The Boar Croc

The skull of Kaprosuchus

Kaprosuchus was an unusual crocodilian. Found in Niger, in the upper part of the Echkar formation, the boar croc is late Cenomanian in age (c 95 Ma). It was almost certainly a terrestrial predator. Evidence for this includes its laterally and somewhat anteriorly positioned orbits, quite unlike the dorsally placed ones of aquatic crocs. Facing forward to an extent, the orbits conferred a degree of stereoscopic vision, a feature known only in land hunters. In addition, the croc's relatively straight, sharp and laterally compressed teeth contrast with those of aquatic taxa.

The most distinctive feature of Kaprosuchus is its large dentition. The croc had three pairs of tusks. Most were in the anterior of its snout, in the manner of a sabertoothed cat. A large retroarticular process enabled the jaws to gape wide enough to accomodate prey--or a part of it--between the long teeth.

Why did Kaprosuchus evolve such enormous teeth? Undoubtedly, its prey was quite large relative to itself. Kaprosuchus was rather small--only about five meters long, with a 50cm skull. Like a sabertoothed cat, it relied on powerful armament to compensate for its modest size. With its weaponry, the boar croc was not only terrestrial but an apex predator. Almost certainly, it attacked contemporary dinosaurs. Nigersaurus and Rebbachisaurus are possibilities, but both may predate Kaprosuchus. Likewise, the hadrosauroid Ouranosaurus appears gone by Echkar time. Perhaps the modest sized sauropod Aegyptosaurus was the boar croc's principal prey. Both lived c 95 m.y.a.

How did Kaprosuchus come to occupy a niche almost invariably filled, in the Mesozoic, by large theropods? The Echkar has yielded the big Carcharodontosaurus, but Kaprosuchus could scarely have lived alongside it. Under normal circumstances, the boar croc could never have competed with a carch. It could only have become top predator if, or when, big theropods were excluded from its environment.

Kaprosuchus owed its existence to a dry spell. Haq (2014) indicates a profound lowstand occurred in late Cenomanian time. Sea levels were very high at the end of the Cenomanian (c 94 Ma) but low just prior to that (c 95 Ma). Although geologically brief, the lowstand was ecologically profound. In Niger, arid conditions caused vegetation and prey populations to fall below the threshold where a big predator (especially an endothermic one) could survive. Adapted to dry habitats, some sauropods persisted. Even a small population of them could've supported the modest-sized, ectothermic croc. Compared to a carcharodontosaur, its requirements were minimal. Facing no competition or threat from carcharodontosaurs, Kaprosuchus evolved to hunt sauropods (probably immature ones, mainly).

How did Kaprosuchus hunt its prey? It was probably an ambush predator. As an ectotherm, Kaprosuchus was unlikely to have chased its quarry, which was endothermic hence had better endurance. In addition, a sauropod was probably too big to overcome except with the element of surprise i.e. ambush. If Aegyptosaurus was a low feeder like Nigersaurus (which seems likely in an arid habitat with insufficient rain for forests) the evolution of Kaprosuchus wouldn't be surprising. Lowering the head and neck made a sauropod highly vulnerable. Hiding in low vegetation of the sort preferred by its prey, Kaprosuchus, like some modern reptiles, often waited for days or weeks for its chance. When a sauropod came near enough, the boar croc leaped at it, mouth agape, and slammed its jaws on its head or neck. Penetrating deeply into the skull, the tusks caused rapid immobilization and death. When biting into cervical vertebrae, the teeth cut the spinal cord, resulting in instant paralysis. Another possible strategy was to rip open the belly and disembowel the sauropod.

No doubt, a degree of stereopsis enabled the hunter to aim its attack more precisely. Once the quarry was subdued it may have provided enough sustanance for several months.

The dorsal position of the croc's external nares and fusion of its nasal bones evolved to withstand the stresses incurred by bite force. The posterior of a Kaprosuchus skull was high. Part of this area accomodated powerful muscles capable of closing the jaw with great force, necessitating some protection anteriorly.

To a considerable degree, the hypertrophied teeth of Kaprosuchus mirrored the special requirements of an ectotherm. The boar croc didn't have the reserves of energy needed for a prolonged gladitorial fight with a dinosaur. Given its limited endurance, it was essential to subdue the prey quickly.* Long teeth evolved to produce a rapid result--preferably wrecking a sauropod's brain or severing its spinal cord--aided by semi-stereopsis to accurately hit these targets.

Kaprosuchus was short-lived, because the arid conditions necessary for its existence were ephemeral. In a geologically short time, sea levels were higher. Environmental conditions reverted to normal; indeed they were soon antithetical to the requirements of the boar croc. Kaprosuchus was an evolutionary dead end and an anomaly. Almost without exception, theropods monopolized the top (land) predator niches of the mid-late Mesozoic. As soon as the arid period was over, big theropods reappeared and the boar croc vanished.

*All reptiles evolved to survive with minimal expenditure of energy. A turtle doesn't repell an attacker by fighting--which is energetically costly-- but by withdrawing into its shell and waiting out the attack. Likewise, by injecting venom into its prey, a poisonous snake can subdue it quickly without having to expend a lot of energy fighting.

Addendum 1

Besides its dentition and other unusual features, Kaprosuchus had head horns. They were probably display structures, and another feature suggests interaction with others of its kind.

Kaprosuchus had a keratinous shield over its premaxillaries. The expanded anterior may have been used for pushing or shoving contests to establish dominance. In view of the size of potential prey, it wouldn't be surprising if Kaprosuchus fought in groups. A single individual could hardly have eaten a sauropod--even a relatively small one. The carcass could've fed a whole family or pack. If Kaprosuchus had an elevated metabolic rate it might've hunted in packs like wolves (as opposed to ambush predation). That may be far fetched however. A number of individuals could've lain in wait for the prey, and helped kill it. In any event, Kaprosuchus was probably somewhat gregarious, in which case there was a hierarchy established by head horns and robust anterior snouts.

Addendum 2

If Kaprosuchus was a sauropod hunter why didn't it evolve larger size? The need for ambush tactics, in an ectotherm, may explain this. Had the boar croc been larger, it would've been better able, in theory, to subdue its quarry. But larger size would've made concealment, hence an ambush strategy, more difficult. Kaprosuchus got the best of both worlds. Disproportionately large teeth conferred the killing power of a huge croc but modest size aided concealment.

Addendum 3

Note I gave a length of 5 meters for the boar croc when the wiki estimate is 6m. I assume a terrestrial croc wouldn't need a long tail for sculling and was generally more compact, hence shorter, than an aquatic one. In fact even 5m might be too long. Perhaps only about 4-4.5m.

Addendum 4

Baurusuchid crocs of the mid late Cretaceous were also fierce terrestrial predators. The radiation of such crocs, like Kaprosuchus, owed much to the hotter climate of low paleolatitudes and opportunities created by the extinction of carchs and certain other theropods. Abelisaurs, however, must have competed successfully with baurusuchids, few of which existed after the Santonian. Both groups probably preyed largely on juvenile sauropods.

Reference Bilal U Haq 2014 Cretaceous Eustasy Revisited Global and Planetary Changes 113