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Tomorrow is the day to see tons of fossils at the Museum of Discovery. Come out and see what Arkansas has to offer in fossils. We have a far more diverse array of fossils than most people realize. Here are a few dinosaurs you will see.
Here are some casts of Allosaurus bones, a foot and a humerus. Allosaurus itself didn’t live here, but its descendant, Acrocanthosaurus, did.
Here is an ischium of an Apatosaur, which did not live here either. But its titanosaur relatives did.
Here are some hands and claws of Allosaurus and Utahraptor, a giant version of Velociraptor.
Not interested in dinosaurs? How about a Smilodon, one of the biggest and most iconic of mammalian predators?
Of course, what I have shown the past few days is only a few things that the University of Arkansas Earth Sciences Department is bringing. The Anthropology Department will also be there with their own fossils. The Arkansas Geological Survey will be there with a cast of the dinosaur, “Arkansaurus” and more. The Virtual Fossil Museum will be present with more than virtual fossils. Not to be outdone, the Museum of Discovery will have a collection of dinosaur skulls on display, which will be part of an upcoming dinosaur exhibit which you won’t want to miss.
So were you able to figure out what Monday’s fossil was? Congratulations to Herman Diaz for not only correctly identifying it, but the relative that lived in Arkansas as well. If you want to see it in person, along with much more, come out to the National Fossil Day event at the Museum of Discovery in Little Rock.
This is the foot of Allosaurus fragilis, which means “different delicate reptile”. It has gone by a lot of names. Depending on the researcher you ask, Antrodemus, Creosaurus, Labrosaurus, Epanterias, and Saurophaganax are all just versions of Allosaurus.
Because of this, it can be difficult to say just how big Allosaurus was. The most famous specimen called “Big Al” is actually on the small side, measuring only about 7.5 m (25 ft), whereas the American Museum of Natural History has a specimen that is almost 10 m (33 ft.). The fossil called Epanterias is a good 12 m (40 ft.). Thus, Allosaurus may have been essentially the same length as Tyrannosaurus rex, although it was more lightly built, so it would not have weighed as much. We should keep in mind though, that every species has a fairly wide range of sizes, so even when we can measure a bunch of living specimens, stating an average size has to come with wide error bars, so take these measurements with a grain of salt.
Allosaurus itself never lived in Arkansas. But a close relative of it did. Acrocanthosaurus is classified as a Carcharodontosaurid, which is a group that is generally considered to by descended from earlier allosaurs. Allosaurus himself lived in the late Jurassic Period, whereas Acrocanthosaurus lived in the early to middle Cretaceous, so the timing lines up with what we know. No bones of Acrocanthosaurus have ever been found in Arkansas, although they have been found in Texas and Oklahoma. What we do have in Arkansas is their tracks. A large trackway was found in Howard County in 2011. This trackway was mostly footprints of large sauropods such as Sauroposeidon, but it also contained theropod trackways, which were identified as being from Acrocanthosaurus.
Acrocanthosaurus was as big as the biggest allosaurs and was known for unusually long spines on the vertebra, especially over the ribcage. Why did it have the spines? While they weren’t as tall as Spinosaurus, but they were longer than would be necessary for strictly muscle attachment, so the best explanation was that it formed part of a display to make it look bigger and more impressive. It was the biggest predator around, so it likely was not for defense, but to intimidate rivals and impress potential mates.
As the biggest predator around, it preyed upon the main herbivores of the day, which in this case were probably juvenile or elderly sauropods (the healthy adults were likely immune from predation simply on account of size). The skulls were more lightly built than tyrannosaurs, so they probably did not munch through bone like tyrannosaurs. They were apparently more selective in their eating. This would have made them very popular with the scavengers of the time as they would have left more behind.