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Unanswered Question about Cretaceous–Tertiary Extinction Event: 2

June 29, 2019 1 comment

In the previous part of this series, I wrote about how our current understanding of K-T extinction event and its immediate aftermath is rather incomplete. For starters, the most popular theory about what caused it is highly inadequate. We also do not understand why certain vertebrates such as “modern” birds aka Neornithes survived, but other sibling groups such as Hesperornithes, Ichthyornithes and Enantiornithes became extinct. This pattern of disappearance is especially odd since Enantiornithes were significantly more successful prior to the K-T event. Moreover, they were almost identical in general appearance, size range, flight characteristics and likely even possessed similar patterns of coloration and markings to birds that are still around.

Yes.. some had teeth within their beaks and many possessed vestigial claws within their wings. But otherwise, they were functionally identical to modern birds and their global distribution in the late-cretaceous strongly suggests that some could fly across oceans. They also occupied a far more diverse range of habitats than the ancestors of modern birds. So how did a group which was significantly more numerous, occupied a larger range of habitats and likely better at flying (at least then) become extinct while the less numerous group occupying a narrower range of habitats and not better at flying survive? This becomes even more relevant once you understand that the most popular “explanation” for who survived the K-T extinction revolves around adult body size, with those below 10-20 kg being more likely to survive than larger ones (YT video).

Since Neornithes and Enantiornithes had a very similar range of body sizes, something else was at work. And it gets better. Consider groups such as Ichthyornithes and Hesperornithes whose members were basically the late-cretaceous versions of seagulls and diving birds. How do pretty successful aquatic bird-like creatures become extinct when that extinction was more devastating to species living on land? Members of these two groups had access to a better, if still diminished, supply of food than their land-dwelling cousins. And as the discovery of the Qinornis fossil in early Paleocene shows, some non-Neornithes birds did survive. Are you starting to appreciate the inadequacy of currently explanations for why certain groups survived, while other didn’t?

Let us move on to the oceans, or more precisely who survived and who didn’t. While Mosasaurs disappeared after K-T extinction, others such as crocodiles, turtles and sharks survived. While some of you might believe that every Mosasaur species in the late Cretaceous was a 40-60 feet long beast, they came in a range of sizes and some such as Carinodens were about the size of alligators. Moreover, they occupied a range of ecological niches, had global distribution and some displayed specialized dentition. To out it another way, they were more numerous and diverse than crocodilians and were successful in many more environments ranging from the deep-water to shallow coastal areas and estuaries.And yet, it is the crocodilians (well.. some of them) who survived while all Mosasaur species went extinct.

And why did marine Turtles and some shark species make it through the K-T extinction? Why were bony fish the least affected by that extinction? Why did nautiloids, octopodes, squids and cuttlefish survive while the equally numerous belemnoids and ammonoids became extinct? Why did Choristodera (e.g Champsosaurus) survive the K-T extinction only to become extinct in the middle-to-late Eocene? If the direct and indirect effects of that comet/asteroid impact caused so much immediate worldwide damage to terrestrial plants (including flowering plants) how did bees survive? Things don’t look so straightforward and easy to explain now, do they? My point is that there are patterns in what survived and didn’t, but let us stop pretending they coalesce into a coherent theory to explain whatever happened at the time.

And why did most mammalian lineages survive the K-T event, but weren’t able to quickly occupy the niches left behind by all those extinct large animals? Why did it take almost 10-15 million years for mammals to finally reach the size we today associate with deer and pigs. And why were the large flightless birds such as Phorusrhacidae and Bathornithidae among the largest terrestrial carnivores in aftermath of that extinction. What accounts for the absence of large terrestrial mammalian predators in the first 15 million years after the K-T event? And ya.. I have a partial explanation for that, which I might write up in a future post. This become more unusual once you realize that the Earth was densely forested, very green and climatically pleasant place to live for about 15 million years after quickly recovering (within a million years) from the K-T extinction.

What do you think? Comments?