Hoplitosaurus, which means "Hoplite lizard," is a type of armored dinosaur closely related to Polacanthus. Its name originates from a partial skeleton discovered in the Lower Cretaceous Lakota Formation of Custer County, South Dakota, dating to the Barremian age. This genus is relatively obscure and has faced challenges in interpretation due to the state of its damaged remains. While there was an attempt to merge it with Polacanthus in the late 1980s to early 1990s, contemporary reviews have recognized Hoplitosaurus as a valid, albeit not well-understood, genus. The largest found specimen was 4.25 metres in length and weighed approximately 670kg (1477 pounds).

The reclassification of Hoplitosaurus identified it as a distinct type of armored dinosaur known as a polacanthid. However, this classification has sparked ongoing discussions regarding its validity as a separate genus, with some suggesting it might be a specimen of a similar dinosaur called Polacanthus, which is primarily found in Western Europe. This might appear perplexing given that Hoplitosaurus fossils have only been found in the United States. Nevertheless, the discovery of a Stegosaurus specimen in Portugal provides evidence that Europe and North America were once connected, and faunal exchanges between these continents were occurring, potentially extending as late as the late Jurassic period.

Regarding Hoplitosaurus, we can determine that as a polacanthid, it would have been a four-legged dinosaur adorned with robust plates of bony armor, known as osteoderms or scutes, running down its back. Additionally, it likely featured an extensive array of spikes, particularly around its shoulders and along its sides, mirroring the characteristics seen in other polacanthid genera. Due to its quadrupedal stance and the inability to rear up on its hind legs, Hoplitosaurus would have primarily fed on low-lying vegetation, making it an obligate browser.