Einiosaurus is a genus of herbivorous centrosaurine ceratopsian dinosaur from the Upper Cretaceous (Campanian stage) of northwestern Montana. The name means 'bison lizard', in a combination of Blackfeet Indian eini and Latinized Ancient Greek sauros; the specific name (procurvicornis) means 'with a forward-curving horn' in Latin. Einiosaurus is medium-sized with an estimated body length at .
History of discovery
Horner's expeditions to Landslide Butte
left|thumb|[[Jack Horner (paleontologist)|Jack Horner led the team that discovered Einiosaurus]]
Einiosaurus is an exclusively Montanan dinosaur, and all of its known remains are currently held at the Museum of the Rockies in Bozeman, Montana. 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.
Horner had not been searching for horned dinosaurs. 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. 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. He investigated two sites, Devil's Pocket and Red Rocks, that however proved to contain too few fossils. For some years, since 1982, Horner had requested from the Blackfeet Indian Tribal Council access to the Landslide Butte site. The field notes of Charles Whitney Gilmore from the 1920s indicated that dinosaur eggs could be found there. The council had consistently turned down his requests because they feared widespread disturbance of the reservation. 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. In early July, the council granted Horner access to the entire reservation.
Early in August, Horner's associate Bob Makela discovered the Dino Ridge Quarry, containing extensive ceratopsid remains, on the land of farmer Ricky Reagan. Continual rainfall hampered operations that year. On 20 June 1986, a crew of sixteen returned to reopen the quarry. A large and dense concentration of bones, a bonebed, was excavated, with up to forty bones per square metre being present. This was interpreted as representing an entire herd that had perished. 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. The skulls had to be removed from a rather steep cliff and weighed about half a tonne when plastered. They were airlifted by a Bell UH-1 Iroquois of the United States Army National Guard into trucks to be transported. The aberrant build of these skulls first suggested to Horner that they might represent an unknown taxon. Unexpectedly benefiting from a grant of $204,000 by the MacArthur Fellows Program, Horner was able to reopen the two bonebed quarries in 1987. That year almost all fossils were removed that could be accessed without using mechanised earth-moving equipment. Also, an additional horned dinosaur skull was excavated from a somewhat younger layer. In 1988, more ceratopsid material was found in a more southern site, the Blacktail Creek North. 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. The same year, Horner himself found more horned dinosaur fossils at the Blacktail Creek North. 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.
Interpretation of the collected fossils
right|thumb|Einiosaurus parietal frill bones, including of holotype specimen (A, MOR 456 8-9-6-1)
At the time of the expeditions, it was assumed that all horned dinosaur fossils found in the reservation belonged to a single species, especially as they came from a limited geological time period, its duration estimated at about half a million years. The abundant new remains seemed to prove that the species was real, also because it clearly differed from the type species of Styracosaurus, Styracosaurus albertensis. The comprehensive taphonomic study by Raymond Robert Rogers from 1990 however, did not commit itself fully to this identification, merely mentioning a Styracosaurus sp.
Horner was an expert on the Hadrosauridae, several sites of which had also been discovered in Landslide Butte, including the juveniles and eggs that were the focus of his research. He had less affinity for other kinds of dinosaurs.
Horner had gradually changed his mind on the subject. While still thinking that a single population of horned dinosaurs had been present, he began to see it as a chronospecies, an evolutionary series of taxa. In 1992, he described them in an article as three "transitional taxa" that had spanned the gap between the older Styracosaurus and the later Pachyrhinosaurus. He deliberately declined to name these three taxa. The oldest form was indicated as "Transitional Taxon A", mainly represented by skull MOR 492. Then came "Taxon B" – the many skeletons of the Dinosaur Ridge Quarry and the Canyon Bone Bed. The youngest was "Taxon C", represented by skull MOR 485 found in 1987 and the horned dinosaur fossils of the Blacktail Creek North. In a 1997 book, Horner referred to the three taxa as "centrosaurine 1.", "centrosaurine 2." and "centrosaurine 3.".
Sampson names Einiosaurus
thumb|right|Subadult Einiosaurus (A, specimen MOR 456 8-8-87-1) and adult holotype (B) skull, with outlines of [[squamosal bones superimposed for comparison]]
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 same abstract named Type C as Achelousaurus horneri. In 1995, Sampson published a larger article, indicating the holotype. The generic name Einiosaurus is derived from the Blackfeet eini, "American bison", and Latinised Greek saurus, "lizard". 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. According to Sampson, the name should be pronounced as "eye-knee-o-saurus". The specific name is derived from Latin procurvus, "bent forwards", and cornu, "horn", referring to the forwards curving nasal horn. In addition, some indeterminate specimens from the Two Medicine Formation – such as fragmentary skull MOR 464 or snout MOR 449 – may belong to Einiosaurus or the two other roughly contemporary ceratopsids Achelousaurus and Styracosaurus ovatus. 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. Both taxa fall within the typical size range of the Centrosaurinae.
Evolution
Horner's hypothesis of anagenesis
thumb|left|Diagram showing evolutionary lineage proposed by Horner et al., 1992
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. Normally, this would be interpreted as a series of invasions, with the new animal types replacing the old ones. But Horner noted that the newer forms often had a strong similarity to the previous types. This suggested to him that he had discovered a rare proof of evolution in action: the later fauna was basically the old one but at a more evolved stage. The various types found were not distinct species but transitional forms developed within a process of anagenesis. This conformed to the assumption, prevalent at the time, that a species should last about two to three million years. A further indication, according to Horner, was the failure to identify true autapomorphies – unique traits that prove a taxon is a separate species. The fossils instead showed a gradual change from basal (or ancestral) into more derived characters. "Taxon B" became Einiosaurus, while "Taxon C" became Achelousaurus. 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 high proto-Rocky Mountains. During the Bearpaw Transgression sea levels were rising, steadily reducing the width of their coastal habitat from about to , leading to stronger selection pressures.
thumb|[[Stratigraphic and temporal relationship of taxa hypothesized as representing an anagenetic lineage by Wilson et al., 2020]]
In 1996, Dodson raised two objections to Horner's hypothesis. Firstly, the possession of just one pair of main spikes seemed more basal than the presence of three pairs, as with Styracosaurus albertensis. This suggested to him that the Einiosaurus–Achelousaurus lineage was a separate branch within the Centrosaurinae. Secondly, he was concerned that Einiosaurus and Achelousaurus were a case of sexual dimorphism, one type being the males, the other the females. This would be suggested by the short geological time interval between the layers their fossils had been found in, which was estimated by him at about 250,000 years. But if the hypothesis were true, it would be perhaps the best example of fast evolution in the Dinosauria. Since Wilson and colleagues found in 2020 that Stellasaurus (Horner's "Taxon A") was intermediate between Styracosaurus and Einiosaurus in morphology and stratigraphy, they could not discount that it was a transitional taxon within an anagenetic lineage. In 2010, Gregory S. Paul assigned E. procurvicornis to the genus Centrosaurus, as C. procurvicornis.
Phylogeny
thumb|[[Phylogenetic analyses have varied in the closeness of the relationship between Einiosaurus and Styracosaurus; here, a skull at the American Museum of Natural History]]
Sampson felt, in 1995, that there was not enough evidence to conclude that Einiosaurus was a direct ancestor of Achelousaurus. Unlike Horner, he decided to perform a cladistic analysis to establish a phylogeny. This showed an evolutionary tree wherein Achelousaurus split off between Einiosaurus and Pachyrhinosaurus, as Horner had predicted. Contrary to Horner's claim, Styracosaurus albertensis could not have been a direct ancestor, as it was a sister species of Centrosaurus in Sampson's analysis. This was confirmed by analyses by Ryan in 2007, Nicholas Longrich in 2010, and Xu et al. in 2010. The same year Horner and Andrew T. McDonald moved Styracosaurus ovatus to its own genus, Rubeosaurus, finding it a sister species of Einiosaurus, while Styracosaurus albertensis was again located on the Centrosaurus branch. They also assigned specimen MOR 492, the basis of "Taxon A", to Rubeosaurus. In 2011, a subsequent study by Andrew T. McDonald in this respect replicated the outcome of his previous one, as did a publication by Andre Farke et al. 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. Wilson and colleagues moved MOR 492 to the new genus Stellasaurus in 2020, which therefore corresponds to "Taxon A". Their study found Rubeosaurus ovatus to be the sister species of Styracosaurus albertensis, and concluded Rubeosaurus to be synonymous with Styracosaurus.
Cladistic analyses develop gradually, reflecting new discoveries and insights. Their results can be shown in a cladogram, with the relationships found ordered in an evolutionary tree. The cladogram below shows the phylogenetic position of Einiosaurus in a cladogram from Wilson and colleagues, 2020. Sampson in 1995 rejected the possibility that the difference in skull ornamentation between Einiosaurus and Achelousaurus represented sexual dimorphism, for three reasons. Firstly, the extensive Einiosaurus bone beds did not contain any specimens with bosses, as would have been expected if one of the sexes had them. Secondly, Einiosaurus and Achelousaurus are found in strata of a different age. Thirdly, in a situation of sexual dimorphism usually only one of the sexes shows exaggerated secondary sexual characters. Einiosaurus and Achelousaurus however, each have developed a distinct set of such traits.
In 2021 a study, Wilson and Scannella pointed out that specimen MOR 591 was of a younger individual age than the Einiosaurus skull MOR 456 8-8-87-1, but of the same size. If MOR 591 could indeed be referred to Achelousaurus, this might indicate this genus reached its adult size more quickly.
Paleoenvironment
alt=Family tree of dinosaurs with a map of their locations below|upright=2|thumb|left|Time-calibrated [[phylogenetic relationships of Ceratopsidae (above), and paleogeographic map of the Late Cretaceous with distribution of ceratopsids (below), following Sampson and colleagues, 2013. Einiosaurus is 18]]
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. The Two Medicine Formation is typified by a warm semiarid climate. Its layers were deposed on the east coast of the Laramidia island continent (which consisted of western North America). The high cordillera in the west, combined with predominantly western winds, would have caused a rain shadow, limiting annual rainfall. Rain would mainly have fallen during the summer, when convection storms flooded the landscape. The climate would thus also have been very seasonal, with a long dry season and a short wet season. Vegetation would have been sparse and a little varied. In such conditions, horned dinosaurs would have been dependent on oxbow lakes for a continuous supply of water and food – the main river channels tending to run dry earlier – and perished in them during severe droughts when the animals concentrated around the last watering holes, causing bone beds to form. 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. The surrounding vegetation might have consisted of about high conifer trees. 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 or lower.
More or less contemporary dinosaur genera of the area included Prosaurolophus, Scolosaurus, Hypacrosaurus, and tyrannosaurids of uncertain classification. 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.
The exact composition of the fauna Einiosaurus was part of is uncertain, as its fossils have not been discovered in direct association with other taxa. 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. As with horned dinosaurs, Horner assumed he had found transitional taxa in other dinosaur groups of the Two Medicine Formation. One of these was a form in between Lambeosaurus and Hypacrosaurus; Today, Hypacrosaurus stebingeri is no longer seen as having evolved through anagenesis because autapomorphies of the species have been identified. Horner saw some pachycephalosaur skulls as indicative for a taxon in between Stegoceras and Pachycephalosaurus;
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References
- Dodson, P., Forster, C.A. and Sampson, S.D. (2004). "Ceratopsidae." In, Weishampel, D.B., Dodson, P., and Osmolska, H. (eds.), The Dinosauria. 2nd Edition, University of California Press.
- Lehman, T. M., 2001, Late Cretaceous dinosaur provinciality: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 310–328.