Harrow and Hillingdon Geological Society
What is a microfossil?
The official definition is the remains of an organism that requires a microscope to study it. It is generally taken to include foraminifera, ostracoda, radiolarian and conodonts but not juveniles or parts of larger animals. It also includes microplankton, coccoliths, spores and pollen. The speaker’s definition was based on all the contents of a processed sample that needs to be examined under the microscope being microfossils. The size range is essentially from 1.4mm (the size of specimen that fits on standard micropalaeontological slides) down to 45μm (generally the lower limit that can be studied using a x40 microscope; below that limit needs a specialised microscope).
Coccoliths are very small – about 10μm across – and require a very specialist microscope. Preparation of pollen, microplankton and spore samples requires the stripping of carbonates with hydrochloric acid, then using fuming hydrofluoric acid in a fume chamber to dissolve silica and silicates. Fuming nitric acid dissolves some of the remaining organic material but does not affect pollen, spores or dinoflagellate cysts. This equipment and process is not within the range accessible to most amateur geologists and the study of these fossils is palynology, rather than micropalaeontology as can be practised by the amateur.
Foraminifera are single-celled animals in the phylum Protista with a chambered shell of calcium carbonate or mineral grains. Hormosinella cements sediment into its shell in what is known as an agglutinated shell. Foraminifera evolved quite quickly and they are most useful in the Tertiary. They are widely distributed and they are the most used economically because drill cuttings are useable samples to identify stratigraphical horizons.
Ostracods are crustaceans, mostly small and with bivalve shells. Most are from 2mm down to about 100μm. They are generally not much use for stratigraphy except when the normal range of microfossils is not available, eg in freshwater deposits, such as the Wealden or Purbeck Beds, where the almost exclusively marine foraminifera are absent. Their main use is in the study of palaeoenvironments since they can be used to determine palaosalinity and palaeotemperature.
These are single-celled organisms with a silica shell, usually less than 50μm.
Conodonts are tiny worm-like protochordates, which are largely soft-bodied. The phosphatic conodonts are found as isolated bits of these animals in the Palaeozoic up to the Rhaetic and they are very useful for zoning.
Echinoderms are generally not considered as microfossils, but many of them such as ophiuroids (brittle stars), crinoids, asteroids and starfish have shells made up of tiny plates, which often separate at death. For example, a brittle star 4cm across has about 5,000 plates in its shell. Brittle stars tend to turn up in large numbers in starfish beds with apparently barren material between – because the shells have broken up and preservation of entire shells is relatively rare. In the Lower Lias, the dominant items in the microfossil residue are brittle star plates.
Holothuroids (sea cucumbers) have no hard skeleton but their leathery skin does contain calcium carbonate sclerites 200μm down to about 40μm in size. The Oxford Clay is a good source of these.
Worms are soft-bodied and fossilisation is rare, though occasionally they are found in nodules. Burrows are often attributed to works but their may not be any direct evidence of what formed the burrow. However, several groups of marine worms are active predators with chitinous jaws. Glycera extends back to the Triassic and they have two jaws with a channel down which venom is injected. Scale-worms decay rapidly. Ragworm jaws are thickenings in the gut lining and the animal everts its gut to use them. There is no fossil record of ragworms – the 2 recorded are misidentifications.
Earthworms feed on decaying vegetation, not on soil but soil is taken in and excreted as a means of burrowing. In calcareous soils they absorb the calcium carbonate, which is toxic to them, and this is ejected as crystalline granules, which are common in Quaternary deposits. Round-backed slugs have a shell under the mantle which is formed of granules and can look similar to earthworm granules but most of the examples in the Quaternary are worm granules.
Squids arms have chitinous hooks, which are quite common in the Jurassic (eg Belemnoteuthis). They are similar to worm jaws but have no cavity running in from the outside. Squids also have a cartilage skull with ear stones (statoliths). Octopus ear stones are never found because they are chalky and not hard and discrete and so they break up. Jurassic examples of squid ear stones were first described as otoliths (fish ear stones), largely because their preservation tends not to be of high quality.
Fish ears contain an aragonite crystal structure attached to nerves, which seems to be related to balance and position but is also related to hearing in the more advanced fish. Vast numbers of fossil fish species are known only from their ear stones (otoliths). Fish evolved a lot towards the end of the Cretaceous and modern fish are very different from those before this period. Jurassic otoliths are therefore difficult to associate with specific fish.
There are quite a lot of species of small fossil bivalve gastropods in the French Eocene, which fed on seaweed.
The opossum shrimp is 12mm long and the balance organ in the tail contains a statolith, generally less than 0.1mm. They were first described as fossils from the Middle Eocene of North America and they can be found in the Barton Beds at Barton. They are used in stratigraphy in a layer in the Miocene in Eastern Europe.
Sponge spicules are formed of spongin, which is proteinaceous, calcium carbonate, tiny spicules which are not preserved, or silica.
Soft corals (octocorals) spicules are all calcium carbonate and there are not many fossil records. They are not easy to recognise.
Sea squirts are primitive chordates with calcium carbonate spicules.
Plant microfossils include calcareous algae from the Middle Eocene, stonewort gyrogonites up to 2mm across from the Wealden and Purbeck Beds and endocarps from aggregates of drupelets such as blackberries.
There are some microfossils which no-one knows what they are, though the suspicion is that they may be algae.
The final example of unusual microfossils was illustrated by a specimen from the fossil forest at Lulworth Cove in Dorset, which comprised salt pseudomorphs with the infilling matrix composed of faecal pellets of isopods.