The Letná Formation is a

Late Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

(

Sandbian The Sandbian is the first stage of the Upper Ordovician. It follows the Darriwilian and is succeeded by the Katian. Its lower boundary is defined as the first appearance datum of the graptolite species '' Nemagraptus gracilis'' around million yea ...

, or in the regional stratigraphy Berounian) geologic formation of the Prague Basin,

Bohemian Massif The Bohemian Massif ( cs, Česká vysočina or ''Český masiv'', german: Böhmische Masse or ''Böhmisches Massiv'') is a geomorphological province in Central Europe. It is a large massif stretching over most of the Czech Republic, eastern Ger ...

in the Czech Republic. The formation crops out in the Czech capital, more specifically at Letná Hill, after which the formation is named. The type locality is located at Malá Strana, Holešovice district.Kříž & Steinová, 2009 The more than thick formation comprises a rhythmic alternation of

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s (

greywacke Greywacke or graywacke (German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lit ...

s and subgreywackes),

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tec ...

s, intercalated with siltstones and shales deposited in marine

flysch Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building epi ...

-like environments. Because of the excellent preservation, including gut remains, of a wide assemblage of early Paleozoic taxa in which

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

s dominate, the formation, which is lean in fossils in many areas but exceptionally rich in what has been interpreted as storm beds, has been designated a '' Konservat-Lagerstätte''.

Description

The Letná Formation was first formally defined in 1966 by Havlíček and Vaněk and crops out in the Prague Basin, a northeast–southwest trending minibasin stretching over between

Rokycany Rokycany (; german: Rokitzan) is a town in the Plzeň Region of the Czech Republic. It has about 14,000 inhabitants. The town centre is well preserved and is protected by law as an urban monument zone. Administrative parts Rokycany is made up o ...

in the southwest and Úvaly in the northeast. The Prague Basin contains an Ordovician to Devonian succession in the larger

.Mikuláš, 1998b, p.143 The formation conformably overlies the Libeň Formation and is overlain by the Vinice Formation.Fatka et al., 2013, p.96 The Letná Formation is the thickest unit in the sedimentary sequence with a thickness of more than between Beroun and

Prague Prague ( ; cs, Praha ; german: Prag, ; la, Praga) is the capital and List of cities in the Czech Republic, largest city in the Czech Republic, and the historical capital of Bohemia. On the Vltava river, Prague is home to about 1.3 milli ...

.Mikuláš, 1998a, p.5Fatka et al., 2013, p.97 The formation consists of a rhythmic alternation of

s (

s and subgreywackes),

s, intercalated with siltstones and shales with

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

lenses in the upper part of the succession, deposited in a shallow marine

environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...

, representing a

deposition in the early Paleozoic. While in most of the layers fossils are absent, certain levels contain a rich assemblage of shallow marine fauna interpreted as storm-induced beds. The concentration of a wide variety of excellently conserved fossils of the Letná Formation has granted it the name of a '' Konservat-Lagerstätte'', giving insight into the diversification of marine ecosystems.Noheljová et al., 2019, p.69 A rather diverse fauna composed of brachiopods, trilobites, echinoderms, conularids, and non-trilobite arthropods has been found in outcrops located between Zdice and Chrustenice. This area probably represented a rather shallow environment in the northwestern part of the basin. The fossils are usually fragmentary and preserved as internal or external moulds associated with angular claystone fragments within quartzose sandstones to siltstones. Accumulations of these fossil remains are commonly found in thick-bedded quartzose sandstones of light-grey and yellow or grey-brown color, which might have been deposited in areas sheltered from wave activity by sand bars. Five communities of marine fauna were identified in the formation. The rich brachiopod dominated '' Drabovia'' Community is associated with shallow water, well-sorted quartzose sandstones. The '' Bicuspina'' Community, which probably represents a slightly deeper environment, is typically found in the poorly sorted greywackes of the higher horizons of the Letná Formation. These two communities constitute the '' Drabovia''-'' Aegiromena'' Fauna. In more proximal environments, this fauna is replaced by the trilobite-dominated '' Dalmanitina''-'' Deanaspis'' Association, which contains '' Birmanites ingens'' and '' Selenopeltis buchi''. The more distal, probably deeper parts of the basin are characterized by a poorly diversified assemblage of trilobites associated with rare

graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

s. Lastly, trilobites of the so-called '' Cyclopygid'' Biofacies and brachiopods of the '' Paterula'' Association are typical for the deepest portions of the Prague Basin. Intertidal (''

Skolithos ''Skolithos'' (formerly spelled ''Scolithus'' or ''Skolithus'') is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow. It is produced by a variety of organisms in shallow marine environmen ...

'' ichnofacies) and near-shore (''

Cruziana ''Cruziana'' is a trace fossil consisting of elongate, bilobed, approximately bilaterally symmetrical burrows, usually preserved along bedding planes, with a sculpture of repeated striations that are mostly oblique to the long dimension. It is fo ...

'' ichnofacies) environments are also evidenced by ichnofossils.Fatka et al., 2013, p.98 Fossilized gut remains have been observed in four of the about twenty trilobite species present in the Letná Formation: ''Birmanites ingens'', '' Dalmanitina socialis'', '' Deanaspis goldfussi'' and '' Selenopeltis buchi''. These four trilobites belong to the ''Dalmanitina''-''Deanaspis'' Association, characterizing rather proximal and shallow environments of the Letná Formation. Most of the non-trilobite arthropods discovered in this formation, including forms with non-mineralized exoskeletons, are also found associated with ''D. goldfussi'' and/or ''D. socialis''. This suggests that the development of conditions conducive to soft-tissue fossilization was not uncommon in the environment represented by the ''Dalmanitina''-''Deanaspis'' Association.Fatka et al., 2013, p.98

Fossil content

The following fossils were reported from the formation:Letná Formation
at Fossilworks.org *

Trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

s ** '' Actinopeltis completa'' ** '' Birmanites ingens''Fatka et al., 2013, p.99 ** '' Calymenella parvula'' ** '' Caryon bohemicus'' ** '' Cekovia transfuga'' ** ''

Colpocoryphe grandis ''Colpocoryphe'' is a genus of trilobites in the family Calymenidae. Morphology * Convex Cephalon * Trapezoidal glabellae, frequently distinctly furrowed and surelevated compared to the axis. * Dorsal furrows slightly convergent forwardly with ...

'' ** '' Dalmanitina socialis'' ** '' Deanaspis goldfussi'' ** '' Eudolatites dubia'' ** '' Eccoptochile (Eccoptochile) clavigera'' ** '' Eccoptochile (Eccoptochiloides) tumescens'' ** '' Kloucekia phillipsi'' ** '' Pharostoma pulchrum'' ** '' Placoparia (Hawleia) grandis'' ** '' Plaesiacomia rara'' ** '' Platycoryphe bohemica'' ** '' Primaspis (Primaspis) primordialis'' ** '' Selenopeltis buchi'' ** '' Stenopareia panderi'' ** '' Vysocania moraveci''Pereira et al., 2017, p.466 ** '' Zeliszkella (Zeliszkella) hawlei'' ** '' Girvanopyge sp.'' ** '' Heterocyclopyge sp.'' ** '' Prionocheilus sp.'' * Non-trilobite

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

s ** '' Duslia insignis'' ** '' Quasicaris bohemica'' ** '' Furca bohemica'' ** '' Furca sp.'' ** '' Zonozoe drabowiensis'' *

Crinoid Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

s ** '' Caleidocrinus multiramus'' * Lingulata ** '' Lingula deleta'' ** '' Orbiculoidea grandis'' ** '' Schizocrania incola'' ** '' Schizotreta sp.'' * Gastropods ** '' Asinomphalus antiqua'' ** '' Bucania draboviensis'' ** '' Bucanopsina calypso'' ** '' Bucanopsis comata'' ** '' Cyrtodiscus procer'' ** '' Holopea antiquata'' ** '' Tritonophon bohemica'' ** '' T. peeli'' * Paragastropoda ** '' Antispira praecox'' * Bivalves ** '' Astarte convergens'' ** '' Concavodonta ponderata'' ** '' Grammysia catilloides'' ** '' Leptodesma patricia'' ** '' Modiolopsis veterana'' ** '' Myoplusia bilunata'' ** '' Praenucula bohemica'' ** '' P. dispar'' ** '' Deceptrix sp.'' ** '' Palaeoneilo sp.'' *

Crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...

s ** '' Nothozoe pollens'' *

Ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typi ...

s ** '' Cerninella bohemica'' ** '' Crescentilla pugnax'' ** '' Cytheropsis testis'' ** '' Hastatellina sp.'' ** '' Trubinella latens'' *

Rostroconchia The Rostroconchia is a class of extinct molluscs dating from the early Cambrian to the Late Permian. They were initially thought to be bivalves, but were later given their own class. They have a single shell in their larval stage, and the adult t ...

** '' Ribeiria apusoides'' *

Blastoidea Blastoids (class Blastoidea) are an extinct type of stemmed echinoderm, often referred to as sea buds. They first appear, along with many other echinoderm classes, in the Ordovician period, and reached their greatest diversity in the Mississi ...

** '' Mespilocystites bohemicus'' ** '' Anatifopsis sp.'' *

Rhynchonellata The Rhynchonellata is a class of Lower Cambrian to Recent articulate brachiopods that combines orders from within the Rhynchonelliformea (Articulata revised) with well developed pedicle attachment. Shell forms vary from those with wide hinge li ...

** '' Svobodaina ellipsoides'' ** '' Jezercia chrustenicensis'' ** '' Drabovinella draboviensis'' ** '' Hirnantia ulrichi'' ** '' Drabovia asperula'' ** '' D. fascicostata'' ** '' D. redux'' ** '' Chrustenopora sp.'' ** '' Onniella sp.'' ** '' Saukrodictya sp.'' *

Craniata A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the m ...

** '' Petrocrania obsoleta'' * Strophomenata ** '' Aegiromena aquila'' ** '' Bicuspina cava'' ** '' B. multicostellata'' ** '' Rafinesquina occulata'' * Tergomya ** '' Barrandicellopsis extenuata'' ** '' Cytherina graegaria'' ** '' Dactylogonia blyskavensis'' ** '' Sinuitopsis neglecta'' * Tentaculita ** '' Conchicolites confertus'' * Edrioasteroidea ** '' Hemicystites bohemicus'' ** '' H. velatus'' * Eocrinoidea ** '' Ascocystites drabowiensis'' ** '' A. cf. micraster'' ** '' Echinosphaerites infaustus'' ** '' Macrocystella bohemica'' ** '' Cystidea obscondita'' ** '' Rhombifera bohemica'' * Stylophora ** '' Mitrocystites mitra'' * Soluta ** '' Dendrocystites sedgwicki'' * Scyphozoa ** '' Anaconularia anomala'' ** '' Archaeoconularia fecunda'' ** '' A. insignis'' ** '' Conularia rugulosa'' ** '' Exoconularia exquisita'' ** '' Metaconularia pyramidata'' ** '' Pseudoconularia grandissima'' ** '' P. nobilis'' * Merostomata ** '' Triopus draboviensis'' *

Polychaeta Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which ar ...

** '' Serpulites sp.'' * Machaeridia ** '' Plumulites folliculum'' * Algae ** '' Dendrocystis barrandei'' * Other ** '' Chasmatoporella havliceki'' ** '' Versispira contraria''

Gallery

Bicuspina multicostellata CRF.jpg, ''Bicuspina multicostellata'' Caleidocrinus multiramus CRF.jpg, ''Caleidocrinus multiramus'' Duslia insignis 34234.JPG, ''Duslia insignis'' Holopeidae - Holopea praestans.JPG, ''Holopea praestans''

References

Bibliography

* * * * * * {{cite LSA , last=Mikuláš , first=Radek , year=1998b , title=Ordovician of the Barrandian area: Reconstruction of the sedimentary basin, its benthic communities and ichnoassemblages , url=http://www.jgeosci.org/content/Mikulas_1998.pdf , journal=

Journal of the Czech Geological Society ''Journal of Geosciences'' (1956-1992 ''Časopis pro mineralogii a geologii'', 1993-2006: ''Journal of the Czech Geological Society'') is a peer-reviewed scientific journal published by the Czech Geological Society and its predecessors since 1956. ...

, volume=43 , pages=143–159 , accessdate=2020-05-29

Description

Fossil content

Gallery

See also

References

Bibliography

Further reading