Keith Stewart Thomson
Auteur de Fossils: A Very Short Introduction
A propos de l'auteur
Œuvres de Keith Stewart Thomson
A Passion for Nature: Thomas Jefferson and Natural History (Monticello Monograph) (2008) 27 exemplaires
Private Doubt, Public Dilemma: Religion and Science since Jefferson and Darwin (2015) 19 exemplaires
Saltwater fishes of Connecticut 4 exemplaires
Étiqueté
Partage des connaissances
- Date de naissance
- 20th Century
- Sexe
- male
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Statistiques
- Œuvres
- 13
- Membres
- 551
- Popularité
- #45,290
- Évaluation
- 3.8
- Critiques
- 7
- ISBN
- 32
- Langues
- 1
As mentioned, Thomson’s discussion of the geographical range of coelacanths is now obsolete; at the time he was writing the only known population was in the Comoros (other than the initial discovery off South Africa). There have since been catches in Indonesia and various places on the east African coast. He raises the interesting question, though, which still applies – before Latimeria, the last known coelacanth fossils were from Cretaceous shallow fresh water deposits in Europe; the living species is from deep marine water in the Indian Ocean. What were coelacanths doing in the meantime? Thomson’s answer hinges on the fact that deep marine deposits are always rare in the stratigraphic record; there were probably deep marine coelacanths all along but they just never showed up as fossils. Thompson also critiques some of the speculations based on the Comoros specimens – most were caught between January and March at depths from 150 to 250 meters so it was assumed that was coelacanth habitat. As it turns out, this is when Comoro fishermen do their deep water fishing – it’s when the water is calm offshore – and since they fish with handlines, 150-250 meters is about all they can handle. Thus the fishing records really don’t say that much; they’re where people fish for coelacanths, not where coelacanths actually live.
The biology of the animal is fascinating; genetic studies have made the point that a coelacanth is more closely related – in terms of genetic distance – to a cow than it is to a salmon. It’s certainly a fish on the outside, but that’s a function of where it lives, not what it’s like inside. Coelacanths have a rostral organ – nobody is quite sure yet what it does but the best bet is it has to do with positioning. Submersible observations of coelacanths show positioning is a strong point; the animal swims with its lobe fins (including the little lobe in the middle of the tail) and can swim in any direction in any position – upside down and backwards if that’s what it needs to do. It has an intercranial joint – the skull is hinged in the middle – and that facilitates its hunting tactics, which are to approach a target (again, in any direction and orientation) and suck the prey in with a sudden gulp. It has a hollow, oil-filled notochord, which compensates for relatively small and weak vertebrae. And it has blood isotonic with sea water, a characteristic it shares with elasmobranch fish; coelacanths concentrate urea and trimethylamine oxide in their blood to a level that would be lethal to most vertebrates and use these to make their total osmotic blood concentrations the same as sea water. (Marine actinopterygian fish have blood osmotic concentrations less than sea water and dispose of excess salt through specialized glands in their gills; freshwater fish have mechanisms to concentrate salt and produce copious quantities of very dilute urine to get rid of excess water).
A light read despite some fairly technical discussion of fish physiology; Thomson is a good explainer. Badly needs a second edition, though, to account for all the coelacanth discoveries in the last 20 years. Well referenced; no photographs but good line drawings, graphs, and tables.… (plus d'informations)