The Gauja Formation is a

Middle Devonian In the geological timescale, the Middle Devonian epoch (from 393.3 ± 1.2 million years ago to 382.7 ± 1.6 million years ago) occurred during the Devonian period, after the end of the Emsian age. The Middle Devonian epoch is subdivided into two ...

fossil locality in

Estonia Estonia, officially the Republic of Estonia, is a country in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the Baltic Sea across from Sweden, to the south by Latvia, and to the east by Ru ...

and

Latvia Latvia, officially the Republic of Latvia, is a country in the Baltic region of Northern Europe. It is one of the three Baltic states, along with Estonia to the north and Lithuania to the south. It borders Russia to the east and Belarus to t ...

. It is named after the Gauja River, where it is exposed along the banks.

Description

The Gauja Formation has a maximum thickness of . It is composed of weakly to moderately cemented layers of fine-grained to very fine-grained

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

. The layer is predominantly light to yellowish-gray in color, but can be pinkish brown or variegated. It is mostly composed of

quartzose Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical fo ...

arenite Arenite (from the Latin ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00245 in) and 2 mm (0.08 in) and containing less than 15% matrix. The related adjective is ''arenaceous''. The e ...

s. It is overlain by the thick Amata Formation and the Plavinas Formation. The Gauja Formation contains two cyclic members. The lower layers are known as the Sietiņi Member and are composed mostly of

with a thin layer of

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

at the top. The upper layers are thicker, and the lower part of it is known as the Lode Member. The Lode Member dates from the

and is composed of light colored sandstone. The area is indicative of a near-shore environment of retreating sea.

Paleontology

The Sietiņi and Lode members have yielded numerous fossils of fishes. Among them are '' Asterolepis'',

Bothriolepis ''Bothriolepis'' (from , 'trench' and 'scale') was a widespread, abundant and diverse genus of antiarch placoderms that lived during the Middle to Late Devonian period of the Paleozoic Era. Historically, ''Bothriolepis'' resided in an array ...

, Psammolepis, Cheirolepis, '' Glyptolepis baltica'', '' Laccognathus panderi'', '' Livoniana multidentata'', Strunius,

Panderichthys ''Panderichthys'' is a genus of extinction, extinct Sarcopterygii, sarcopterygian (lobe-finned fish) from the late Devonian period, about 380 Myr, Mya. ''Panderichthys'', which was recovered from Frasnian (early Late Devonian) deposits in Latvia, ...

and '' Megadonichthys kurikae''Ahlberg et al., 2000 The first fossils of young

placoderms Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos plate' and δέρμα 'dermaskin') are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Dev ...

ever discovered were found in the Gauja formation. Ranging in size from as small as 10.4 mm to as big as 38 mm (0.4 to 1.4 in) long, these fossils are important for our understanding of

ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the ovum, egg to adult. The term can also be used to refer to t ...

in extinct

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

Only large

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

remains and

miospore Cryptospores are microscopic fossilized spores produced by embryophytes (land plants). They first appear in the fossil record during the middle of the Ordovician period, as the oldest fossil evidence for the colonization of land by plants. A simila ...

s are known from this member. Examples of which include '' Hostinella'', ''

Archaeopteris ''Archaeopteris'' is an extinct genus of progymnosperm tree with fern-like leaves. A useful List of index fossils, index fossil, this tree is found in Stratum, strata dating from the Upper Devonian to Lower Carboniferous (), the oldest fossils b ...

'', '' Retusotriletes rugulatus'', and '' Ancyrospora''.

Palynological Palynology is the study of microorganisms and microscopic fragments of mega-organisms that are composed of acid-resistant organic material and occur in sediments, sedimentary rocks, and even some metasedimentary rocks. Palynomorphs are the mic ...

studies indicate that the Middle to Upper Devonian boundary may exist in this member. Archaeopteris reconstruccion

References

Bibliography

* {{cite journal , first1=P.E. , last1=Ahlberg , first2=E. , last2=Lukševis , first3=E. , last3=Mark-Kurik , year=2000 , title=A near-tetrapod from the Baltic Middle Devonian , journal=

Palaeontology Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geo ...

, volume=43 , issue=3 , pages=533–548, doi=10.1111/j.0031-0239.2000.00138.x , bibcode=2000Palgy..43..533A , s2cid=129654491 Geologic formations of Europe Geology of Estonia Geology of Latvia Devonian System of Europe Givetian Stage Sandstone formations Shallow marine deposits Devonian southern paleotropical deposits Fossiliferous stratigraphic units of Europe Paleontology in Estonia Paleontology in Latvia

Description

Paleontology

See also

References

Bibliography