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Ostracods

ostracod Acanthocythereis




Introduction
Ostracods are by far the most complex organisms studied within the field of micropalaeontology. They are Metazoa and belong to the Phylum Arthropoda (as trilobites), Class Crustacea (as lobsters and crabs). An important distinguishing feature Ostracods share with other arthropods is the bilateral symmetry of their body form. The paired body parts are enclosed in a dorsally hinged carapace composed of low magnesium calcite, which is what is commonly preserved in the fossil record. They are found today in almost all aquatic environments including hot springs, caves, within the water table, semi-terrestrial environments, in both fresh and marine waters, within the water column as well as on (and in) the substrate. In fact almost anywhere that's wet, even if only for a brief period!
History of Study
The oldest generic names given to ostracods are Cypris and Cythere by Muller in the 1770's and 80's, these are now commonly used as suffixes and prefixes in ostracod nomenclature. In the 1860's Sars classified ostracods as an order divided into four suborders, Podocopa, Myodocopa, Cladocopa and Platycopa. In the early 20th Century workers in the Appalachians in the U.S.A classified Palaeozoic ostracods. In 1958 Pokorny combined these two classifications and in 1961 an Anglo American treatise modified Pokorny's work to give the foundation of today's classification system. It was not until Pokorny's work that the fossil and living classifications were linked.
Range
Ostracod-like organisms (bivalved arthropods) are recorded from the Cambrian, but it is uncertain whether these can be classified as true ostracods. Myodocopid and podocopid forms are recorded from the Ordovician. All these early forms are marine, the first freshwater forms (Darwinulacea and Carbonita) occur in the Carboniferous and by the Jurassic ostracods are common in freshwater environments. Between the Silurian/Devonian and the present there are big gaps in the fossil record of planktonic marine forms, which is thought to reflect weak calcification of the carapace.
geologic time scale diagram click to view larger version
Classification
The Class Ostracoda is separated from other Crustacea by their laterally compressed body, undifferentiated head, seven or less thoracic limbs and the bivalved, perforate carapace lacking growth lines. The living ostracods are further classified in many cases by variations in their appendages and other soft parts. Although exceptionally well preserved fossil ostracods with the soft parts intact have been found these are very rare and therefore the morphological features (see below) of the carapace have become vital in fossil ostracod classification. The Ostracoda have been divided into five Orders, the extant Podocopida and Myodocopida and the extinct Phosphatocopida, Leperditicopida and Palaeocopida (however, the latter groups may well not be ostracods in the strict biological sense).
Applications
Since the fossil record of planktonic marine ostracods is so patchy, biostratigraphic uses of ostracods based on benthic forms are limited to specialised marine environments for example in the Jurassic of the North Sea. In the marine environment benthic ostracods are utilised for palaeoenvironmental reconstructions. Freshwater and brackish facies commonly contain abundant ostracods which are used for environmental studies and for biostratigraphic zonations, for instance in non-marine sediments from Mongolia and China.
Biology
diagram showing ostracod valve features click to view larger version
Several morphological features of ostracods are at times preserved in the fossil forms and have been utilised in their classification. The ostracod carapace is usually ovate, kidney-shaped or bean-shaped, it is divided into a right and left valve, one being, commonly slightly larger than the other partially overlapping it, and hinged at the dorsal margin. The hinge is an important feature in terms of taxonomy and classification. Four basic types of hinge are recognised:
  • The adont hinge is the simplest, without teeth or sockets, often forms part of a contact groove on the larger valve and a corresponding ridge on the smaller valve.
  • The merodont hinge is composed of a tooth and socket at each end of a groove or ridge structure (complementary negative and positive structures in left and right valves).
  • The entomodont hinge differs from the merodont hinge style by having a coarsely crenulated anterior portion of the median groove/ ridge element.
  • The amphidont hinge has a more complex median structure with an anterior tooth and socket.
When alive the two valves of the carapace are closed by adductor muscles, these are normally connected to the inner surface of the valve at a point just anterior of the valve centre, they frequently leave scars on the valves inner surface and either a subcentral tubercle (a sort of boss) or a sulcus (an elongate shallow depression) on the outside. The podocopid ostracods produce a calcified overlapping flange around the ventral margin called a duplicature.
Ostracods sense their surroundings using sensilla (hairs or bristles) which project through the carapace via pore canals, at the margins these are called marginal pore canals. Some shallow water ostracods have eyes and their carapaces have clear eyespots or raised eye tubercles.
Ostracods can reproduce sexually and asexually (parthenogenesis). Ostracods show sexual dimorphism, that is males and females of the same species have carapaces of differing size and shape.They grow by moulting up to nine growth stages (instars). Because of this sexual dimorphism and the ontogenetic variations of ostracods great care must be taken with taxonomy, as a single species may have a series of juvenile stages as well as two adult morphotypes.
The ecology of ostracods is often reflected in the shape and structure of their carapaces hence making them useful palaeoenvironmental indicators. Freshwater ostracods in general tend to have smooth, thin, weakly calcified simple bean-shaped carapaces. They feed on a wide range of food stuffs including diatoms, bacteria and detritus. Pelagic ostracods also tend to have thin, smooth shells and may have long powerful swimming appendages or antennules which have led to the formation of rostral incisures at the anterior of the carapace to allow freer movement of these appendages. Benthic ostracods are commonly detritivores or filter feeders, they either burrow into the substrate, in which case their carapaces tend to be smooth, small, robust and sometimes elongated. Epifaunal types may have flattened ventral surfaces sometimes with projecting alar wings, frills, keels or lateral spines. Those found on coarser substrates in higher energy environments tend to have more robust heavily ribbed or reticulated carapaces.
Life Cycle
Ostracods like other Crustacea moult between growth stages (called an instar), this process is known as ecdysis. There are usually nine instars between egg and adult. This fact has extremely important implications for palaeontological studies. For example, if an assemblage contains a mix of instars it is relatively safe to assume the material is in situ (a biocoenosis, a true reflection of the living assemblage). Ostracods also have a variety of complex reproductive strategies, including brood care of eggs within the carapace (e.g. Darwinula), desiccation-resistant eggs (known to survive in a dry state only to hatch on immersion in water years later), sexual and asexual strategies including parthenogenesis which in Darwinula is thought to be the only method of reproduction utilised. Parthenogenesis is asexual reproduction via the female only, there appears to have been no male Darwinula's for many thousands if not millions of years!
Preparation Technique
WARNING: Please remember all preparation techniques require the use of hazardous materials and equipment and should only be caried out in properly equiped laboratories, wearing the correct safety clothing and under the supervision of qualified staff.
Ostracod carapaces range in size from approximately 100 microns up to several millimetres, and they are commonly prepared in the same way as foraminifera with careful washing with hydrogen peroxide and/or washing soda and sieving using a standard 63 micron sieve. Several washes may be required to break down well indurated material and care should be taken when washing through the sieve to prevent breakage of the specimens. The cleaned residue can then be dried, sieved into fractions (generally 63-125 microns, 125-250 microns, 250-500 microns and greater than 500 microns) and "picked". Care must be taken to clean all sieves and materials used between the preparation of each sample to prevent contamination.
Observation Techniques
Once fossil samples have been prepared ostracod carapaces can be picked from any remaining sediment using a fine brush and a reflected light, binocular microscope. The best method is to scatter a fine dusting of sieved sediment on to a black tray divided into squares, this can then be scanned under the microscope and any ostracods preserved in the sediment can be picked out with a fine brush (preferably a 000 sable paint brush). The picked specimens can then be mounted in card slides divided into numbered squares with sliding glass covers. Gum tragocanth was traditionally used to attach the specimens to the slides but modern office type water soluble paper adhesives are now used. Ostracods are large enough to be observed live in wet preps under microscopes and sometimes with the naked eye. Almost any relatively still water will contain ostracods and samples can be collected especially by scraping them from the surface of water plants or sediment.
Images
The following images are of a representative selection of ostracods aimed at giving a general overview of the different morphotypes. Each specimen is given a generic and, if possible, a species name followed by its age range, the site location from which the sample was obtained and its size in microns. LM (Light Microscope) SEM (Scanning Electron Microscope). Typical and selected marker species are illustrated from each main period of the geological column in which ostracods occur. Click on an image to view a larger version. Because Ostracods are such a diverse taxa they have been split into two groups: freshwater and marine forms.


Freshwater
Potamocypris sp.
?-Recent
Mooghaun Lough, County Clare, Eire
~600 microns right valve SEM
Cyclocypris sp.
?-Recent
Mooghaun Lough, County Clare, Eire
~500 microns left valve SEM
Cypria ophtalmica (Jurine)
?-Recent
Mooghaun Lough, County Clare, Eire
650 microns left valve SEM
Ilyocypris sp.
?-Recent
Mooghaun Lough, County Clare, Eire
800 microns right valve SEM
Metacypris cordata Brady and Robertson
?-Recent
Mooghaun Lough, County Clare, Eire
520 microns right valve SEM
Darwinula stevensoni Brady and Robertson
?-Recent
Mooghaun Lough, County Clare, Eire
780 microns right valve SEM
Cyclocypris ovum (Jurine)
?-Recent
Mooghaun Lough, County Clare, Eire
480 microns right valve SEM
Limnocythere sanctipatricii (Brady and Robertson)
?-Recent
Mooghaun Lough, County Clare, Eire
850 microns female left valve SEM
Ilyocypris sp.
?-Recent
Mooghaun Lough, County Clare, Eire
750 microns right valve SEM
Cyclocypris laevis Muller
?-Recent
Mooghaun Lough, County Clare, Eire
580 microns dorsal view carapace SEM
Pseudocandona rostrata (Brady and Norman)
?-Recent
Mooghaun Lough, County Clare, Eire
1000 microns left valve SEM
Pseudocandona rostrata (Brady and Norman)
?-Recent
Mooghaun Lough, County Clare, Eire
1000 microns left valve internal view SEM
Candona candida (Muller)
?-Recent
Mooghaun Lough, County Clare, Eire
1100 microns female right valve SEM
Herpetocypris sp.
?-Recent
Mooghaun Lough, County Clare, Eire
2000 microns female right valve SEM
Herpetocypris reptans (Baird)
?-Recent
Mooghaun Lough, County Clare, Eire
2000 microns internal view right valve SEM
Limnocythere inopinata (Baird)
?-Recent
Mooghaun Lough, County Clare, Eire
630 microns female left valve SEM
Limnocythere sanctipatricii (Brady and Robertson)
?-Recent
Mooghaun Lough, County Clare, Eire
840 microns female left valve SEM
Candona neglecta (Sars)
?-Recent
Merja Sidi Bou Rhaba, Northwestern Morocco
1300 microns SEM
Cyprideis torosa (Jones)
?-Recent
Merja Sidi Bou Rhaba, Northwestern Morocco
SEM
Cyprideis torosa (Jones) detail of sieve pores with setae
?-Recent
Merja Sidi Bou Rhaba, Northwestern Morocco
SEM
Potamocypris sp. (Brady)
?-Recent
Merja Sidi Bou Rhaba, Northwestern Morocco
SEM
Ilyocypris gibba (Ramdohr)
?-Recent
Merja Sidi Bou Rhaba, Northwestern Morocco
820 microns SEM
Marine
Heterocyprideis sorbyana (Jones)
?-Recent
Skagen Core, Jutland, Denmark
adult right valve SEM
Heterocyprideis sorbyana (Jones)
?-Recent
Skagen Core, Jutland, Denmark
adult right valve internal view SEM
Rabilimis mirabilis Brady
?-Recent
Skagen Core, Jutland, Denmark
adult left valve SEM
Cytheropteron pseudomontrosiense Whatley and Masson
?-Recent
Skagen Core, Jutland, Denmark
adult left valve internal view SEM
Cytheropteron pseudomontrosiense Whatley and Masson
?-Recent
Skagen Core, Jutland, Denmark
juvenile left valve SEM
Normanicythere leioderma (Norman)
?-Recent
Skagen Core, Jutland, Denmark
adult left valve SEM
Cluthia cluthae (Brady, Crosskey and Robertson)
?-Recent
Skagen Core, Jutland, Denmark
adult right valve SEM
Bairdia subdeltoidea (von Munster)
Lower Miocene-Pliocene
Pissouri, Cyprus, Eastern Mediterranean
adult right valve SEM
Cistacythereis pokornyi hellenica Uliczny
Pliocene-Recent
Pissouri, Cyprus, Eastern Mediterranean
adult left valve SEM
Australoecia poteroacuta Coles and Whately
Upper Eocene-Upper Oligocene
Pissouri, Cyprus, Eastern Mediterranean
right valve SEM
Eucytherura complexa (Brady)
Pliocene-Recent
Pissouri, Cyprus, Eastern Mediterranean
right valve SEM
Aurila punctata punctata (von Munster)
Late Miocene-Recent
Pissouri, Cyprus, Eastern Mediterranean
left valve SEM
Cythere texana Stadnichenko
?Eocene?
Pin Oak Creek, Bastrop County, Texas, USA
left valve (femle) SEM
Cythere texana Stadnichenko
?Eocene?
Pin Oak Creek, Bastrop County, Texas, USA
left valve internal view (male) SEM
Cytheridea (Clithrocytheridea) cf. oliveri (Cushman)
?Eocene?
Pin Oak Creek, Bastrop County, Texas, USA
left valve (female) SEM
Cytheridea (Clithrocytheridea) cf. oliveri (Cushman)
?Eocene?
Pin Oak Creek, Bastrop County, Texas, USA
left valve internal view (female) SEM
Semicytherura Sp.
Late Palaeocene
Itori Borehole, Eastern Dahomey Basin, Nigeria
560 microns (length) SEM
Paleocosta olurebei (Reyment)
Late Palaeocene
Araromi Borehole, Eastern Dahomey Basin, Nigeria
760 microns (length) SEM
Buntonia ioruba Reyment
Late Maastrchtian-Late Palaeocene
Araromi Borehole, Eastern Dahomey Basin, Nigeria
590 microns (length) right valve female SEM
Stigmatocythere teiskotensis (Apostolescu)
Palaeocene
Araromi Borehole, Eastern Dahomey Basin, Nigeria
810 microns (length) ventral view female SEM
Costa levigata
Palaeocene
Itori Borehole, Eastern Dahomey Basin, Nigeria
780 microns (length) left valve female SEM
Actinocythereis asanmamoi Reyment
Late Palaeocene
Gebekebo Borehole, Eastern Dahomey Basin, Nigeria
660 microns (length) left valve internal view female SEM
Brachycythere armata Reyment
Late Maastrichtian (latest Cretaceous)
Araromi Borehole, Eastern Dahomey Basin, Nigeria
810 microns (length) dorsal view SEM
Brachycythere armata Reyment
Late Maastrichtian (latest Cretaceous)
Araromi Borehole, Eastern Dahomey Basin, Nigeria
810 microns (length) ventral view SEM
Parascypris sp.
Late Maastrichtian (latest Cretaceous)
Araromi Borehole, Eastern Dahomey Basin, Nigeria
960 microns (length) ventral view SEM
Phacorhabdotus pokornyi Paulson
Late Cretaceous-Palaeocene
Eastern Gulf Coast, U.S.A
580 microns (length) left valve SEM
Cytherelloidea crafti Sexton
Late Cretaceous?
Eastern Gulf Coast, U.S.A
700 microns (length) left valve SEM
Bythocypris windhami Butler and Jones
Early Cretaceous-Recent?
Eastern Gulf Coast, U.S.A
920 microns (length) left valve SEM
Carinocythereis priddyi Smith
Cretaceous?
Eastern Gulf Coast, U.S.A
670 microns (length) left valve SEM
Cythereis dallasensis Alexander
Cretaceous?
Eastern Gulf Coast, U.S.A
840 microns (length) left valve SEM
Antibythocypris bipunctata
Maastrichtian?
Eastern Gulf Coast, U.S.A
360 microns (length) left valve SEM
Antibythocypris bipunctata
Maastrichtian?
Eastern Gulf Coast, U.S.A
360 microns (length) internal view right valve SEM
Cytheropteron gaudaloupense Crane
Cretaceous-Recent?
Eastern Gulf Coast, U.S.A
660 microns (length) left valve SEM
Cytheropteron furcalatum Alexander
Cretaceous-Recent?
Eastern Gulf Coast, U.S.A
680 microns (length) SEM
Brachycythere ledoforma porosa Crane
Cretaceous-Recent?
Eastern Gulf Coast, U.S.A
680 microns (length) SEM
Cytheropteron harrissi Skinner
Cretaceous-Recent?
Eastern Gulf Coast, U.S.A
690 microns (length) SEM
Ovocytheridea reniformis van den Bold
Cenomanian-Turonian (Cretaceous)
Nkalagu Borehaole, Eastern Dahomey Basin, Nigeria
1110 microns (length) SEM
Schuleridea kellawaysii
Bathonian-Callovian (Jurassic)
North Leigh Borehole, Oxfordshire, U.K
590 microns (length) internal view left valve SEM
Nophrecythere cruciata intermedia (Lutze)
Callovian (Jurassic)
Cumnor, Oxfordshire, U.K
690 microns (length) left valve SEM
Polycope plumhoffi Bate and Coleman
Hettangian-Bathonian (Jurassic)
Trunch Borehole, Norfolk, U.K
250 microns (length) SEM
Nanacythere minor Michelsen
Sinemurian (Jurassic)
De Lutte borehole, Eastern Netherlands
310 microns (length) left valve SEM
Gramannicythere bachi bachi (Gramann)
Sinemurian-Lower Pliensbachian (Jurassic)
Trunch Borehole, Norfolk, U.K
340 microns (length) right valve SEM
Gramannicythere bachi bachi (Gramann)
Sinemurian-Lower Pliensbachian (Jurassic)
Trunch Borehole, Norfolk, U.K
340 microns (length) internal view right valve SEM
Liasina lanceolata (Apostolescu)
Upper Sinemurian-Lower Toarcian (Jurassic)
De Lutte borehole, Eastern Netherlands
680 microns (length) right side SEM
Kinkelinella intermedia (Gramann)
Sinemurian (Jurassic)
Trunch Borehole, Norfolk, U.K
640 microns (length) left valve SEM
Kinkelinella intermedia (Gramann)
Sinemurian (Jurassic)
De Lutte borehole, Eastern Netherlands
450 microns (length) female carapace SEM
Rhombocythere penarthensis Anderson
Norian Rhaetian (Triassic)
Well 6608/11-1 Norweigen North Sea
740 microns (length) internal view right valve SEM
Rhombocythere penarthensis Anderson
Norian Rhaetian (Triassic)
Well 6608/11-1 Norweigen North Sea
740 microns (length) left valve SEM


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