The fossil record can be a frustrating tease. Although it’s wonderful that paleontologists are assembling an ever more complete understanding of prehistoric life, every ancient species contains mysteries within mysteries. It’s one thing to discover the remains of a fossil organism. It’s another to understand the biology, ecology, evolution, and extinction of that species.
Herpetologist Wilfred Neill considered this fact in the introduction of The Last of the Ruling Reptiles – his 1971 ode to alligators, crocodiles, gharials, and their extinct cousins. Invoking the heyday of the crocodyliformes, Neill lamented:
What would we not give for a firsthand look at the Mesozoic Era, that remote time when reptiles, often of great size and grotesque mien, walked the earth, swam in the fresh waters and the seas, even glided through the air on leathery wings. But we are vouchsafed no such look, and we must cast about among the sciences for scraps of information that might supplement paleontological data.
The past provides essential context for the present, but, in that classic aphorism which all geology and paleontology students are relentlessly taught, the present is the key to the past. To understand the biology of prehistoric species, we often have to look for analogs among living organisms. In the case of fearsome marine crocodiles that slipped through the Jurassic seas, that means turning to orcas for clues.
Paleontologists have known about the Mesozoic’s marine crocs for over a century and a half. They were among the first prehistoric celebrities discovered by early 19th century naturalists. But these sea crocs weren’t like the semiaquatic ambush predators we know today. The Late Jurassic’s marine crocodyliformes lacked the classic bony armor enfolded in the skin of modern species – the loss streamlined their bodies and allowed species such as Geosaurus to glide through the water, propelled by paddle-like limbs and a crescent-shaped tail fin. Nor did Geosaurus and kin have the classic crocodile skull profile. In addition to rings of bone that supported their eyes – a feature absent among modern crocs – and enlarged salt glands to cope with the chemistry of the oceans, Mesozoic sea crocs often had relatively deep skulls filled with teeth that most closely resembled the dental armaments of Tyrannosaurus rex.
To specialists, these unusual seagoing crocs are called metriorhynchids. Exactly how many genera and species of metriorhynchid paleontologists have discovered is a complicated issue. Many species were named from teeth or skeletal fragments – a taxonomic explosion that researchers have been trying to untangle as they review the record of these aberrant archosaurs. The latest effort to stabilize the metriorhynchid family tree was published by University of Edinburgh paleontologist Mark Young and co-authors this week in PLoS One.
The focus of the paper is a redescription of Dakosaurus maximus – a particularly deep-skulled sea croc – and Plesiosuchus manselii, a larger and shallower-snouted relative that was previously thought to be a species of Dakosaurus. The paper is a smorgasbord of anatomical details, but it’s not only that. Both species of sea croc were contemporaries that may have shared the same habitat around 150 million years ago. Within a sea full of carnivorous marine reptiles – including even larger fish-bodied ichthyosaurs and massive-jawed pliosaurs – how did such similar crocodiles divvy up the ecological space?
We can’t observe the ecology of Dakosaurus and Plesiosuchus directly. The best paleontologists can do is try to reconstruct the details of their habitat and gauge the predaceous powers of these crocs. Based on skull anatomy and tooth damage, it looks like the sharp-toothed neighbors were adapted to take on different prey.
Dakosaurus had a truly frightening smile. Serrated, interlocking teeth set into this croc’s deep skull could have sliced prey into manageable chunks, and the archosaur’s deep skull would have been structurally resistant to the stresses created by large, struggling victims. Young and colleagues hypothesize that the 15-foot-long Dakosaurus was capable of taking on prey as large as itself, but the damage to the croc’s teeth hint that the carnivore didn’t only clamp its jaws around big game. The crowns of Dakosaurus teeth are often spalled and broken. Marine biologists have seen this before. Today’s killer whales sometimes snarf prey through suction feeding – creating an underwater vacuum to catch whole squid and fish. As the victims are pulled into the mouth, they abrade the whale’s teeth. Perhaps Dakosaurus used the same trick.
Despite being a larger croc that was presumably capable of taking on even larger prey, though, Plesiosuchus had a very different skull shape and didn’t share the same pattern of dental damage. This croc had a longer, shallower snout that was less well-suited to dealing with thrashing victims, yet the creature had a large optimal gape – what the researchers define as “the gape at which multiple teeth come into contact with a prey item.” Why, then, didn’t Plesiosuchus suffer from more broken teeth?
Young and colleagues suspect that Dakosaurus and Plesiosuchus might have analogs among the orcas of the North Atlantic. Some of these whales are suction-feeding experts, and the damage to their teeth resembles the wear seen in Dakosaurus. But there are larger killer whales in the same waters that don’t share these pathologies. These bigger orcas are specialists at killing other whales. Perhaps, like the orcas, Plesiosuchus targeted especially large victims, although there’s no evidence that the archosaur would have had the assistance of an entire croc pod in such an exploit.
Dakosaurus and Plesiosuchus are contradictory crocs. Plesiosuchus had a weak skull for a large-prey specialist, and the croc with such a reinforced skull – Dakosaurus – may have relied on sucking up smaller fish, only occasionally supplemented by bigger prey. And, lacking soft tissues, we can’t even be sure that Dakosaurus was capable of suction-feeding. The skulls of these sea crocs hint that they were targeting different prey and had their own ways of acquiring flesh, but, for now, we’re left with Neill’s lament that we can’t observe these hypercarnivores in action.
In time, though, the secrets of these crocs may be drawn out from their skeletons. By looking to modern whales, and carefully reconstructing the soft tissue anatomy of the sea crocs, paleontologists may be able to figure out how the fantastic marine monsters fed. Ultimately, orcas may be the key to estimating the destructive powers of crocodiles who perfected the same techniques millions of years before.