The Serina Series: Episode II: Crocodilians

Question

In the first episode of this series, Cats, I asked if it's possible for all 600 million house cats to evolve different niches if they were the only amniotes in a terraformed world. Needless to say, it's going to be a blue moon kind of extremely difficult because the cats' specialties in eating meat are that extreme.

So scratch the kitties. Instead, let's look to a terraformed world full of a different group of amniotes: Crocodilians. There are two separate criteria for this question.

1. It must be from a species that we humans have bred captively for non-zoo purposes.
2. It must be from a species that is listed in the wild either as Least Concern or Near Threatened.

These two criteria have reduced an already small list into this even smaller list:

• American alligator
• Nile crocodile
• Morelet's crocodile
• Freshwater crocodile
• Saltwater crocodile
• New Guinea crocodile

Long ago, different branches of crocodylomorphs had occupied the niches that would later be filled by their competitors, the dinosaurs. Creatures like bipedal Effigia and Postosuchus, sail-backed Arizonasaurus and toothless Shuvosaurus ruled the world during the Trassic period only to disappear during the extinction event at the beginning of the Jurassic period, 201 million years ago. In their absence, the previously established dinosaurs could now claim the world as their empire for the next 135 million years, taking on a wide variety of shapes, sizes and niches as a result.

Now, the crocodilians listed above have no relation to the Triassic crocs who looked and acted like dinosaurs, but in a terraformed world where there are no other amniotes (there are still fish and amphibians around, as these reptiles can tackle a wide variety of prey items), would they have evolved and diversified into the terraformed world's "neo-dinosaurs"?

Answer 1

Perhaps

According to this paper lovingly called Alligators and Crocodiles Have High Paracellular Absorption of Nutrients, But Differ in Digestive Morphology and Physiology which seemed to be written about the feasibility of farming Crocodilians, the American alligator (Alligator mississippiensis, a species directly named in the paper), can be raised with planted based diet. Crocodiles cannot.

Alligators also grow well on a variety of different diets, including feeds containing inexpensive carbohydrates and plant protein (Kercheval and Little 1990; Staton et al. 1990a,1990b). Crocodiles, however, seem to require a narrower range of foods, and only thrive when fed a diet of lean animal protein (Garnett and Murray 1986;Read 2000; Peucker and Jack 2006; Webb et al.2013).

It's highly unlikely, but theoretically possible that they could diversify into the different necessary roles, provided the planet was uniquely terraformed with the proper vegetation. You will need to do a deeper dive into the research paper and it's sources to find the actual plants and mixtures required for this.

Answer 2

Highly Unlikely

Assuming the terraformed world has an atmosphere and vegetation similar to Anthropocene Earth, I don't think they would. Carnivorous megafauna needed to eat herbivorous megafauna to get aquire calories required to survive, and they in turn needed megaflora. Megaflora was a product of the climate of the Mesozoic.

In terms of crocodiles themselves, they are very good at hunting in the water, but terrible at hunting on land(waiting for something to come up for a drink doesn't count). They also appear to have a hard maximum on size. Anyone from Florida can tell you, gators run faster than you, but they can't turn while they run. These animals haven't changed very much over the last 50 million years, and they is because they are the apex predator of the river/delta/estuary ecological niche. While they had greater physiology early on, their ancestors evolved into their current niche due the dinosaurs having controlling the land.