Riparovenatormilnerae

Introduction: The Early Cretaceous Wessex Formation of England has yielded the remains of numerous dinosaurs, including just recently between 2013 and 2017, a new species of Spinosaurid called Riparovenator, named by Chris Barker and associates in 2021. Despite their heads outwardly resembling those of crocodiles, spinosaurids like Riparovenator may have actually hunted more like giant wading birds, which has earned them the nickname, "Croc-Faced Hell-Herons". There has been much debate about the feeding habits of spinosaurids, but if these theropods were indeed “Hell-Herons” rather than fully aquatic predators like crocodiles, then Riparovenator, which means “Riverbank Hunter”, is the perfect name for this still rather enigmatic animal.

Discovery: The bones of Riparovenator milnerae were discovered, along with those of a second spinosaurid, between 2013 and 2017, in the Wessex Formation on the Isle of Wight in southwest England, which used to be part of the old Anglo-Saxon Kingdom of Wessex, hence the name of the formation. These rocks date to about 128 million years ago in the Early Cretaceous Period. The spinosaurid bones were initially referred to as Baryonyx, a spinosaurid known from the same time period but from southeast England (Sussex). However, in 2021, these remains were found to represent two new spinosaurids, Ceratosuchops and Riparovenator, named by Chris T. Barker, David William Elliot Hone, Darren Naish, Andrea Cau, Jeremy A.F. Lockwood, Brian Foster, Claire E. Clarkin, Philipp Schneider and Neil John Gostling. Riparovenator is Latin meaning “Riverbank Hunter” from rīpārius, "of the river bank", and vēnātor, "hunter". This name was given because it is thought by some that spinosaurids hunted aquatic prey along the edges of rivers and lakes like a giant heron (more on that soon!). The species name honors paleontologist Angela Milner, who was one of the scientists who described Baryonyx, and who passed away the same year Riparovenator was described. The holotype of Riparovenator (catalogued under the following specimen numbers: IWCMS 2014.95.6, 2014.96.1, 2014.96.2, 2020.448.1, 2020.448.2, and 2014.96.3) consists of premaxillary elements, a braincase, and a partial lacrimal and prefrontal bone. Additional referred material includes a nasal fragment (IWCMS 2014.95.7) and a series of 22 caudal (tail) vertebrae (IWCMS 2020.447.1-39).

Evolution and Description: Riparovenator was a spinosaurid, a specialized clade of megalosaurs that survived the Jurassic and thrived in the early Cretaceous. They typically had long crocodile-like snouts, though they were laterally compressed like most theropod skulls instead flattened top to bottom like a crocodile’s. They had robust arms with large claws on their three-fingered hands. Though some researchers had posited that at least some spinosaurids were quadrupedal, this has been found to be unlikely by most studies, so they were probably bipedal like other theropods, though their hind limbs were a bit on the short side by theropod standards. Their teeth were conical and lacked serration, like a crocodilian’s. Riparovenator was probably no different in these regards from other spinosaurs. It was a member of the subfamily Baryonichinae, so it probably looked more like Baryonyx than spinosaurine spinosaurids like Spinosaurus. So as a baryonychine spinosaurid, Riparovenator probably lacked the prominent sail that adorned the backs of many spinosaurines. Like other spinosaurids though, it probably did have small display crests on its head. Riparovenator was small for a spinosaurid, but still a large theropod, likely measuring around 8.5 meters (28 ft) in length.

Ecology: The Wessex Formation was a dry “Mediterranean” climate habitat with gallery forests forming around rivers. Around the river, Riparovenator would have probably been an apex predator, a role it would have shared with its close relative Ceratosuchops, which lived at the same time and place. Baryonyx, is known from Sussex, slightly east of Riparovenator’s home in Wessex, but may have also overlapped in range with Riparovenator, in which case it also may have shared the title of apex predator of the river’s edge. Having multiple similarly sized and closely related predators suggests there may have been some sort of niche partitioning between them. While they all would have been predators of predominantly aquatic prey, each species may have had a specific prey preference that differed from each other, thus helping them to avoid competition. They were probably sight-based predators, perhaps also relying on sensitive touch receptors in their snout, as studies on brain casts of spinosaurids show that they probably didn’t have the most extraordinary sense of smell or hearing. Their brains were also fairly typical of other non-coeluosaurian theropods, so their intelligence and behavior were probably similar to a crocodile’s a lizard’s.

Riparovenator, like spinosaurids in general, was probably a river’s edge predator, like a heron. Their conical teeth were great from grabbing fish or other slippery aquatic prey, but would have poor at slicing through flesh, so it is thought that they mostly hunted aquatic prey, and left the hunting of large terrestrial prey, such as Iguanodon, to other theropods, in the case of the Wessex Formation, the moderately large charcharodontosaur Neovenator, and the medium-sized early tyrannosaur Eotyrannus. There has been some debate as to whether spinosaurids were fully aquatic predators that actually swam after prey, or if they merely patrolled the water’s edge and waded into the shallows and caught prey like a heron. Both sides have put forth good arguments in the last couple decades, but there is a bit more support for the heron hypothesis, and this seems to be more of the view of the authors who named Riparovenator, “the Riverbank Hunter”. Interestingly, while they may have waded like herons, spinosaur head movements in catching prey may have been more-crocodile like. They lack the binocular vision and strong s-curve necks of herons, and so probably didn’t dart their heads forward to catch prey like one. Instead, they probably relied on quick lateral (side to side) movements of their jaws to catch prey as it swam by, much like a crocodilian. So their nickname “Croc-Faced Hell-Herons” may in fact be quite accurate. They may have swallowed food whole, like a heron or pelican, or may have used their large hand claws to tear apart larger prey.

Extinction and Legacy: With so little material, it is impossible to say what the temporal range of Riparovenator was. But spinosaurids thrived near wetlands in Europe, Asia, Africa, and South America, throughout the Early and mid-Cretaceous, so it is likely that Riparovenator, or its descendants, continued on in southern England for quite some time before going extinct. Spinosaurids went extinct around 93 million years ago, around the same time as carcharodontosaurs and the marine ichthyosaurs, indicating a widespread global ecological shift as the Earth moved into the Late Cretaceous. What caused this shift isn’t entirely clear, but it was probably due to a combination of localized extinctions, compounded by isolation due to the separating continents, as well as major changes in climate. The bones of Riparovenator are held in the Dinosaur Isle, a museum in Sandown on England’s Isle of White.