Desdichado
Hero
As a kid, I always liked dinosaurs. I think, when I was only about four or five, the very first time my folks took me to a public library (that I remember) I picked up a dinosaur book and have been hooked very since. All through the 70s, when I was a youngster, I read dinosaur books (mostly all written in the 50s and 60s, if not earlier) I followed up on the dinosaurs, learning their jaw-breaking names by heart. I think most little boys like dinosaurs, but subsequently grow out of that phase; I, on the other hand, did not. Later, therefore, I learned that all while I was reading the classic story of lumbering, cold-blooded swamp monster ala Brontosaurus or Trachodon (both names no longer used) that literally at that same time a raging academic debate about the validity of that model was going on.
It all started with the discovery of Deinonychus antirrhopus by John Ostrom in 1969. This animal was clearly athletic, active and quite likely not "cold-blooded." It was also very similar to very early birds like Archeopteryx lithographica; so similar in fact that it really should be placed in the same family as a closely related animal. Maniraptoriform dinosaurs, like the "raptors" are often described as the ancestors of the birds.
Of course, there's a problem with this. The earliest bird, aforementioned A. lithographica, occurs in the fossil record many tens of millions of years before the earliest "raptor." That means that animals that had bird-like metabolism and anatomy had to have existed even before that. The "raptors" might even be secondarily flightless; descendents of early birds like A. lithographic rather than ancestors. They may well have been feathered rather than scaly. This raises the question; at what point in the development of dinosaur and dinosaur-like creatures did "warm-bloodedness" develop? Of course, despite what I learned as a kid, cold-blooded and warm-blooded are not binary points, on and off, but rather endpoints of a somewhat complicated and complex spectrum of metabolic strategies.
Parsimony, that is, applying Occam's Razor to the closest living relatives of dinosaurs, is ambiguous on the subject. Birds are descendents of dinosaurs, and are clearly warm-blooded, but that's not necessarily any kind of evidence that dinosaurs themselves were. Crocodilians are the closest relatives to dinosaurs amongst reptiles; in fact, crocodilians are not very closely related to any other reptile at all. They are closest to lacertilians (lizards and snakes) but that's somewhat like saying humans are closely related to the platypus. Although crocodilians are cold-blooded, that may be partially a response to their lifestyle; they have a number of anatomical adaptations that clearly pave the way towards warm-bloodedness, including a four-chambered heart and an air-sac studded skeleton system to improve respiration, features that they share with birds, by the way. Since crocodilians evolved as "dinosaur uncles"; they are not in the direct line of descent as dinosaurs, but are close relatives to the direct antecedents of dinosaurs, it is parsimonious, in fact, difficult to concieve of dinosaurs that did not share at least these two features.
Some crocodilians and other "thecodont" archosaurs of the type that later led to dinosaurs did not share a sluggish, water-bound ambush predator lifestyle, as todays crocodilians do. In fact, Terristrisuchus was a true crocodilian with very long, slender legs, an upright gait, and other features that lead scientists to believe that it was a fast, terrestrial (or even arboreal) animal. Also, pterosaurs were like "dinosaur brothers" -- both evolved from small, very similar archosaurian "protodinosaurs." Pterosaurs were clearly warm-blooded, there doesn't seem to be any way to describe them as actual flying creatures otherwise. They are also known from several fossils to have featured a hair-like integuement, an anatomical feature that almost would guarantee warm-bloodedness, as it would be absurdly maladaptive to a cold-blooded creature to be hairy or feathery.
Some fairly recent dinosaur finds further support the idea that some dinosaurs at least were warm-blooded; the feathered dinosaurs of the Liaoshang region of China that have been found just in the last ten years or so being notable examples. Sinosauropteryx is a clearly feathered small dinosaur, for example, that is not particularly closely related to birds, leading one to conclude from parsimony that at least most of the meat-eating dinosaur lineages (the therapods) were warm-blooded.
A more interesting discovery was the early Cretacous finds on "Dinosaur Cove" in Australia. Australia was an island continent at the time, and closer to the south pole than it is now. It would have been dark for much of the year, and although the Mesozoic was warmer climatically than Earth is now, it would still have featured winter temperatures that were at or below freezing. Therefore, to find small dinosaurs like Leaellynasaura that have no common ancestor going back to the very earliest of dinosaurs with animals like the "raptors" is somewhat surprising. Although hibernation isn't impossible to rule out, its difficult to imagine how hypsilophodontids, and small ones at that, could have survived in such a place, without the possibility of migration, if they weren't also warm-blooded, and also probably covered in some kind of down-like or hair-like or feather-like covering.
So, although it's possible that warm-bloodedness developed twice in dinosaurs, once in the ornithopod lineage, and once again in the therapod lineage that fortuitously survives today in the form of birds, parsimony would suggest that dinosaurs may have always been warm-blooded. Certainly their ancestors had the right anatomical equipment to develop warm-bloodedness, both major lineages of dinosaurs had probable warm-blooded representatives, and close sister groups of the dinosaurs, the pterosaurs, were almost certainly warm-blooded as well. The Pterosaurs and "protopterosaurs" (also known as basal Ornithodirans) and the proto-dinosaurs (lagosuchians) all fail to be true dinosaurs only on a few details of their anatomy. Therefore, it seems reasonable to postulate that warm-bloodedness developed before true dinosaurs actually did. Protodinosaurs like Marasuchus lilliensternis are good candidates for truly warm-blooded creatures, being small and probably quite active, as well as being closely related to protopterosaurs like Sharovipteryx and Scleromochlus.
What does all this mean, if the dinosaurs were all warm-blooded? Not too much if you've already become accustomed to the idea of fast, "dino-ninjas" as portrayed in the Jurassic Park movie. However, at the very least, the appearance of many dinosaurs may have to be reinvisioned. Likely most small to medium-sized dinosaurs had some type of warming integuement; maybe not strictly feathers or hair, but something that served the same purpose, a downly-like filament. Hatchlings of all species, even the big ones, may also have been downy or "furry" for the same reason, growing out of their pelts as they matured. Certainly we know from skin impressions that large dinosaurs had rough, pebbly hides, but that doesn't make them any different from large mammals such as elephants or rhinoceroses today.
Anyway, clearly off-topic, but it was bouncing around in my head -- anybody have any comments? Anyone actually follow dinosaur research?
It all started with the discovery of Deinonychus antirrhopus by John Ostrom in 1969. This animal was clearly athletic, active and quite likely not "cold-blooded." It was also very similar to very early birds like Archeopteryx lithographica; so similar in fact that it really should be placed in the same family as a closely related animal. Maniraptoriform dinosaurs, like the "raptors" are often described as the ancestors of the birds.
Of course, there's a problem with this. The earliest bird, aforementioned A. lithographica, occurs in the fossil record many tens of millions of years before the earliest "raptor." That means that animals that had bird-like metabolism and anatomy had to have existed even before that. The "raptors" might even be secondarily flightless; descendents of early birds like A. lithographic rather than ancestors. They may well have been feathered rather than scaly. This raises the question; at what point in the development of dinosaur and dinosaur-like creatures did "warm-bloodedness" develop? Of course, despite what I learned as a kid, cold-blooded and warm-blooded are not binary points, on and off, but rather endpoints of a somewhat complicated and complex spectrum of metabolic strategies.
Parsimony, that is, applying Occam's Razor to the closest living relatives of dinosaurs, is ambiguous on the subject. Birds are descendents of dinosaurs, and are clearly warm-blooded, but that's not necessarily any kind of evidence that dinosaurs themselves were. Crocodilians are the closest relatives to dinosaurs amongst reptiles; in fact, crocodilians are not very closely related to any other reptile at all. They are closest to lacertilians (lizards and snakes) but that's somewhat like saying humans are closely related to the platypus. Although crocodilians are cold-blooded, that may be partially a response to their lifestyle; they have a number of anatomical adaptations that clearly pave the way towards warm-bloodedness, including a four-chambered heart and an air-sac studded skeleton system to improve respiration, features that they share with birds, by the way. Since crocodilians evolved as "dinosaur uncles"; they are not in the direct line of descent as dinosaurs, but are close relatives to the direct antecedents of dinosaurs, it is parsimonious, in fact, difficult to concieve of dinosaurs that did not share at least these two features.
Some crocodilians and other "thecodont" archosaurs of the type that later led to dinosaurs did not share a sluggish, water-bound ambush predator lifestyle, as todays crocodilians do. In fact, Terristrisuchus was a true crocodilian with very long, slender legs, an upright gait, and other features that lead scientists to believe that it was a fast, terrestrial (or even arboreal) animal. Also, pterosaurs were like "dinosaur brothers" -- both evolved from small, very similar archosaurian "protodinosaurs." Pterosaurs were clearly warm-blooded, there doesn't seem to be any way to describe them as actual flying creatures otherwise. They are also known from several fossils to have featured a hair-like integuement, an anatomical feature that almost would guarantee warm-bloodedness, as it would be absurdly maladaptive to a cold-blooded creature to be hairy or feathery.
Some fairly recent dinosaur finds further support the idea that some dinosaurs at least were warm-blooded; the feathered dinosaurs of the Liaoshang region of China that have been found just in the last ten years or so being notable examples. Sinosauropteryx is a clearly feathered small dinosaur, for example, that is not particularly closely related to birds, leading one to conclude from parsimony that at least most of the meat-eating dinosaur lineages (the therapods) were warm-blooded.
A more interesting discovery was the early Cretacous finds on "Dinosaur Cove" in Australia. Australia was an island continent at the time, and closer to the south pole than it is now. It would have been dark for much of the year, and although the Mesozoic was warmer climatically than Earth is now, it would still have featured winter temperatures that were at or below freezing. Therefore, to find small dinosaurs like Leaellynasaura that have no common ancestor going back to the very earliest of dinosaurs with animals like the "raptors" is somewhat surprising. Although hibernation isn't impossible to rule out, its difficult to imagine how hypsilophodontids, and small ones at that, could have survived in such a place, without the possibility of migration, if they weren't also warm-blooded, and also probably covered in some kind of down-like or hair-like or feather-like covering.
So, although it's possible that warm-bloodedness developed twice in dinosaurs, once in the ornithopod lineage, and once again in the therapod lineage that fortuitously survives today in the form of birds, parsimony would suggest that dinosaurs may have always been warm-blooded. Certainly their ancestors had the right anatomical equipment to develop warm-bloodedness, both major lineages of dinosaurs had probable warm-blooded representatives, and close sister groups of the dinosaurs, the pterosaurs, were almost certainly warm-blooded as well. The Pterosaurs and "protopterosaurs" (also known as basal Ornithodirans) and the proto-dinosaurs (lagosuchians) all fail to be true dinosaurs only on a few details of their anatomy. Therefore, it seems reasonable to postulate that warm-bloodedness developed before true dinosaurs actually did. Protodinosaurs like Marasuchus lilliensternis are good candidates for truly warm-blooded creatures, being small and probably quite active, as well as being closely related to protopterosaurs like Sharovipteryx and Scleromochlus.
What does all this mean, if the dinosaurs were all warm-blooded? Not too much if you've already become accustomed to the idea of fast, "dino-ninjas" as portrayed in the Jurassic Park movie. However, at the very least, the appearance of many dinosaurs may have to be reinvisioned. Likely most small to medium-sized dinosaurs had some type of warming integuement; maybe not strictly feathers or hair, but something that served the same purpose, a downly-like filament. Hatchlings of all species, even the big ones, may also have been downy or "furry" for the same reason, growing out of their pelts as they matured. Certainly we know from skin impressions that large dinosaurs had rough, pebbly hides, but that doesn't make them any different from large mammals such as elephants or rhinoceroses today.
Anyway, clearly off-topic, but it was bouncing around in my head -- anybody have any comments? Anyone actually follow dinosaur research?
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