Sunday, May 17, 2015

Deinonychus: Beast of the Week

Today we will be looking at a well-loved, and extremely important dinosaur.  Check out Deinonychus antirrhopusDeinonychus was a meat-eating dinosaur that lived in what is now the United States, including Wyoming, Montana, Oklahoma, and even parts of the East coast, including Maryland, during the Early Cretaceous Period, between 115 and 108 million years ago.  It was relatively small, measuring about eleven feet long from snout to tail.  The name, Deinonychus, translates to "Terrible Claw" in reference to the second digit on each of its feet, which possessed an enlarged, crescent-shaped talon, which was retractable, like a switchblade.

Deinonychus about to get you.  Reconstruction by Christopher DiPiazza.

Starting in the 1800s, when the first dinosaur was seriously recognized by science, the image of dinosaurs was that of hulking, oafish lizard-like creatures.  That all changed when the bones of Deinonychus were unearthed in the 1960s, however.  It was the first dinosaur that really made paleontologists stop and reassess their views on the way dinosaurs lived, by showing signs of being a fast-moving, light-weight predator that shared more characteristics with birds than it did with other reptiles, like lizards and crocodiles.  Since then, many more bird-like dinosaurs have been discovered, of course.  Most people now, especially younger dinosaur lovers, would see small, bird-like dinosaurs as a commodity in their books and toy collections, but one shouldn't forget that Deinonychus was society's first taste of this amazing kind of dinosaur.

Deinonychus skeletal mount on display at the American Museum of Natural History in New York.

When alive, despite its size, Deinonychus possessed weapons which would have made it a formidable predator.  The most obvious were the feet, each possessing a second digit with an almost five inch long "killer claw" which could be held above the ground when not in use to prevent wear, and swung forward for stabbing, when attacking.  Each of the hands was equipped with three long fingers, each tipped with a hook-shaped claw, as well.  Inside the mouth, Deinonychus had many small, blade like teeth, which were serrated.  The jaws of this dinosaur were designed for slicing off bite-sized pieces of meat.  Deinonychus' tail was long (took up about half of its total body length) and had small bony rods running down its length.  We call these structures ossified tendons, which are present in a lot of different kinds of dinosaurs.  They would have made the tail stiff, like a fishing pole, and would have helped the dinosaur make sharper turns while running.  Although only bones have ever been found, it is likely Deinonychus was covered in feathers just like a bird, based on more well-preserved remains of other dinosaurs which were extremely closely related to it. 

Deinonychus food diagram

There are a few ideas as to exactly how Deinonychus hunted.  The first, and most well-known is that Deinonychus hunted in groups to kill larger prey.  The idea is that these predators could have used their powerful hind legs to jump onto larger prey and clung on using their front claws and used tails for balance.  Then they would have used their deadly claws on their feet to bicycle kick into their prey's body, essentially disemboweling it until it collapsed.  This pack-hunting theory is further supported by an amazing discovery of dinosaur fossils which includes the bones of the larger plant-eating dinosaur, Tenontosaurus, with Deinonychus teeth marks on its bones.  One could argue that there is a strong chance the Tenontosaurus was already dead and that the Deinonychus were merely scavenging it, but bones from Deinonychus were found nearby as well!  This suggests that the smaller meat-eaters may have indeed attacked it, and in self defense the larger plant-eater managed to take out some of its attackers before it died.  It's a cool idea, and has been recreated in art countless times, (Seriously, poor Tenontosaurus' whole identity has been reduced to "Deinonychus food" in most books and other media.) but still really can't be totally proven.  Some who oppose this idea argue that perhaps the Deinonychus were not pack hunters, were all drawn to the plant-eater's dead body, and killed a few of each other as they fought over the meat.

The second idea of Deinonychus' hunting behavior delves into its killer claw more deeply.  Believe it or not, many modern birds also actually have an enlarged second digit talon on their feet.  The ones that do, like most hawks and eagles, use this claw to pin down prey (alive or dead) to stabilize it as they tear bite-size chunks of meat off with their sharp beaks.  It is very possible that dinosaurs like Deinonychus could have hunted mostly smaller prey, and used their talons for the same purpose. The video below I took at my job of our Eurasian Hawk demonstrating this technique on a dead mouse.  



Eggs that are believed to have belonged to Deinonychus have also been discovered.  It all started when the rock from which a Deinonychus skeleton was extracted were examined more closely.  It was discovered that they contained dinosaur eggshells.  The next question was whether or not the Deinonychus was eating the eggs, which could have belonged to another dinosaur, or if it the eggs were its own and it was protecting them.  Soon after, tiny bones, called gastralia, were discovered with the eggshells.  Gastralia, or belly ribs, as they are sometimes called, are found on the underside of a dinosaur's torso.  This suggested that the Deinonychus' chest and belly were in contact with the eggs, and it very well may have been incubating them, much like parent birds do today.  Not only does this support the idea that Deinonychus was guarding its own eggs, it also suggests that Deinonychus was endothermic! ("warm-blooded")  Think about it.  Only an animal that gave off its own body heat would brood eggs to keep them warm.  Ectothermic ("cold-blooded") animals, like lizards and crocodiles, rely on the sun, decomposing nesting material, or other outside sources of warmth to incubate their eggs. (There are exceptions, like some pythons who incubate their eggs, creating warmth with muscle friction, but this is an exception, not a norm.)  What a great find!

That is all for this week!  As always feel free to comment below or on our facebook page!

References

Fowler, D. W.; Freedman, E. A.; Scannella, J. B.; Kambic, R. E. (2011). Farke, Andrew Allen, ed. "The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds". PLoS ONE 6 (12): e28964. doi:10.1371/journal.pone.0028964. PMC 3237572. PMID 22194962.

Makovicky, P.J.; Grellet-Tinner, G. (2000). "Association between a specimen of Deinonychus antirrhopus and theropod eggshell". In Bravo, A.M. and T. Reyes. First international symposium on dinosaur eggs and babies,Isona i Conca Dellà Catalonia, Spain, 23–26 September 1999. pp. 123–128.

Maxwell, W. D.; Ostrom, J.H. (1995). "Taphonomy and paleobiological implications of TenontosaurusDeinonychus associations". Journal of Vertebrate Paleontology 15 (4): 707–712. doi:10.1080/02724634.1995.10011256.

Ostrom, John Harold (1970). "Stratigraphy and paleontology of the Cloverly Formation (Lower Cretaceous) of the Bighorn Basin area, Wyoming and Montana". Bulletin of the Peabody Museum of Natural History 35: 1–234.

Ostrom, J. H. (1969). "Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana". Peabody Museum of Natural History Bulletin 30: 1–165.

Ostrom, J.H. (1976). "On a new specimen of the Lower Cretaceous theropod dinosaur Deinonychus antirrhopus". Breviora 439: 1–21.

Turner, Alan H.; Makovicky, Peter J.; Norell, Mark A. (2007). "Feather quill knobs in the dinosaur Velociraptor". Science 317 (5845): 1721. doi:10.1126/science.1145076. PMID 17885130.

Xu, X.; Zhou, Z.; Wang, X.; Kuang, X.; Zhang, F. & Du, X. (2003). "Four-winged dinosaurs from China". Nature 421 (6921): 335–340. doi:10.1038/nature01342. PMID 12540892.

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