Owner Comments:
At first glance, a coin featuring a Spectacled Caiman would appear out of place in a set such as this. After all, it’s a set about birds and other theropod dinosaurs, which are very different animals than crocodilians such as the Spectacled Caiman. However, the demise of all pterosaurs and dinosaurs aside from avians in the end-Cretaceous extinction 66 million years ago has resulted in crocodilians and avians being each other’s closest living relatives. This can be a rather surprising fact for many people, so I will explain this here.
Dinosaurs, pterosaurs and crocodilians are all members of a distinct order of reptiles known as Archosaurs. There are two distinct groups of Archosaurs: Avemetatarsalia, which is the dinosaur and pterosaur line and Psuedosuchia, which is the crocodilian line. In looking at Archosaurs as a whole, it is clear that they have evolved several traits in certain groups that set them apart from the other reptile orders; Testudines (turtles) and Squamates (lizards and snakes). The first trait is endothermy in the dinosaur and pterosaur groups. This has led Archosaurs to have very different circulatory systems than other reptiles, the most notable difference being that while turtles, lizards and snakes all have hearts that are functionally three-chambered, all Archosaurs have four-chambered hearts, similar to what is found in mammals, the other major endothermic animal group.
The three-chambered heart of turtles, lizards and snakes allows for the mixing of oxygenated and deoxygenated blood, while the four-chambered heart of dinosaurs, and presumably those of the extinct pterosaurs as well, prevents this mixing from occurring. Crocodilians have an interesting mix of traits here. While their hearts are four-chambered like those of dinosaurs, they have unique valves in their heart that are cog-like, which can allow for the mixing of oxygenated and non-oxygenated blood when the animal needs it do so. This re-vamping of the Archosaurian circulatory system indicates that crocodilians evolved ectothermy secondarily, and that their ancestors were likely warm-blooded like dinosaurs and pterosaurs.
The reasons for the evolution of ectothermy in crocodilians is presently unknown, but it does give them some advantages over their endothermic cousins, particularly in the area of food requirements. Crocodilians need to eat much less for their body mass and much less often than their dinosaurian cousins, which gives them a distinct advantage over them in times of scarcity. This is likely one of the big reasons that crocodilians made it across the Cretaceous-Paleogene boundary relatively unscathed while dinosaurs were nearly obliterated and the pterosaurs were fully obliterated. However, the endothermic dinosaurs have the advantage as far as geographic range as they can withstand much colder temperatures than the cold-blooded crocodilians.
Crocodilians and dinosaurs also share a unique respiratory system featuring highly efficient, unidirectional lungs though crocodilians do not have the system of air sacs throughout the body that dinosaurs do. This respiratory system is amazing in that it prepared different Archosaurs to adapt to very different lifestyles and environments. Scientists believe that this respiratory system was key in both the takeover of the skies by flying theropods and pterosaurs and the conquest of aquatic ecosystems by crocodilians.
Another interesting connection comes in the form of beta-keratin. It has been discovered that crocodilian embryos actually produce the exact same form of beta-keratin in the egg that is used by dinosaurs to produce feathers, but the gene is suppressed by the time the crocodilian hatches, producing the scaly hide found on crocodilians. Theropod dinosaurs and crocodilians are the only living animals to produce this unique form of beta-keratin. It is also interesting to note that the presence of this beta-keratin in crocodilians may indicate that the evolution of feathers goes back further in time than is presently thought, and that feathers may not have been unique to theropods back in the Mesozoic. Hair-like structures known as pycnofibers are known from some species of pterosaurs and at least two Ornithischian dinosaur fossils are known with what is currently termed "feather-like structures". It is unknown whether or not these structures are truly analogous to theropod feathers, but the presence of feather beta-keratin in crocodilians makes this a fascinating possibility.
Genetic studies have also backed up the close relationship between dinosaurs and crocodilians, finding many common genetic markers between the two groups though dinosaurs have evolved much more rapidly than crocodilians over the course of time. While both the Avemetatarsalia and the Pseudosuchia evolved in the Triassic, the Psuedosuchians evolved first, appearing in the fossil record 250 million years ago. The earliest Avemetatarsalians date to around 245 million years ago. Despite this, avian theropods are actually the older extant group, with the oldest avian dating to around 130 million years ago, in the early Cretaceous. Modern crocodilians do not appear in the fossil record until the late Cretaceous around 83.5 million years ago. In absolute terms, the two groups aren’t that closely related but the end Cretaceous event decimated all other Archosaurs, leaving these two families as the only Archosaurian survivors. Since that event, both avians and crocodilians have become vital members of a huge variety of ecosystems all over our planet.