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The Devil’s Tail: What the Fossils say


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After such an impact, in accordance with the general laws of nature, the following phenomena will occur: cloudy skies, then rain, high storms and hurricanes, the lakes and rivers will overflow their margins, the ocean will flood from the east to the west the lowlands, later also the mountains…[]
The complete destruction of all creatures living on land will be the consequence; and when after thousand of years the water will be evaporated into the ether, new animal species will have made multiple attempts to colonize the land again and without doubt after thousands of years some of these attempts will be successful.
Franz von Paula Gruithuisen (1774-1852) ” Ueber die Natur der Kometen … ” (1811)

Fig.1. Caricature in a pamphlet by A. Lund about the 1857 comet that supposedly should destroy earth June, 13 of the very same year (in the end the comet decided not even to appear, image in public domain).

A single cause – the impact of the Chicxulub (according to some interpretations meaning “the devil’s tail“) meteorite – for the faunal turnover at the end of the Cretaceous should explain all the observed changes in the fossil record. According to the most popular scenario the mass extinction was caused mainly by the consequences of the impact: the shock wave and fire storms were soon followed by the release of large quantities of gas from the vaporized rocks, rich in carbonates and sulphates. The gases reacted with water vapour to form acid rain and the dust in the atmosphere blocked solar irradiance, causing a “nuclear winter” and inhibiting the photosynthetic activity of land plants and especially the phytoplankton in the oceans. Without plants, soon the entire food chain collapsed.
A number of major organisms groups disappeared, like nonavian dinosaurs, marine and flying reptiles, ammonites, the large reef-building bivalves or rudists, but also mammals and modern avian dinosaurs suffered drastic losses. In contrast other large terrestrial vertebrates like reptiles and especially amphibians, considered vulnerable to chemical or thermal change of the environment, suffered few losses. Also insects, widespread organisms now and then, show almost no changes. Land plants show mixed results; some research suggested that after the destruction of the vegetation cover by fire and inhibited photosynthetic activity soon ferns proliferated in the devastated landscape. However this increase of fern spores in the sediments, also referred as fern-spike, is know only from some sites in North America and one location in New Zealand, it is not (yet?) found in sediments of Eurasia.
In the marine realm the calcareous nannoplankton (as Coccolithophorids) and planktonic foraminifera (with calcareous shells) diminished significantly, however benthic foraminifera, diatoms, dinoflagellates and radiolarians (with silica shells) show almost no response. Brachiopods show a pronounced decline of diversity, however molluscs (excluding the rudists) like bivalves and gastropods show a weak decline over time.
The disappearance of organisms with calcareous shells was considered a result of acid rain and modified chemistry of oceans and lakes. However this scenario is in conflict with the survival of vulnerable animals like amphibians. Also many bivalves, gastropods and probably echinoderms (the fossil record of this group is still poorly studied) – organisms protected by calcareous shells or plates – show no significantly changes.
According to the impact-hypothesis the phototrophic phytoplankton survived the diminished solar irradiance and drop in temperatures by forming resting spores, however radiolarians thrived during the Cretaceous-Palaeogene boundary and those organisms don’t produce spores!
The record of fossil fishes shows that almost 90% of the families survived, this is hard to explain if we assume that the base of the food chain, the phytoplankton, was heavily reduced by the consequences of the impact. Strangely marine apex predators, like sharks and large reptiles, experienced a major to total collapse.

Considering only some groups, like planktonic foraminifers, nannoplankton, brachiopods, ammonites and large marine reptiles, their decline could be explained by superficial effects of the impact on the oceans of earth. However other organisms, like radiolarians, most molluscs, echinoderms and fishes, thrived in the same moment and the same environment. The same pattern is observed in the terrestrial realm, some groups suffered heavy losses, some show a decline but survived into the Palaeogene and some show no changes at all.
In summary the response of organisms at the end of the Cretaceous show a mixed pattern, hard to explain with only one cause or the generalized scenario of an impact.
A major problem is to determinate in which time period all these various groups experienced decline or complete extinction. Locations with a complete stratigraphic transition of the Cretaceous-Palaeogene boundary are rare and most studied sites (especially terrestrial deposits) are located within North America, supposedly most affected by the nearby impact.  The available worldwide profiles are still to sparse and fragmentary to decide if the meteorite was the only cause for a worldwide and sudden mass extinction. However there is possibly also another example, less prominent than the devil’s tail, to reevaluate the significance of past impacts on earth’s biosphere (to be continued…).

Bibliography:

ARCHIBALD, J.D. (1996): Dinosaur Extinction and the End of an Era: What the Fossils Say Critical Moments in Paleobiology and Earth History Series. Columbia University Press: 240
PROTHERO, D. (2006): After the Dinosaurs: The Age of Mammals (Life of the Past). Indiana University Press: 384

David Bressan About the Author: Freelance geologist dealing with quaternary outcrops interested in the history and the development of geological concepts through time. Follow on Twitter @David_Bressan.

The views expressed are those of the author and are not necessarily those of Scientific American.



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