Ostracods - Minia

Ostracods

By

Omar Mohamed

Introduction:

Ostracods are by far the most complexorganisms studied within the field ofmicropalaeontology. They are Metazoa andbelong to the Phylum Arthropoda (as trilobites),Class Crustacea (as lobsters and crabs). Animportant distinguishing feature Ostracodsshare with other arthropods is the bilateralsymmetry of their body form. The paired bodyparts are enclosed in a dorsally hinged carapacecomposed of low magnesium calcite, which iswhat is commonly preserved in the fossil record.They are found today in almost all aquaticenvironments including hot springs, caves,within the water table, semi-terrestrialenvironments, in both fresh and marine waters,within the water column as well as on (and in)the substrate. In fact almost anywhere that'swet, even if only for a brief period!

• History of Study The oldest generic names given to ostracods are Cyprisand Cythere by Muller in the 1770's and 80's, these are now commonlyused as suffixes and prefixes in ostracod nomenclature. In the 1860's Sarsclassified ostracods as an order divided into four suborders, Podocopa,Myodocopa, Cladocopa and Platycopa. In the early 20th Century workersin the Appalachians in the U.S.A classified Palaeozoic ostracods. In 1958Pokorny combined these two classifications and in 1961 an AngloAmerican treatise modified Pokorny's work to give the foundation oftoday's classification system. It was not until Pokorny's work that the fossiland living classifications were linked.

Range

Ostracod-like organisms (bivalved arthropods) are recorded from theCambrian, but it is uncertain whether these can be classified as trueostracods. Myodocopid and podocopid forms are recorded from theOrdovician. All these early forms are marine, the first freshwater forms(Darwinulacea and Carbonita) occur in the Carboniferous and by theJurassic ostracods are common in freshwater environments. Between theSilurian/Devonian and the present there are big gaps in the fossil record ofplanktonic marine forms, which is thought to reflect weak calcification ofthe carapace.

Classification

Kingdom ANIMALIA

Phylum CRUSTACEA

Class OSTRACODA

The Class Ostracoda is separated from other Crustacea by their laterallycompressed body, undifferentiated head, seven or less thoracic limbs andthe bivalved, perforate carapace lacking growth lines. The living ostracodsare further classified in many cases by variations in their appendages andother soft parts. Although exceptionally well preserved fossil ostracodswith the soft parts intact have been found these are very rare andtherefore the morphological features (see below) of the carapace havebecome vital in fossil ostracod classification. The Ostracoda have beendivided into five Orders, the extant Podocopida and Myodocopida and theextinct Phosphatocopida, Leperditicopida and Palaeocopida (however, thelatter 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 tospecialised marine environments for example in the Jurassic of the North Sea.In the marine environment benthic ostracods are utilised forpalaeoenvironmental reconstructions. Freshwater and brackish faciescommonly contain abundant ostracods which are used for environmentalstudies and for biostratigraphic zonations, for instance in non-marinesediments from Mongolia and China.

• Biology

• 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.

◆Entomodont

• 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). Ostracodsshow 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 Techniques

• 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.

◆Freshwater Ostracods

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

Darwinula stevensoni Brady and Robertson

?-Recent

Mooghaun Lough, County Clare, Eire

780 microns right valve SEM

Metacypris cordata Brady and Robertson

?-Recent

Mooghaun Lough, County Clare, Eire

520 microns right valve SEM

Pseudocandona rostrata (Brady and Norman)

?-Recent

Mooghaun Lough, County Clare, Eire

1000 microns left valve internal view SEM

Limnocythere sanctipatricii (Brady and

Robertson)

?-Recent

Mooghaun Lough, County Clare, Eire

840 microns female left valve SEM

◆Marine Ostracods

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

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

Stigmatocythere teiskotensis (Apostolescu)

Palaeocene

Araromi Borehole, Eastern Dahomey Basin,

Nigeria

810 microns (length) ventral view female SEM

Actinocythereis asanmamoi Reyment

Late Palaeocene

Gebekebo Borehole, Eastern Dahomey Basin,

Nigeria

660 microns (length) left valve internal view

female SEMCarinocythereis priddyi Smith

Cretaceous?

Eastern Gulf Coast, U.S.A

670 microns (length) left valve SEM