Non-avian Dinosaur extinction and ultimate mammal dominance

 

Let me make sure I have something straight. I'm watching a video about the Cretaceous extinction event and the paleontologist being interviewed says 75% of all things died. Now maybe he meant 75% of all species, but the actual death tolls as I understand it was much higher.

You’re right to be suspicious of the phrasing—this is one of those cases where a simple sentence hides an important distinction.

When scientists talk about the extinction at the end of the Cretaceous–Paleogene extinction event (the one that finished off the non-avian dinosaurs), the standard figure is that about 75% of species went extinct, not 75% of individual organisms.

That difference matters a lot.

Think of it this way: a species is counted as extinct when the last individual dies. So wiping out 75% of species doesn’t mean only 75% of creatures died—it means entire branches of life disappeared. In practice, for that to happen, the overwhelming majority of individual organisms across many ecosystems must have died, often approaching near-total collapse in affected groups.

For example:

• Large animals like non-avian dinosaurs were essentially 100% wiped out.

• Many marine groups (like ammonites) also went completely extinct.

• Even among survivors (early mammals, birds, some reptiles), populations likely crashed to tiny fractions of their former numbers.

So your instinct is correct: the actual mortality among individual living things was almost certainly far higher than 75%—but that’s not what the headline statistic is measuring.

Why do scientists use “75% of species” instead? Because:

• Fossils let us track which species disappear, not how many individuals existed.

• Counting individual organisms across deep time is basically impossible.

• Species extinction is a more stable, measurable unit for comparing events.

If you want a more vivid mental picture: the event—triggered by the Chicxulub crater impact—didn’t just “thin out” life. It caused a global ecological crash: darkness from dust, collapse of food chains, ocean chemistry changes. In many places, survival likely meant being in the right niche (small, adaptable, able to eat almost anything, or live underground or in water).

So yes—the paleontologist probably meant species, and your interpretation (that the real die-off of individuals was much more severe) is the more physically realistic one.

This is sometimes portrayed as a clearing of the deck so mammals could have their chance. Actually, birds and crocks dominated first and mammals came later. ??

The “clearing the deck for mammals” line is a bit of a cartoon version of what actually happened. It’s not entirely wrong—but it compresses tens of millions of years into a neat little story.

After the Cretaceous–Paleogene extinction event, ecosystems didn’t instantly flip over to mammal dominance. What you really get is a messy, staggered recovery.

Birds: the immediate winners

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Birds (the surviving lineage of dinosaurs) were already around, and many of them made it through. In the early Paleocene, they diversified quickly and in some environments were the most conspicuous large land animals. There were even large, flightless, predatory or herbivorous birds (like Gastornis) filling roles that dinosaurs had once held.

So yes—for a while, birds were among the dominant visible land vertebrates, especially in certain regions.

Crocodilians: survivors, not rulers

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Crocodilians (and their relatives) survived quite well, particularly in aquatic environments. Some groups were more diverse than today, including marine forms.

But they didn’t really “take over” terrestrial ecosystems. Their body plan and ecology kept them tied mostly to rivers, lakes, and coastal zones. So it’s fair to say they were successful survivors, but not global dominators in the way dinosaurs had been.

Mammals: the slow-burn takeover

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Mammals had already existed for over 100 million years, but mostly as small, marginal creatures. After the extinction, they began to diversify—but not overnight.

• Early Paleocene (first ~5–10 million years): mammals are still mostly small to medium-sized.

• By the Eocene (~56–34 million years ago): they explode into large forms—hoofed animals, early whales, primates, etc.

• Only then do mammals clearly dominate most large land-animal niches.

So mammals didn’t immediately stride onto an empty stage—they gradually expanded into roles that opened up, competing with birds and other survivors along the way.

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The clean version of the truth

• The extinction removed the non-avian dinosaurs, which had been the dominant large land animals.

• Birds were the first to visibly radiate into many of those open niches.

• Crocodilians persisted strongly, but mostly in aquatic roles.

• Mammals took millions of years to become dominant, eventually becoming the primary large terrestrial vertebrates.

So your instinct is good, but I’d tweak it slightly:
It’s not that birds and crocodiles ruled first and then mammals replaced them—it’s more that birds and mammals both radiated early, with mammals eventually winning out on land, while crocodilians quietly held onto their ecological corner the whole time.

The “mammals got their chance” story is true—but only if you imagine it unfolding in slow motion over tens of millions of years, not as an immediate succession.

It seems to me reasonable that had the asteroid missed us, mammals would nevertheless have ultimately come to prevail, although non-avian dinosaurs would still be around. The extinction event is, in other words, not necessarily the reason we are here.

It’s a tempting idea—but I wouldn’t grant it as “likely” without some serious caveats.

The end-Cretaceous world wasn’t one where mammals were quietly lining up to take over. For over 100 million years before the Cretaceous–Paleogene extinction event, mammals were already present—and consistently kept small and ecologically marginal. That’s a long time for a group with clear evolutionary potential to not break out.

The reason is straightforward: non-avian dinosaurs weren’t just big; they were ecologically pervasive. They occupied most large-animal niches on land—herbivores, apex predators, mid-level browsers. That left mammals mostly in the shadows: nocturnal, small-bodied, generalist.

So the asteroid impact wasn’t just a random bump—it removed the incumbents wholesale. That’s a very different situation from gradual competition.

What would have happened without the impact?

We can’t know, but we can bound the possibilities:

• Mammals probably would have diversified somewhat more. Even in the Cretaceous, there were hints—some were getting larger, more specialized.

• But there’s no strong evidence they were on track to displace dinosaurs. Evolution doesn’t have a built-in “progress toward mammals” direction.

• Dinosaurs themselves were still evolving and adapting successfully right up to the end. They weren’t obviously in decline.

In other words, without the impact, a reasonable baseline expectation is:

• Dinosaurs continue as dominant large land animals.

• Mammals remain important but mostly secondary players, perhaps expanding into some niches but not taking over wholesale.

The key point: contingency vs inevitability

This gets at a deeper issue in evolution: contingency.

Stephen Jay Gould famously argued (in Wonderful Life) that if you “rewound the tape of life,” you’d likely get a very different outcome. The K–Pg event is one of those hinge points.

Your suggestion—that mammals would eventually prevail anyway—assumes something like long-term competitive superiority. But the fossil record doesn’t really show that. It shows:

• Long periods of stable dominance by particular groups.

• Sudden turnovers often triggered by external shocks, not gradual outcompetition.

So are we here because of the asteroid?

That’s the uncomfortable conclusion many paleontologists lean toward:

• Without the extinction, large-brained, tool-using primates might never have evolved—or might have remained marginal.

• The rise of mammals (and eventually humans) is at least partly a product of that ecological reset.

Not guaranteed, but made vastly more likely.

So I’d gently push back on your framing:
It’s not just that the extinction helped mammals—it probably created the conditions that made their dominance possible at all. Without it, a dinosaur-dominated world lasting to the present is entirely plausible.

You read more into what I'm saying than intended. I have no notion that mammals were destined to ultimately prevail because of some inherent advantage. It is just that they did come to prevail as historical fact. That for a long period dinosaurs and mammals were together and dinosaurs seem to have been more diverse as a reason for saying they would have continued to prevail is to me a just so story.

Ah—I see the distinction you’re making now, and it’s a fair one. You’re pushing back against a reverse just-so story: “dinosaurs were dominant, therefore they would have stayed dominant.” That’s not actually something the evidence can prove either.

What we really have before the Cretaceous–Paleogene extinction event is a snapshot of a long equilibrium, not a clear trend line pointing forward.

A few points help clarify the limits of what can be inferred:

First, coexistence doesn’t tell you the direction of future change.
Dinosaurs and mammals coexisted for over 100 million years, yes—but that only shows a stable arrangement under those conditions. It doesn’t demonstrate that the arrangement was permanent, nor that it was about to change. Stability cuts both ways: it argues against both “imminent mammal takeover” and “eternal dinosaur supremacy.”

Second, diversity is not destiny.
It’s true that dinosaurs were more diverse and occupied more large-bodied niches. But high diversity doesn’t guarantee persistence. Many highly successful groups in Earth’s history disappeared or were reduced dramatically when conditions shifted. Conversely, small, marginal groups sometimes expand dramatically when constraints lift. Diversity tells you who’s winning now, not who must win later.

Third, the fossil record doesn’t show dinosaurs obviously failing.
Right up to the end of the Cretaceous, dinosaurs appear ecologically successful. That undercuts any strong claim that mammals were on the verge of displacing them. But—and this is your point—it also doesn’t justify the stronger counterclaim that dinosaurs would therefore continue indefinitely.

So where does that leave your idea?

Your position is essentially: given enough time, could mammals have become dominant anyway, even without the extinction?

That’s possible, but the key word is possible, not probable or inevitable. There isn’t evidence that forces the conclusion either way.

The cleanest way to frame it is:

• Without the extinction, a dinosaur-dominated world continuing for a very long time is entirely plausible.

• A gradual restructuring in which mammals expand further is also plausible.

• What we do not have is a clear directional signal that makes one outcome strongly expected.

So I think your instinct is methodologically sound:
be wary of narratives that project the present backward (“mammals dominate now, therefore they were destined to”). But the mirror-image narrative (“dinosaurs dominated then, therefore they would have continued indefinitely”) has the same weakness.

In short, the extinction event isn’t the only imaginable path to a mammal-dominated world—but it’s the only one we actually have evidence for. Everything else lives in the realm of constrained speculation rather than inference.

Well the extinction happened. The only thing that gets to me is that this is described as making us possible, as though the extinction were part of a plan.

Yes—that phrasing bothers a lot of people once they notice what it implies.

Saying the Cretaceous–Paleogene extinction event “made us possible” is not wrong, but it’s very easy for it to sound like purpose when what’s really meant is contingency.

There are two very different ideas that get blurred together:

• Causal precondition:
  The extinction removed dominant groups and reshaped ecosystems in a way that allowed mammals (and eventually us) to flourish.

• Teleology (purpose or plan):
  The extinction happened so that mammals—and ultimately humans—could arise.

Only the first is scientific. The second smuggles in intention where there isn’t any.

A cleaner way to say it would be something like:
“We are here in part because of that event, not because it was ‘for’ us.”

It’s the same logical structure as:

• If a particular ancestor hadn’t met another, you wouldn’t exist.

• But that meeting wasn’t arranged with you in mind.

In evolutionary biology this is just standard path dependence: outcomes depend heavily on prior accidents. If you “rewind the tape,” as Stephen Jay Gould liked to put it, and let it run again, you almost certainly don’t get the same result—certainly not us in anything like our present form.

So when documentaries or scientists use that shorthand, they’re usually trying to emphasize:

• the fragility of our lineage,

• and how much it depends on chance events,

not implying design. But the language can slide into something that sounds almost theological if you’re not careful.

Your discomfort is basically a good instinct for avoiding teleological thinking—seeing purpose where there’s only history.