Our Archaeopteryx show has bedazzling fossils – the only Archaeopteryx skeleton in the New World, complete with clear impressions of feathers. Plus frog-mouthed pterodactyls, fast-swimming Sea Crocs, and slinky land lizards. Today we learn about the Geosaurus.
Geosaurus – Shark-Tailed Sea Croc
Speediest of the ocean-going crocodilians
Some creatures of the Late Jurassic lagoon were up & coming evolutionary clans – the teleosts, for example, were just beginning their takeover of the marine ecosystem. Other groups were Darwinian ultra-conservatives, living fossils in the Jurassic, changing slowly or not at all. The Chimaeras are a fine example.
And then there were a few very special cases – Late Jurassic critters that had reached the apogee of Natural Selection, the highest development of their race. Best representative of this phenomenon:
The Super-Swimmer Croc, Geosaurus.
The earliest crocs of the Triassic were land animals, roughly fox-sized with long legs. In the Early Jurassic, crocs went into rivers and lagoons. That’s not a surprise. All living crocodilians swim well in freshwater, and a few – the Florida Croc and the Australian Salt Water Croc – will go out beyond the surf and navigate between oceanic islands.
But…..no modern-day croc is super-specialized for life in the high seas. None have the double-lobed tail of the sort we see in big, fast sharks, like the White Shark. Open-water sharks have a characteristic double-lobe tail. The vertebral column takes a sharp bend upwards to support the upper lobe of the tail. The lower lobe is made of tough skin and connective tissue. You can see the double-lobed tail configuration in our Archaeopteryx show in the hybodont sharks, a family common in the Jurassic.
To compete with such speedy sharks, a croc would have to evolve a double lobed tail. No crocs did – except one extraordinarily graceful clan, the geosaurs.
Our exhibit is graced with one of the finest geosaur specimens ever dug. This awesome Solnhofen skeleton demonstrates how evolution had transformed a “normal” river & lagoon crocodile into a reptilian torpedo, an open water predator that matches a shark in efficiency.
Geosaur evolution made a sacrifice unusual among the crocodilians – it traded in armor for velocity. All early crocs from the Triassic and earliest Jurassic had thick bone plates over the back and neck and all over the throat and belly. All modern day crocs too carry extensive armor plate. This armor is useful when crocs are attacked by land predators or by other crocs. Most of the sea-going crocs of the Jurassic and Cretaceous kept some armor. Case in point: armor was carried by the teleosaurs, big sea-crocs who were the apex predators at Solnhofen and most other sites in the Mid and Late Jurassic. There is excellent evidence that large Jurassic dinosaur meat-eaters did indeed attack teleosaurs.
The geosaurs went a different way. They went skinny-dipping.
Geosaur skin was totally devoid of bone armor plates. They were naked. This development made the geosaur body lighter and more flexible.
Fast-swimming demands a specialized flipper for steering. The “normal” croc has long front legs and very long hind legs. The hind legs have wide webbed feet and assist the tail in propulsion underwater. All modern crocodilians and most fossil species keep this arrangement.
The geosaur limb equipment evolved in a unique way. Those long, strong hind legs were retained. But the fore-limbs were transformed into short flippers that worked like the diving planes of a submarine. No other croc clan did this with their front limb.
Impressive….but the outstanding geosaur specialization was the tail. “Normal crocs” have a deep, strong tail that bends down just a little bit at the end. The geosaurs went far beyond “normal” – they evolved a tail almost identical in profile to that of a modern tiger shark or a Jurassic hybodont. The geosaur tail possessed two lobes, one bigger than the other in shark-fashion.
Take a good look at our geosaur…..notice something strange?
The tail is upside down! The vertebral column bends down, not up the way it does in sharks. Mummified geosaurs show that the upper lobe was made from tough skin and connective tissue, just like the lower lobe of sharks. The hydrodynamics of the upside-down tail worked just as well as the right-side-up shark tail.
Here’s a wonderful example of how evolution works: Natural Selection is opportunistic. It operates on what is already there. “Normal crocs” already had a slight down bend of the vertebral column. For “normal crocs” to evolve a right-side-up version of a shark tail was almost impossible. But evolution took the simpler path by emphasizing the downward bend and then adding the upper lobe.
No croc of any age matched the swimming efficiency of geosaurs (although the Cretaceous Hyposaurus, from my home state of New Jersey came close). Most other croc groups are distant seconds. Therefore, the Late Jurassic was the high point of croc-natatory prowess (look it up; “natatory”, a good adjective).
Why? Why didn’t some later croc group evolve upside-down shark tails as specialized as those of geosaurs? We don’t know. My guess is that sharks evolved so fast in the Cretaceous that crocs were pushed out of the open-water/fast-swimming niches.
One more thought – geosaurs probably had to crawl onto sandy beaches to build nests and lay eggs. Their tiny flipper-like fore limbs would have been a big disadvantage – mom geosaurs must have been far more vulnerable to land predators than “normal crocs.”