Marginocephalians, primarily from TEXT, see also authors in:

Currie, P. J. and K. Padian (eds.) 1997. Encyclopedia of Dinosaurs. Academic Press, San Diego

Farlow, J. O. and M. Brett-Surman (eds.) 1997. The Complete Dinosaur. Indiana University Press, Bloomington

Group named by Sereno 1986, Natl. Geogr. Res. 2: 234-256; in recent years there has been a growing interest in the origin of the ceratopsians.

 HETERODONTOSAURIDS: TEXT, Chapter 23

Completely known taxon:

Heterodontosaurus, TEXT: figs. 23.1 - 23.3

mixed flood fan and eolian facies in South Africa

Why are descendants not in southern hemisphere?

Liassic form too specialized to be basal thyreophoran, reveals early morphologic divergence of ornithischians

• small, narrow beaks (selective cropping)
• dentary rotates within predentary
• caniniform dentary tooth (use as in musk deer?)
• neural spines confined to base of tail
• nine well-ossified, cuboidal carpals, well ossified manus (digging?)
• tibia fused distally to fibula and calcaneum-astragalus
• three distal tarsals fused to metatarsus

• obturator process on ischium
• pleurokinetic skull
• absence of ossified tendons
• elongate prepubic process

• akinetic skull (Marginocephalia)
• pmx tooth row below mx tooth row (Hypsilophodontidae)
• pmx contacts lacrimal (Psittacosauridae)
• jugal boss (Ceratopsia)
• canine teeth (Goyocephale)
• facetted, continuous, inclined grinding surface (Ceratopsia)
• manus and shoulder ceratopsian-like
• manus: 2-3-4-3-2 (IV, V divergent)

• vomers contact maxilla (mx meet on midline) rather than premaxilla (pmx excluded from internal nares)
• posteriorly projecting par-sq shelf (absent in psittacosaurs, Leptoceratops)
• acetabulae more widely separated than dorsal borders of ilia
• no obturator process on ischium (nor in thyreophorans)
• loss of pubic symphysis

• frontal and parietal thickened
• anterior and medial walls of orbit fully ossified
• squamosal and jugal with horns, smaller osteoderms on skull
• squamosal broadly expanded on occiput
• small caniniform teeth in premaxilla (first dentary tooth is large and caninifrom in Goyocephale)
• channelled and grooved zygapophyses on dorsals
• ischium contacts pubic peduncle, small pubis excluded from acetabulum (as in nodosaurs)
• basket of ossified tendons surrounds tail distally
• flat-headed Homalocephalidae -open supratemporal fenestera (STF); the group is represented in North America by a new pachycephalosaur from the late Campanian of New Mexico (Williamson and Sealey 1999 JVP 19(3 supplement) p. 84A)
• thick-headed Pachycephalosauridae -closed STF in some genera
• flat-headed forms seen as progressive, leading toward a group all with high domes (cf. protoceratopsids merging with ceratopsids) [Forster and Sereno "The Complete Dinosaur" 1997]

• Relatively large heads, traditionally seen as useful for butting
• No rostral bone
• Occipital condyle slopes ventroposteriorly, dorsal vertebrae horizontal
• Small forelimb (1/4 length hindlimb)
• Rib articulations indicate very broad back
• Six sacral vertebrae
• Basal caudal neural spines long, nearly vertical (as in protoceratopsids)
• Pubis slightly expanded anteriorly (as in ceratopsids)
• Ischium slightly curved (as in ceratopsids)

There are only four pachycephalosaur specimens with significant postcranial material, two from Asia and two from North America. "Pachy" is the most complete pachycephalosaur skeleton from North America, and the second most complete specimen in the world (Homalocephale from Mongolia is more complete, and from a much smaller animal). It was collected by Michael Triebold from the Hell Creek Formation. "Pachy" is estimated to have been 3.45 m long and weighed 260 kg (by comparison, Willo is estimated to have been 3.9 m long and weighed 300 kg). Additional attributes of "Pachy:"

• a large, heavy-headed animal body held horizontally, chest cavity very wide relative to its depth
• zygapophyses of the neck smooth, those of dorsals are longitudinally ridged and grooved
• base of the neck curves anterodorsally
• neck is elsewhere unknown; the posture of neck and structure of zygapophyses suggest that head-butting was not important, the large, spinous head may have served to deliver side blows (as in giraffe), to discourage carnivory, and/or in display posturing
• external surfaces of the ribs are fluted
• it was associated with a sub-fauna of small dinosaurs, separate from megafauna

• rostral present
• vaulted palatal surface of premaxilla
• skull with narrow beak and flaring jugals
• jugal enlarged with inclined ridge, deeper under orbit than under infratemporal fenestrae
• incipient frill dominated by parietal (squamosal in pachycephalosaurs)

Psittacosauridae

Protoceratopsidae

Ceratopsidae

The oldest ceratopsian is Chaoyangsaurus of Middle or late Jurassic age (Bathonian on the basis of ostracods) from northeastern China (Zhao et al. 1999 JVP 19: 681-691). It is a fragmentary skull exhibiting the following ceratopsian characters:

• rostral bone (broad as in psittacosaurs)
• jugal that flares well beyond skull roof (the lateral projection is present in Heterodontosaurus)
• deep infraorbital ramus of jugal
• wide ventral process of predentary

Similarities to Heterodontosaurus include:

• chisel-shaped tooth crowns with denticles restricted to crown apex; authors suggest that ceratopsian-heterodontosaurid relationships should be re-examined

Completely known taxa: Psittacosaurus, TEXT: fig. 28.8; see also Russell and Zhao 1996 CJES 33: 637-648)

> 120 specimens are known, specifically diverse (1.5 m or less in length), these are the "Desert beavers" of early Cretaceous dune-dominated environments of Central Asia

Psittacosaurs are "living fossils" with skull and hand specializations. The skull is powerful; the brain is very small. The diameter of the ring of sclerotic plates is the smallest known among dinosaurs. The dentitions are slicing; gastroliths often number more than 50.

• short preorbital region of skull
• small, elevated nostrils
• nasals extend below nares anteriorly, contacting rostral
• premaxilla contacts lacrimal, prefrontal (na, mx contact separating pmx from la, prf in ceratopsids - but not in Heterodontosaurus and some iguanodonts and hadrosaurs)

• absence of premaxillary teeth
• manal digit IV with one phalanx, digit V absent (only three functional digits)

Postcranial skeleton with powerful forelimbs, short body and tail relative to those of primitive ornithischians.

There are six sacrals, limb proportions are like those of cursorial ornithopods (hypsilophodonts)

Neoceratopsia: A small skull and skeleton seems to be closest to the ancestry of all higher ceratopsians (Neoceratopsia) and a small, archaic protoceratopsid of early Cretaceous age (Barremian-Aptian ? 120 myr) from Inner Mongolia and named Archaeoceratops by Dong and Azuma (1997, Sino-Japanese Silk Road Dinosaur Expedition, China Ocean Press, Beijing, p. 68-89). The attributes they describe include:

• the muzzle is low, unlike in psittacosaurs, and the external nares are small unlike in ceratopsids
• there are 2-3 premaxillary teeth (among protoceratopsids, only Protoceratops has two premaxillary teeth)
• maxillary teeth are like those of protoceratopsids, dentary teeth are as in primitive ornithopods.
• absence of a frill (the back of the skull is like that of Leptoceratops)
• lower border of lower jaw is straight (as in Montanaceratops), not bowed (as in Leptoceratops)
• the cervicals are evidently not fused
• there are but 6 sacral vertebrae, as in Leptoceratops (more occur in other protoceratopsids: 7 in Microceratops and 8 in Protoceratops and Montanaceratops)
• caudal spines are elongated, as in protoceratopsids
• the ischium is straight (in Leptoceratops and Protoceratops it is gently curved)

• there is no trace of nasal horn
• the palpebral fused to prefrontal
• the squamosal does not extend behind the quadrate cotylus
• the infratemporal fenestra narrow anteroposteriorly
• the frill is extremely abbreviated.

(1 - 2.5 m long, late Cretaceous)

Completely known taxa:

Leptoceratops

Protoceratops

May have been capable of bipedal locomotion

Autapomorphies (Dodson, TEXT)

• circular antorbital fossa
• inclined parasaggital process of palatine
• maxillary sinus communicating with antorbital fossa

• Small, elongate external nares

• No hint of brow horns or frill ornamentation

• Syncervicals (4, as in ceratopsids)

• Six to eight sacrals

• Ischium gently decurved

• Bluntly tapering unguals

• Tail dilated, with long caudal spines (cf. pachycephalosaurs)

The route toward the Ceratopsidae (Chinnery and Weishampel 1998 JVP 18: 569-585):

Basal neoceratopsians:

• presence of a frill
• ventral border mandible either very curved or straight
• fusion of first three cervical vertebrae in adults
• presence of parietal fenestrae

• tooth enamel on lateral upper and medial lower teeth
• angular anterodorsal orbital rim
• large antorbital fossa
• paroccipital processes 40% basal skull length

• jaw articulation at or above level of tooth row
• highly curved ventral border of mandible
• glenoid fossa primarily on coracoid

• curved maxillary tooth row
• vertical notch wear pattern

• presence of a nasal horn
• predentary less than 67% length of dentary
• squamosal and jugal contact above infratemporal fenestra
• eight or more sacrals
• partially everted iliac blade

• small infratemporal fenestra
• large nares
• complete fusion of cervicals 1-3

For another view of protoceratopsian classification (You and Dodson 2000 JVP 20(3, supplement): 80A)

Protoceratopsidae (including Archaeoceratops, Breviceratops, Bagaceratops and Protoceratops) are characterized by

• a narrow rostral
• concave ventral edge of premaxilla
• small, elliptical and high-positioned nares
• posterodorsally extended squamosal and fenestrated frill

• wide rostral bones
• convex ventral edges of premaxillae
• loss of premaxillary teeth
• round, low positioned nares
• downward processes of squamosals and round posterior margins of the frills

Leptoceratops (new species) bonebeds occur in the Two Medicine of Montana, new species (Chinnery and Trexler 1999 JVP 19(3 supplement), p. 38A:

 Ceratopsidae: TEXT, Chapter 29, pp. 613-616

(4 - 8 m long, late Cretaceous)

• folded frontals
• squamosal with strong postquadratic ala
• teeth with split roots
• a cohesive group, perhaps exclusively North American
• existence of Asian ceratopsids not convincing (Dodson 1996)

• large nasal openings
• at least three teeth in vertical columns
• semi-erect orientation of forelimbs

• vertical orientation of occlusal surface
• spherical occipital condyle

• Fully quadrupedal, huge skulls
• Abundantly horned, frilled
• Beak browsers as well as spherical occipital condyle
• Sprawling forefeet, huge olecranon, deltopectoral crest (Langston specimen)
• Expanded and squared distal end scapula
• Not gallopers, cf. lack of differentiation of dorsal series
• Chest narrow, posterior dorsals broad
• Number of separate cervicals 6
• Number of sacrals 10
• Everted ilium
• Distally dilated pubis
• Tail rat-like, ventral location tv. proc.
• Skin impressions

A. Isolated ceratopsid teeth from the Aptian-Albian (latest Early Cretaceous of Utah and Maryland (Chinnery et al. 1998 New Mexico Mus. Sci. Nat. Hist. Bull. 14: 297-302)

B. Zuniceratops with a fenestrate frill, prominent postorbital horns, teeth showing shear-function and a decurved ischium. Small animal. Primitive characteristics include single-rooted teeth and a single replacement tooth in the dental battery ? Turonian age, about 90 myr. Ceratopsids of a slightly older age (early Cenomanian) are known from Uzbekistan (Khodzhakul Formation, Asiaceratops) (Wolfe and Kirkland 1998 New Mexico Mus. Sci. Nat. Hist. Bull. 14: 303-317)

In the last 25 million years of the Cretaceous, ceratopsians grow to elephantine proportions; the were small creatures the size of large rodents during the previous 40 million years.

Centrosaurinae

Completely known taxa:

Centrosaurus

Styracosaurus

• short squamosals
• deep skulls
• circular nares with projection from nasal-premaxilla contact posteriorly
• nose horns usually dominate, hooks and processes on parietals

Getty et al. 1997 (JVP Abs., p. 48A) multi-event flooding, 90% Centrosaurus

Pachyrhinosaurus horn "pads" (Dodson 1996, p. 176), derivation of genus (Dodson 1996, p. 196): "Styracosaurus" one pair of parietal spikes instead of three (6 specimens)

+200,000 yrs Einiosaurus (Blackfoot for buffalo reptile)

+250,000 yrs Achelousaurus (Achelous, mythical Greek form-changer)

Chasmosaurinae

Completely known taxa:

Anchiceratops

Chasmosaurus

Pentaceratops

Triceratops

• complex narial opening with numerous fenestrae and bony processes
• large, conical epijugal

Triceratops

• Skulls are common but only one relatively complete skeleton has been found (Gartska et al. Dinofest Abstracts 1996)
• a composite mount in the Minnesota Museum features a head 2.2 m long, a total length of 7.9 m length (26 feet) and a height at the hips of 2.9 m hips (9.5 feet)
• the skull of Torosaurus is up to 2.35 m long

• long skulls
• elliptical nares
• brow horns usually dominate
• horns as "display" structures, holed squamosals

T. prorsus (less common, more derived)

• closed frontal fontanelle
• relatively short supraorbital horns (cf. basal skull length horn/skull 0.61 or less)
• short, convexly rounded and deep rostrum (anterior to nasal horn)

Diceratops, fenestrate frill, long, thin squamosals

Oxygen isotopes suggest that frill serves as a heat-shedding device (Reese’s work)
 
 

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