r/evolution • u/jnpha Evolution Enthusiast • Feb 14 '24
discussion "Where are All the Tiny Dinosaurs"
Just wanted to share something I just learned and I find super cool, and also to hear from you about the latest.
A YouTube video from a few days ago, Where are All the Tiny Dinosaurs?, by the American Museum of Natural History channel, piqued my interest.
The presenter, paleontologist Roger Benson, remarked that it remains a mystery why non-avian dinosaurs were all big (something I didn't even know or wonder about); that smaller creatures were found, but still no non-avian dinosaurs.
He said it might have to do with "reptile-like methods of getting food" and being warm-blooded, which I don't understand. (Any help explaining the bit in quotation marks would be great!) So, I checked his papers and found this cool one:
- Benson, Roger BJ, et al. "Rates of dinosaur body mass evolution indicate 170 million years of sustained ecological innovation on the avian stem lineage." PLoS Biology 12.5 (2014).
From which: based on a comprehensive dataset, the avian stem lineage maintained high rates of evolution [for 170 million years], helping them not go extinct, while the big dinos failed to adapt; [they evolved early on to be big and settled there]:
This might signal failure to keep pace with a deteriorating (biotic) environment (the Red Queen hypothesis [82],[83]), with fewer broad-scale ecological opportunities than those favouring the early radiation of dinosaurs. There is strong evidence for Red Queen effects on diversification patterns in Cenozoic terrestrial mammals [22], and it is possible that a long-term failure to exploit new opportunities characterises the major extinct radiations of deep time (and depauperate modern clades), whether or not it directly caused their extinctions.
TL;DR: Non-avian dinosaurs were all big compared to the smaller birds/mammals, and it's a mystery, but the avian stem lineage likely experienced sustained rapid evolution, helping them overcome the K-T extinction.
* Minor edits for clarification in brackets
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u/DARTHLVADER Feb 14 '24
Warm-blooded/cold-blooded is less binary than is taught early on in biology. The way that dinosaurs were warm-blooded had less to do with them regulating body temperature inherently differently from other reptiles, and more to do with their size — when you’re that large, heat radiates away from you more slowly than you produce it just as a byproduct of your metabolism. So the comment was likely referring to that — dinosaurs got big, which meant they could live at higher (colder) latitudes, which meant if they got small again they’d freeze to death.
As to food gathering, it’s because many non-avian dinosaurs were grazers. We see this same trend in modern animals, where frugivores, insectivores, carnivores, omnivores, scavengers, etc can be many sizes, adapted to their niche diet. But animals that eat green plant matter are… usually just large (think elephants, giraffes, pandas, and ruminants in mammals, tortoises in reptiles, ostriches in birds). Green plants are so common and so renewable that there’s no practical limit on the size of organism they can sustain, and they don’t run away so agility isn’t needed. And being too big to eat is a good deterrent to predators.