''Avaceratops'' 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 ...

of small herbivorous

ceratopsia Ceratopsia or Ceratopia ( or ; Ancient Greek, Greek: "horned faces") is a group of herbivore, herbivorous, beaked dinosaurs that thrived in what are now North America, Asia and Europe, during the Cretaceous Period (geology), Period, although ance ...

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 ...

s which lived during the late

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 ...

in what are now the Northwest

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. Most fossils come from the

Judith River Formation The Judith River Formation is a fossil-bearing geologic formation in Montana, and is part of the Montana Group. It dates to the Late Cretaceous, between 79 and 75.3 million years ago, corresponding to the "Judithian" land vertebrate age. It was ...

Discovery and naming

The first remains of ''Avaceratops'' were found by Eddie Cole in the

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 ...

, in 1981, on land of the Careless Creek Ranch, owned by rancher Arthur J. Lammers."Avaceratops." In: Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. ''The Age of Dinosaurs''. Publications International, LTD. p. 129. . They were preserved scattered throughout the remains of a prehistoric stream bed. This ''Avaceratops'' specimen was likely buried in the sandbar after its body was swept downstream by the current. The finds, displayed in Cole's fossil shop, were in October 1981 inspected by Peter Dodson who in July 1982 during a visit with Cole to the site discovered additional bones, which from 1984 were excavated by Anthony Fiorillo. The fossil was formally named and described by Dodson in 1986, as 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 ...

''Avaceratops lammersi''. It was the first ceratopsid named since '' Pachyrhinosaurus'' in 1950. The genus was named after Ava Cole, Eddie's wife. The specific epithet honors the Lammers family. In 1990 George Olshevsky emended the name to ''A. lammersorum'', with the specific name in the genitive plural because it referred to several people. However, Dodson objected against this change, arguing the genitive singular might also refer to a single family name. In 1990 Thomas Lehman renamed ''A. lammersi'' into '' Monoclonius lammersi''. This alternative name has found no acceptance. 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 ...

, ANSP 15800, consists of a partial skeleton containing the lower skull, a left lower jaw, vertebrae, a complete shoulder girdle and most elements of forelimbs and hindlimbs. The type specimen might represent a juvenile or a subadult individual. While Dodson in 1986 was inclined to consider it almost fully grown, a 2019

histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...

analysis concluded that the animal was still experiencing the kind of rapid growth that would be expected from a centrosaurine in its first few years of life.

Kenneth Carpenter Kenneth Carpenter (born 21 September 1949) is an American paleontologist. He is the former director of the USU Eastern Prehistoric Museum and author or co-author of books on dinosaurs and Mesozoic life. His main research interests are armore ...

made a reconstruction of the skull, a cast of which was by Leroy Glenn combined with restored parts of the postcranial skeleton to create a mount that in 1986 was displayed in the

Academy of Natural Sciences The Academy of Natural Sciences of Drexel University, formerly the Academy of Natural Sciences of Philadelphia, is the oldest natural science research institution and museum in the Americas. It was founded in 1812, by many of the leading natur ...

Philadelphia Philadelphia ( ), colloquially referred to as Philly, is the List of municipalities in Pennsylvania, most populous city in the U.S. state of Pennsylvania and the List of United States cities by population, sixth-most populous city in the Unit ...

; a copy of this skeletal mount was donated to the Upper Musselshell Valley Historical Museum in Harlowton. In 1993 Paul Penkalski referred two earlier found

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...

s to ''Avaceratops'', USNM 4802 and USNM 2415, of larger individuals. In 1999, Penkalski and Dodson described a second skull, MOR 692, again of a larger individual. This specimen includes the upper skull, with the nose and brow horns. However, the assignment of the latter to ''Avaceratops'' has been questioned due to differences in relative chronological position, skull morphology, and lack of intermediate forms; thus, ''Avaceratops'' is only represented by the type specimen.

Description

''Avaceratops'' was originally by Dodson considered to have been an exceptionally small species. He estimated the holotype length at and assumed it had almost attained adult size. The maturity of the animal was disputed by a 2019 histological analysis. In addition, a second skull (which does not necessarily belong to ''Avaceratops''), MOR 692, indicates a body length of . Paul in 2010 estimated the weight of a four-metre-long animal at one tonne.Paul, G.S., 2010, ''The Princeton Field Guide to Dinosaurs'', Princeton University Press p. 264 Another change in the image of ''Avaceratops'' brought about by MOR 692 concerned the brow horns. These were first thought to be rather short, although this was purely speculative, the holotype not having preserved them. The new skull showed postorbital horn cores with a length of . ''Avaceratops'' has a distinctive frill at the back of the skull. The squamosal, the element at the front of the frill side, is large with a continuously curving instead of a stepped edge. A raised area at the base of the squamosal divides it into two equal halves, whereas more derived species have an enlarged top part. The squamosal is separated from the

parietal bone The parietal bones ( ) are two bones in the skull which, when joined at a fibrous joint known as a cranial suture, form the sides and roof of the neurocranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four bord ...

at the rear of the skull by a small indentation. The parietals however, show no indentation at the midline of the frill rear. Also the parietals are probably lacking '' fenestrae'' which are typical of many other genera except ''

Triceratops ''Triceratops'' ( ; ) is a genus of Chasmosaurinae, chasmosaurine Ceratopsia, ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cretaceous Period (geology), period, about 68 to 66 million years ago on the island ...

'', resulting in a solid frill, although damage to the holotype allows for a small opening to be present.

Classification

''Avaceratops'' was by Dodson in 1986 assigned to the

Ceratopsidae Ceratopsidae (sometimes spelled Ceratopidae) is a family of ceratopsian dinosaurs including ''Triceratops'', ''Centrosaurus'', and ''Styracosaurus''. All known species were quadrupedal herbivores from the Upper Cretaceous. All but one species are k ...

within the

Ceratopsia Ceratopsia or Ceratopia ( or ; Ancient Greek, Greek: "horned faces") is a group of herbivore, herbivorous, beaked dinosaurs that thrived in what are now North America, Asia and Europe, during the Cretaceous Period (geology), Period, although ance ...

(both names being derived from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

for 'horned face'), a group of herbivorous dinosaurs with

parrot Parrots (Psittaciformes), also known as psittacines (), are birds with a strong curved beak, upright stance, and clawed feet. They are classified in four families that contain roughly 410 species in 101 genus (biology), genera, found mostly in ...

-like beaks which thrived in what are now

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

and

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

, during 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. Apart from being a ceratopsian, little is certain about ''Avaceratops's'' taxonomic position. Because most of the skeleton is only known from a juvenile, and juveniles tend to express ancestral traits more strongly, a

cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

analysis would likely indicate a too basal, low, position in the evolutionary tree. For this reason, ''Avaceratops'' has often been excluded from such analyses. Penkalski and Dodson in 1999 concluded that ''Avaceratops'' most likely occupied a basal position within the Centrosaurinae but that it was also possible it was a basal member of the Ceratopsinae or just outside the

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 ...

formed by both groups. A phylogenetic analysis performed by Sampson et al. (2013) found that ''Avaceratops'' was the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

of the new genus '' Nasutoceratops'', described and named in

2013 2013 was the first year since 1987 to contain four unique digits (a span of 26 years). 2013 was designated as: *International Year of Water Cooperation *International Year of Quinoa Events January * January 5 – 2013 Craig, Alask ...

. This clade containing both ''Avaceratops'' and ''Nasutoceratops'' was named the Nasutoceratopsini in 2016. The

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

presented below follows a phylogenetic analysis by Chiba ''et al.'' (2017):

Paleobiology

''Avaceratops'', like all ceratopsians, was a

herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...

. During the Cretaceous, flowering plants were "geographically limited on the landscape", so it is likely that this dinosaur fed on the predominant plants of the era:

fern The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

cycad Cycads are seed plants that typically have a stout and woody (ligneous) trunk (botany), trunk with a crown (botany), crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants o ...

s and

conifer Conifers () are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a sin ...

s. It would have used its sharp ceratopsian beak to bite off the leaves or needles. The

habitat In ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...

of ''Avaceratops'' was heavily forested and wet.

References

* https://web.archive.org/web/20050824122458/http://www.vertpaleo.org/jvp/19-692-711.html (online abstract of the preceding) * https://web.archive.org/web/20090531083004/http://www.dinosaurvalley.com/Visiting_Drumheller/Kids_Zone/Groups_of_Dinosaurs/index.php * '' The Humongous Book of Dinosaurs'', published by Stewart Tabori & Chang

Notes