Meet Nanuqsaurus hoglundi. Based on its 25-inch-long skull, its body was probably half the length of a Tyrannosaurus Rex's, but this petit tyrannosaurid was an apex predator of the Arctic.
Cranial fragment fossils of Nanuqsaurus hoglundi (Nanuqsaurus means "polar bear lizard") were collected from the Kikak-Tegoseak Quarry in North Slope, Alaska, in 2006, but it's a paper released today in PLoS that identifies the dinosaur as a member of the tyrannosaur clan, sharing a recent common ancestor with Tyrannosaurus rex, Albertosaurus sarcophagus, and Gorgosaurus libratus. Researchers studied the cranial bones of Nanuqsaurus hoglundi and found that it shared commonalities with other tyrannosaurs, meaning that this smaller creature was likely T-Rex's northern cousin.
Figure from A Diminutive New Tyrannosaur from the Top of the World, PLoS: Relative size of Nanuqsaurus hoglundi (A), compared to other tyrannosaurs (B-E) and the lower latitude individual Troodon formosus (F) and a North Slope Troodon (G).
The study authors believe that Nanuqsaurus hoglundi developed a smaller body than other tyrannosaurs because the seasonal changes in light in their Arctic home put greater constraints on their food supply, limited just how big they could get. The presence of a tyrannosaur in the Arctic is itself rather interesting, hinting at more we can learn about both dinosaur diversity in the Late Cretaceous Arctic and the physical and regional diversity of tyrannosaurs. Even at its smaller size, though, I wouldn't want to meet one of these dinosaurs in a dark, snowy alley.