Microinvertebrates

Ostracodes

Ostracodes are small crustaceans enclosed within a bean-shaped, two-part shell. They live in all aquatic environments, have a fossil record extending from the Cambrian, and are useful indicators of past salinity.

Morphology

Living ostracodes secrete a calcareous carapace formed of two slightly overlapping, ovate, hinged valves. Most ostracodes are less than 2 mm in length, although some Palaeozoic species reached 80 mm and a few living forms are 20-30 mm in length. Carapaces can be heavily calcified and ornamented with ribs, ridges, or tubercles or can be smooth and featureless.

The soft parts are completely contained within the valves. Ostracodes are simple crustaceans with a reduced number of appendages that are mainly used for locomotion (swimming and walking) and feeding (Figs 13.10 and 13.11). The valves are closed by adductor muscles, which leave scars on the interior surface (Fig. 13.12). Slender canals containing sensory bristles, or setae, perforate the carapace. Clear eye spots may also be developed on the carapace to enhance the ostracode's sensory perception. The most important sensory organs in living ostracodes are the sensory hairs that occur on the appendages and the valves. Reproductive organs occupy a large proportion of the internal volume, and sexual dimorphism is common. Males usually have a greater length: height ratio. Some Palaeozoic females had a brood pouch that

Fig. 13.12 The inner face of the valve of a female freshwater ostracode.

Second antennae: Eye: large in Muscles: used

Second antennae: Eye: large in Muscles: used

forms and for stability in used for feeding used for locomotion benthic ostracodes and cleaning forms and for stability in used for feeding used for locomotion benthic ostracodes and cleaning

Male ostracode Female ostracode

Male ostracode Female ostracode

formed a distinctive swelling on the external surface of the carapace (Fig. 13.13). The eggs of living ostracodes are usually shed freely rather than being brooded by the female. Some freshwater species produce eggs that are very resistant to dessi-cation. Some living ostracodes use bioluminescence to attract mates. Low flashes of bluish light are produced by external secretions. Males of the reef-dwelling genus Vargula are able to synchronize their flashes, resulting in a dramatic display.

Ecology/paleoecology

Ostracodes can be benthic or pelagic. Pelagic ostracodes are found only in the marine environment whereas as benthic ostracodes live in marine or fresh water. Carapace structure, shape, and sculpture varies according to substrate type, salinity, temperature, and depth. Therefore, ostracodes are useful indicators of paleoenvironment.

Freshwater ostracodes tend to have thin, ovate, featureless carapaces. Marine benthic ostracodes are more heavily calcified with certain features that can be related to the nature of the substrate. For example, ostracodes that crawl on soft, fine-grained sediment can have a flattened ventral surface sometimes with lateral projections to distribute the weight of the carapace.

Ostracodes are one of the most abundant groups of animals in brackish waters and are very useful in paleoenvironmental reconstructions, particularly in the Quaternary. Four groups of ostracodes have been identified in brackish water environments based on their salinity tolerance:

1 Holeuryhaline - species that freely inhabit fresh water, sea water and brackish water.

2 Marine-brackish species.

3 True brackish species.

4 Brackish-freshwater species.

Salinity is the main factor controlling the distribution of ostra-codes. Ostracodes are affected both by the concentration and the variability of the salinity. Cyprideis is an important species in the identification of brackish water sediments. The occurrence of an indicative species, the proportion of ostracodes in the entire assemblage, and comparisons of the proportions of marine, freshwater and brackish species in an assemblage provide more detailed information regarding changes in the environment through time.

The shape of specialized pore canals in ostracodes is known to change in response to salinity levels. As salinity increases, the outline of the canal changes from round to oval to irregular. Analysis of the proportions of the different pore canal shapes has been used to reconstruct Quaternary marginal marine environments. Size and carapace structure can also be used to infer salinity. Marine and freshwater species tend to be smaller in brackish water. In conditions of raised salinity, carapaces tend to be more heavily calcified and show an increase in ornamentation, although in extreme environments (e.g., hypersaline) ostracodes tend to be thin shelled.

Sea level variation in marginal marine environments can be documented using ostracodes. Estuarine and brackish water assemblages are often used in the reconstruction of Pleistocene and Holocene sea level variation. Ostracodes have been used to identify submerged shorelines and differentiate between habitats associate with marginal marine environments.

Evolutionary history

The oldest ostracodes are reported from the early Cambrian, although the group was short lived. New forms radiated in the Ordovician, continuing through the rest of the Palaeozoic. The main groups became extinct at the end of the Permian but a further radiation occurred in the Jurassic and ostracodes persist to the present day.

Orsten fossils

In some areas, notably around Sweden and other Baltic states, limestone nodules are found in bituminous alum shales (rich in heavy hydrocarbons and phosphate). These unusual rocks probably formed on sea beds with limited or absent oxygen, and often record mass kills of animals that lived on or near the sea floor. The hydrocarbons come from incompletely decayed tissue, and the phosphate from the molecules in mitochondria that power cells, ATP and ADP (adenosine tri- and diphosphate).

The most common Orsten deposits are of Cambrian age, but they range up to the early Cretacous in their stratigraphic distribution. Dissolution of these nodules leaves a residue of exceptionally preserved microfossils. Only cuticle-bearing animals are preserved, with none greater than 2 mm in maximum dimension. Phosphate-rich fluids encrusted or impregnated the cuticle, leaving a 3-dimensional though usually hollow, fossil.

Orsten fossils are highly biased, coming mainly from arthropod groups, but they preserve, with extreme fidelity, the external form of embryos, larvae and a range of different molt stages for these species. Orsten fossils have thrown light on the function and evolution of arthropods, including trilobites and ostracodes. In Cambrian examples, stem-group arthropods have been recovered, which may show what the ancestors of all modern arthropods looked like. Intriguingly, even at this early stage in the Phanerozoic, arthropods have a high diversity, at a high taxonomic level. This is taken by some authors to suggest that the origin of the phylum lies well into the Proterozoic, consistent with molecular data, but at odds with the current fossil record from that time period (see p. 136).

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