Wat zijn otolieten?

Otoliths are the ear-bones of fishes. As fossils they are only known in the subclass Teleostei (class Osteichthyes). Originating from the Upper-Jura the Teleostei develop quickly in the Cretaceous. From Upper-Cretaceous to recent times they are the main constituent of the fish-fauna. Nowadays they have reached the height of their development. Otoliths are the best remains to base systematics of fossil Teleostei on and also for quantitative analyses, mainly because of the fact that other remains are much more seldom found. The balancing-organ consists of three bended canals, each discharging by means of a widening (ampulla) into the utriculus. Under the utriculus lies the sacculus, which has a recess (lagena). Utriculus, sacculus and lagena all contain an earbone, called respectively asteriscus, sagitta and lapillus (see fig. 1). Almost only the sagitta is found as a fossil, being ordinarily the larger one. By means of the otoliths the fish can determine its linear acceleration. The outline of an otolith is in most cases more or less circular or oval, sometimes with protuberances. The margins are smooth, knotted or dentate. The less recognizable outside can bear tubercles which can be strongly marked or weakly developed. The inside is the most important. The so-called sulcus acusticus (see fig. 2) is very characteristic. The outline is also important for determination. Sometimes there can be difficulties because of the fact that the limits of variability are not satisfactorily known. About the way of life of tertiairy fishes we can get much information by studying the recent representatives of the same group. Of course great care must be taken when making comparisons. A separation of the habitat is possible after: 1. way of life: pelagic (necton or plancton) and benthonic 2. sea-depth: litoral, sublitoral, bathyal and abyssal 3. climate: tropic, subtropic, temperate, boreal and arctic. In one sample usually the remains are found of fishes from several habitats (thanatocoenosis or dead community). This contains autochthon and allochthon elements. The original habitat can usually be found by means of the thanatocoenosis. Otoliths can have great stratigraphical importance in the Kenozoicum for fishes are mainly pelagic and therefor have a wide horizontal dispersion. Moreover their evolutionspeed is rather high, so that they have a small vertical range. In consequence of their small size they are easily obtainable from bore-samples.