When I was a kid, Spinosaurus was one of my favorite dinosaurs. There was something so wonderfully odd about a massive predator with a sail on its back, but the trouble was that no one had a good idea what this animal looked like.
Spinosaurs have been known to paleontologists since 1820. The trouble was that the first remains ever found—two teeth discovered in England—were not very informative, and were originally interpreted as making up part of a crocodile's smile.
It was not until about one hundred years later, in 1915, that the German paleontologist Ernst Stromer described and named Spinosaurus itself from bones found in the Cretaceous rock of Egypt. It was clearly a massive animal, comparable in size to Tyrannosaurus, but the skeleton Stromer found was very fragmentary. All he had were parts of the sail, ribs, tail and lower jaw. Even worse, these fossils were destroyed during WWII when Allied bombers attacked Munich. Stromer's scientific descriptions survived, but no other remains of Spinosaurus had been found. Thus paleo-artists were faced with a conundrum: the strange nature of Spinosaurus made it quite popular, but there was no way to know what the whole animal looked like. In many cases—such as a John Sibbick restoration I recall from my childhood—Spinosaurus was restored as a creature similar to Allosaurus with a sail on top.
The discovery of a related dinosaur in 1986 would help solve the mystery. In that year paleontologists Alan Charig and Angela Milner described the partial skeleton of a predatory dinosaur with a large claws on its forelimbs and an elongated, crocodile-like snout they called Baryonyx. (It is probable that the teeth found in the 19th century belong to this dinosaur.) It was a dinosaurian predator unlike almost every other, except one—details of its teeth and lower jaw corresponded to the long-last remains of Spinosaurus. Further discoveries, such as Irritator (named in 1996) and Suchomimus (named in 1998), strengthened this connection, and it was confirmed by new discoveries of partial Spinosaurus skulls and jaws over the past 15 years. Rather than being an oddball, Spinosaurus was the first-recognized member of a group of crocodile-snouted dinosaurs.
Paleontologist Tor Bertin has just reviewed our present knowledge of this group of dinosaurs in PalArch's Journal of Vertebrate Paleontology. Despite what we have come to know about them in the past few decades, they are still mysterious, rare dinosaurs.
As is presently understood, the spinosaurs can be split into two subgroups: the Spinosaurinae (Spinosaurus and Irritator) and the Baryonychinae (Baryonyx and Suchomimus ). (There were most certainly others, but many spinosaur remains are so fragmentary that it is difficult to tell whether they belong to a distinct species or one already known.) The differences between them are subtle. The baryonychines, for example, have a greater number of teeth in their jaws, as well as having larger teeth in the front of the jaw and teeth that are a little more blade-like. There were other slight differences in the skull, as well, but since teeth are the most commonly discovered parts of spinosaurs, they are the most useful parts of the skeleton for making comparisons among spinosaurs from different places.
That brings us to an interesting hypothesis of Bertin's. Spinosaurs have been found in Europe, Africa, Asia, South America and, according to a presentation at this year's Society of Vertebrate Paleontology conference, Australia. They were wide-ranging dinosaurs that existed from the Late Jurassic through the Late Cretaceous. Given how widely they were distributed over space and time, Bertin proposes that spinosaurs might have been present in North America, too.
Bertin bases his case on a collection of spinosaur teeth recently found in the Late Cretaceous rock of China. During this time a land connection allowed the dispersal of dinosaurs from Asia to North America, resulting in a sweeping correspondence between the Late Cretaceous dinosaurs on both continents. Given this consistency, it is certainly possible that spinosaurs also crossed over into western North America, although definitive proof that they did so has not yet been recognized. Paleontologists working in North America should keep a watchful eye out for spinosaurs, Bertin suggests, especially since many spinosaur teeth have been mistaken for those of crocodiles in the past!
Frustratingly, the dearth of good spinosaur skeletons has made it very difficult to study the group. There is good evidence, from gut contents and from bones with teeth embedded in them, that spinosaurs fed on fish, pterosaurs and young dinosaurs, but the mechanics of their jaws and the range of prey they could have tackled require further study. (Likewise, even though Spinosaurus was restored with long arms in Jurassic Park III, no one has yet found a Spinosaurus arm bone. We don't know whether it had relatively small and short arms like other large predatory dinosaurs or whether it retained the strong, relatively large arms seen in its relatives like Baryonyx.) In general it appears that the spinosaurs were fish-eaters that hunted near the water's edge and snapped up whatever other prey they could catch, but, outside of this general statement, we still have much to learn about the lives of these strange dinosaurs.
Bertin, Tor. (2010). A Catalogue of Material and Review of the Spinosauridae. PalArch's Journal of Vertebrate Paleontology, 7 (4), 1-39