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66 million years ago, the dinosaur empire was in its death throes when its final nail in the coffin came hurtling down from the sky. A clump of rock the size of Mount Everest smashed into the Gulf of Mexico, bringing about the end of 70-75% of all species. Oh, the dinosaurs are still alive, don't get me wrong, but the glory days of their empire died when that bolide hit the Earth.

Now, it is a popular trope among the speculative evolution subgenre to imagine what life was like if Chicxulub never struck the planet and the dinosaur empire was left to continue. Results vary from the sub-par Dougal Dixon book The New Dinosaurs to the more elaborate, more detailed and more enjoyable Speculative Dinosaur Project.

But I would like to take this a little differently. In this alternate Earth, Chicxulub hit the Gulf of Mexico not at the end of the Cretaceous period 66 million years ago, but the end of the Jurassic period, 145 million years ago. Back then, the dinosaur empire was at its prime, with dinosaurs conquering every corner of the globe, occupying virtually every niche imaginable, from sparrow-sized Epidexipteryx to the extra-long sauropod Diplodocus.

The key element that the survivors of the fall of the dinosaur empire in our timeline had was their size. No one larger than 25 kilograms (or 55 pounds) stood a chance. As far as I know, any of the nonavian dinosaurs at the end of the Cretaceous exceeded that maximum limit, and that was the cause of their downfall. The dinosaurs at the end of the Jurassic, on the other hand, were noticeably different, with different lineages, different environmental preferences, different diets, different size ranges (some species at the time could certainly top the intended maximum survival limit of 25 kilograms) and different methods of adaptation.

Based on our knowledge of the fauna of the late Jurassic period (163.5 million years ago to 145 million years ago), would any of the dinosaurs within that timeframe survive a Chicxulub-sized impact, thus ensuring the continued survival of the dinosaur empire?

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  • $\begingroup$ At the end of jurassic there already was a mass extinction (cause unknown) adding an impact event on top will likely wipe out far more than the original chicxulub. also there is no evidence for a decline 66 million years ago. there is a reduction in fossilization, but we see that whenever you have environmental change. $\endgroup$
    – John
    Commented Oct 4, 2019 at 2:46
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    $\begingroup$ @John What mass extinction? This is the first I've heard of it. $\endgroup$
    – JohnWDailey
    Commented Oct 4, 2019 at 2:56
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    $\begingroup$ @John There was no extinction event at the end of the Jurassic. Are you thinking perhaps of the Triassic-Jurassic Extinction? $\endgroup$
    – Arkenstein XII
    Commented Oct 4, 2019 at 3:07
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    $\begingroup$ @ArkensteinXII Its not one of the big extinction events but there is one, there are quite a lot of extinction events in earths history .paleontology.fandom.com/wiki/Mass_extinction $\endgroup$
    – John
    Commented Oct 4, 2019 at 4:12
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    $\begingroup$ There are plenty of small dinosaurs, many well under 25kg, they just don't get as much attention. Gasparinisaura, Yamaceratops, not to mention many troodontids, just for a few examples from the late cretaceous $\endgroup$
    – John
    Commented Oct 4, 2019 at 4:24

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Large animals are first in a row to die in such a catastrophic events. They need a lot of food per day and thus need high average density of eatable biomass. Just take a look at how dinosaus started: Permian-Triassic extinction event. All proto-mammals larger than a fat mouse gone extinct and those who survived were unable to evolve to "full size" faster than dinosuars did. So most of land gaints would most likely gone extinct.

On the other hand in Jurassic period there were much grater variety in small to "middle" (up to elephant mass) dinosuars that would certainly survive and would not allow mammals to get their heads off the ground. This would lead to more intense evolution among dinos and it hard to say what new superspecies would look like.

But moving that impact back in time would not deny epoch change: continetal drift still going on. and thus it would not deny dinosuars extinction - it would just change the species that would extinct and make this Cretaceous–Paleogene extinction a little bit longer. Even some small nonavian dinosuars would leave in Australia continent up to first humans (who will hunt them to extinction)

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  • $\begingroup$ @Alexander indistinguishable is pushing it there was already major modifications to dentitions, posture, metabolism and likely skin covering $\endgroup$
    – John
    Commented Oct 4, 2019 at 20:17
  • $\begingroup$ @John "pushing", I agree. My point should have been that proto-mammals were still reptiles. $\endgroup$
    – Alexander
    Commented Oct 4, 2019 at 20:46
  • $\begingroup$ @Alexander Technically mammals are still reptiles, or reptiliomorphs at the very least. $\endgroup$
    – Arkenstein XII
    Commented Oct 4, 2019 at 21:05
  • $\begingroup$ @Alexander & ArkensteinXII technically whether mammals are reptiles depends on whether you consider reptilia to mean amniota or sauropsids. , mammals are not part of sauropsida, they are part of amniota which is a larger clade. Reptilia is not clearly defined clade. but under current leanings reptilia would be sauropsida in which case no mammal has ever been a reptile. $\endgroup$
    – John
    Commented Oct 5, 2019 at 0:32
  • $\begingroup$ "Reptile" is a cladistically annoying word. A less ambiguous sentence would be "if you looked at a Permian-Triassic proto-mammal, you'd say 'that's basically a lizard'". $\endgroup$
    – IndigoFenix
    Commented Oct 10, 2019 at 9:32
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There are two major factors to be considered. The first is that at the end of the Jurassic it is reasonable to assume that the dinosaurs were in a state of decline as they were at the end of Cretaceous when the Chicxulub impactor delivered its death blow. So based on this it is probable that the dinosaurs wouldn't survive the end of Jurassic and impactor combination.

However, the other factor needs to be taken into account. Continental drift will result in a different geographic arrangement. If the impactor crashed at the same point on the surface of the Earth where Mexico is or was when the Chicxulub impactor arrived. Then at the end of the Jurassic, it is likely there will be different landmasses or oceans at that point.

This can result in a somewhat different outcome from the effects of the impactor's impact. There are specific outcomes involving matter injected from the crash site that led to the mass extinction.

Further research would be necessary to determine what was at the impact site, what types of matter would be thrown up, and the kinds of consequences this entails.

In summary, the most probable outcome would be the mass extinction of dinosaurs at the end of the Jurassic instead of the Cretaceous. This conclusion has to be tempered with a more considered evaluation of the probable consequences of the impactor colliding at what is likely to be effectively a different site from what Mexico was at the end of the Cretaceous.

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