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The KT extinction event is well-known as the demise of the dinosaurs. But today we have about 10000 bird, 11000 non-avian reptiles, and 5000 mammal species. That means almost 50% of reptile species are dinosaurs and mammals are a distant third, which is still pretty good! In fact, birds refilled empty niches left behind by the extinction event as did mammals and other reptiles. So was the asteroid really a dinosaur killer?

Consider the ratio of dinosaurs:reptiles:mammal species from the Jurassic to today. I see broadly thee possibilities:

  1. It was much more biased, such as 4:2:1, in the Jurassic era, with the asteroid evening things out.
  2. It was similar to today, but the asteroid initially hit dinosaurs exceptionally hard, leaving it as perhaps 0.25:2:1 after which bird speciation brought it back to 2:2:1. Note that birds are very deep within Dinosauria, with every other branch extinct.
  3. The asteroid didn't change the proportions very much, but wiped out almost every large species as most extinctions do. The pruning of the dinosaurs was more spectacular due to a small number of large species in the fossil record, but in terms of total species counts they were hit no (more) harder than other vertebrates.

(2) is the classic "dinosaur killer" we know and love, while (3) means we should instead think of the asteroid as killing off giants and leaving the meek to inherit the Earth. Is there reasonably strong evidence as to which scenario unfolded?

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    $\begingroup$ I think small fossils are relatively rare because they are more likely to degrade, be scavenged etc. I can't find any evidence, but at the time of the KT debate I don't know how many small species (say <2-3 kg) were known, so the demise might have seemed more dramatic. $\endgroup$
    – bob1
    Mar 2, 2022 at 1:07

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