Where did the thalattosuchians go?

Crocodilians, which include crocodiles, gharials, caimans and alligators, are represented by relatively few species today. Not only are these animals not particularly speciose, they are also quite similar in appearance and habit. Looking at them, they are unmistakably crocodile-esque, with their toothy smiles and elongate bodies embedded with rows of bony scales. Crocodilians are generally ambush predators that hunt in the water or at the water’s edge. While some species mostly hunt fishes, the stereotypical image of crocodilians lying in wait for an unwitting animal to take a drink is not uncommon. Though they seem quite comfortable in water, crocodilians have a sprawling, somewhat awkward posture on land. Nonetheless, they will frequently leave the water to rest and bask in the sun.

Once upon a time, there was quite an impressive diversity in the forms of crocodilian-like animals, many of which appear to have had very different lifestyles from modern species. One group of these animals was the thalattosuchians, which were particularly specialized for living in the ocean. Some thalattosuchians superficially looked quite similar to modern crocodilians, particularly teleosaurids, which includes Platysuchus and Steneosaurus.

Others were more obviously adapted for an oceanic lifestyle, and would have looked quite different from your typical crocodile or alligator. These species include GeosaurusRhacheosaurus and Dakosaurus, known collectively as metriorhynchids. If you look closely at Geosaurus, you can see that the limbs were more fin-like in shape, and both Geosaurus and Rhacheosaurus preserve outlines of a tail fin, demonstrating evidence of their strong swimming abilities.

Thalattosuchians appear in rocks estimated to be 199.3 million  years old, and then seemingly disappeared 93.9 million years old [1]. Despite this long and successful history, for better or worse, they can no longer be found in our seas.


Questions for Creationists

Where did the thalattosuchians go? Why would God create them only to have them completely disappear? Since they lived in oceans, shouldn’t they have survived Noah’s flood? Why do modern crocodilians have very similar lifestyles whereas the other crocodilians seemed to have lived very differently?


1. Paleobiology database

Photo credit

Slender-snouted crocodile, gharial, black caiman, chinese alligator, Platysuchus, Steneosaurus, Dakosaurus, Geosaurus, Rhacheosaurus, metriorhynchid reconstructions


2 thoughts on “Where did the thalattosuchians go?

  1. Ted Lawry

    Interesting post. I particularly like that the sea crocodiles are clearly crocs, but were also clearly highly adapted to the sea with fins replacing limbs and a fish-like tail. They are a croco-fish! Do creationists say that sea crocs were members of the croc “Kind?” In that case how can they object to saying whales evolved from land mammals. But saying that sea crocs weren’t crocs makes a mockery of the whole idea of kind! Great post!

    1. Hi Ted: thanks for your comment! Excellent point about the croco-fish/whale evolution disparity! To be fair, thalattosuchians certainly look more croc-y than whales look like hoofed mammals, so it’s probably easier to imagine that they could be part of the same kind.

      Regardless, I’m not sure that creationists have ever assigned thalattosuchians to a kind. Frequently they’ll suggest that a kind is equivalent to a taxonomic level of family or sometimes higher, but the taxonomic position of a group of organisms is relatively arbitrary even from the point of scientists. So are thalattosuchians part of the crocodilian kind? Does each crocodilian family constitute a kind and then thalattosuchians were a different kind?

      Either way, these guys, and many of the other animals I try to highlight, aren’t usually (ever?) mentioned by creationists. It makes one wonder if creationists have a grasp of the enormous amount of diversity of life out there and if they’ve really considered why so many things have disappeared.

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