Alcovasaurus, alternatively known as Miragaia longispinus, is a genus of herbivorous stegosaurian dinosaur that lived in the Late Jurassic.

In 1914, the find was named and described as Stegosaurus longispinus by Charles Whitney Gilmore on the basis of holotype UW 20503 (originally UW D54), a partial postcranial skeleton of an adult individual consisting of forty-two vertebrae, a fragmentary sacrum, two ischia, a portion of one pubis, the right femur, several ribs and four dermal tail spines.

[6] Unfortunately, the type specimen of this species was damaged in the late 1920s when the water pipes of the University of Wyoming's museum burst.

[1] Although the validity of Stegosaurus longispinus was disputed because the long dermal spines were likely to be a product of ontogeny or sexual dimorphism,[8] the amateur freelance paleontologist Roman Ulansky decided that the long tail spines were sufficient to remove S. longispinus from Stegosaurus and place it in a new genus, "Natronasaurus".

Galton and Carpenter also referred a very large round spike base to the species, found by Cliff Miles in Wyoming in the 1990s.

This was based on a description of a new specimen, MG 4863, of Miragaia longicollum preserving the first relatively complete tail known from a dacentrurine stegosaur.

[2] Gilmore first diagnosed S. longispinus from other Stegosaurus species by the presence of very long dermal spikes, distal caudal vertebral centra rounded in anterior/posterior view, vestigial transverse processes on distal caudal vertebrae, and centra with mushroom-shaped dorsal extensions.

Galton & Carpenter found this trait difficult to confirm from the extant photographic evidence, but it would make Alcovasaurus differ from all known stegosaurians with four spikes, which always show a shorter rear pair.

[1] However, a 2017 phylogeny of stegosaurids by Thomas Raven and Susannah Maidment found that Alcovasaurus lacked the fusion between the trochanters of the femur seen in adult eurypodans (stegosaurians and ankylosaurians), which means that it cannot be confidently placed as a stegosaur.

[16] In 2019, Costa and Mateus re-interpreted A. longispinus in the context of newly recognized specimens of Miragaia found in Portugal.

These remains allowed them to recognize A. longispinus as a close relative of Miragaia, within the stegosaurid clade Dacentrurinae.