Excessively Interested In Dinosaurs

Marsh guardian 🍃🌱

Utahraptor day!! I had a lot of fun with this one <3

Admiring Disney's Dinosaur (2000)

Ik not too many are fond of this movie but I am due to how much the graphics were mesmerizing

Firstly, what is warm-bloodedness? This is a term not widely used in scientific literature anymore, since it's kind of nebulous and encompasses three different thermoregulatory phenomena:

Endothermy - ability to create and internally control one's own heat

Homeothermy - ability to maintain a consistent internal body temperature

Tachymetabolism - maintaining a high metabolism rate

It's possible to show a variety of combinations of these three phenomena, and it's also possible for the same organism to change between different states. Some cold-blooded animals show commonly warm-blooded features, or have found other ways to achieve thermoregulation [1, 2]. So, 'warm-bloodedness' is a pretty complex thing. In addition to that, a lot of these things are hard to assess in fossils, since we see only static snapshots, mostly of bone rather than the organs which would give better indications. But there are some things the fossil record and evolutionary biology can potentially tell us.

First, growth rates. Generally, it takes a lot of energy to grow, and especially to grow fast. Dinosaurs generally seem to have grown rapidly during their youth to reach adult size quickly (comparatively to reptilian growth rates), which can be seen through analysis of bones [3, 4]. This can be taken as evidence of tachymetabolism and endothermy, as the ability to generate massive energy for growth would imply an ability to generate massive energy for thermoregulation, too. However, this is not consistent across all dinosaurian groups, and analysis shows a variety of intermediate comparative rates, similar to mesothermic animals, so it's probably complicated [5].

Secondly, posture. Dinosaurs famously walk upright, with legs directly under the body, unlike the sprawled posture of most modern reptiles. In modern animals, only endotherms show upright posture and gait - no modern ectotherm has an upright posture [6, pg 23 onward]. Additionally, many dinosaurs had limb ratios which suggest adaptations to running and high-speed movement. This suggests an active lifestyle, which would have demanded higher metabolic rates and higher energy needs, which could enable endothermy [7].

Third, habitat. We have evidence for dinosaurs that lived in the cold polar regions of the planet - for this, they would have had to be capable of endothermy, otherwise they would not have survived the low ambient temperatures [8]. This implies that at least some dinosaurs were endothermic.

One of the most well known common features of warm-bloodedness is the four chambered heart. Fun fact, crocodiles also have four-chambered hearts!! This makes them an anomaly in the world of cold-bloodedness. It could therefore be possible that endothermy evolved early in the archosaur line (the larger family group dinosaurs evolved from) [9, 10] - this would imply that endothermy was in fact basal to Dinosauria. There is evidence for very active early crocodilians, and it is suggested that modern crocodilians have 'reverted' to ectothermy due to selection pressures of their hunting methods [11]. If this is true, perhaps dinosaurs were basally endothermic, but some later reverted to ectothermy for various reasons.

Incidentally, it's not necessarily accepted when exactly endothermy evolved in terms of mammalian lines, either. There seems to be some evidence for endothermy in early synapsids in the late Permian [12], some theories suggest a late-Triassic origin [13], and some suggest true endothermy didn't evolve until crown-group mammals in the mid Jurassic [14]. Conversely, some people think it may in fact be basal to all amniotes [15], with current ectothermy being a secondarily evolved characteristic! So who got there first? We don't know!!

So, to conclude an hour's worth of poking around at various sources - there is a lot of evidence to support that at least some groups of dinosaur were endothermic, particularly the coelurosaurs from which birds evolved. Certain groups meet various criteria for assessing 'warm-bloodedness', although not all do. There is some evidence to suggest that archosaurs evolved endothermy early in their lineage, and therefore that endothermy could in fact be basal to Dinosauria as a group.

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