Tracking the Course of Evolution

THE K-T EXTINCTION (cont.)

by Richard Cowen

NOTE: This is page 3 of a three-page document.

Environmental Sex Determination
Most catastrophic scenarios are so severe that it's difficult to see how some groups of animals survived. Many living reptiles have environmental sex determination (ESD). The sex of an individual with ESD is not determined genetically, but by the environmental temperatures experienced by the embryo during a critical stage in development. Often, but not universally, the sex that is larger as an adult develops in warmer temperatures. This pattern probably evolved because, other things being equal, warmer temperatures promote faster growth and therefore larger final size (at least for ectotherms). Female turtles are larger than males because they carry huge numbers of large eggs, so baby turtles tend to hatch out as females if the eggs develop in warm places and as males in cooler places. (This makes turtle farming difficult.) Crocodiles and lizards are just the reverse. Males are larger than females because there is strong competition between males, so eggs laid in warmer places tend to hatch out as males. ESD is not found in warm-blooded, egg-laying vertebrates (birds and monotreme mammals), and it didn't occur in dinosaurs if they too were warm-blooded.
ESD is found in such a wide variety of ectothermic reptiles today that it probably occurred also in their ancestors. If so, a very sudden change in global temperature should have caused a catastrophe among ectothermic reptiles at the K-T boundary. But it did not. Crocodilians and turtles were hardly affected at all by the K-T boundary events, and lizards were affected only mildly.

High-Latitude Dinosaurs
Late Cretaceous dinosaurs lived in very high latitudes north and south, in Alaska and in South Australia and Antarctica. These dinosaurs would have been well adapted to strong seasonal variation, including periods of darkness and very cool temperatures. An impact scenario would not easily account for the extinction of such animals at both poles.

Birds
The survival of birds is the strangest of all the K-T boundary events, if we are to accept the catastrophic scenarios. Smaller dinosaurs overlapped with larger birds in size and in ecological roles as terrestrial bipeds. How did birds survive while dinosaurs did not? Birds seek food in the open, by sight; they are small and warm-blooded, with high metabolic rates and small energy stores. Even a sudden storm or a slightly severe winter can cause high mortality among bird populations. Yet an impact scenario, according to its enthusiasts, includes "a nightmare of environmental disasters, including storms, tsunamis, cold and darkness, greenhouse warming, acid rains and global fires." There must be some explanation for the survival of birds, turtles, and crocodiles through any catastrophe of this scale, or else the catastrophe models are wrong.
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Where Are We?
It is clear that at least the extreme "impact winter" models are wrong. It's not clear that impact hypotheses or volcanic hypotheses can explain satisfactorily the extinction patterns we see in the fossil record. There are nagging fears that we are overstating the effects of the impact because the results are so clear in North America, close to the impact site.

OCEANOGRAPHIC CHANGE
An impact or a gigantic eruption that might otherwise have caused only a regional extinction might have caused the global K-T extinction by inducing longer-term climatic changes. These changes would be best recorded in ocean sediments and marine fossils. Tropical reef communities were drastically affected in the K-T extinctions, as were microplankton in the surface waters of the ocean. The pattern of marine K-T extinctions is consistent with a massive breakdown in normal marine ecology.
Oxygen isotope measurements across the K-T boundary suggest that oceanic temperatures fluctuated markedly in Late Cretaceous times and through the boundary events. Furthermore, carbon isotope measurements across the K-T boundary suggest that there were severe, rapid, and repeated fluctuations in oceanic productivity in the 3 m.y. before the final extinction, and that productivity and ocean circulation were suppressed for at least several tens of thousands of years just after the boundary, and perhaps for 1 or 2 m.y. afterward. These changes could have devastated terrestrial ecosystems as well as marine ones. Steve D'Hondt has suggested that climatic change is the connection between the impact and the extinction: the impact upset normal climate, with long-term effects that lasted much longer than the immediate and direct consequences of the impact.
There were survivors: hardly any major groups of organisms became entirely extinct. Even the dinosaurs survived in one sense (as birds). In particular, planktonic diatoms survived well, possibly because they have resting stages as part of their life cycle. They recovered as quickly as the land plants emerged from spores, seeds, roots, and rhizomes. The sudden interruption of the food chains on land and in the sea may well have been quite short, even if full recovery of the climate and full marine ecosystems took much longer. D'Hondt et al. suspect that normal surface productivity was re-established in the oceans after a few thousand years at most. However, it took about three million years for the full marine ecosystem to recover, probably because so many marine predators (crustaceans, molluscs, fishes, and marine reptiles) had disappeared, and had to be replaced by evolution among surviving relatives.
We still do not have an explanation for the demise of the victims of the K-T extinction, while so many other groups survived. We do not know whether it was the impact alone, or the combination of the impact and the plume volcanism, that caused the extinction, and we do not know the linkages between the physical events and the biological and ecological effects. It would be astonishing if the impact played no role, and it would be astonishing if the volcanism played no role.
The unusual severity of the K-T extinction, its global scope, and the sudden and dramatic biological features such as the fern-spore spike may have happened because an asteroid impact and a gigantic eruption occurred when global ecosystems were particularly vulnerable to a disturbance of oceanic stability. We will probably gain a better perspective on the K-T boundary as we gather more information about the Late Permian and Late Devonian extinctions. It looks increasingly probable that the Permian extinction was linked with a massive plume eruption, and this may mean that mass extinctions need either an external (impact) trigger or an internal (volcanic) one, and in addition they also require a tectonic or geographic setting that made the global ecosystem vulnerable.
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Tracking the Course of Evolution

THE K-T EXTINCTION (cont.)

by Richard Cowen

Further Reading