Acrocanthosaurus atokensis: The Mandatory Giant Theropod Post of the summer.
Size: 40 feet (12 meters) in length.
Time Period: The Aptian and Albian stages of the Early Cretaceous Period.
Locale: The type specimen was unearthed in the Antlers Formation of Oklahoma, though additional locations will be discussed.
Name: The generic name means “high-spined lizard,” in reference to this animal’s high vertebrae. The specific name refers to Atoka county, in which this dinosaur was first discovered.
I assume that when I say huge, North American theropod, a certain Tyrannosaurus rex jumps to mind. The next best thing would be Allosaurus or the latter’s possible synonym, Saurophaganax. However, the huge, North American theropod that I plan on discussing here is Acrocanthosaurus, first discovered in the 1940s.
The most notable feature of Acrocanthosaurus (which even made it into the animal’s genus name) is the row of tall neural spines projecting from its back, neck, tail, and hips. They were over two times the size of the vertebrae that they projected from, and some had prominent depressions on the sides. Many other prehistoric animals had such structures, such as Dimetrodon and Spinosaurus, the latter of which had neural spines that were 11 times taller than the vertebrae supporting them. The spines could have supported a sail like that of a Dimetrodon, or may have instead supported a raised hump on its back like that of a bison. The latter makes as much sense as the more popularized former option when one considers that, like a bison, Acrocanthosaurus had some room for muscle attatchment on or at the base of its spines. The ridge or sail created could have been involved in fat storage, temperature control, communication, or sexual display. No matter what the case, Acrocanthosaurus’s spines made it very unique and, dare I say, incredibly cool-looking.
The arms of Acrocanthosaurus were equipped with eagle-like talons, and were probably used to assist in the catching of prey. When the Acrocanthosaurus had used its large head and jaws to bite down on prey, the muscular forelimbs would retract towards the body, holding the theropod’s future meal closer to it. The stiffness of the claws and the hand in general may have been an adaption for gripping onto prey without worrying (if dinosaurs could worry) about dislocating a finger. This is an interesting concept, and I’d like to see similar research about other theropod hunting styles.
The history of Acrocanthosaurus when it comes to classification is somewhat jumbled. Tall neural spines from Europe were discovered after Acrocanthosaurus, and were labeled Altispinax. Later, a few paleontologists included the neural spines in the genus Acrocanthosaurus before realizing that they didn’t belong in either genus. They were then assigned to Becklespinax, remaining the only remains from this enigmatic genus. Later sources tell you that Acrocanthosaurus was a ‘carnosaur,’ and a member of the wastebasket group that was home to such disparate animals as Dilophosaurus, Tyrannosaurus, and Ceratosaurus at the time. Within this group, the animal was often advertised as a spinosaur because of its high neural spines. Still other, slightly less farfetched publications asserted its status as an allosaurid, possibly because of the moderate resemblance and distribution. Now, we know that Acrocanthosaurus was a ‘carnosaur’ in the sense that it was a member of the Allosauroidea. More specifically, it was a carcharodontosaurid, and a relative of the stupendously proportioned Giganotosaurus and Carcharodontosaurus.
Since its initial discovery in Oklahoma, different remains of Acrocanthosaurus, or animals like it, have been popping up around the United States. In the 1990s, more complete specimens of Acrocanthosaurus were found in the Twin Mountains Formation of Texas. Acrocanthosaurus was also found to be the only truly large theropod in the Cloverly Formation of Wyoming, and its remains were found near the huge shoulder blade of a Sauroposeidon, one of the sauropods that it may have eaten. Speaking of which, there is a trackway in Texas showing theropod footprints alongside those of a sauropod. The former may have belonged to Acrocanthosaurus and the latter to Sauroposeidon. The Acrocanthosaurus’s footprints indicate that it tried to jump on the side of the sauropod, but the trackway ends before we can see what goes on. Outside of Texas, Wyoming, and Oklahoma, more dubious remains have been found that could belong to Acrocanthosaurus. A tooth found in Arizona, as well as various tooth marks on sauropod bones, have been attributed to Acrocanthosaurus, and several teeth from the Arundel Formation of Maryland may belong to an eastern member of the genus Acrocanthosaurus. If this is the case, Acrocanthosaurus had a wide distribution and was also very successful.
Lastly, this dinosaur’s presence, along with that of some others, may suggest an interesting dispersal pattern for the whole of the Carcharodontosauridae. Neovenator, the basalmost known member of the Carcharodontosauridae’s sister taxon, is known from Europe, as is Concavenator, a basal carcharodontosaurid. It can be proposed that the entire group originated in Europe, and that they then spread to Asia and North America, and from these continents to Gondwana. The theory needs some backing up (thanks to the find of a really basal carcharodontosaur in the Jurassic of Africa), but it could be sound. At the very least, the neovenatorids probably had a Laurasian origin.
Wow, that post was longer than I intended it to be. It’s fitting that it is, because it’s the last post I’ll create in two weeks. I apologize for the spotty update rate of the blog, but I’m short on ideas. This was my last big gamble at a last big post. Thanks for the attention and see you in a bit.