A protoceratopsid is a dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 of the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Protoceratopsidae. The name 'protoceratopsid' is derived from Greek

Ancient Greek

Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 for 'first horned face'. They resembled and were closely related to, ceratopsids

Ceratopsidae

Ceratopsidae is a speciose group of marginocephalian dinosaurs including Triceratops and Styracosaurus...

 but were generally smaller and more primitive. Protoceratopsids have so far been found exclusively in the Late Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 of Asia

Asia

Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

 (Nemegt Basin

Nemegt Basin

The Nemegt Basin is situated in the northwestern part of the Gobi Desert, in the Ömnögovi Province in the southern part of Mongolia. It is known locally as the valley of the dragons, since it is a source of many fossil finds, including dinosaurs, dinosaur eggs and trace fossils.The main geological...

), dating to between about 99.6 and 70.6 million years ago. A typical protoceratopsid is Protoceratops andrewsi.

The taxon Protoceratopsidae was introduced by Walter W. Granger

Walter W. Granger

Walter Willis Granger was an American vertebrate paleontologist who participated in important fossil explorations in the United States, Egypt, China and Mongolia.-Early life and career:...

 and William King Gregory

William King Gregory

William King Gregory was an American zoologist, renowned as a primatologist, paleontologist, and functional and comparative morphologist. He was an expert on mammalian dentition, and a leading contributor to theories of evolution...

 in May 1923 as a monotypic family for Protoceratops andrewsi. Granger and Gregory recognized Protoceratops close relationship to other Ceratopsia

Ceratopsia

Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...

, but considered it primitive enough to warrant its own family, and perhaps suborder. Protoceratopsidae was later expanded to include all ceratopsians that were too advanced to be psittacosaurids, but too primitive to be ceratopsids. In 1998, Paul Sereno

Paul Sereno

Paul Callistus Sereno is an American paleontologist from the University of Chicago who discovered several new dinosaur species on several continents. He has conducted excavations at sites as varied as Inner Mongolia, Argentina, Morocco, and Niger...

 defined Protoceratopsidae as the stem-based clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 including "all coronosaurs closer to Protoceratops than to Triceratops

Triceratops

Triceratops is a genus of herbivorous ceratopsid dinosaur which lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago in what is now North America. It was one of the last dinosaur genera to appear before the great Cretaceous–Paleogene...

." Sereno's definition ensures that Protoceratopsidae is monophyletic, but probably excludes some dinosaurs that have been traditionally thought of as protoceratopsids (for example, Leptoceratops

Leptoceratops

Leptoceratops , was a primitive ceratopsian dinosaur genus from the Late Cretaceous Period of what is now Western North America, at the same time as its giant...

 and Montanoceratops

Montanoceratops

Montanoceratops was a genus of small ceratopsian dinosaur. It lived during the early Maastrichtian of the late Cretaceous Period...

). The latter genera are now often classified in a mostly North American family Leptoceratopsidae

Leptoceratopsidae

The family Leptoceratopsidae, its name derived from the type genus Leptoceratops, is a group of several small neoceratopsian genera which appear not to belong to the clade Protoceratopsidae. They resembled, and were closely related to, other ceratopsids, but all discovered species are generally...

.

Sereno (2000) included three genera in Protoceratopsidae: Protoceratops

Protoceratops

Protoceratops is a genus of sheep-sized herbivorous ceratopsian dinosaur, from the Upper Cretaceous Period of what is now Mongolia. It was a member of the Protoceratopsidae, a group of early horned dinosaurs...

, Bagaceratops

Bagaceratops

Bagaceratops, meaning "small-horned face" , is a genus of ceratopsian dinosaur that lived in what is now Mongolia around 80 million years ago, during the Late Cretaceous...

, and Graciliceratops

Graciliceratops

Graciliceratops is a Ceratopsian dinosaur first described by paleontologist Paul Sereno in 2000. It is known from the Late Cretaceous period and its fossils were found in Mongolia...

. Derived characters shared by these dinosaurs include a narrow strap-shaped paroccipital process, a very small occipital condyle, and an upturned dorsal margin of the predentary. In Protoceratops and Bagaceratops (and also in the non-protoceratopsid Leptoceratops), there is a blade-shaped parietal sagittal crest

Sagittal crest

A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull of many mammalian and reptilian skulls, among others....

 (Sereno 2000: 505). Several other more recently recognized genera may also be protoceratopsids. In 2003, Vladimir Alifanov named, but did not define, a new ceratopsian family Bagaceratopidae

Bagaceratopidae

Bagaceratopidae is a family of neoceratopsian dinosaurs. It was named by Alifanov in 2003 but no definition has been proposed. Because of lacking of the definition, Bagaceratopidae was considered inactive by Paul Sereno in 2005...

 to include Bagaceratops

Bagaceratops

Bagaceratops, meaning "small-horned face" , is a genus of ceratopsian dinosaur that lived in what is now Mongolia around 80 million years ago, during the Late Cretaceous...

, Platyceratops

Platyceratops

Platyceratops is a dinosaur from the Late Cretaceous, during the Campanian Age, about 75-72 million years ago. Its fossils have been found in Mongolia. Its skull is larger than Bagaceratops; it has been referred to Bagaceratopidae or the Neoceratopsia...

, Lamaceratops

Lamaceratops

Lamaceratops,"Lama Horned Face", is a dinosaur from the Late Cretaceous period. It is classified as a ceratopsian, a herbivorous "frilled" or armored dinosaur similar to, but smaller than, Bagaceratops. It had a horn on the front of its face, much like most of the later Ceratopsids.The fossils of...

 and Breviceratops

Breviceratops

Breviceratops was a ceratopsian dinosaur from Late Cretaceous Mongolia. It was related to Protoceratops. The fossils originally described by Maryanska and Osmolska in 1975 and placed in Protoceratops, were moved to the new genus by Kurzanov in 1990.The type species is Breviceratops...

. However, applying Sereno's phylogenetic definition, Alifanov's Bagaceratopidae appears to be a subclade of Protoceratopsidae.