Achelousaurus

Horner's expeditions to Landslide Butte in Montana All known Achelousaurus specimens were recovered from the Two Medicine Formation in Glacier County, Montana during excavations conducted by the Museum of the Rockies, which still houses the specimens. The discoveries came about by an accidental chain of events. In the spring of 1985, paleontologist John "Jack" R. Horner was informed that he would no longer be allowed to exploit the Willow Creek site, where he had studied the Maiasaura Egg Mountain nesting colony for six years. Having already made extensive arrangements for a new field season, he was suddenly forced to seek an alternative site. Horner had always been intrigued by the field diaries of Charles Whitney Gilmore who had reported the discovery of dinosaur eggs at Landslide Butte in 1928, but never published on them. In this locality, Gilmore had employed George Fryer Sternberg to excavate skeletons of the horned dinosaurs Brachyceratops and Styracosaurus ovatus. led the team that discovered Achelousaurus. A. horneri was named after him That summer, Horner obtained the permission of the Blackfeet Indian Tribal Council to prospect for fossils on Landslide Butte, which is part of the Blackfeet Indian Reservation; it was the first paleontological investigation there since the 1920s. In August 1985, Horner's associate Bob Makela discovered a rich fossil site on the land of the farmer Ricky Reagan, which was called the Dinosaur Ridge Quarry and contained fossils of horned dinosaurs. On 20 June 1986, Horner and Makela returned to the Blackfeet Indian Reservation and resumed work on the Dinosaur Ridge Quarry, which proved to contain, apart from eggs, more than a dozen skeletons of a horned dinosaur later named Einiosaurus. In August 1986, at a nearby site – the Canyon Bone Bed on the land of Gloria Sundquist, east of the Milk River – Horner's team discovered another Einiosaurus bone bed. Part of the discoveries made on this occasion was an additional horned dinosaur skull, specimen MOR 492, that later would be referred to (i.e., formally assigned to) Rubeosaurus, the genus name in 2010 given to Styracosaurus ovatus. Raymond Robert Rogers, who was studying the stratigraphy of the bone beds, referred to it as a Styracosaurus sp. (of undetermined species) in 1989. Styracosaurus ovatus – though sometimes considered an invalid nomen dubium – had already been found in the area by G. F. Sternberg and was an obvious candidate. Horner, an expert on the Hadrosauridae family, had less affinity for other kinds of dinosaurs. Meanwhile, Horner had come to a more complex view of the situation. He still thought that the fossil material had been part of a single population but concluded that this had developed over time as a chronospecies evolving into a series of subsequent taxa. In 1992, Horner, David Varricchio, and Mark Goodwin published an article in Nature based on the six-year field study of sediments and dinosaurs from Montana. They proposed that the expeditions had uncovered three "transitional taxa" spanning the gap between the already known Styracosaurus and Pachyrhinosaurus. For the moment, they declined to name these 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 and the horned dinosaur fossils of the Blacktail Creek. In a 1997 book, Horner referred to the three taxa as "centrosaurine 1.", "centrosaurine 2." and "centrosaurine 3.". Sampson names Achelousaurus loses his horn to Hercules on an Attic krater Sampson had continued his studies of the material since 1989. In 1994, in a talk during the annual meeting of the Society of Vertebrate Paleontology, he named "Taxon C" as a new genus and species, Achelousaurus horneri. Although an abstract was published containing a sufficient description, it did not identify a holotype, a name-bearing specimen. In 1995, in a subsequent article, Sampson indicated specimen MOR 485 as the holotype specimen of Achelousaurus horneri. The generic name consists of the words Achelous, the name of a Greek mythological figure, and saurus, which is Latinized Greek for lizard. Achelous (Ἀχελῷος) is a Greek river deity and a shapeshifter who was able to transform himself into anything. During a fight with Hercules, the mythical hero, Achelous took the form of a bull, but lost the battle when one of his horns was removed. This allusion is a reference to the supposedly transitional traits of the dinosaur and the characteristic loss of horns through ontogenetic and phylogenetic development, and thus through individual change and evolution. The holotype specimen MOR 485 was collected by Hostetter and Ray Rogers from the Landslide Butte Field Area about northwest of Cut Bank. In 1995 Sampson described it as the partial skull of an adult animal including the nasal and supraorbital (region above the eye socket) bosses (roundish protuberances instead of horns), and the parietal bones. Specimen MOR 571 includes a partial skull and lower jaws with associated ribs and vertebrae of an adult. In addition, some indeterminate specimens from the Two Medicine Formation – such as fragmentary skull MOR 464 or snout MOR 449 – may belong to Achelousaurus or the two other roughly contemporary ceratopsids Einiosaurus and Styracosaurus ovatus. The subadult specimen MOR 591 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. ==Description==

General build Achelousaurus is estimated to have been long with a weight of . In the tooth sockets, new teeth grew under the old ones, each position housing a column of teeth posed on top of each other. Achelousaurus had 25 to 28 such tooth positions in each maxilla (upper jaw bone). Distinguishing traits In 1995, when describing the species, Sampson gave a formal list of four traits that distinguish Achelousaurus from its centrosaurine relatives. Firstly, adult individuals have nasal bones with a boss on top that is relatively small and thin, and heavily covered with pits; secondly, adult individuals do not have true horns above the eye sockets but relatively large bosses with high ridges; thirdly, not yet fully grown individuals, or subadults, have true horncores (the bony part of the horns) above the eye sockets with the inward facing surface being concave; and fourthly, the parietal bones of the neck shield have a single pair of curved spikes sticking out from the rear margin to behind and to the outside. Adult Achelousaurus skulls had a rugose, heavily pitted boss on the snout or nasal region, where many other ceratopsids had a horn. The large spikes of Achelousaurus correspond to "Process 3" spikes of other centrosaurines and were similar to those of Einiosaurus, though curved more to the sides, similar to Pachyrhinosaurus. at least in the third position. In 2020, it was denied that these processes were separate ossifications. In the most mature individuals, the front-most P6 and P7 processes would be less imbricated relative to each other, rotated around their longitudinal axes. ==Evolution==

Horner's hypothesis of anagenesis 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 . This led to stronger selection pressures, In 2010, Gregory S. Paul assigned A. horneri to the genus Centrosaurus, as C. horneri. Phylogeny have varied in the closeness of the relationship between Einiosaurus, Achelousaurus, 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 Achelousaurus was a direct descendant of Einiosaurus. 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 Achelousaurus in a cladogram from Wilson and colleagues, 2020. , the possible ancestor of Einiosaurus and Achelousaurus'' skull casts positioned in a phylogenetic tree, in the Natural History Museum of Utah, with Achelousaurus (cast of MOR 485) third from the upper left row (number 03) known by 2024 ==Paleobiology==

Function of skull ornamentation value 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. All three main known specimens have syncervicals consisting of three fused neck vertebrae; the trait could have been inherited from a smaller ancestor using a stiffer neck for burrowing or food acquisition. Social behavior It has been claimed that ceratopsian dinosaurs were herding animals, due to the large number of known bone beds containing multiple members of the same ceratopsian species. In 2010, Hunt and Farke pointed out that this was mainly true for centrosaurine ceratopsians. Horner assumed that the horned dinosaurs at Landslide Butte lived in herds which had been killed by drought or disease. Dodson concluded that the fact that the Achelousaurus bone beds were monospecific (containing only one species) confirmed the existence of herds. Metabolism and growth There has long been debate about the thermoregulation of dinosaurs, centered around whether they were ectotherms ("cold-blooded") or endotherms ("warm-blooded"). Mammals and birds are homeothermic endotherms, which generate their own body heat and have a high metabolism, whereas reptiles are heterothermic ectotherms, which receive most of their body heat from their surroundings. A 1996 study examined the oxygen isotopes from bone phosphates of animals from the Two Medicine Formation, including the juvenile Achelousaurus or Einiosaurus specimen MOR 591. δ18O values of phosphate in vertebrate bones depend on the δ18O values in their body water and the temperature when the bones were deposited, making it possible to measure fluctuations in temperature for each bone of an individual when they were deposited. The study analyzed seasonal variations in the body temperature and differences in temperature between skeletal regions, to determine whether the dinosaurs maintained their temperature seasonally. A varanid lizard fossil sampled for the study showed isotopic variation consistent with it being an heterothermic ectotherm. The variation of the dinosaurs, including MOR 591, was consistent with them being homeothermic endotherms. The metabolic rate of these dinosaurs was likely not as high as that of modern mammals and birds, and they may have been intermediate endotherms. ==Paleoenvironment==

of Ceratopsidae (above), and paleogeographic map of the Late Cretaceous with distribution of ceratopsids (below), following Sampson and colleagues, 2013. Achelousaurus is 19 Achelousaurus 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. Achelousaurus specimens are found in the uppermost member of the formation, the Flag Butte Member, which spans 77 to 74.8 Ma. 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. Achelousaurus 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, Einiosaurus 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 Achelousaurus was part of is uncertain, as its fossils have not been discovered in direct association with other taxa. Its intermediate anagenetic position suggests that Achelousaurus shared its habitat with forms roughly found in the middle or at the end 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; these have not been consistently referred to a new genus. Finally, Horner thought there was a taxon present that was transitional between Daspletosaurus and Tyrannosaurus. In 2017, tyrannosaurid remains from the Two Medicine Formation were named as a new species of Daspletosaurus: Daspletosaurus horneri. The 2017 study considered it plausible that D. horneri was a direct descendant of D. torosus in a process of anagenesis, but rejected the possibility that D. horneri was the ancestor of Tyrannosaurus. ; it is uncertain whether the two ceratopsids were contemporaneous Other ceratopsians from the Two Medicine Formation include Einiosaurus and Stellasaurus. 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. 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. The skull of Achelousaurus was more than twice as strong than that of Einiosaurus in its bending strength and torsion resistance. This might have indicated a difference in diet to avoid competition. The bite strength of Achelousaurus, measured as an ultimate tensile strength, was 30.5 newtons per square millimeter (N/mm2) at the maxillary tooth row and 18 N/mm2 at the beak. By comparison, it was 10.3 N per square millimeter (N/mm2) and 6.40 N/mm2, respectively, for Einiosaurus. Wilson and colleagues found that since the Two Medicine centrosaurines were separated stratigraphically, they were therefore possibly not contemporaneous. The indeterminate specimen TMP 2002.76.1 is from the Dinosaur Park Formation and, if it belongs to Achelousaurus, the genus would be the stratigraphically oldest known pachyrhinosaurine taxon. Both animals occur right below the marine shales of the Bearpaw Formation, but due to longitudinal differences, TMP 2002.76.1 is about 500,000 years older than the Achelousaurus fossils from the Two Medicine Formation. ==See also==