Torosaurus, meaning “perforated lizard,” is a genus of ceratopsid dinosaur from the late Maastrichtian stage of the Late Cretaceous Period (70 to 65 million years ago). Torosaurus is often mis-referred to as “bull” (from the Latin word taurus) lizard. But its actual name comes from the Greek word toreo, meaning “to perforate or pierce.” Its name is in reference to the window-like holes found in the elongated frill, which have traditional served to distinguish it from the solid frill of Triceratops.

In 1891, two years after Triceratops was named, a pair of ceratopsian skulls with elongated frills bearing holes were found in southeastern Wyoming. Paleontologist Othniel Charles Marsh coined the genus Torosaurus for them. Similar specimens have since been found in Wyoming, Montana, South Dakota, North Dakota, and Utah in the United States, and in Saskatchewan, Canada. Fragmentary remains have also been found in Texas and New Mexico that could be possibly be identified as belonging to Torosaurus.

There are two known species of Torosaurus: T. latus and T. utahensis. A third species, T. gladius, was subsequently regarded as a mis-assignment.

T. utahensis was originally described as Arrhinoceratops utahensis in 1946, but a review in 2005 showed that it did belong to the genus Torosaurus. But further studies suggested it could have actually been an Arrhinoceratops or a new genus, as dinosaurs found in the Hell Creek Formation and southern “Alamosaurs fauna” rarely overlap and were probably separated by a geographic barrier.

In 2010, research on dinosaur growth and development concluded that Torosaurus may not represent a distinct genus at all, but rather a mature form of Triceratops. Findings were published on the examination of 38 skull specimens. The conclusion was later challenged in 2011. Currently, Torosaurus is still considered its own genus.

Torosaurus had one of the largest skulls of any known land animal. The frilled skull reached 8.5 feet long. Torosaurus would have measured close to 30 feet long from head to tail, and would have weighed an estimated 4.4 to 6.6 tons.

Torosaurus, like its close relative Triceratops, was a herbivore. During the Cretaceous flowering plants were limited in their geographical distribution. It is likely Torosaurus fed on the more abundant ferns, cycads and conifers, using their sharp beaks to bite off leaves or needles.