At least fifteen individuals of varying ages are represented by three adult skulls and hundreds of other bones from two low-diversity, monospecific (one species) bonebeds, which were discovered by Jack Horner in 1985 and excavated from 1985 to 1989 by Museum of the Rockies field crews.

In the spring of 1985 he had been informed by landowner Jim Peebles that he would no longer be allowed access to the Willow Creek "Egg Mountain" site where during six years a nesting colony of Maiasaura had been excavated.

[1] This forced Horner to find an alternative site because supplies had already been bought for a new summer season and fourteen volunteers and students expected to be employed by him.

However, one of its members, Marvin Weatherwax, had earlier in 1985 observed that an excavation by Horner of a mosasaurid in the Four Horns Lake had caused only limited damage to the landscape.

[3] Early in August, Horner's associate Bob Makela discovered the Dino Ridge Quarry, containing extensive ceratopsid remains, on the land of farmer Ricky Reagan.

[6] In late August 1986, Horner and preparator Carrie Ancell on the land of Gloria Sundquist discovered a second horned dinosaur site, at one mile distance from the first, called the Canyon Bone Bed, in which two relatively complete skulls were dug up.

[14] In the second week of June 1989, student Scott Donald Sampson in the context of his doctoral research with a small crew reopened the Canyon Bone Bed, while Patrick Leiggi that summer with a limited number of workers restarted excavating the Dino Ridge Quarry.

[16] In 1990, the expeditions were ended because the reservation allowed access to commercial fossil hunters who quickly strip-mined sites with bulldozers, through a lack of proper documentation greatly diminishing the scientific value of the discoveries.

[21] This reflected the fact that the expedition members had started to take the possibility into account that the species was completely new to science, informally referring to it as "Styracosaurus makeli" in honour of Bob Makela, who had died in a traffic accident in June 1987.

[26] In 1994, Sampson, in a talk during the annual meeting of the Society of Vertebrate Paleontology, named Horner's "Taxon B" as a new genus and species, Einiosaurus procurvicornis.

Although an abstract was published containing a sufficient description, making the name valid, it did not yet identify by inventory number a holotype, a name-bearing specimen.

The name was chosen to honour the Blackfeet tribe but also to reflect the fact that ceratopsids were, in Sampson's words, "the buffalo of the Cretaceous", living in herds and having a complex life.

[33] The subadult specimen MOR 591 from the uppermost Two Medicine Formation was assigned to Achelousaurus in 1995 and henceforward, but in 2021, John Wilson and Jack Scannella stated that it could also possibly belong to Einiosaurus.

The nasal horn has a base that is long from front to rear, is transversely flattened, and is strongly curved forwards in some adult specimens.

It is typically portrayed with a low, strongly forward and downward curving nasal horn that resembles a bottle opener, though this may only occur in some adults.

[36] In 1992, the study by Horner et al. tried to account for the fact that within a limited geological period of time (about half a million years) there had been a quick succession of animal communities in the upper Two Medicine Formation.

[39] The animals were living on a narrow strip on the east-coast of Laramidia, bordering the Western Interior Seaway and constrained in the west by the 3 to 4 kilometres (2 to 2.5 mi) high proto-Rocky Mountains.

Horner disagreed: he saw the gradual morphological changes as clear proof that, in this case, the evolution of one taxon into another, without a splitting of the populations, could be directly observed.

[43] Subsequent studies have sought to determine the precise relationships within this part of the evolutionary tree, with conflicting results regarding the question whether Styracosaurus albertensis or Einiosaurus might have been in the direct line of ascent to Achelousaurus.

[33] In 2011, a subsequent study by Andrew T. McDonald in this respect replicated the outcome of his previous one,[46] as did a publication by Andre Farke et al.[47] In 2017, J.P. Wilson and Ryan further complicated the issue, concluding that MOR 492 ("Taxon A") was not referable to Rubeosaurus and announcing that yet another genus would be named for it.

[55] In 1995, Sampson noted that earlier studies had found that the horns and frills of ceratopsians most likely had a function in intraspecific display and combat, and that these features would therefore have resulted from sexual selection for successful mating.

In contrast, ceratopsine ceratopsids, such as Triceratops and Torosaurus, are typically found singly, implying that they may have been somewhat solitary in life, though fossilized footprints may provide evidence to the contrary.

[60][34] In 2010, a study by Julie Reizner of the individuals excavated at the Dino Ridge site concluded that Einiosaurus grew quickly until its third to fifth year of life after which growth slowed, probably at the onset of sexual maturity.

[34] Einiosaurus is known from the Two Medicine Formation, which preserves coastal sediments dating from the Campanian stage of the Late Cretaceous Period, between 83 and 74 million years ago.

[62] The brown paleosol in which the horned dinosaurs were found – a mixture of clay and coalified wood fragments – resembles that of modern seasonally dry swamps.

[63] Einiosaurus ate much smaller plants, though: a 2013 study determined that ceratopsid herbivores on Laramidia were restricted to feeding on vegetation with a height of 1 m (3.5 ft) or lower.

As proven by tooth marks, horned dinosaur fossils in the Landslide Butte Field Area had been scavenged by a large theropod predator, which Rogers suggested were Albertosaurus.

According to Horner and colleagues in 1992, its intermediate anagenetic position suggests that Einiosaurus shared its habitat with forms roughly found in the middle of the time range of its formation.

In addition, remains of other indeterminate and dubious centrosaurines, including Brachyceratops, are known from the formation and though they may represent younger stages of the three valid genera, this is not possible to demonstrate.

[70][38] Whereas Horner assumed that Einiosaurus and Achelousaurus were separate in time, in 2010 Donald M. Henderson considered it possible that at least their descendants or ancestors were overlapping or sympatric and thus would have competed for food sources unless there had been niche partitioning.