''Acristavus'' (meaning "non-crested grandfather") is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

saurolophine Saurolophinae is a subfamily (biology), subfamily of hadrosaurid dinosaurs. It has since the mid-20th century generally been called the Hadrosaurinae, a group of largely non-crested hadrosaurs related to the crested sub-family Lambeosaurinae. How ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

. Fossils have been found from the

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountai ...

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

and

Wahweap Formation The Wahweap Formation of the Grand Staircase–Escalante National Monument is a geological Formation (geology), formation in southern Utah and northern Arizona, around the Lake Powell region, whose strata date back to the Late Cretaceous (Campan ...

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

, United States. The

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''A. gagslarsoni'' was named in 2011. Unlike nearly all hadrosaurids except ''

Edmontosaurus ''Edmontosaurus'' ( ) (meaning "lizard from Edmonton"), with the second species often colloquially and historically known as ''Anatosaurus'' or ''Anatotitan'' (meaning "duck lizard" and "giant duck"), is a genus of hadrosaurid (duck-billed) din ...

'', ''Acristavus'' lacked ornamentation on its skull. The discovery of ''Acristavus'' is paleontologically significant because it supports the position that the ancestor of all hadrosaurids did not possess cranial ornamentation, and that ornamentation was an adaptation that later arose interdependently in the subfamilies Saurolophinae and

Lambeosaurinae Lambeosauridae /ˌlæmbiəˈsɔːraɪniː/ (meaning 'lambe's lizards') is an extinct group of crested hadrosauroid dinosaurs. Description Size Uncertainty surrounds the size of lambeosaurs from the European continent. Hadrosaurs found there, ...

. It is closely related to ''

Brachylophosaurus ''Brachylophosaurus'' ( or ) is a genus of hadrosaurid dinosaur that lived during the Late Cretaceous period of western North America. It was first named in 1953 by Charles Mortram Sternberg for a skull and skeleton he discovered in 1936 in th ...

'' and ''

Maiasaura ''Maiasaura'' (from the Greek ''μαῖα'', meaning "midwife" and ''σαύρα'', the feminine form of ''saurus'', meaning "reptile") is a large herbivorous saurolophine hadrosaurid ("duck-billed") dinosaur genus that lived in the area currently ...

'', and was assigned to a new

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

called

Brachylophosaurini Saurolophinae is a subfamily (biology), subfamily of hadrosaurid dinosaurs. It has since the mid-20th century generally been called the Hadrosaurinae, a group of largely non-crested hadrosaurs related to the crested sub-family Lambeosaurinae. How ...

Discovery and occurrence

The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

specimen of ''Acristavus'', MOR 1155, was recovered at the Two Medicine Formation, in Teton County, Montana. The specimen was collected in 1999 by C. Riley Nelson in well-

indurated In materials science, friability ( ), the condition of being friable, describes the tendency of a solid substance to break into smaller pieces under stress or contact, especially by rubbing. The opposite of friable is indurate. Substances tha ...

tan colored calcareous sandstone that was deposited during the

stage Stage, stages, or staging may refer to: Arts and media Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly Brit ...

of the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

period, approximately 79 million years ago. MOR 1155 consists of an almost complete skull with associated postcrania including eleven cervical vertebrae, three incomplete dorsal vertebrae, a proximal caudal vertebra, several dorsal ribs, the left humerus, the left ulna, the right sternal, the left pubis, the left femur, the left tibia, two left metatarsals, five left pedal phalanges and one right pedal phalanx. A second specimen UMNHVP 16607 assigned to ''Acristavus'' in 2011, was excavated from the Smokey Mountain Road locality in the Reynolds Point Member of the Wahweap Formation in Utah. It was collected by C. R. Nelson in 2000, in lithified, yellow sandstone and its age was estimated to be 80.69 Ma, with a range of uncertainty of 81.05-80.19 Ma. UMNHVP 16607 consists of a partial articulated skull, including both lacrimals, a complete braincase, and a cervical vertebra.

Description

Distinguishing anatomical features

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism. According to Gates (2011), ''Acristavus'' can be distinguished based on the following characteristics: * an enlarged caudodorsal brow of the postorbital and the basioccipital, which extends caudally well beyond the dorsal border of the foremen magnum to a greater extent than in other hadrosaurine species * a deep laterally squared frontonasal suture * swelling of the interdigitate suture between the prefrontal and frontal * a deep depression is present on the lateral surface of the postorbital jugal process

Classification

''Acristavus'' was first assigned to the

, in a basal position, by Gates et al. (2011). All subsequent

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analyses have confirmed this assignment. Brachylophosaurines are derived members of the group

Saurolophinae Saurolophinae is a subfamily (biology), subfamily of hadrosaurid dinosaurs. It has since the mid-20th century generally been called the Hadrosaurinae, a group of largely non-crested hadrosaurs related to the crested sub-family Lambeosaurinae. How ...

. Other brachylophosaurins include ''

'', ''

'', and potentially ''

Wulagasaurus ''Wulagasaurus'' (meaning "Wulaga lizard", in reference to the discovery locality) is a genus of saurolophine hadrosaurid dinosaur from the Late Cretaceous of Heilongjiang, China. Discovery Its remains were found in a bonebed in the middle ...

''. Gates concluded that ''Acristavus'' and ''Maiasaura'' shared a sister-

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

relationship but more recent analysis by Prieto-Márquez (2013) shows that ''Maiasaura'' is more closely related to the more derived ''Brachylophosaurus''. The following cladogram is based on the 2013 phylogenetic analysis by Prieto-Márquez (the relationships within Lambeosaurinae and between basal hadrosauroids are not shown):

Paleoecology

Habitat

The Wahweap Formation has been radiometrically dated as being between 81 and 76 million years old. During the time that ''Acristavus'' lived, the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...

was at its widest extent, almost completely isolating southern

Laramidia Laramidia was an island continent that existed during the Late Cretaceous period (99.6–66 Year#SI prefix multipliers, Ma), when the Western Interior Seaway split the continent of North America in two. In the Mesozoic era, Laramidia was an island ...

off from the rest of North America. The area where dinosaurs lived included lakes, floodplains, and east-flowing rivers. The Wahweap Formation is part of the

Grand Staircase The Grand Staircase is an immense sequence of sedimentary rock layers that stretches south from Bryce Canyon National Park and Grand Staircase–Escalante National Monument, through Zion National Park, and into Grand Canyon National Park. C ...

region, an immense sequence of sedimentary rock layers that stretch south from Bryce Canyon National Park through Zion National Park and into the Grand Canyon. The presence of rapid sedimentation and other evidence suggests a wet, seasonal climate.

Paleofauna

''Acristavus'' shared its

paleoenvironment Paleoecology (also spelled palaeoecology) is the study of interactions between organisms and/or interactions between organisms and their environments across geologic timescales. As a discipline, paleoecology interacts with, depends on and informs ...

with other dinosaurs, such as the

lambeosaur Lambeosauridae /ˌlæmbiəˈsɔːraɪniː/ (meaning 'lambe's lizards') is an extinct group of crested hadrosauroid dinosaurs. Description Size Uncertainty surrounds the size of lambeosaurs from the European continent. Hadrosaurs found there, ...

'' Adelolophus hutchisoni'', the

ceratopsian Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Asia and Europe, during the Cretaceous Period, although ancestral forms lived earlier, in the Late Ju ...

Diabloceratops ''Diabloceratops'' ( ) is an extinct genus of centrosaurine ceratopsian dinosaur that lived approximately 81.4-81 million years ago during the latter part of the Cretaceous Period in what is now Utah, in the United States. ''Diabloceratops'' wa ...

eatoni'', unnamed ankylosaurs and pachycephalosaurs, and the

theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...

Lythronax ''Lythronax'' () is a genus of tyrannosaurid dinosaur that lived in North America around 81.9-81.5 million years ago during the Late Cretaceous period. The only known specimen was discovered in Utah in the Wahweap Formation of the Grand Stairca ...

argestes'', which was likely the

apex predator An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hig ...

in its ecosystem. Vertebrates present in the Wahweap Formation at the time of ''Acristavus'' included

freshwater fish Freshwater fish are fish species that spend some or all of their lives in bodies of fresh water such as rivers, lakes, ponds and inland wetlands, where the salinity is less than 1.05%. These environments differ from marine habitats in many wa ...

bowfin The ruddy bowfin (''Amia calva'') is a ray-finned fish native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being one of only two surviving species ...

s, abundant

ray Ray or RAY may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), the bony or horny spine on ray-finned fish Science and mathematics * Half-line (geometry) or ray, half of a line split at an ...

s and

sharks Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

, turtles like ''

Compsemys ''Compsemys '' is an extinct genus of prehistoric turtles from the Late Cretaceous and Paleocene of North America and possibly Europe. The type species ''C. victa'', first described by Joseph Leidy from the Hell Creek Formation in Montana in 1856 ...

'', crocodilians, and

lungfish Lungfish are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, inc ...

. A fair number of mammals lived in this region, which included several genera of

multituberculates Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, a ...

cladotheria Cladotheria is a clade (sometimes ranked as a legion) of mammals. It contains modern therian mammals (marsupials and placentals) and several extinct groups, such as the "dryolestoids", amphitheriids and peramurids. The clade was named in 1975 ...

ns,

marsupials Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a ...

, and

placental Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished ...

insectivores A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant which eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores wer ...

. These mammals were more primitive than those that lived in the area that is now the

Kaiparowits Formation The Kaiparowits Formation is a sedimentary geological formation, rock formation found in the Kaiparowits Plateau in Grand Staircase–Escalante National Monument, in the southern part of Utah in the western United States. It is over 2800 feet (8 ...

Trace fossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

s are relatively abundant in the Wahweap Formation, and suggest the presence of

crocodylomorphs Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. Extinct crocodylomorphs were considerably more ...

, as well as

ornithischian Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek st ...

and

dinosaurs. In 2010 a unique trace fossil was discovered that suggests a predator-prey relationship between dinosaurs and primitive mammals. The trace fossil includes at least two fossilized mammalian den complexes as well as associated digging grooves presumably caused by a

maniraptora Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to ''Ornithomimus velox''. It contains the major subgroups Avialae, Dromaeosauridae, Troodontidae, ...

n dinosaur. The proximity indicates a case of probable active predation of the burrow inhabitants by the animals that made the claw marks. Invertebrate activity in this formation ranged from fossilized

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

burrows in

petrified logs Petrified wood (from Ancient Greek meaning 'rock' or 'stone'; literally 'wood turned into stone'), is the name given to a special type of ''fossilized wood'', the fossilized remains of terrestrial vegetation. ''Petrifaction'' is the result o ...

to various

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

s, large

crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...

s,Kirkland, James Ian. "An inventory of paleontological resources in the lower Wahweap Formation (lower Campanian), southern Kaiparowits Plateau, Grand Staircase-Escalante National Monument, Utah." Abstracts with Programs - Geological Society of America, vol.37, no.7, pp.114, Oct 2005. and a wide diversity of

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

s and

ostracods Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant) have been identified,Brandão, S.N.; Antonietto, L.S; Nery, D.G.; Santos, S.G.; Karano ...

.Williams, Jessica A J; Lohrengel, C Frederick. ''Preliminary study of freshwater gastropods in the Wahweap Formation, Bryce Canyon National Park, Utah.'' Abstracts with Programs - Geological Society of America, vol. 39, no. 5, pp.43, May 2007

References

{{Taxonbar, from=Q1932513 Saurolophinae Dinosaur genera Campanian dinosaurs Taxa named by Jack Horner Fossil taxa described in 2011 Dinosaurs of the United States