and Hunt suggested that it was very likely the track was made by a T. rex, which would make it the first known footprint from this species. The track was made in what was once a vegetated wetland mudflat. It measures long by wide. A second footprint that may have been made by a Tyrannosaurus was first reported in 2007 by British paleontologist Phil Manning, from the Hell Creek Formation of Montana. This second track measures long, shorter than the track described by Lockley and Hunt. Whether or not the track was made by Tyrannosaurus is unclear, though Tyrannosaurus is the only large theropod known to have existed in the Hell Creek Formation. A set of footprints in Glenrock, Wyoming dating to the Maastrichtian stage of the Late Cretaceous and hailing from the Lance Formation were described by Scott Persons, Phil Currie and colleagues in 2016, and are believed to belong to either a juvenile T. rex or the dubious tyrannosaurid Nanotyrannus lancensis. From measurements and based on the positions of the footprints, the animal was believed to be traveling at a walking speed of around 2.8 to 5 miles per hour and was estimated to have a hip height of to . A follow-up paper appeared in 2017, increasing the speed estimations by 50–80%. Description Size T. rex was one of the largest land carnivores of all time. One of the largest and the most complete specimens, nicknamed Sue (FMNH PR2081), is located at the Field Museum of Natural History in Chicago. Sue measured long, was tall at the hips, and according to the most recent studies, using a variety of techniques, maximum body masses have been estimated approximately . A specimen nicknamed Scotty (RSM P2523.8), located at the Royal Saskatchewan Museum, is reported to measure in length. Using a mass estimation technique that extrapolates from the circumference of the femur, Scotty was estimated as the largest known specimen at in body mass. Not every adult Tyrannosaurus specimen recovered is as big. Historically average adult mass estimates have varied widely over the years, from as low as , to more than , with most modern estimates ranging between and . Skull The largest known T. rex skulls measure up to in length. Large fenestrae (openings) in the skull reduced weight, as in all carnivorous theropods. In other respects Tyrannosaurus's skull was significantly different from those of large non-tyrannosaurid theropods.