November 26, 2012
Stegosaurus was a weird dinosaur. We’ve known that for well over a century, but, as Darren Naish has often pointed out, Stegosaurus was strange even compared to its Jurassic relatives. The dinosaur’s arrangement of broad, alternating plates is a departure from the arrangements of smaller plates, back spikes and accessory spines seen on many other stegosaurs, including the perplexingly well-armed Gigantspinosaurus sichuanensis.
Ornamented with a double row of short, narrow plates along its back, the roughly 160-million-year-old Gigantspinosaurus generally resembled other stegosaurs from Late Jurassic Asia, such as Tuojiangosaurus. But, as you might be able to guess from the dinosaur’s name, the feature that immediately sets Gigantspinosaurus apart from similar species is a enormous hooked spine that jutted out from behind the shoulder blade. These striking spikes were found close to their life position on the first skeleton of this dinosaur to be found–erroneously attributed to Tuojiangosaurus, before being redescribed as Gigantspinosaurus in 1992–although their exact orientation isn’t entirely clear. Did the shoulder spikes curve straight backward, or were they tiled slightly upwards? And, more significantly, how did such prominent ornaments evolve? No one knows.
As yet, we know relatively little about the natural history of Gigantspinosaurus. The dinosaur has a name, and skin impressions have helped researchers restore what the stegosaur looked like, but many aspects of the spiky herbivore’s biology remain mysterious. In the grand scheme of stegosaur evolution, though, the ornamentation of Gigantspinosaurus has sometimes been taken as evidence that similar forms had shoulder spikes. In addition to paired spikes along its tail, the Late Jurassic stegosaur Kentrosaurus possessed an extra pair of spikes along its side. These were originally placed over the hips, but, due to the discovery of Gigantspinosaurus, some researchers have argued that the spikes truly belong at the shoulders.
Frustratingly, paleontologists have yet to find a Kentrosaurus skeleton with side spikes in place. But the discovery of Gigantspinosaurus doesn’t necessarily mean that its cousin Kentrosaurus had the same arrangement. Among stegosaurs, the two genera were relatively distantly related, and it’s entirely possible that more than one side spike arrangement evolved. As paleontologist Heinrich Mallison has argued, the hips of Kentrosaurus seem to possess areas where the spikes could have articulated, and this arrangement would be consistent with the dinosaur’s ornamentation pattern–small plates at the front give way to spikes along the stegosaur’s back and tail. Indeed, the side spikes on Kentrosaurus more closely resemble the same structures along the dinosaur’s back and tail and the shoulder spike of Gigantspinosaurus. If Kentrosaurus had plates up front and serially homologous spikes along the back, then why shouldn’t the hip spikes remain a reasonable hypothesis? Together, Gigantspinosaurus and Kentrosaurus might represent different alternatives in the stegosaur armory.
April 24, 2012
Spinosaurus was one of my favorite childhood dinosaurs. The carnivore’s enigmatic sail was certainly eye-catching, and that immense billboard set the predator apart from the other huge theropods. But the Spinosaurus I grew up with isn’t around anymore. The creature I knew was based on a partial skeleton discovered by German paleontologist Ernst Stromer in 1912, but was destroyed by an Allied bombing raid during World War II. With only photographs left, paleontologists and artists filled in the missing parts of the spinosaur’s anatomy on the basis of other large, carnivorous dinosaurs. The end result was something like an Allosaurus with a sail.
The blunt-skulled Spinosaurus faded away as paleontologists found new specimens of closely related dinosaurs. The long-snouted Baryonx, discovered in England in 1983, showed that spinosaurs had huge hand claws, crocodile-like skulls. And despite the group’s name, some lacked sails. With this new search image in place, paleontologists began to turn up multiple new spinosaurs from Africa, South America, Australia and now southeast Asia.
Earlier this week, paleontologist Ronan Allain and co-authors described the partial skeleton of a new spinosaur in the journal Naturwissenschaften. The dinosaur, named Ichthyovenator laosensis, appears to be the first definite spinosaur known from Asia. (A few probable spinosaur teeth have been uncovered, hinting that there are skeletons still waiting to be found.) Exactly how long ago this dinosaur roamed Laos is unclear. While Ichthyovenator was discovered in Early Cretaceous rock, the deposits could be anywhere from about 125 to 112 million years old.
If the reconstruction presented by Allain and colleagues is correct, Ichthyovenator was an unusual spinosaur. In other sail-backed forms, such as Spinosaurus and Suchomimus, the great ornament is created by neural spines that rise to a peak and gradually slope downwards. But Icthyovenator might have had a more wavy sail that dipped downwards at the hips before briefly rising again, creating the appearance of two smaller sails.
We still don’t know why spinosaurs had sails to start with, so why Ichthyovenator displayed a different arrangement is doubly perplexing. And equally frustrating is the fact that the skull of Ichthyovenator remains unknown. More than anything else, the distinctive skulls of these dinosaurs set them apart from other theropods, but no skull bones or even teeth were found with this dinosaur. This makes the name Ichthyovenator—”fish hunter”—a hypothesis that has yet to be confirmed by additional evidence. Spinosaurs have often been cast as specialized fish hunters that may have hunted along prehistoric rivers and lakes. Ichthyovenator is expected to have shared this way of life, but we as yet know little of this dinosaur’s biology.
Allain, R., Xaisanavong, T., Richir, P., & Khentavong, B. (2012). The first definitive Asian spinosaurid (Dinosauria: Theropoda) from the early cretaceous of Laos Naturwissenschaften DOI: 10.1007/s00114-012-0911-7