Hesperosaurus (Greek for "western lizard"); pronounced HESS-per-oh-SORE-us
Woodlands of North America
Late Jurassic (155 million years ago)
Size and Weight:
About 20 feet long and 2-3 tons
Short, wide head with small brain; relatively blunt, oval-shaped plates on back; quadrupedal posture
Stegosaurs--the spiked, plated dinosaurs--first evolved in Asia during the middle to late Jurassic period, then crossed over to North America a few million years later, where they prospered up until the cusp of the ensuing Cretaceous period. That would explain the "in-between" features of one of the first identified North American stegosaurs, Hesperosaurus, with its wide, round, mushroom-shaped dorsal plates and unusually short and blunt head (earlier stegosaurs from Asia possessed smaller skulls and less ornate plates, while the skull of Stegosaurus, which followed Hesperosaurus by about five million years, was much more narrow).
Ironically, the near-complete skeleton of Hesperosaurus was discovered in 1985 during an excavation of its much more famous cousin. Initially, the near-complete skeleton of Hesperosaurus was interpreted as an individual, or at least a species, of Stegosaurus, but by 2001 it was classified as a separate genus. (Just to show that paleontology is not set in stone, a recent re-examination of Hesperosaurus' remains led to the conclusion that Hesperosaurus was actually a Stegosaurus species after all, and the authors recommended that the closely related stegosaur genus Wuerhosaurus should also be so assigned. The verdict is still out, and for the time being, Hesperosaurus and Wuerhosaurus retain their genus status.)
However you choose to classify Hesperosaurus, there's no mistaking the distinctive plates on this dinosaur's back (about a dozen roundish, short structures significantly less pointed and dramatic than the comparable plates on Stegosaurus) and its spiked tail, or "thagomizer." As with Stegosaurus, we don't know for sure why Hesperosaurus evolved these features; the plates may have aided in intra-herd recognition or served some kind of signaling function (say, turning bright pink in the presence of raptors and tyrannosaurs), and the spiked tail may have been wielded in combat by males during mating season (the winners earning the right to pair with females) or used to inflict puncture marks on curious predators.
Speaking of mating, once recent study of Hesperosaurus (published in 2015) speculates that this dinosaur was sexually dimorphic, the males differing anatomically from the females. Surprisingly, though, the author proposes that female Hesperosaurus possessed narrower, pointier plates than the males, whereas most of the sexual differentiation in large animals (both millions of years ago and today) favors the males of the species! To be fair, this study has not been widely accepted by the paleontology community, perhaps because it's based on too few fossil specimens to be considered conclusive