August 15, 2012
Carnotaurus was a weirdo. Not only did this 26-foot predator of Argentina’s Late Cretaceous have prominent horns jutting from its short, deep skull, but, since the time of the dinosaur’s discovery in 1985, paleontologists have been puzzled by the strange arms of the theropod. Despite having absolutely huge shoulder bones, Carnotaurus had wimpy arms that were even stubbier than those on the oft-ridiculed tyrannosaurs. Stubby forelimbs go all the way back to the beginning of the lineage that Carnotaurus belonged to–the abelisaurids–but this ancient South American predator took the reduction to extremes.
Among the relatively short-armed tyrannosaurs, at least, the evolution of small arms is often associated with developing big, well-muscled heads. As tyrannosaur heads became larger and heftier, their arms became smaller to compensate. The idea is that it’s all about balance–if you have a huge head and beefy arms, you’re going to fall on your face. (Sorry, Trogdor.) So far as I know, no one has actually tracked these evolutionary trends, but it remains the prevailing hypothesis. An in-press Acta Palaeontologica Polonica paper about the neck of Carnotaurus forwards a similar explanation for the puny arms of abelisaurids.
The study, written by paleontologist Ariel Méndez, compares the neck vertebrae of Carnotaurus with the same bones in the dinosaur’s close cousin from Cretaceous Madagascar, Majungasaurus. Both were big, short-snouted predators with strange head ornaments, but, as Méndez points out, the neck of Carnotaurus is much more heavily built. For example, the neck vertebrae of Carnotaurus are much wider, with the last bone in the series being as wide as the dinosaur’s skull. In Majungasaurus, the last neck vertebra is only about half the width of the skull (although it should be noted that the Majungasaurus neck vertebrae were inflated in size by about 20 percent to match the neck of a subadult to an adult skull).
So what do these differences mean? Unfortunately, Méndez does not include a full muscular reconstruction in the study but notes that the bony differences almost certainly indicate different muscle arrangements. In general, it seems that Carnotaurus was a more robust animal than Majungasaurus, although increased power may have come with a cost of reduced flexibility between the base of the neck and the tail. Méndez, referring to previous research, also points out that having more heavily-built skulls and necks may be associated with smaller forelimbs. Indeed, while skulls are often the focus of feeding studies, recent research on a variety of carnivores–such as Tyrannosaurus, the sabercat Smilodon and the modern Komodo dragon–have affirmed the importance of neck muscles to feeding. Even carnivores with relatively weak bites, such as sabercats and Komodo dragons, receive a great deal of extra power from their neck muscles while feeding. Perhaps the same was true of Carnotaurus.
Yet the stouter neck of Carnotaurus doesn’t actually explain why this dinosaur had tiny arms. After all, Majungasaurus also had the robust shoulder girdle-vestigial arm combination, yet its neck is clearly not as heavily built as in Carnotaurus. More than that, big shoulders and smallish arms seem to go all the way back to early abelisaurids, such as the recently-described Eoabelisaurus. Although the hefty head and neck-small arms idea makes sense, the idea has yet to be rigorously tested against the actual history of dinosaurs such as abelisaurids and tyrannosaurs. Why huge, powerful carnivores had puny arms remains an evolutionary puzzle.
Méndez, A. (2012). The cervical vertebrae of the Late Cretaceous abelisaurid dinosaur Carnotaurus sastrei Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0129
May 24, 2012
Some dinosaur lineages are more famous than others. I can say “tyrannosaur” and most anyone immediately knows what I’m talking about: a big-headed, small-armed predator similar to the notorious Tyrannosaurus rex. The same goes for “stegosaur,” and of course it helps that Stegosaurus itself is the famous emblem of this bizarre group. But public understanding hasn’t kept up with new discoveries. In the past two decades, paleontologists have identified various dinosaur lineages vastly different from the classic types that gained their fame during the Bone Wars era of the late 19th century. One of those relatively obscure groups is the abelisaurids: large theropod dinosaurs such as Carnotaurus with high, short skulls and ridiculously stubby arms that make T. rex look like Trogdor the Burninator. And paleontologists Diego Pol and Oliver Rauhut have just described an animal close to the beginning of this group of supreme predators—a dinosaur from the dawn of the abelisaurid reign.
Pol and Rauhut named the dinosaur Eoabelisaurus mefi. Discovered in roughly 170-million-year-old Jurassic rock near Chubut, Argentina, the mostly complete dinosaur skeleton is about 40 million year older than the next oldest abelisaurid skeleton. Eoabelisaurus, placed in context with other theropod dinosaurs of the same era, represents a time when predatory dinosaurs were undergoing a major radiation. Early members of many terrifying Cretaceous predators such as the tyrannosaurs and abelisaurids had already appeared by the Middle to Late Jurassic.
Not all of these Jurassic predators looked quite like their later Cretaceous counterparts. Jurassic tyrannosaurs such as Juratyrant and Stokesosaurus were relatively small predators, unlike their bulky, titanic relatives from the Late Cretaceous. Eoabelisaurus was a little closer to what was to come.
Despite being many tens of millions of years older than relatives such as Carnotaurus and Majungasaurus, the newly described dinosaur displays some tell-tale features that characterize the group. While a significant portion of the dinosaur’s skull is missing, the head of Eoabelisaurus had the short, deep profile seen among other abelisaurids. And this dinosaur already had distinct forelimbs. Much like its later relatives, Eoabelisaurus had a strange combination of heavy shoulder blades but wimpy forelimbs, with a long upper arm compared to the lower part of the arm. The dinosaur’s condition was not as extreme as in Carnotaurus—a dinosaur whose lower forelimbs were so strange that we have no idea what, if anything, Carnotaurus was doing with its arms—but they were still comparatively small and tipped with little fingers good for wiggling but probably useless in capturing prey.
And with a 40-million-year gap between Eoabelisaurus and its closest kin, there are plenty of other abelisaurids to find. The question is where they are. Is their record so poor that very few were preserved? Or are they waiting in relatively unexplored places? Now that the history of these blunt-skulled predators has been pushed back, paleontologists can target places to look for the carnivores.
Pol, D., Rauhut, O. (2012). A Middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs. Proceedings of the Royal Society B, 1-6 : 10.1098/rspb.2012.0660
May 16, 2012
Dinosaurs and mini-golf: The two complement each other. Granted, dinosaurs probably wouldn’t have been very good at the pastime—imagine how hard it would be for Carnotaurus to use a putter—but they make for excellent fairway decor. And in some places, the dinosaurs remain even after the mini-golf course has closed. Paleontologist Joe Peterson sent in this example: a Tyrannosaurus standing over a closed course in Oshkosh, Wisconsin. Maybe it’s just the position of the hands, but the tyrant seems to be begging. “MOAR TASTY TOURISTS, PLZ?”
Have you seen a dinosaur or other prehistoric creature in an unusual place? Please send a photo to firstname.lastname@example.org.
January 24, 2012
A few months ago, I wrote about a big, carnivorous dinosaur with what may have been the wimpiest arms of all time. No, not Tyrannosaurus, but a very distantly related predatory dinosaur from Cretaceous South America called Carnotaurus. Despite this dinosaur’s massive, beefy shoulderblade, the arm of Carnotaurus was little more than a nub that would have barely stuck out from the body. And, according to a recent fossil find from Madagascar, Carnotaurus wasn’t alone in having ridiculously tiny forelimbs.
Carnotaurus belonged to a group of theropods called abelisaurids. Among them were large predators that spread through the southern portion of the Cretaceous world, including Majungasaurus from Madagascar. (This dinosaur got a brief publicity boost thanks to the first episode of the sensationalistic show Jurassic Fight Club.) This was another hefty carnivore with bizarre head ornamentation. As demonstrated in a new Journal of Vertebrate Paleontology paper by researchers Sara Burch and Matthew Carrano, Majungasaurus also had truly vestigial arms.
Tiny arms are a common abelisaurid feature. Majungasaurus was expected to share this feature with other closely related dinosaurs, but a lack of fossil evidence prevented paleontologists from seeing what the forelimb of this animal really looked like. That changed in 2005, when paleontologists discovered a nearly complete and mostly articulated skeleton of Majungasaurus, including elements from the entire forelimb and shoulder girdle. (Among the lot was a furcula, or the equivalent of a wishbone, which is the first time this bone has been found in an abelisaurid.)
When viewed together, the forelimbs of this animal look like an evolutionary joke. A large humerus connects to a broad shoulder girdle, but the lower part of the arm—from the radius and ulna down to the dinosaur’s four fingers—is composed of short, stout bones that altogether make up less than a third of the length of the upper arm bone. And the fingers were short, stubby, and lacked sharp claws.
But what may be even stranger is that the arms of Majungasaurus were probably capable of a relatively wide range of motion. The connection between the humerus and the shoulder girdle was more flexible than in many other theropod dinosaurs, and Burch and Carrano suggest that the wrist of Majungasaurus, too, could probably be extended quite far. Conversely, though, the paleontologists note that the fingers were probably relatively stiff and the dinosaur lacked the ability to move them very much, so perhaps the dinosaur used its hand as a single unit—like a dinosaurian mitten. That’s assuming that Majungasaurus was doing anything with its arms at all. This dinosaur’s arms and hands had become so reduced that it is difficult to imagine what they could have possibly done with them other than impotently flap them around. We may never know for sure.
Burch, S., & Carrano, M. (2012). An articulated pectoral girdle and forelimb of the abelisaurid theropod Majungasaurus crenatissimus from the Late Cretaceous of Madagascar Journal of Vertebrate Paleontology, 32 (1), 1-16 DOI: 10.1080/02724634.2012.622027
September 22, 2011
Tyrannosaurus gets a lot of guff for having relatively small, two-fingered hands, but that isn’t really fair. Though small, the arms of Tyrannosaurus and other big tyrannosaurs were robust and heavily muscled, hinting that the dinosaurs may have used their arms like meat hooks while tangling with struggling prey. So let’s have no more of this “Tyrannosaurus had sissy arms” nonsense. If we’re going to poke fun at any dinosaur for having wimpy forelimbs, it should probably be Carnotaurus.
While tyrannosaurus were among the most formidable predators in North America and Asia during the Late Cretaceous, in South America the same roles were often played by a different breed of theropod dinosaur known as abelisaurids. Of these, Carnotaurus is probably the most famous—the fact that this “meat-eating bull” had two horns sticking out of its short, deep skull gave it an instant appeal. As fearsome as Carnotaurus looked, though, it’s hard not chuckle at the dinosaur’s arms—the hand and lower part of the forelimb were so reduced in size that some paleontologists have viewed them as vestigial structures that have almost entirely lost their ability to function in acquiring prey. In a new paper published in Palaeontology, researcher Javier Ruiz and colleagues reexamine the strange arms of this dinosaur and how they compare to those of other abelisaurid predators such as Majungasaurus and Aucasaurus.
As pointed out by Ruiz and co-authors, the arms of Carnotaurus have a robust lower portion, made up of the radius and ulna, that is about a quarter of the length of the upper arm bone (the humerus). The hand itself has four fingers, and unlike in the other abelisaurids considered in the paper, the fourth metacarpal bone is the biggest bone in the hand. This small and peculiar difference helps set Carnotaurus apart, but the comparisons among this dinosaur, Majungasaurus and Aucasaurus may also add some new information about how the arms of these dinosaurs got to be so wimpy.
In the big picture of theropod evolution, the abelisaurid dinosaurs belong to an even larger group called ceratosaurs. Earlier representatives of this group such as Limusaurus and Ceratosaurus already had relatively short and stubby hands in the Jurassic, and it appears that the hands of abelisaurids followed this evolutionary trend. The question is why this reduction in limb size happened. We can come up with “just so” stories in an attempt to explain the trend, but testing the idea is another matter entirely and something that is not touched on in the paper by Ruiz and collaborators. Equally perplexing is why the hand of Carnotaurus was so small while the other arm bones were thick and powerful-looking, even compared to other abelisaurids. We don’t yet have a good answer for why this should be so. For now, Ruiz and colleagues conclude that the hands of the odd abelisaurids were as odd and diverse as the different arrangements of crests, horns and bumps which adorned their skulls. How the structures related to the lives of the animals themselves will require further study.
RUIZ, J., TORICES, A., SERRANO, H., & LÓPEZ, V. (2011). The hand structure of Carnotaurus sastrei (Theropoda, Abelisauridae): implications for hand diversity and evolution in abelisaurids Palaeontology DOI: 10.1111/j.1475-4983.2011.01091.x