This is a pretty hot topic in paleontology. Ancestral crocs, as a matter of fact, weren’t the super-advanced, ultra-tough monsters we know. They were a pretty diverse group including the little 2 metre long dino-like, two legged
Terrestrisuchus, which was a land dweller, looking quite a lot like a cross between a lizard and an Allosaurus.
In the Jurassic croc species included marine crocs, some looking like icthyosaurs, and one gharial-type actual croc, and a miniature two foot long, thing called
Bernissartia, much like the modern types, appeared in Europe. Later, in the Cretaceous, a giant croc,
Deinosuchus, was the largest ever crocodile at 15 metres long, three times bigger than the average saltwater croc of today.
Scientific American explains the heart of the debate:
It is a longstanding mystery why dinosaurs became and remained so plentiful for more than 180 million years. The traditional theory: dinosaurs suddenly replaced other land animals because of special traits that gave them an evolutionary advantage, such as being warm-blooded, nimble and able to occupy varied habitats. This new research presents a fresh mathematical analysis of previous fossil data that indicates that ancestors of modern-day crocodiles had as diverse body types as early dinos, with whom they co-existed for some 30 million years.
Although the data doesn't directly contradict the idea of dinosaur superiority, the authors say it is likely that these crocodilians were even more successful than dinosaurs, the latter of which may have survived major extinctions due to sheer luck.
This is a little iffy.
Crocs are, and were, ultra successful. There’s a bit of a dichotomy, however, in seeing them as an alternative to the dinosaurs. The crurotarsans were undeniably a
diverse group as this list from Wikipedia indicates.
However, this argument has a flaw, or possibly several. Part of the problem is what you call a non-dinosaur, and what you call a crurotarsan.
The crurotarsans were also beneficiaries of evolution, in the form of crocodiles, which are one of the all time most successful species.
The non-crocodilians were a pretty diverse group, and apparently in terms of classifications, crurotarsans are supposed to include Archosaurs.
That’s begging the question, more than a bit, if they're included. It's not clear from the Scientific American article if this is the case, but as a classification, it's very misleading.
Archosaurs are the ancestral reptile group. They
predate both the Triassic groups. They also included a few croc-like creatures, including
Champosaurus. Other supposed crurotarsan species include
Desmatosuchus, a thecodont, which is yet another class of reptile.
Then there's the "more is better" argument:
Brusatte and researchers from the University of Bristol in England expanded this research by analyzing the existing fossil record to show crurotarsans may have even been more successful than dinos. First, the team constructed a new family tree to separate the dinosaurs from the croc ancestors. They then assembled a database of 65 dinosaur and crurotarsan species that included over 400 skeletal features, such as whether they had beaks or shorter arms than legs.
If dinosaurs were more fit for the environment, they should have had a higher rate of evolution and more diverse body types. Instead the researchers found that the two groups evolved at similar rates and that the crurotarsans had a wider range of body types, suggesting that they had actually adapted to more lifestyles and ecological niches.
No.
This is where the logic breaks down, badly.
Evolution isn’t some sort of even-handed balance, affecting all species equally. To evolve, certain conditions have to apply, and some species take advantage while others can’t. Forms of evolution are also highly variable.
The dinosaurs were considered to have been well adapted to a huge global drought which occurred in the Triassic. It doesn’t follow that the non-dinosaurs, like the mammal-like reptiles, or the crurotarsans, were necessarily superior species.
The information presented indicates the crurotarsans died out on land specifically
because they couldn't adapt to an environmental change, and the dinosaurs, obviously, could.
Even if the Archosaurs aren't included, this argument has some large holes in it. This particular extinction, as far as is known, wasn’t some sudden event like an asteroid, as the proponents of the theory seem to think. It was a long drawn out thing, a big environmental shift, and evolutionary pressures would have been intense.
The croc species of crurotarsans were much better off, obviously, in their habitat, than the land dwellers. They had a working food supply.
The dinosaurs went on to dominate the land, rather noticeably unchallenged by
any other species, including mammals, amphibians, and other reptile types.
The mistake is assuming that crurotarsans and dinosaurs were interchangeable as land dwellers.
The basic non-croc forms of crurotarsans, (even if you allow for the Archosaurs as being crurotarsans, which frankly I don’t), were pretty much basic reptiles, lizard form. Some added features, but not in the class or range of the big Jurassic dinosaurs. Their morphology is very basic, and even allowing for letting in everything and everything as a crurotarsan, many of them are just varieties of crocs.
There were some big ones, some strange shapes, some large predators, but developmentally, you don’t see the emergence of the drastically different classes.
This is an interesting “what if” theory, but really, it’d be nice to pin down the subject a lot better, expand the evolutionary findings to include known Triassic conditions, and define what's classified as crurotarsans for the benefit of those trying to understand the issues.
And leave the Archosaurs out of the equation.
