Speculative Biology, of a Sorts

[Torvosaurus]

I understand and agree with the KT extinction event, of course. From that, no terrestrial creature greater than 55 pounds in weight survived, which I agree with. But what continues to sit at the back of my mind is the why behind no non-avian dinosaurs surviving. Most dinosaurs are large, obviously, and (relatively) easy to fossilize, but smaller creatures have some issues with fossilization, part of that being their small size itself.

So why did birds survive but no smaller, non-avian dinosaurs did? I'm sure there were coelurosaurs in the smaller range, less than 50 pounds and likely far smaller, for example.

So if these smaller dinos had survived, how would they evolve? Birds evolved into primary roles on islands (Madagascar and New Zealand) and island continents (South America), and possibly elsewhere (Bathornithids in North America, maybe?). So I'd like to hear thoughts on how non-avian birds would have developed.

Torvo

[Ajax88]

Being able to fly likley helped immensely. In a world upended by disaster, being able to freely move hundreds of miles a day to find new sources of food and water is a major advantage.

[Protopatch]

I understand and agree with the KT extinction event, of course. From that, no terrestrial creature greater than 55 pounds in weight survived, which I agree with. But what continues to sit at the back of my mind is the why behind no non-avian dinosaurs surviving. Most dinosaurs are large, obviously, and (relatively) easy to fossilize, but smaller creatures have some issues with fossilization, part of that being their small size itself.

So why did birds survive but no smaller, non-avian dinosaurs did? I'm sure there were coelurosaurs in the smaller range, less than 50 pounds and likely far smaller, for example.

So if these smaller dinos had survived, how would they evolve? Birds evolved into primary roles on islands (Madagascar and New Zealand) and island continents (South America), and possibly elsewhere (Bathornithids in North America, maybe?). So I'd like to hear thoughts on how non-avian birds would have developed.

Torvo

This is an interesting as well as tricky question raised here.
A few branches of Avialae survived the K/T event at the expense of Dinosaurs, the main difference between them being that the former could fly. And we commonly assume that their flight ability helped them to find food more easily than their unfortunate relatives or even, to seek for the few remaining areas of more lenient climate. Nevertheless, Pterosauria didn't make it neither, so there must be an explanation in their metabolism as well.
Considering the weight criterion, I would like to speculate on the evolution of Deinonychosauria or Alvarezsaurida, for example, but do you possibly think about other specific clades/genera/species ?

[Torvosaurus]

Sure birds could fly, but not mammals (at least not that we know of, with bats appearing app. 55mya). We know that mammals were far more sophisticated during the late Cretaceous than originally believed and weren't just a few mouse-like creatures (ohh, the good old days of grade school with mice-like critters gnawing on t-rex remains, and birds were "not" related to dinosaurs 🙂). If mammals survived, then small dinosaurs should have been able to.

As far as what non-avian dinosaurs could have survived, anything small enough to survive through the "nuclear" winter. Smaller Cretaceous-era dinosaurs, mainly. Maybe a pterosaur as well. Show that it was small enough and could have lived at the dinosaur's end time, and you're good to go.

Torvo

[Gwangi]

Sure birds could fly, but not mammals (at least not that we know of, with bats appearing app. 55mya). We know that mammals were far more sophisticated during the late Cretaceous than originally believed and weren't just a few mouse-like creatures (ohh, the good old days of grade school with mice-like critters gnawing on t-rex remains, and birds were "not" related to dinosaurs 🙂). If mammals survived, then small dinosaurs should have been able to.

As far as what non-avian dinosaurs could have survived, anything small enough to survive through the "nuclear" winter. Smaller Cretaceous-era dinosaurs, mainly. Maybe a pterosaur as well. Show that it was small enough and could have lived at the dinosaur's end time, and you're good to go.

Torvo

Mammals couldn't fly but I think what helped them survive was a combination of small size and lifestyle. They would have probably been living underground and feeding on various seeds and invertebrates, the same thing many birds feed on for that matter. Even if most of the plant life is gone there would still be plenty of seeds lying around and seeds have a long shelf life.

[Torvosaurus]

Okay, I'm not arguing any of the points made, by any means. I agree 100%. But...

If one or two or three species had survived, what would have become of them? That's not, IMO, that big of a stretch. They would have been in competition with mammals, obviously. Maybe they evolved into larger dinosaurs over time, or maybe they became extinct within a hundred thousand years, or by the end of the Paleocene, perhaps. I'm curious how other people see small, non-avian dinosaurs evolving in the Cenozoic, had they survived.

Torvo

[stargatedalek]

I think part of the confusion comes from how you're framing the question, as one of competition between surviving dinosaurs and mammals. I think looking to competition with birds might give us a clearer picture, as they were more similar in niches at the time. Birds could simply travel much further and so were better suited to surviving a low resource environment. Mammals that competed with small dinosaurs likely also got outcompeted by flying birds, only the smallest of mammals that lived underground lasted.

As for Pterosaurs, that I would simply posit to diet. Most small pterosaurs were insectivorous or piscivorous, not herbivorous or omnivorous like many small birds would have been.

[Libraraptor]

The definitive answer is still pending, if there ever will be one. I personally wouldn't be surprised when non-avian dinosaurs will be found in layers after the great mass extinction. That would be a great scientific sensation, wouldn't it?

[dinofelid]

A few branches of Avialae survived the K/T event at the expense of Dinosaurs, the main difference between them being that the former could fly. And we commonly assume that their flight ability helped them to find food more easily than their unfortunate relatives or even, to seek for the few remaining areas of more lenient climate. Nevertheless, Pterosauria didn't make it neither, so there must be an explanation in their metabolism as well.

It's also interesting that the Enantiornithes, a large group of birds that were apparently very successful and populous at the time of the KT extinction, also went completely extinct along with the non-avian dinosaurs. I found a paper here which at the end offers some speculations about the reasons:

It is unlikely that a single factor can explain the extinction of enantiornithines. Most likely a combination of important physiological differences allowed neornithines to survive when enantiornithines and other stem birds and non-avian dinosaurs all died off. Although elevated growth rates evolved in at least some Late Cretaceous enantiornithine lineages, development was still slow compared to crownward birds. The lack of phenotypic flexibility of the digestive system suggested by the absence of gizzard stones in all known specimens may indicate enantiornithines were unable to feed on detritus like seeds. The half-buried morphology of their nests may also have been an important factor, limiting their ability to recover clutches and indicating relatively longer incubation periods (egg-turning facilitates protein absorption, reducing incubation time). Precocial enantiornithine juveniles may also have had unusual molting patterns which may have made them susceptible to the sudden decrease in temperatures associated with the nuclear winter that followed the impact. Together with major losses in habitat due to forest fires, these physiological limitations may have brought the demise of this successful avian lineage.

The first few issues, namely relatively slower growth rate and half-buried nests and lack of ability to survive on seeds, might also have applied to small non-avian dinosaurs.

[Protopatch]

I'm taking this thread a bit off the initial topic, but this should remain within the realm of speculative biology
Here is an interesting article about the (disputable) theory of an oceanic dispersal for certain species of dinosaurs, released just one year ago :

https://evolutionnews.org/2024/05/fossil-friday-did-giant-dinosaurs-swim-across-oceans/

However, it still keeps the door open for speculation regarding the Mesozoic paleogeography and, as a corollary, the unexpected distribution of certain species that have been uncovered in areas where they weren't supposed to be found.
I'm thinking about a temporary land connection which might have appeared during a low tide event and which could have allowed dinosaurs to cross different landmasses, usually separated by oceans or seas.

What would be your guess ?^^

[carliro]

In the end it boiled down to chance. Many mammals were also affected by the KT event, especially metatherians.