Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"
Today’s picture is of an African Dwarf Crocodile, Osteolaemus tetraspis. I’ve already written an entry about this species, but the picture there has been cropped. Here you can see the full painting. The entry, should you care for a fact or few, is here:
So rather than repeat anything I've written there, just a few more (hopefully) interesting comments, about crocodiles and their wider context.
I've not spoken there about the relationships of crocodilians to other animals. Most people still think of crocodiles as being 'reptiles'. The word 'reptile' has by now almost lost any meaning when it comes to categorizing species. Crocodiles are in fact much more closely related to birds than they are to lizards and snakes. But crocodiles, birds, lizards and snakes, and perhaps also turtles and tortoises, are related to each other though this relationship goes back to very ancient times. Mammals are more distant relatives of this ancient group … mammal origins go back again to creatures that looked very lizard–like and would be considered 'reptiles' by many people today, though in terms of relationships, they fall outside of the real lizard and snake group.
The dinosaurs of old were also related to the crocodilians – but their relationship was at the same distance from crocodilians as those of birds. Birds are considered to be dinosaurs today. So it is inaccurate to think of crocodiles as being close relatives of dinosaurs rather than being close relatives of birds. Rather one should think of crocodilians as being close relatives of 'birds and other dinosaurs'. Crocodiles shared an ancestor with birds and other dinosaurs going back to about two hundred and fifty million years ago. Their ancestry with lizards, snakes, turtles and many other groups that had died out by now goes back even further than that.
As such, crocodiles have aspects that can be thought of as more 'advanced' than those of scaly reptiles like lizards and snakes. Lizards have sideways–facing limbs and usually crawl on their bellies. Snakes have no choice in the matter of course. Crocodiles also drag themselves along on their bellies most of the time they're moving on land, but also can lift their bodies up for a 'high walk', and smaller crocodiles can even gallop with a bounding motion. This style of locomotion is considered more advanced even though it has originated over two hundred million years ago in ancient crocodile–like animals. But the 'crawling along on the belly' style is even older: the very first backboned animals coming onto land over three hundred and fifty million years ago moved like that. A 'higher' mode of walking evolved many separate times: in crocodiles and related species, in dinosaurs and birds, in mammals, and even in lizards. Monitor lizards for instance do a very crocodile–like high walk, while many smaller lizards can also lift their bodies as they run. A few lizards can even run on their back legs.
Crocodiles also have parental care. The mother will guard the eggs, escort the hatchlings to water, and watch over them for a while. This too is closer to birds than to lizards and snakes.
Furthermore, crocodiles have more efficient lungs and hearts than lizards and snakes do. The lungs of a crocodile have more internal partitions, increasing the area for oxygen uptake and carbon dioxide expulsion. The heart is essentially three–chambered like that of lizards and snakes, but partitions reduce the amount of mingling of oxygenated and de–oxygenated blood. Therefore, crocodiles have more efficient metabolisms. Nevertheless, they are still ectothermic, meaning they don't generate much of their own body heat, instead absorbing heat from their environment. They are also not very active: most of the time they just lie there. They can go without food for very long … more than a year in some cases. This is an actual advantage of having a slow metabolism, and this may even be why crocodiles are still with us while just about all other large land animals from the times of the dinosaurs became extinct by whatever catastrophe killed them off.
This is something that fascinates me. Crocodiles come from a section of animals that included very sophisticated and high–functioning beings. We all know that birds are perhaps the best adapted and successful of all the living backboned creatures inhabiting the land rather than the oceans. They are warm–blooded, most practice parental care, and they have extremely elaborate anatomical structures. Feathers are perhaps the most notable – and think of the diversity of kinds of feathers that exist. Internally, they have air sacs going through almost their entire bodies, even into their bones, that are extensions of their lungs increasing breathing efficiency. They have interesting adaptations of their muscles and skeletal systems. They are incredibly diverse, from the insect–sized bee hummingbird which can beat its wings a hundred times per second, to the giant flightless ostrich. They include penguins, pelicans and parrots, flamingoes, seagulls and sparrows. And birds are dinosaurs – just one group of them. Now add to this all the non–avian dinosaurs that existed over the entire time of their range from the late Triassic to the end of the Cretaceous and see what an incredibly huge and diverse group you have. This group was flourishing from the Jurassic to the Cretaceous.
By all accounts, many dinosaurs other than birds also were extremely sophisticated. The majority if not all of the dinosaurs were probably warm–blooded with fast metabolisms. Some of the giant types might have slowed down metabolically as they became very big, but probably started out with fast metabolisms to help them grow rapidly. Many appear to have practiced parental care. Many dinosaurs had feathers, and many show evidence of having had internal air sacs too. And many had strange and highly modified anatomical structures, to the extent that today we still don't know what a lot of it was for. This includes strange frills and hollow crests on their heads, and a great variety of plates and spikes on their bodies. Some dinosaurs definitely were slow, but some were certainly very fast. All of them had efficient styles of locomotion with the body carried right above the legs and feet. Many of them could effectively walk and run on their hind legs.
So it's strange to think of these amazing creatures dominating the world in the late Cretaceous, while their relatives the crocodiles remained much more conservative in build. The crocodiles held over a body type and metabolism that was 'primitive' for the group to which they and the dinosaurs belonged, and evolved relatively slowly (but see next paragraph). They settled for a more low–key existence, lounging in rivers and swamps for most of the time while the dry land was filled with dinosaur colour and noise, action and drama. Then came the asteroid (if that is indeed what happened) and suddenly the very sophistication of the non–avian dinosaurs brought their downfall. If it was an asteroid that hit the Earth, then probably the cause of the catastrophe was the huge cloud of dust and debris it would have kicked into the atmosphere when it disintegrated itself as well as a vast amount of the Earth's crust upon impact. This cloud would have obscured the sun over probably the entire surface of the planet, and might have done so for many months to a year or longer. This would have caused a massive die–off of plants and a collapse of ecosystems all over the land and in most of the ocean. The dinosaurs with their fast metabolisms would have needed lots of food, which suddenly was unavailable. Only the birds – being small, needing much less – survived. As did the ancestors of today's crocodiles and alligators. But everything that was big and had intensive needs died. Indeed, apart from the crocodiles and alligators, everything bigger than a cat died out at the end of the Cretaceous. It must have been quite a catastrophe. But the primitive, laid–back, low–key crocodiles could handle it. Even in places where temperatures dropped catastrophically from the sun being blacked out for months, they might have been able to survive in frozen–over pools, with just their nostrils sticking out, like some alligators can do even today. They might have survived for a year or two without eating, as some crocodiles can do even today. So when at last the clouds dissipated and the sun broke through again, clean rain fell once again and the seeds of new plants sprouted and grew again, they were still there to resume their lives while the big, flamboyant dinosaurs and so many other things were gone forever.
So in a sense crocodiles can thank their slow rate of evolution for their current success. They survived the Cretaceous extinction, and could again diversify afterwards. But they did not diversify as much as the mammals and birds did, which is why today there are only a few crocodile species. Their conservative nature is a mixed blessing: they survive but don't thrive as much as the things that take more evolutionary chances. But it would be wrong to think of crocodiles as not evolving or taking such chances at all. Indeed, over the course of more than two hundred million years, crocodiles and their relatives have included a very diverse assemblage of types. The interesting thing is that within the crocodilian assemblage it is also the more conservative types that have proven to be the most stable and persistent, while the more sophisticated and more experimental types were usually the ones more vulnerable to extinction.
The term 'crocodile' is not easy to apply historically when we go back to the very first crocodile ancestors that we could term such. There was a vast variety of primitive crocodile–like things, and it is not clear exactly which one of them led to modern crocodiles. All of them were fairly small, and some were clearly living on dry land while others seem to have been semi–aquatic. The land–living types soon evolved more efficient limbs and ankles, allowing the 'high walk' I mentioned earlier, by which they could get around faster and more efficiently. Some of these went on to give rise to the dinosaurs; other resulted in vaguely crocodile–like animals, sometimes quite large, like the predatory Rauisuchians and the herbivorous Aetosaurs. These were off the main branch of crocodilian evolution, though. The larger, more inclusive group is called the Crocodylomorpha or 'crocodile–forms'. A great diversity of small types exited … some with long slender limbs, some that were probably bipedal similar to small dinosaurs. Some were vegetarians. On the other extreme the Pristichampsids and Sebecosuchians were large, fast hunters living on land, with deep skulls and dinosaur–like flattened, blade–like teeth. Then there were ocean–living Thalattosuchians also, adapted to the point where their front and back feet were turned into flippers, and they probably had shark–like fin extensions to their tails. But the group to which modern crocodiles belong, the Eusuchia, came into origin in the late Cretaceous, about 100 million years ago. These included modern–crocodile–like forms from the outset, but again also featured types like nothing that exists today. There were for instance the Stomatosuchids which had very flat, broad skulls and might have had pelican–like throat sacs; there were the Mekosuchines which dwelled mainly on land; there was even a small kind that might have been able to climb trees. This line, which lived in Australia and on surrounding islands, only became extinct very recently with the arrival of humans in these places, causing a huge loss of contemporary crocodile diversity.
I will try and make drawings of many of these extinct crocodiles and crocodile–forms, for a future article (or several). For now, if you want just a glimpse of all this ancient diversity, have a look at Darren Naish's article here. There are some black–and–white sketches. You might also check out this one on a dissection of a Dwarf Crocodile. Darren goes into the scientific terminology a lot but from what I'd mentioned above I think you will be able to find your way. He also argues for the existence of two or possibly three different species of dwarf crocodiles. Other very interesting anatomical features include that some of the dwarf crocodile's lower teeth poke holes into its upper jaw; the possibility that the bony scutes might help with the support of the body through muscles attached to them; the extreme complexity of the musculature of the hind limbs and tail; how the latter even has implications for how the tails of dinosaurs should be reconstructed.
I hope this has given you a thing or two to think about, regarding the context within which every species that exists today … relationships, history, and how what we learn from one kind of animal, can tell us things about many other kinds as well. There's lots more to say about crocodiles, living and extinct, and I hope to soon cover some of that in further articles.