Acrocanthosaurus: Beast of the Week

This week we are checking Acrocanthosaurus atokensis!  Acrocanthosaurus was a meat-eating dinosaur that lived in what is now the United States, including Oklahoma, Texas, Wyoming, and even as far east as Maryland, during the Early Cretaceous Period, between 125 and 100 million years ago.  Acrocanthosaurus measured about thirty eight feet (11.5 meters) long from snout to tail, and was likely the top predator in its environment.  The name Acrocanthosaurus, translates to "High Spine Lizard" and is in reference to this dinosaur's particularly tall neural arches, which gave it a distinctive ridge down its back.

Watercolor reconstruction of Acrocanthosaurus by Christopher DiPiazza.

Acrocanthosaurus' spine was interesting in that we haven't found another theropod dinosaur with anything similar.  It is often compared to the unrelated, Spinosaurus' sail-like structure, but Acrocanthosaurus' runs all the way from the back of the skull to about midway down the tail, rather than just the back like in Spinosaurus.  Acrocanthosaurus also has a close relative from Spain, Concavenator, which has a triangular-shaped hump over its hips, but again, this is concentrated to one area of the spine. The purpose of this ridge-like look is a mystery.  Some believe it would have formed a hump rather than a ridge, like what you might see on a modern bison's skeleton.  (Which also have extended neural arches, but mostly over the shoulders.)   As of now the safe answer could always be that Acrocanthosaurus possessed this ridge down its spine for display purposes to other members of its species.  Maybe they were larger in males than in females and played a role in mate selection?  Perhaps it was significantly shorter in juveniles?  There is still a lot we don't know!

Bison skeleton.  Note the tall neural arches above the shoulders.  The living animal has a hump there.

The skull of Acrocanthosaurus is long, low, and narrow in the front, but the back of the lower jaw become increasingly deep, suggesting there was a lot of muscle there in life, allowing this dinosaur to bite down hard.  The teeth of Acrocanthosaurus were slightly curved and serrated on both the front and back edges.  They were also thicker than the very blade-like teeth of its later relatives, like Giganotosaurus and Cacharodontosaurus, suggesting Acrocanthosaurus was more of a generalist hunter.  This is also further supported by the fact that Acrocanthosaurus' range was so widespread across what is now the United States, which would have consisted of multiple different kinds of habitats, and therefore likely different kinds of prey.

Acrocanthosaurus skull from the North Carolina Museum of Natural Sciences.

The front limbs of Acrocanthosaurus were short, but powerful, and had a decent range of motion to them.  Each hand possessed three hooked claws.  It is difficult to say exactly how Acrocanthosaurus would have utilized these arms since they were so short, but they may have played a part when it was going after large prey, like a sauropod, at close quarters.

Acrocanthosaurus skeletal mount on display at the North Carolina Museum of Natural Sciences.

When alive, Acrocanthosaurus, would have coexisted with many other dinosaurs, including the much smaller predator, Deinonychus, the ornithopod, Tenontosaurus, and a few different kinds of large sauropods, like Sauroposeidon and Astrodon on the east coast.  In fact, there is a dinosaur trackway in Texas consisting of large theropod prints that are believed to have been made by Acrocanthosaurus along with those of a large Sauropod.  These tracks show, since at times the theropod prints overlap the sauropod ones, that those of the theropod were made later, and could have possibly been following the larger herbivore.  At one point some scientists believe the theropod even attacked the sauropod, since the meat-eater tracks skip for a while where they finally intersect. (did it latch onto the side of its intended prey?)  This hypothesis can't be fully proven, however.  Even though no bones from either dinosaur were found with this trackway, it is suggested the predator was Acrocanthosaurus because the size, location, and age of the tracks.

Photograph of the tracks believed to have been from Acrocanthosaurus possibly stalking a sauropod in the field in Texas.  You can now see these tracks in person on display at the American Museum of Natural History in New York City.

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

References

Currie, Philip J.; Carpenter, Kenneth. (2000). "A new specimen of Acrocanthosaurus atokensis (Theropoda, Dinosauria) from the Lower Cretaceous Antlers Formation (Lower Cretaceous, Aptian) of Oklahoma, USA". Geodiversitas 22 (2): 207–246.

Lockley, Martin G. (1991). Tracking Dinosaurs: A New Look at an Ancient World. Cambridge: Cambridge University Press. p. 252pp.

Senter, Phil; Robins, James H. (2005). "Range of motion in the forelimb of the theropod dinosaur Acrocanthosaurus atokensis, and implications for predatory behaviour". Journal of Zoology 266 (3): 307–318.

Stovall, J. Willis; Langston, Wann. (1950). "Acrocanthosaurus atokensis, a new genus and species of Lower Cretaceous Theropoda from Oklahoma". American Midland Naturalist (American Midland Naturalist, Vol. 43, No. 3) 43 (3): 696–728.

Thomas, David A.; Farlow, James O. (1997). "Tracking a dinosaur attack". Scientific American 266 (6): 48–53.

Rhamphorhynchus: Beast of the Week

Rhamphorhynchus was a pterosaur that lived in what is now Germany during the late Jurassic period, between 150 and 148 million years ago.  The adults measured about four feet (1.2 meters) long from beak to tail and had wingspans of about six feet (1.8 meters) wide.  When alive Rhamphorhynchus would have eaten meat, mostly in the form of fish and other marine life.  The genus name translates to "Beak Snout" from Greek.  Rhamphorhynchus muensteri is by far the most well known species of Rhamphorhynchus, known from many individual specimens, however, there is also a species known from a single wing fossil found in what is now Dorset, England, called Rhamphorhynchus etchesi.

Rhamphorhynchus muensteri life reconstruction in watercolors by Christopher DiPiazza.

Rhamphorhynchus belonged to the group of pterosaurs that existed from the Triassic to the early Cretaceous periods, characterized by their proportionally smaller heads short legs, and long narrow tails, called rhamphorhynchoids. (named after Rhamphorhynchus, itself.)  In addition to their wings, which form from flexible skin attached from the tip of their fourth finger (which was extremely long and sturdy) to about the ankle, pterosaurs from this group also sported a membrane between their legs, which likely aided in flight.  Pterosaurs from this group also sported teeth, and Rhamphorhynchus was no exception.  The teeth of Rhamphorhynchus, were particularly long, slightly curved, and interlocked when the jaws were closed.  The tip of its jaws also curved into each other and were beaklike.  This seems to be an adaptation for grasping fish, cephalopods, and other slippery marine prey.  

Rhamphorhynchus skeleton on display at the Royal Ontario Museum that showcases preserved soft tissue, like wing membranes and the tail vane.

At the tip of its tail, Rhamphorhynchus sported a fan-like structure, called a tail vane, which may have helped it steer during flight.  Since Rhamphorhynchus is known from many individuals of different stages of maturity when they died with soft tissue preserved, we know that the shape of this structure changed as the animal aged.  The youngest individuals sported narrow, oval-shaped vane, slightly older individuals had vanes that were more pointed and diamond-like, while in the most mature individuals the tip of the vane flared out into an almost sideways triangle shape.  Because of this change it is possible the vane may have had a role in some sort of communication, like sexual display within the species, as well.  Originally, scientists thought Rhamphorynchus consisted of several more species than the two listed at the top of this post, grouping them based on size and tail vane shape.  More recently, however, it was determined they more likely mostly the same species at different developmental stages of life. 

Rhamphorhynchus fossil with imprints of the wing membranes and tail vane cast on display at the American Museum of Natural History in New York.

Since Rhamphorhynchus is known from many specimens of various sizes, paleontologists were able to study and predict how it would have grown as it aged when alive.  Thanks to a study in 2012, examining various specimens of different stages when they died, it was determined that like many modern reptiles, Rhamphorhynchus would have been able to survive on its own soon after hatching, meaning it was likely able to fly right away, and would have grown rapidly early in life.  This growth spurt seemed to slow down at about three years of age, which also seems to be when they were fully mature.  

The environment that Rhamphorynchus lived in would have been a series of islands with lagoons in a relatively shallow sea.  It likely would have hunted over the water, snatching small fish and other marine animals off the surface of the water while flying.  It would have coexisted with fellow pterosaur, Pterodactylus, and the dinosaurs Archaeopteryx and Compsognathus.  

References

Bennett, S. C. (1995). "A statistical study of Rhamphorhynchus from the Solnhofen Limestone of Germany: Year-classes of a single large species". Journal of Paleontology. 69 (3): 569–580.

Prondvai, E.; Stein, K.; Ősi, A.; Sander, M. P. (2012). Soares, Daphne (ed.). "Life history of Rhamphorhynchus inferred from bone histology and the diversity of pterosaurian growth strategies". PLOS ONE. 7 (2): e31392.

O'Sullivan, Michael; Martill, David M. (June 2015). "Evidence for the presence of Rhamphorhynchus (Pterosauria: Rhamphorhynchinae) in the Kimmeridge Clay of the UK" Proceedings of the Geologists' Association. 126 (3): 390–401.