| Achelousaurus | |
|---|---|
.jpg) | |
| Skull ofholotype specimen MOR 485 (with reconstructed parts in blue-gray), at theMuseum of the Rockies,Montana | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ceratopsia |
| Family: | †Ceratopsidae |
| Subfamily: | †Centrosaurinae |
| Tribe: | †Pachyrhinosaurini |
| Clade: | †Pachyrostra |
| Genus: | †Achelousaurus Sampson, 1994 |
| Species: | †A. horneri |
| Binomial name | |
| †Achelousaurus horneri Sampson, 1994 | |
Achelousaurus (/əˌkiːloʊˈsɔːrəs,ˌækɪˌloʊəˈsɔːrəs/[1]) is agenus ofcentrosaurineceratopsiddinosaur that lived during theLate Cretaceous Period of what is now North America, about 77 to 74.8 million years ago. The first fossils ofAchelousaurus were collected inMontana in 1987, by a team led byJack Horner, with more finds made in 1989. In 1994,Achelousaurus horneri was described and named byScott D. Sampson; the generic name means "Achelous lizard", in reference to the Greek deityAchelous, and thespecific name refers to Horner. The genus is known from a few specimens consisting mainly of skull material from individuals, ranging from juveniles to adults.
A large centrosaurine,Achelousaurus supposedly was about 6 m (20 ft) long, with a weight of about 3 t (3.3 short tons). As a ceratopsian, it walked on all fours, had a short tail and a large head with a hooked beak. It had a bony neck-frill at the rear of the skull, which sported a pair of long spikes, which curved towards the outside. AdultAchelousaurus had rough bosses (roundish protuberances) above the eyes and on the snout where other centrosaurines often had horns in the same positions. These bosses were covered by a thick layer ofkeratin, but their exact shape in life is uncertain. Some researchers hypothesize that the bosses were used in fights, with the animals butting each other's heads, as well as for display.
Within theCeratopsia,Achelousaurus lies within thecladePachyrostra (or "thick-snouts"). It has been suggested that it was the direct descendant of the similar genusEiniosaurus (which had spikes but no bosses) and the direct ancestor ofPachyrhinosaurus (which had larger bosses). The first two genera would betransitional forms, evolving throughanagenesis fromStyracosaurus. There has been debate about this theory, with later discoveries showing thatAchelousaurus is closely related toPachyrhinosaurus in the groupPachyrhinosaurini.Achelousaurus is known from theTwo Medicine Formation and lived in the island continent ofLaramidia. As a ceratopsian,Achelousaurus would have been a herbivore and it appears to have had a highmetabolic rate, though lower than that of modern mammals and birds.
All knownAchelousaurus specimens were recovered from theTwo Medicine Formation inGlacier County, Montana during excavations conducted by theMuseum of the Rockies, which still houses the specimens. The discoveries came about by an accidental chain of events.[2] In the spring of 1985,paleontologistJohn "Jack" R. Horner was informed that he would no longer be allowed to exploit theWillow Creek site, where he had studied theMaiasauraEgg Mountain nesting colony for six years.[3] 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 ofCharles Whitney Gilmore who had reported the discovery of dinosaur eggs at Landslide Butte in 1928, but never published on them.[4] In this locality, Gilmore had employedGeorge Fryer Sternberg to excavate skeletons of the horned dinosaursBrachyceratops andStyracosaurus ovatus.[5] That summer, Horner obtained the permission of theBlackfeet Indian Tribal Council to prospect for fossils on Landslide Butte, which is part of theBlackfeet 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.[6] On 20 June 1986, Horner and Makela returned to the Blackfeet Indian Reservation and resumed work on the Dinosaur Ridge Quarry,[7] which proved to contain, apart from eggs, more than a dozen skeletons of a horned dinosaur later namedEiniosaurus. In August 1986, at a nearby site – the Canyon Bone Bed on the land of Gloria Sundquist, east of theMilk River – Horner's team discovered anotherEiniosaurus 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 toStyracosaurus ovatus.[8][9]
During the field season of 1987 (early July), volunteer Sidney M. Hostetter located another horned dinosaur skull near the Canyon Bone Bed, specimen MOR 485.[10] By the end of August, it had been secured and was driven on agrain truck to the Museum of the Rockies inBozeman.[11] On 23 June 1988, another site was discovered in the vicinity – the Blacktail Creek North.[12] In the summer of 1989, graduate studentScott D. Sampson joined the team, wanting to study the function of the frill display structures in horned dinosaurs.[13] At the end of June 1989, Horner, his son Jason and his head preparator Carrie Ancell discovered horned dinosaur specimen MOR 591, a subadult skull and partial postcranial skeleton, near the Blacktail Creek.[14]
It was initially assumed that all the horned dinosaur material recovered by the expeditions could be assigned to a single "styracosaur" species distinct fromStyracosaurus albertensis, as the fossils represented a limited geological time period, then estimated at half a million years.[15]Raymond Robert Rogers, who was studying thestratigraphy of the bone beds, referred to it as aStyracosaurus sp. (of undetermined species) in 1989.[16]Styracosaurus ovatus – though sometimes considered an invalidnomen dubium[17] – had already been found in the area by G. F. Sternberg and was an obvious candidate.[15] But also the possibility was taken into account that the finds were of a species new to science. This species was informally named "Styracosaurus makeli" in honor of Bob Makela, who had died in a traffic accident just days before the discovery of specimen MOR 485.[18] In 1990, this name, as an invalidnomen nudum,[19] appeared in a photo caption in a book by Stephen Czerkas.[20]
Horner, an expert on theHadrosauridae family, had less affinity for other kinds of dinosaurs.[15] In 1987 and 1989, horned dinosaur specialistPeter Dodson was invited to investigate the newceratopsian finds.[15] In 1990, the fossil material was seen by Dodson as strengthening the case for the validity of a separateStyracosaurus ovatus, to be distinguished fromStyracosaurus albertensis.[21]
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 achronospecies evolving into a series of subsequenttaxa. In 1992, Horner, David Varricchio, and Mark Goodwin published an article inNature 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 knownStyracosaurus andPachyrhinosaurus. 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.[22][23] In a 1997 book, Horner referred to the three taxa as "centrosaurine 1.", "centrosaurine 2." and "centrosaurine 3.".[24]
Sampson had continued his studies of the material since 1989. In 1994, in a talk during the annual meeting of theSociety of Vertebrate Paleontology, he named "Taxon C" as a new genus and species,Achelousaurus horneri. Although anabstract was published containing a sufficient description, it did not identify aholotype, a name-bearing specimen.[25] In 1995, in a subsequent article, Sampson indicated specimen MOR 485 as the holotype specimen ofAchelousaurus horneri. Thegeneric name consists of the wordsAchelous, the name of aGreek mythological figure, andsaurus, which isLatinized Greek for lizard.Achelous (Ἀχελῷος) is a Greek river deity and ashapeshifter who was able to transform himself into anything. During a fight withHercules, 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 throughontogenetic andphylogenetic development, and thus through individual change and evolution.[22] Dodson, in 1996, praised the generic name for being original and intelligent.[15] The specific name honors Jack Horner, for his research on the dinosaurs of the Two Medicine Formation in Montana. Sampson also named "Taxon B" as the genusEiniosaurus in the same article whereinAchelousaurus was described. He said paleontologists needed to be cautious when naming new ceratopsian genera because their intraspecific variation (i.e., variation within a species) might be mistaken for interspecific differences (between species). Until 1995, only one new genus of centrosaurine dinosaur had been named sincePachyrhinosaurus in 1950, namelyAvaceratops in 1986.[22]Achelousaurus thus holds particular importance for being one of the few ceratopsid genera named in the late twentieth century.[26]
The holotype specimen MOR 485 was collected by Hostetter and Ray Rogers[27] from the Landslide Butte Field Area about 40 km (25 mi) northwest ofCut Bank. In 1995 Sampson described it as the partial skull of an adult animal including thenasal andsupraorbital (region above the eye socket) bosses (roundish protuberances instead of horns), and theparietal bones.[22] Additionally, MOR 485 preserves some bones of the skull rear and sides, which in 2009 were listed by Tracy L. Ford as a rightsquamosal bone, the left squamosal, bothmaxillae, bothlacrimal bones, bothquadrate bones, bothpalatine bones, thebraincase and the basioccipital bone.[28] In 2015, Leonardo Maiorino reported that as part of the same specimen a fragmentary lower jaw has been catalogued as MOR 485-7-12-87-4.[29] A right squamosal bone from another adult individual was recovered from the same Canyon Bone Bed site as MOR 485 (and catalogued under the same number), but only reported in 2010.[30] Two other specimens were collected on the Blacktail Creek, 35 km (22 mi) to the south of Cut Bank and referred toAchelousaurus by Sampson in 1995. Specimen MOR 591 is a partial skull and an about 60% complete skeleton of a sub-adult specimen that includes the vertebral column, pelvis,sacrum and afemur.[22] It also includes lower jaws, catalogued as MOR 591-7-15-89-1.[31] Both skull and lower jaws are nearly complete, lacking only the braincase andoccipital region.[28] MOR 591 is smaller than the holotype with a skull base length of about 60 cm (24 in).[32] Specimen MOR 571 includes a partial skull and lower jaws with associated ribs and vertebrae of an adult.[22] The skull consists of only the parietals, and the lower jaws are limited to their upper rear bones, thesurangulars andarticulars.[28] A fifth specimen is MOR 456.1, a subadult.[33] None of the specimens were of an advanced individual age.[34] According to Andrew McDonald and colleagues, theAchelousaurus finds represented single individuals, not bone beds.[35]
In addition to fossils that have been unequivocally assigned toAchelousaurus, some other material has been found of which the identity is uncertain. Acentrosaurine ceratopsid specimen with bosses from theDinosaur Park Formation (specimen TMP 2002.76.1) found in 1996 was suggested to belong to a new taxon in 2006, but may instead belong toAchelousaurus orPachyrhinosaurus. Since it is missing the parietal bones, which are used todiagnose centrosaurines, it is not possible to assign it to any genus with confidence.[34][36] In 2006, it was also proposed thatMonoclonius lowei, a dubious species based on a skull (specimen CMN 8790) from the Dinosaur Park Formation, could be a sub-adult specimen ofStyracosaurus,Achelousaurus orEiniosaurus, with which it is roughly contemporaneous.[37] In addition, some indeterminate specimens from the Two Medicine Formation – such as fragmentary skull MOR 464[38] or snout MOR 449 – may belong toAchelousaurus or the two other roughly contemporary ceratopsidsEiniosaurus andStyracosaurus ovatus.[8] The subadult specimen MOR 591 was assigned toAchelousaurus in 1995 and henceforward, but in 2021, John Wilson and Jack Scannella stated that it could also possibly belong toEiniosaurus.[39]
Achelousaurus is estimated to have been 6 m (20 ft) long with a weight of 3 t (3.3 short tons).[40] The skull of an adult individual (holotype specimen MOR 485) was estimated to have been 1.62 m (5.3 ft) long. This puts it in the same size-range as other members of theCentrosaurinae subgroup ofceratopsians that lived during theCampanian age. It was about as large as its close relativeEiniosaurus, but with a much heavier build.[22]Achelousaurus approached the robustness of one of the largest and most heavily built horned dinosaurs known:Triceratops.[41]
As a ceratopsid,Achelousaurus would have been aquadrupedal animal with hoofed digits and a shortened, downwards swept tail. Its very large head, which would have rested on a straight neck, had a hooked upper beak, very large nasal openings, and long tooth rows developed into dental batteries that contained hundreds of appressed and stacked individual teeth.[22][40] 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).[42]
In 1995, when describing the species, Sampson gave a formal list of fourtraits that distinguishAchelousaurus 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 beingconcave; 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.[22]
Besides these unique characteristics, Sampson pointed out additional differences with two very closely related forms. The frill spikes ofAchelousaurus are more outwards oriented than the spikes ofEiniosaurus, which are medially curved; the spikes ofAchelousaurus are nevertheless less directed to the outside than the comparable spikes ofPachyrhinosaurus.Achelousaurus also differs fromPachyrhinosaurus in its smaller nasal boss that does not reach the frontal bones at its rear. Apart from the skull, no features of the skeleton are known that distinguishAchelousaurus from other members of the Centrosaurinae.[22]
Horned dinosaurs mainly differ from each other in their horns, which are located on the snout and above the eyes, and in the large skull frill, which covers the neck like a shield.Achelousaurus exhibited the build ofderived ("advanced") centrosaurines, which are typified by short brow horns or bosses, combined with elaborate frill spikes. The general frill proportions are typically centrosaurine, with a wide rounded squamosal bone at the side, which expanded towards the rear.[43] It also shares the typical frill curvature with a top surface that isconvex from side to side and concave from front to rear.[44]
AdultAchelousaurus skulls had a rugose, heavily pitted boss on the snout or nasal region, where many other ceratopsids had a horn.[22] Such a boss is often called "pachyostotic", i.e. consisting of thickened bone.[45] But describing it as a thick "boss" can be misleading: in fact, it forms a wide depression with a thin bone floor and irregular excavations, though it is less depressed than withPachyrhinosaurus.[34] The nasal boss covered about two-thirds of the top surface of the nasal bones.[8] The boss was similar to that seen in the related genusPachyrhinosaurus, though narrower, shorter, and less high.[22] It covered 27% of the total skull length, was 30% longer than the nostril-eye socket distance and was about twice as long as the eye socket.[46] Its rear edge did not reach the level of the eye socket.[34] The nasal boss extended forward, where it fused onto the nasal andpremaxilla bones (of the upper jaw) at the front of the snout,[22] though the nasal bone itself did not fuse with the premaxilla.[34] The boss of specimen MOR 485 furthermore had an excavation (or cavity) at its front end. The horn core that formed the boss may have developed by either becoming procurved (i.e. bent forward) during growth, like the horn of the relatedEiniosaurus, until it fused onto the nasal bone; or from a simple, erect horn, which later extended its base forward over the snout region, as inPachyrhinosaurus.[22] The nasal bone formed the top of a large bony nostril. From the rear edge of that nostril a sharp process stuck out to the front.[8] The snout was – compared to that ofEiniosaurus – relatively wide at the level of the rear nostrils. The lacrimal bone, in front of the eye socket, was thickened, mainly on the inner surface while the outer surface was featureless apart from a crater-like excavation.[47]
Adult skulls also possess large, rugose, and oval bosses on the supraorbital region above the eyes, instead of the horns of other ceratopsids. The supraorbital bosses extended from thepostorbital bone forward to incorporate the triangularpalpebral andprefrontal bones, and had high transverse ridges around the middle, which were thick at their base and thin towards their top.[22] The palpebral bones strongly stood out, forming an "antorbital buttress". The fused prefrontals did not reach the nasal boss,[22] forming a distinctive transverse saddle-shaped groove separating the nasal boss from the supraorbital bosses.[48] This groove extended backwards, separating the supraorbital bosses from each other and forming a T-shape in top view.[34] These bosses were similar to those ofPachyrhinosaurus, but with taller ridges and more pronounced rugosities. The long and low supraorbital horncores of the sub-adult specimen MOR 591 were similar to those of sub-adultEiniosaurus andPachyrhinosaurus. They had a concave surface on the inner side as withPachyrhinosaurus; ridges on the postorbital bones were present that may indicate a beginning transition to bosses.[22]
The skull roof ofAchelousaurus had a midline cavity, with an opening at the top called the frontalfontanelle, a feature found in all ceratopsids, which have a "double" skull roof formed by the frontal bones folding towards each other between the brow horn bases. This cavity formedsinuses that extended below the supraorbital bosses, which were therefore relatively thin internally, being 25 mm (1 in) thick from the outside to the cavity roof. This cavity appears to have partially closed over as an animal aged, with only the rear part of the fontanelle being open in the adult specimen MOR 485.[22]
Like that of all other ceratopsids, the skull ofAchelousaurus had a parietosquamosal frill or "neck shield", which was formed by the parietal bones at the rear and the squamosal bones at the sides. The parietal is one of the main bones used to distinguish centrosaurine taxa from each other and resolve relationships between them, whereas the squamosal is very similar across taxa.[22] InAchelousaurus, the squamosal bone was much shorter than the parietal. Of its inner margin the rear portion formed a step in relation to the front part, with the suture between the squamosal and the parietal showing a kink to behind at the level of the rearsupratemporal fenestra, a typical centrosaurine trait.[8] The squamosal and thejugal bone, by touching each other, excluded thequadratojugal from the edge of the lateraltemporal fenestra, i.e. the opening at the rear of the skull side.[49]
The frill ofAchelousaurus had two conspicuous large spikes that were directed backwards and curved to the sides away from each other. During the 1990s, it was increasingly understood that such spikes on the parietals were not random growths but specific traits that could be used to determine the evolution of horned dinosaurs, if only it could be analyzed how they corresponded among species. Sampson, in the paper describingAchelousaurus in 1995, therefore introduced a generalized numbering system for such parietal processes, counting them from the midline to the side of the frill.[22] This was applied to the Centrosaurinae as a whole in 1997.[50] The large spikes ofAchelousaurus correspond to "Process 3" spikes of other centrosaurines and were similar to those ofEiniosaurus, though curved more to the sides, similar toPachyrhinosaurus.[22] They were shorter and thinner than the corresponding spikes ofStyracosaurus.[51] Between these spikes, on both sides of the central frill notch, were two small tab-like processes ("Process 2") that were directed towards the midline.[22] Innermost "Process 1" spikes, as present inCentrosaurus, are lacking withAchelousaurus.[51] The frill had two large paired openings, the parietal fenestrae, with a midline parietal bar between them. A linear row of rounded swellings ran along the top of the parietal bar, which may behomologous to the spikes and horns in the same area of somePachyrhinosaurus specimens. A row of relatively small processes ran along the parietal shield margin from the "Process 3" spikes outwards,[22] for a total per side of seven. They were largely equal in size, causing the P4 process to be reduced in comparison to the P3.[52] These lower processes appear to have been capped by epoccipitals, bones that lined the frills of ceratopsids.[22] InAchelousaurus these epoccipitals, which start as separate skinossifications orosteoderms, fuse with the underlying frill bone to form spikes,[53] at least in the third position.[54] 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.[52]
The bosses on the skull ofAchelousaurus may have been covered in akeratinous sheath in life, but their shape in a living animal is uncertain.[22] In 2009, the paleontologist Tobin L. Hieronymus and colleagues examined correlations between skull morphology, horn, and skin features of modern horned animals, and examined the skull of centrosaurine dinosaurs for the same correlates. They proposed that the rugose bosses ofAchelousaurus andPachyrhinosaurus were covered by thick pads of cornified (or keratinized) skin, similar to the boss of modernmuskoxen (Ovibos moschatus). The nasal horncore of adultAchelousaurus had an upward slant and its upper surface had correlates for a thickepidermal (outer layer of skin) pad that graded into correlates for a cornified sheath on the sides. A thick pad of epidermis may have grown from the V-shaped pitted notch at the tip of the nasal horncore. The growth direction of the nasal pad would have been towards the front. The supraorbital bosses may have had a thick pad of epidermis, which grew at a sideways angle similar to the curved horncores ofCoronosaurus, as indicated by the orientation of the "fins" or ridges on the bosses. That the supraorbital bosses lacked asulcus (or furrow) at their bases indicates that their horn pads stopped at the wrinkled edges of the bosses. The pitting might indicate a softer growing layer connecting the hard inner bone with the hard horn sheath. In addition, correlates for arostral scale in front of the nasal boss and scale rows along the parietal midline and supraorbital-squamosal region were identified.[55]
In 1992, the study by Horneret al. tried to account for the fact that within a limited geological period of time (about half a million years) there had been a quicksuccession 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 laterfauna was basically the old one but at a more evolved stage. The various types found were not distinct species buttransitional forms developed within a process ofanagenesis. 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 trueautapomorphies – unique traits that prove a taxon is a separate species. The fossils instead showed a gradual change frombasal (or ancestral) into more derived characters.[23]
The horned dinosaurs discovered by Horner exemplified this phenomenon. In the lowest layers of the Two Medicine Formation, 60 m (200 ft) below the overlayingBearpaw Formation, "Transitional Taxon A" was present. It seemed to be identical toStyracosaurus albertensis, differing from it only in the possession of just a single pair of parietal spikes. The middle layers, 45 m (150 ft) below the Bearpaw, contained "Transitional Taxon B" that also had a single spike pair but differed in the form of its nasal horn that curved to the front over the anterior branches of the nasal bones. In the upper strata, 20 m (65 ft) below the Bearpaw, "Transitional Taxon C" had been excavated. It too had a spike pair but now the nasal horn was fused with the front branches. The upper surface of the horn was elevated and very rough. The orbital horns showed coarse ridges. Subsequently, "Taxon A" was namedStellasaurus,[9][52] "Taxon B" becameEiniosaurus, while "Taxon C" becameAchelousaurus.[23] In 1992, Horneret al. did not name these as species for the explicit reason that the entire evolutionary sequence was seen as representing a grade of transitional ceratopsians betweenStyracosaurus albertensis, known from theJudith River Formation, and the derived, hornlessPachyrhinosaurus from theHorseshoe Canyon Formation, which had the spike pair and bosses on the nose and above the eyes, as well as additional frill ornamentation.[23]
Horner thought he had found the mechanism driving this evolution, elaborating on ideas he had developed even before he had investigated Landslide Butte.[56] The animals were living on a narrow strip on the east-coast of Laramidia, bordering theWestern Interior Seaway and constrained in the west by the 3 to 4 kilometres (2 to 2.5 mi) highproto-Rocky Mountains. During the Bearpaw Transgression sea levels were rising, steadily reducing the width of their coastal habitat from about 300 km (200 mi) to 30 km (20 mi).[57] This led to strongerselection pressures,[23] the severest forAchelousaurus which lived during the phase that the coastal strip was at its narrowest.[58] The lower number of individuals that the smaller habitat could have sustained constituted apopulation bottleneck, making rapid evolution possible.[15] Increasedsexual selection would have induced changes in the sexual ornamentation such as spikes, horns and bosses.[23] A reduced environmental stress by lower sea levels on the other hand, would be typified byadaptive radiation. That sexual selection had indeed been the main mechanism would be proven by the fact that young individuals of all three populations were very similar: they all had two frill spikes, a small nasal horn pointing to the front, and orbital horns in the form of slightly elevated knobs. Only in the adult phase did they begin to differ. According to Horner, this also showed that the populations were very closely related.[59]
Horner did not perform an exactcladistic analysis determining the relationship between the three populations. Such an analysis calculates which evolutionary tree implies the lowest number of evolutionary changes and therefore is the most likely. He assumed that this would result in a tree in which the types were successive branches. Such a tree would, as a consequence of the method used, never show a direct ancestor-descendant relationship. Many scientists believed such a relation could never be proven anyway. 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. Evolutionists in general would be too hesitant to recognize this.[60] Such a transition is called anagenesis; he posited that, if the opposite,cladogenesis, could not be proven, a scientist was free to assume an anagenetic process.[23]
Basing himself on revised data, Sampson in 1995 estimated that the layers investigated represented a longer period of time than the initially assumed 500,000 years: after the deposition of Gilmore'sBrachyceratops quarry, 860,000 years would have passed, and after theEiniosaurus beds 640,000 years, until the maximal extent of the Bearpaw transgression. He did not adopt Horner's hypothesis of anagenesis but assumedspeciation took place, with the populations splitting. These time intervals were still short enough to indicate that the rate of speciation must have been high, which might have been true of all centrosaurines of the late Campanian.[22]
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 withStyracosaurus albertensis. This suggested to him that theEiniosaurus–Achelousaurus lineage was a separate branch within the Centrosaurinae. Secondly, he was concerned thatEiniosaurus andAchelousaurus were a case ofsexual 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.[15]
In 2010, Horner admitted that specimen TMP 2002.76.1 seemed to indicate thatAchelousaurus was not descended fromEiniosaurus, as it preceded both in age, and yet had a nasal boss. But he stressed that even if the lineages split off, its ancestor might have resembledEiniosaurus. Furthermore, it might still be possible thatEiniosaurus was a direct descendant ofRubeosaurus. Also, the process of rapid displacements and extinctions of species could in his opinion still be elegantly explained by a westward expansion of the Bearpaw Sea.[8]
The process of anagenesis was affirmed by Wilson and Scannella in 2016, who studied the ontogenetic changes in horned dinosaurs. They compared a smallEiniosaurus specimen, MOR 456 8-8-87-1, withAchelousaurus specimen MOR 591. Both proved to be quite similar, with the main differences being a longer face in MOR 456 8-8-87-1, and a sharper supraorbital horncore in MOR 591. They concluded thatAchelousaurus was likely the direct descendant ofEiniosaurus. The more adultEiniosaurus individuals approached theAchelousaurus morphology. The differences between the two taxa would have been caused byheterochrony – differential changes in the speed the various traits developed during the lifetime of an individual.[32] Since Wilson and colleagues found in 2020 thatStellasaurus (Horner's "Taxon A") was intermediate betweenStyracosaurus andEiniosaurus in morphology and stratigraphy, they could not discount that it was a transitional taxon within an anagenetic lineage.[52]
In 1995, Sampson formally placedAchelousaurus in theCeratopsidae, more precisely the Centrosaurinae.[22] In all analyses,Einiosaurus andAchelousaurus are part of the clade Pachyrhinosaurini. By definition,Achelousaurus is a member of the cladePachyrostra (or "thick-snouts"), in which it is united withPachyrhinosaurus.[61] In 2010,Gregory S. Paul assignedA. horneri to the genusCentrosaurus, asC. horneri.[40] This has found no acceptance among other researchers, with subsequenttaxonomic assessments invariably keeping the generic nameAchelousaurus.[62][63][61][64]
Sampson felt, in 1995, that there was not enough evidence to conclude thatAchelousaurus was a direct descendant ofEiniosaurus. Unlike Horner, he decided to perform a cladistic analysis to establish aphylogeny. This showed an evolutionary tree whereinAchelousaurus split off betweenEiniosaurus andPachyrhinosaurus, as Horner had predicted. Contrary to Horner's claim,Styracosaurus albertensis could not have been a direct ancestor, as it was asister species ofCentrosaurus in Sampson's analysis.[22]
Subsequent studies have sought to determine the precise relationships within this part of the evolutionary tree, with conflicting results regarding the question whetherStyracosaurus albertensis orEiniosaurus might have been in the direct line of ascent toAchelousaurus. In 2005, an analysis by Michael Ryan and Anthony Russell foundStyracosaurus more closely related toAchelousaurus than toCentrosaurus.[65] This was confirmed by analyses by Ryan in 2007,[66] Nicholas Longrich in 2010,[67] and Xu et al. in 2010.[68] The same year Horner and Andrew T. McDonald movedStyracosaurus ovatus to its own genus,Rubeosaurus, finding it a sister species ofEiniosaurus, whileStyracosaurus albertensis was again located on theCentrosaurus branch. They also assigned specimen MOR 492, the basis of "Taxon A", toRubeosaurus.[8] In 2011, a subsequent study by Andrew T. McDonald in this respect replicated the outcome of his previous one,[62] as did a publication by Andre Farke et al.[63] In 2017, J.P. Wilson and Ryan further complicated the issue, concluding that MOR 492 ("Taxon A") was not referable toRubeosaurus and announcing that yet another genus would be named for it.[9] Wilson and colleagues moved MOR 492 to the new genusStellasaurus in 2020, which therefore corresponds to "Taxon A". Their study foundRubeosaurus ovatus to be the sister species ofStyracosaurus albertensis, and concludedRubeosaurus to be synonymous withStyracosaurus.[52]
BeforeAchelousaurus was described,Pachyrhinosaurus canadensis had been considered a solitaryaberrant form among centrosaurines, set apart from them by its unusual bosses.Achelousaurus gave evolutionary context to the Canadian species, while expanding the temporal and geographical range for what came to be seen as "pachyrhinosaurs."[61] In all analyses,Achelousaurus andPachyrhinosaurus weresister groups. In 2008, another closely related species was named,Pachyrhinosaurus lakustai. In that study, the term "Pachyrhinosaurs" was used for the clade consisting ofAchelousaurus andPachyrhinosaurus.[69] WhenPachyrhinosaurus perotorum was described in 2012, the clade name Pachyrostra was coined, uniting the two genera;Achelousaurus is the basalmost pachyrostran. Shared derived traits (orsynapomorphies) of the group are an enlarged nasal ornamentation and a change of the nasal and brow horns into bosses.[61] At the end of the Campanian, there seems to have been a trend of pachyrostrans replacing other centrosaurines.[34] Also in 2012, the clade Pachyrhinosaurini was named, consisting of species more closely related toPachyrhinosaurus orAchelousaurus than toCentrosaurus. Apart fromEiniosaurus andRubeosaurus, this includedSinoceratops andXenoceratops, according to a 2013 study.[70]
Cladistic analyses develop gradually, reflecting new discoveries and insights. Their results can be shown in acladogram, with the relationships found ordered in an evolutionary tree. The cladogram below shows the phylogenetic position ofAchelousaurus in a cladogram from Wilson and colleagues, 2020.[52]
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In 1995, Sampson noted that earlier studies had found that the horns and frills of ceratopsians most likely had a function inintraspecific display and combat, and that these features would therefore have resulted from sexual selection for successful mating.[22] Likewise, in 1997 Horner concluded that such ornamentation was used by males to establish dominance and that females would have preferred well-equipped males as their offspring would then inherit these traits, conferring a reproduction benefit.[58] Dodson thought that in the Centrosaurinae in general the display value of the frill had been reduced compared to the nasal and supraorbital ornamentation.[71] Sampson in 1995 rejected the possibility that the difference in skull ornamentation betweenEiniosaurus andAchelousaurus represented sexual dimorphism, for three reasons. Firstly, the extensiveEiniosaurus bone beds did not contain any specimens with bosses, as would have been expected if one of the sexes had them. Secondly,Einiosaurus andAchelousaurus are found in strata of a different age. Thirdly, in a situation of sexual dimorphism usually only one of the sexes shows exaggeratedsecondary sexual characters.Einiosaurus andAchelousaurus however, each have developed a distinct set of such traits.[22]
Hieronymus, in 2009, concluded that the nasal and supraorbital bosses were used for butting or ramming the head or the flank of a rival. The bone structure indicates that the bosses were covered by cornified pads as in modern muskoxen, suggesting dominance fights similar to those of members of theCaprinaesubfamily. In the latter group, an evolutionary transition can be observed, where the originally straight horns become more robust, padded, and increasingly curved downwards. The evolution from horncores into bosses in Centrosaurinae would likewise have reflected a change in fighting technique, from clashing to high-energy head-butting. Head-butting would have been an expensive and risky behavior. Opponents would have engaged this way only after assessing each other's strengths visually. For this reason, Hieronymus considered it unlikely that the bosses served forspecies recognition as this was already guaranteed by the innate species-specific display rituals preceding a real – instead of a ritual – fight. The bosses would have evolved for actual combat, part of asocial selection in which individuals competed for scarce resources such as mates, food and breeding grounds.[55]
Previously it had been suggested that the fusion of the first three neck vertebrae, such as seen in the mature specimen MOR 571, might have been apaleopathology, an instance of the diseasespondyloarthropathy, but in 1997 it was concluded that it was more likely a normal ontogenetic trait, the vertebrae growing together to form a so-called "syncervical" to support the heavy head.[72] All three main known specimens have syncervicals consisting of three fused neck vertebrae;[73][74] the trait could have been inherited from a smaller ancestor using a stiffer neck for burrowing or food acquisition.[75]
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.[30] Horner assumed that the horned dinosaurs at Landslide Butte lived in herds which had been killed by drought or disease.[76] Dodson concluded that the fact that theAchelousaurus bone beds were monospecific (containing only one species) confirmed the existence of herds.[71]
There has long been debate about thethermoregulation of dinosaurs, centered around whether they wereectotherms ("cold-blooded") orendotherms ("warm-blooded"). Mammals and birds arehomeothermic endotherms, which generate their own body heat and have a highmetabolism, whereas reptiles areheterothermic ectotherms, which receive most of their body heat from their surroundings. A 1996 study examined theoxygen isotopes from bonephosphates of animals from the Two Medicine Formation, including the juvenileAchelousaurus orEiniosaurus 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. Avaranid 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.[77][39]
In 2021 a study by Wilson and Scannella pointed out that specimen MOR 591 was of a younger individual age than theEiniosaurus skull MOR 456 8-8-87-1, but of the same size. If MOR 591 could indeed be referred toAchelousaurus, this might indicate this genus reached its adult size more quickly.[39]
Achelousaurus is known from the Two Medicine Formation, which preserves coastal sediments dating from the Campanian stage of the LateCretaceous 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.[34][78]
The Two Medicine Formation is typified by a warmsemiarid climate. Its layers were deposed on the east coast of the Laramidia island continent (which consisted of western North America). The highcordillera in the west, combined with predominantly western winds, would have caused arain shadow, limiting annual rainfall. Rain would mainly have fallen during the summer, whenconvection 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 onoxbow 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, causingbone beds to form.[79] The brownpaleosol in which the horned dinosaurs were found – a mixture of clay andcoalified wood fragments – resembles that of modern seasonally dry swamps. The surrounding vegetation might have consisted of about 25 m (80 ft) high conifer trees.[80]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 1 m (3.5 ft) or lower.[81]
More or less contemporary dinosaur genera of the area includedProsaurolophus,Scolosaurus,Hypacrosaurus,Einiosaurus andtyrannosaurids of uncertain classification. As proven by tooth marks, horned dinosaur fossils in the Landslide Butte Field Area had been scavenged by a largetheropod predator, which Rogers suggested wereAlbertosaurus.[82]
The exact composition of the faunaAchelousaurus was part of is uncertain, as its fossils have not been discovered in direct association with other taxa. Its intermediate anagenetic position suggests thatAchelousaurus 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 betweenLambeosaurus andHypacrosaurus;[23] in 1994 he would name itHypacrosaurus stebingeri.[83] Today,Hypacrosaurus stebingeri is no longer seen as having evolved through anagenesis because autapomorphies of the species have been identified.[84] Horner saw somepachycephalosaur skulls as indicative for a taxon in betweenStegoceras andPachycephalosaurus;[23] these have not been consistently referred to a new genus. Finally, Horner thought there was a taxon present that was transitional betweenDaspletosaurus andTyrannosaurus.[23] In 2017, tyrannosaurid remains from the Two Medicine Formation were named as a new species ofDaspletosaurus:Daspletosaurus horneri.[85] The 2017 study considered it plausible thatD. horneri was a direct descendant ofD. torosus in a process of anagenesis, but rejected the possibility thatD. horneri was the ancestor ofTyrannosaurus.[86]
Other ceratopsians from the Two Medicine Formation includeEiniosaurus andStellasaurus. In addition, remains of other indeterminate and dubious centrosaurines, includingBrachyceratops, are known from the formation and though they may represent younger stages of the three valid genera, this is not possible to demonstrate.[45][52] Whereas Horner assumed thatEiniosaurus andAchelousaurus were separate in time, in 2010 Donald M. Henderson considered it possible that at least their descendants or ancestors were overlapping orsympatric and thus would have competed for food sources unless there had beenniche partitioning. The skull ofAchelousaurus was more than twice as strong than that ofEiniosaurus in its bending strength andtorsion resistance. This might have indicated a difference in diet to avoid competition. The bite strength ofAchelousaurus, measured as anultimate 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, forEiniosaurus.[87] Wilson and colleagues found that since the Two Medicine centrosaurines were separated stratigraphically, they were therefore possibly not contemporaneous.[52]
The indeterminate specimen TMP 2002.76.1 is from the Dinosaur Park Formation and, if it belongs toAchelousaurus, the genus would be the stratigraphically oldest known pachyrhinosaurine taxon.[34] Both animals occur right below the marineshales of the Bearpaw Formation, but due to longitudinal differences, TMP 2002.76.1 is about 500,000 years older than theAchelousaurus fossils from the Two Medicine Formation.[36]
This one has to be pronounced ak-e-LOH-uh-SAWR-us, NOT a-KEE-lo-SAWR-us, as I recall hearing. The name derives from Greek Akheloos (the two o's being pronounced separately), which Latin rules turn into Achelous (divided as a-che-lo-us, short e, long o and short u), again pronounced in four syllables with the accent on the next-to-last, that is the one with the long o. All sources I have checked indicate that ak-e-LOH-us is the accepted English pronunciation of the Latin name. Since the scientific name Achelousaurus was formed by arbitrarily combining Achelous and saurus instead of using the stem-form Achelo-, the "u" needs to pronounced.
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