation. Both genera have been treated as 'rhyniophytes', although other studies regard their placement as uncertain.
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 mainly known from fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s found in Victoria, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. Originally the rocks in which they were found were considered to be late Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
in age; more recently they have been found to be Early Devonian (Pragian, around ). Specimens consist only of incomplete leafless stems, some of which bore groups of 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 fused, at least at the base.
Source of fossils
Yarravia was named in 1935 by Lang and Cookson, based on fossils found at a location they called the 'Yarra Track', near to the town of Wood's Point, Victoria, Australia. The shaleShale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
in which they were found, now called the Wilson Creek Shale, was for many years regarded as of late Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
age. The dating was based on graptolite
Graptolite
Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...
fossils which were identified as Monograptus
Monograptus
Monograptus is a genus of the Order Graptoloidea. This particular genus is the last stage of the graptoloid evolution before its extinction in the early Devonian. Characteristics of monograptus include: One uniserial stipe, with very elaborate thecae...
uncinatus, which is of early Ludlow
Ludlow
Ludlow is a market town in Shropshire, England close to the Welsh border and in the Welsh Marches. It lies within a bend of the River Teme, on its eastern bank, forming an area of and centred on a small hill. Atop this hill is the site of Ludlow Castle and the market place...
age (around ). These were subsequently described as a new species, M. thomasi, which proved to be of Early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
age (Pragian, around ). Rocks of Silurian age do occur in the region and the fossil plant Baragwanathia
Baragwanathia
Baragwanathia is a genus of extinct plants of the division Lycopodiophyta of Late Silurian to Early Devonian age , fossils of which have been found in Australia, Canada and China.-Description:...
has been found from both late Silurian and the Early Devonian strata. However, Yarravia was found only in the Devonian shales. Tims and Chambers say that the material described by Lang and Cookson was from near Yea
Yea, Victoria
Yea is a town in Victoria, Australia. It is in the Shire of Murrindindi local government area. Located north-east of Melbourne via the Melba Highway, Yea sits at the junction with the Goulburn Valley Highway, and above sea-level. At the 2006 Census, Yea had a population of 1,052.- History :The...
, some distance from the location originally given.
Fossils collected near Rebreuve
Rebreuve-Ranchicourt
Rebreuve-Ranchicourt is a commune in the Pas-de-Calais department in the Nord-Pas-de-Calais region of France.-Geography:Rebreuve-Ranchicourt is situated about southwest of Béthune and southwest of Lille, at the junction of the D341 and the D57 road...
, Pas-de-Calais Department, northern France, were named Yarravia minor by Danzé-Corsin in 1956. The strata in which they were found were initially considered Pragian, but are now thought to be Emsian (from around ).
Description
Only the uppermost parts of Yarravia have been found in Australia. The stems (axes) were bare, around 2 to 2.5 mm in diameter. The longest pieces were up to 7.5 cm in length. No examples of branching were seen. The internal structure of the stems is not preserved.The sporangia were borne in a group at the ends of stems. The specimens are flattened, so that the original shape is not entirely clear, but up to five or six upright elongated sporangia appear to have been radially arranged on a base formed by a widening of the stem; there may have been a central space. At the base the sporangia are fused together, but their tips may be free. The entire structure can be described as a 'synangium'. Blackened material inside the sporangia was assumed to have been spores.
Two species were described from the Australian material. In Y. oblonga, the synangium (the group of fused sporangia) was longer than wide, being about 7 mm long by 1.25 mm wide. The tips of the sporangia were free and appear to consist of tissue not containing spores. The synangium of Y. subsphaerica was significantly larger and as long as wide, being about 1 cm in length and width. The tips of the sporangia were less prominent.
Taxonomy
The genus Yarravia was named in 1935 by Lang and Cookson, based on fossils found in an Australian locality they called the 'Yarra Track'. Two species were named, Y. oblonga and Y. subsphaerica, referring to the shape of the sporangia.Phylogeny
The relationships of Yarravia are uncertain. The genus HedeiaHedeia
Hedeia is a genus of early land plant of uncertain affinity. It comprises erect axes terminating in clusters of sporangia....
, of similar age, which is known from both Australia and China, had a similar radial cluster of sporangia at the tips of stems; the two genera are distinctive in this way compared to plants from other regions. It has been suggested that Yarravia may simply be Hedeia in a different state of preservation. Both genera have been treated as 'rhyniophytes', although other studies regard their placement as uncertain.