els || dinosauria (dinosaurs)

Dinosauria (Dinosaurs)Kenneth Carpenter, Prehistoric Museum, Price, Utah, USA

Dinosaurs are a group of extinct vertebrates that lived

from 230 to 65 million years ago (Ma). They originated in

southern Pangaea during the Middle Triassic from a small,

meter-long bipedal archosaur (the group that includes

crocodiles and pterosaurs). Within a few million years

dinosaurs rapidly diversified into two major groups

(Ornithischia and Saurischia) based on pelvic structure

and three main clades or subgroups (Ornithischia, Sauro-

podomorpha and Theropoda). Further diversification

resulted in the various dinosaur classic types, such as tyr-

annosaurs, apatosaurs, stegosaurs, ankylosaurs and cer-

atopsians. These various types reflect the two locomotor

styles, bipedal and quadrupedal, as well as the three

dietary groups, herbivorous, carnivorous and omnivor-

ous. Dinosaurs spread globally and by the Jurassic appear

on all the continents, including ice-free Antarctica. They

also spread into a variety of niches, including flight. The

extinction of this seemingly successful group 65 Ma is still

the subject of much debate. Hypothetical causes include

disease, volcanism, sea level drop, asteroid impact, or a

combination of these. The only survivor of the ‘Great

Extinction’ are the birds, or avian dinosaurs.

Introduction

Traditionally, dinosaurs were considered as reptilesbecause of the type of openings in their skulls and thestructure of their pelvis. More recently, dinosaurs areconsidered transitional between crocodiles and modernbirds. This new interpretation is based partly on the dis-covery of feathers and hair-like structures on small and

medium-sized dinosaurs. Feathers have traditionally beenconsidered exclusively an avian character, thus their dis-covery on various dinosaurs has added to the growing bodyof evidence that some dinosaurs, specifically theropods, aremore closely related to birds than they are to crocodiles.See also: Aves (Birds); Reptilia (Reptiles)

Basic Design

There is a common misperception that anything large andextinct is a dinosaur, especially if it is a skeleton. Butdinosaurs are a specific group of animals defined by a set ofcriteria, some of which are esoteric, including the absenceof a postfrontal bone in the skull, an elongate crest on thehumerus for muscles, three or fewer bones in the fourthfinger of the hand and one in the fifth finger (if it is presentat all), three or more vertebrae in the pelvis, an open hip-socket coupled with a ball-like head on the femur thatplaced the legs beneath the body, a prominent forwardprojecting crest on the tibia, a hinge-like ankle, andstanding on the toes, rather than flat-footed. Althoughsome of these features may occur in other closely relatedreptiles (e.g. pterosaurs, Marasuchus), only in the dino-saurs do all of these features occur. See also: Pterosauria(Pterosaurs)Dinosaurs have also beendivided into twomajor groups,

or orders, the Saurischia and Ornithischia. Originally, thisdivision was based on differences in the pelvis, but morerecent studies have added other characters as well. Insaurischians, the three bones of the pelvis (ilium, pubis andischium) form a triangle (Figure 1a), whereas in ornithis-chians, they form a rectangle (Figure 1b). Saurischians alsohave a small opening below the nasal opening in the skull,and have long neck (cervical) ribs that lie parallel with theneck vertebrae. Ornithischians, on the other hand, have anaccessory bone at the tip of the lower jaws called a pre-dentary, triangular cheek teeth that are largest in the centreof the tooth row, and a tall process behind the tooth row onthe lower jaws called the coronoid process to which theadductor muscles attach.

Introductory article

Article Contents

. Introduction

. Basic Design

. Diversity

. Habitats and Abundance

. Habits and Lifestyles

. Oddities within the Group

. Synopsis

Online posting date: 15th June 2011

eLS subject area: Evolution & Diversity of Life

How to cite:Carpenter, Kenneth (June 2011) Dinosauria (Dinosaurs). In: eLS.John Wiley & Sons, Ltd: Chichester.

DOI: 10.1002/9780470015902.a0001545.pub2

eLS & 2011, John Wiley & Sons, Ltd. www.els.net 1

http://dx.doi.org/10.1038/npg.els.0001548




Diversity

Saurischians include the predominately carnivorous,bipedal theropods and herbivorous, quadrupedal sauro-pods (‘brontosaurs’). Ornithischians were all herbivorous,and include bipedal ornithopods, and quadrupedal hornedceratopsians, armour-plated stegosaurs and ankylosaurs(Figure 2 and Figure 3).

Theropods are the most studied group of dinosaurs,consequently, we know themost about them. These are thearchetypical carnivorous dinosaurs having a mouth-full ofsharp, blade-like, serrated teeth and long grasping hands.The most primitive theropods, such as Eoraptor and Her-rarasaurus from the Late Triassic of Argentina, are small,less than 2m in length. They were not the top predators inthe ecosystem, nor were they very abundant. Moreadvanced theropods, called ‘neotheropods’, appearedbefore the end of the Triassic and these show the trendtowards increased size and modifications making themmore efficient predators. The neotheropods have a hingedmid-jaw joint that is thought by some palaeontologists tohave served as a shock absorber while biting prey. Tostrengthen the arms while grappling with struggling prey,the paired clavicles are fused into a single structure calledthe furculum or ‘wish-bone’. This structure essentially tiedthe two shoulders together so as to transfer the stress ofholding a struggling prey to the body (Lipkin and Car-penter, 2008). The hands are modified as well, with thenumber of digits reduced to four or less that are equippedwith raptor-like claws. The pelvis is enlarged to accom-modate larger and more powerful leg muscles, and this inturn required that the pelvis be braced by more sacralvertebrae (up to five). The foot was also modified byreducing the first and fifth digits so that all the weight wascarried by the central three toes. See also: DinosaurLocomotion

The earliest of these neotheropods include Coelophysisfrom the Late Triassic and Dilophosaurus with the pairedcrests on its head. Dilophosaurus and related forms from

Antarctica, Africa and China were large (4–6m long),making them the top predators of that time. Moreadvanced neotheropods, called averostrans, appear glob-ally by the Late Jurassic. They are characterised by asym-metrical teeth at the front of the mouth and furtherenlargement of the pelvis with a ‘hook-like’ blade at thefront of the ilium (the upper pelvis bone). One of theseaverostrans, the ceratosaurs, increased to seven thenumberof vertebrae bracing the pelvis, and grew to 8m or more inlength. Ceratosaurs are the dominant predator in thesouthern hemisphere during the Late Cretaceous andinvolve into some bizarre forms, such Carnotaurus, whichtook arm reduction to the extreme so that the stumpy armswere useless for grasping prey. Another, larger group ofaverostrans, the tetanurans, includes many of the famousthereopods, such asVelociraptor andTyrannosaurus. Theyhave interlocking tail vertebrae that keep the tail stiff tostabilise the body while running. Also, the teeth no longerextend to below the eye socket, but ends farther forwards;some, the ornithomimosaurs, are actually toothless.Another group, the spinosaurs, is characterised by tallspines on the vertebrae that produce a sail on the back, andelongated snouts with long conical teeth thought to indi-cate a fish diet.Tetanurans as a whole increased the relative length of

their hands and arms (especially those closest to the birdlineage – more below), and developed long, slender hindlimbs (especially the lower leg and foot). One group, thetyrannosaurs, however, went the other extreme, bydeveloping short, but powerful arms, and large skulls withbone-crushing teeth. One of the earliest tyrannosaurs,Guanlong, from the Jurassic of China had a crest on itsskull, and long, grasping arms.A later relative,Dilong fromthe Early Cretaceous of China, has fuzz-like protofeathers,which has led to the suggestion that perhaps hatchlingTyrannosaurus may have retained a fuzzy coat (see dis-cussion by Norell and Xu, 2005). Protofeathers were firstfound in Sinosauropteryx, a small theropod from China(Figure 4).One subset of the tetanurans is the maniraptorans, from

which the modern bird probably descended. They showtwo important adaptations that would be expected ofprecursors of flight: greatly elongated arms and hands, anda large bony sternum for muscles that pull the arms downor inwards towards the body. In addition, they have a arcedwrist-bone that allows the hand to fold towards the fore-arm. In birds, this enables the wings to fold and suggeststhat some of these theropods, such as Velociraptor andOviraptor, to fold their arms in bird-like fashion. Onerecently discovered group, the alvaresaurs, are so bird-like,that they were originally thought to be flightless birds.They have small skulls with tiny teeth, short armswith a large thumb. Mononykus has a single, large digitthought to be used to dig into ant or termite nests. Ovir-aptors shortened the typical long tails of theropods byeliminating some of the vertebrae, as well as fusing those atthe end into a mass called a pygostyle, a feature seen inbirds today.

(a) (b)

Figure 1 Dinosaurs have traditionally been divided into two groups based

primarily on the pelvis. Three bones form the pelvis, the horizontal ilium

(yellow), the forward projecting pubis (green) and the rearward projecting

ischium (purple). In saurischians, the three bones typically form a triangle

(a), whereas in ornithischians, they typically form a rectangle (b).

Dinosauria (Dinosaurs)

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The deinonychosaurs include the sickle-clawed Veloci-raptor of ‘Jurassic Park’ fame. As the movie implied, theseare indeed some of the largest brained dinosaurs relative tobody size. We know how the claws were used based on aspecimen of Velociraptor ‘fighting’ with its prey, Proto-ecratops. This pair of remarkable specimens shows theVelociraptor clutching the head of theProtoceratops, whileimpaling the Protoceratops in the throat. The shouldersocket of deinonychosaurs faces outwards, which allowsthe arms to move in a flapping direction, a necessary pre-cursor to flapping flight, as well as gliding. One such gliderwas Microraptor, which had long feathers not only on itsarms but also along its legs and tail. Archaeopteryx, fromthe Late Jurassic, is the best known of the ‘transitional’theropod-bird dinosaurs. As has been long noted, withoutits covering of feathers, it would have been called a smalltheropod. It has small conical teeth in a long, pointedsnout, three long fingered hand and a long bony tail. Thecrow-sized Archaeopteryx in fact looks like a miniature

version of Deinonychus or Velociraptor, including aretractable sickle claw on the hind foot.Archaeopteryx hasasymmetrical flight feathers on its wings, but how efficientof a flier it was remains controversial. Later birds from theEarly Cretaceous show a reduction in the number of tailvertebrae, fusion into a long pygostyle, and an increase inthe number of sacral vertebrae. Some toothed forms flewalongside toothless species showing that the appearance ofmore advanced birds did not mean the immediate extinc-tion of more primitive species.The other saurischian group is the sauropodomorphs, or

sauropod-forms.These include the classic long-necked, longtailed, quadrupeds. Some were the largest land animals thatever existed, with lengths of 30–45m andweights of 45 000–140 000 kg (Amphicoelias). But sauropodomorphs did notstart out large. The earliest ones, called prosauropods, wereonly a few metres long. They were characterised by smallheads relative to body size, proportionally long necks, andleaf-shaped teeth edgedwith large denticles suitable only for

a

bc

d f

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Figure 2 The major dinosaur groups include the sauropodomorphs (a), represented by the sauropod Brachiosaurus; Theropoda (b) by Tyrannosaurus;

Ornithopoda (c) denoted by the hadrosaur Edmontosaurus; the Ceratopsia (d) depicted by Triceratops; the Pachycephalosauria (e) by Pachycephalosaurus;

Stegosauria (f) by Stegosaurus and the Ankylosauria (g) by Euoplocephalus. All to scale.



eating plants.Panphagia is one of the earliest prosauropods,appearing in the earlyLateTriassic ofArgentina. By the endof the Triassic and Early Jurassic, prosauropods attainedmuch larger sizes, with Lufengosaurus from China reaching

9m, and weighing 1700 kg. The melanosaurids occupy thetransition between prosauropods and sauropods (somepalaeontologists consider them the earliest sauropods).They were much larger than prosauropods, being 10–13m

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ThyreophoraMarginocephalia

SauropodomorphaTheropoda

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Ornithischia Saurischia

Figure 3 Cladogram showing the most current thinking about the relationship of the dinosaurs (after Sereno, 1999). Unlike the traditional evolutionary

trees that were based on superficial resemblances of species and the geological age of the fossils, cladograms rely upon shared advanced or derived

characters to unite dinosaur species regardless of geological age. The reason for the change in analysis is to concede that the fossil record is very incomplete

and that the fossil record may not preserve the first occurrence of an advanced character. For example, the closest dinosaur relative to the Late Jurassic bird

Archaeopteryx (�152 Ma), are the troodontids, typified by Troodon and Saurornitholestes from the Late Cretaceous (�75 Ma).

Figure 4 Small theropod Sinosauropteryx showing a coat of fine, hair-like protofeathers. Note the light and dark bands on the tail. Also, some of the internal

organs are visible.



long, and may have weighed over 2000kg. Being so large,they must have been quadrupeds. To deal with this mass,they increased the number of sacral vertebrae from three ofprimitive prosauropods, to four.

This trend of increase in size and increasing the numberof sacral vertebrae is continued with the sauropods. Inaddition, sauropods loose the denticles on the teeth andbroaden them into spoon-shaped structures. This toothform is further modified into slender pencil-shaped teeth inseveral different lineages independently. The larger bodysize required increased food consumption. However,because the heads did not become larger relative to bodysize, the heads only served to gather leaves, not to processthem. Consequently, advanced sauropods, called the neo-sauropods, restrict the teeth to the front of the mouth andhave shifted the nostrils towards the top of the skull wherethey would not be in the way. Digestion took place in thelarge intestine by fermentation bacteria. To aid in gather-ing food, the necks became longer, and inMamenchisaurushalf of its 30m lengthwas neck!See also:DinosaurFeeding

The neosauropods have been further divided into thediplodocoids and the macronarians. Diplodocoids, such asDiplodocus and Nigersaurus, have pencil-shaped teeth, thefront of the snout is broad and squared-off, and most havelong whip-like tails. These sauropods were widespread inAfrica, South America and North America, and rare else-where else. The other sauropods groups, the macronarians(‘big noses’), are characterised by enlarged nostril openingsin the skull (as big or bigger than the eyesockets), andelongated upper hand bones (metacarpals). Camarasaurusfrom the Late Jurassic of North America is the most com-mon sauropod found as fossils (Figure 5), and may haveformed herds. The brachiosaurids are the tallest sauropodsbecause their elongated forelimbs and elongatednecks make them the ‘giraffe dinosaurs’. In fact, one ofthem is called Giraffatitan. The longest living group of

macronareans is the titanosaurs. These appear in the LateJurassic and survive until the end of the Cretaceous. Theyare characterised by stocky (robust) lower arm bones, theulna and radius, very wide hips, six vertebrae in the pelvis,and in some cases, a loss of digits in the hand. Titanosaurswere the dominant herbivorous dinosaurs of the southerncontinents throughout the Cretaceous. Argentinasauruswas the largest dinosaur of the Late Cretaceous.The other major dinosaur group based on hip structure

is the ornithischians, or the ‘bird-hipped’ dinosaurs(although as was discussed above, birds are actually des-cendants of saurischian dinosaurs). They are characterisedby a triangular wedge of bone between the tips of the lowerjaw, called a predentary, and five or more pelvic vertebrae.The teeth indicate a herbivorous diet, either to slice, grindor pulp plants. Most of them also have teeth inset from theedges of the jaws suggesting a fleshy cheek to keep the foodin themouth.Many ornithischians have bony rods, ossifiedtendons, embedded in the muscles of the back and tail thatserved to keep the body horizontal.The earliest ornithischians appear in the Late Triassic

and include Lesothosaurus and Eocursor. These small, twometre or less, animals had small hands and long legs sug-gesting that they were primarily bipeds. Before the end ofthe Late Triassic and into the Early Jurassic, however,ornithischians began to diversify and spread globally. Thearmored dinosaurs, or thyreophorans, are characterised bybody armour formed in the skin analogous to that on theback of crocodiles. The twomore famous of these armoreddinosaurs include the plated Stegosaurus and armouredAnkylosaurus. One of the earliest of these is Scutellosaurusfrom the Early Jurassic of western North America. This1.5m long biped is very similar to other primitiveornithischians, such as Lesothosaurus from the LateTriassic of Africa. Scelidosaurus, also from the EarlyJurassic, is much larger, about 3–4m long and with

Figure 5 The sauropod Camarasaurus from the Upper Jurassic Morrsion Formation shows many of the characteristics of sauropods: small head, long neck,

long tail and elephant-like legs.




proportionally larger, cone-shaped armour. It is thoughtby some palaeontologist to be the earliest ankylosaurs,whereas some believe it shares an ancestry with bothstegosaurs and ankylosaurs.

Although tall, triangular plates characterise Stego-saurus, most stegosaurs had smaller plates arranged inpairs, as well as long spikes. Primitive stegosaurs, such asHuayangosaurus from theMiddle Jurassic of China, have aspike-like structure on the shoulders. Such structures arelost in themore advanced stegosaurus, such asStegosaurus.The body armour probably had multiple functions,including species recognition, thermoregulation, and sex-ual and threat display. The sideways projecting tail spikesserved for defence (Figure 6), as evidenced by a tail vertebraofAllosaurus bearing a partially healed puncture that fits atail spike perfectly (Carpenter et al., 2005). Stegosaursranged from the 4m long, 750 kg Chunkingosaurus, to the5m, 2000 kg Stegosaurus.

In ankylosaurs, the armour covered the body in a loosepavement, with smaller discs of bone interspersed in thegaps between the much larger plates (Figure 7). The evo-lution of ankylosaurs is punctuated with a progressivewidening of the hips and belly to accommodate anincreasingly larger fermenting gut. A similar increase in thesize of the pelvis and gut was independently developed bytitanosaurid sauropods and ceratopsians. Presumablythese changes accompanied a change towards a harsher(more fibrous?) diet that required a quick passage of a large

amount of food as occurs in elephants today. Amongankylosaurs, the 3m, 500 kg Struthiosaurus was one of thesmallest, whereas the 6m, 4000 kg Ankylosaurus was thelargest. See also: Dinosaur Growth – Egg to Adult

Figure 6 The armour plated Stegosaurus is characterised by tall, triangular plates on its back and tail. The purpose of the plates is controversial, with

suggestions ranging from radiators to control body heat to sexual and threat display.

Figure 7 Gastonia shows the characteristic protective body armour. The

armour formed within the skin much like that of modern crocodiles.




Ankylosaurs can be divided into three groups: theprimitive polacanthids characterised by simple teeth, aflange located low on the sides of the shoulder blade, andlong, triangular, grooved spines on the neck and shoulders.These appear in theMiddle Jurassic andappear tohavebeenreplaced in the Early Cretaceous by the other two families,the nodosaurids and ankylosaurids. Gargoyleosaurus andMymoorapelta are known from the Upper Jurassic Morri-son Formation, where they lived alongside Stegosaurus,Allosaurus and Apatosaurus. The last are Polacanthus fromEngland and Gastonia from North America. Nodosauridsare characterised by large solid spines or spikes on the neckand shoulders, whereas the ankylosaurids are noted for theirclub of bone formed by fused armour at the end of the tail.This club is attached to a handle formed by the interlockingof the tail vertebrae and by wrapping them in bony tendons.The most common nodosaurid is Edmontonia, representedby several partial skeletons from North America. The mostfamous ankylosaurid isAnkylosaurus, but this is a very rareanimal. Much more common is Pinacosaurus from Chinaand Mongolia. It is represented by numerous skeletons ofboth juveniles and adults. Both nodosaurids and ankylo-saurids appear to have gone extinct a million years or lessbefore the end of the Cretaceous.

One of the chief prey of theropods were the bipedalornithopods. These dinosaurs were built horizontally, likethereopods, with a long tail counterbalancing the bodyover the hind legs. However, unlike theropods, someornithopods (hadrosaurs and large iguanodonts) retained

long forelimbs that allowed them to assume a quadrupedalposture occasionally. The back and tail were stiffened bylong bony rods in themuscle, much like the long rods in thedrumstick of a turkey. Ornithopods first appear in theMiddle Jurassic of China and persisted globally (exceptIndia, which was an island continent for all of the Cret-aceous) until the end of the Mesozoic. They are charac-terised by a premaxillary tooth row or beak that is situatedlower than the cheek teeth, a jaw joint below the level of thecheek teeth as well, hinged skull bones that allow the facialbones to more in different directions while chewing, andnarrow frontal bones on the skull roof.The most primitive ornithopods are the hypsilopho-

donts. Generally, these are all small (1–3.5m), with simple,leaf-shaped teeth, a long, slender tail, and long, slenderhind limbs; running was their only defence from predators.Oryctodromeus is known to have dug burrows. The moreadvanced ornithopods are the iguanodontids and hadro-saurids, which together form the Iguanodontia. Besidetheirmuch larger body sizes (up to 10mormore), they havegreatly enlarged nostril openings in the skull, teeth modi-fied for grinding and hand becoming columnar for weightbearing. In addition, there are six or more vertebrae in thesacrum, probably owing to the increase weight. InIguanodon and its closest relatives, the ‘thumb’ claw ismodified into an immobile spike of uncertain function.Aproto-thumb spike is seen inCamptosaurus from theLateJurassic. It has an immobile thumb ending in a pointedclaw (Figure 8). Iguanodonts are a very diverse groupduring

Figure 8 Ornithopods, typified by Camptosaurus, lacked protective structures on the head or body. Ornithopods may have travelled in herds for group

protection.



the Early Cretaceous, when they underwent a profounddegree of evolution. Some, such as Eolambia and Equiju-bus, were originally thought to be hadrosaurs, but morerecent work has shown them to lie in a transitional zone.

True hadrosaurs are known from Europe, SouthAmerica, but mostly from Asia and North America. Theyunderwent further modification from their iguanodontancestry with development of a ‘duck bill’ (hence theirpopular name of duck-billed dinosaurs), increase in thenumber of tooth positions, development of a broadgrinding surface formed by closely packed teeth, and moremobile jaw permitting extensive chewing. These changes inthe mouth suggest that hadrosaurs cropped low growingplants, but had more efficient food processing than seen inankylosaurs. Thus, they lack the ankylosaur’s large fer-menting gut. One branch of hadrosaurs, the lambeosaur-ines, are characterised by elaborate crests formed byexpansion of the premaxillary and nasal bones. Thesestructures are known to have formed during growthbecause the hatchlings lacked such structures. Cory-thosaurus has a cocks-comb looking crest, whereasParasaurolophus has a trombone-like crest. Anotherbranch, the hadrosaurines, tends not to have crests, theirbeaks are exceptionally wide, and their nostril openings aregreatly expanded in size as well. The most commonhadrosaurine, Edmontosaurus, may have formed large

herds because it occurs in fossil deposits containing onlythat species. Presumably, a portion or all of the herd waskilled and buried.One other bipedal ornithischian group, the pachyce-

phalosaurs, used to be considered an ornithopod because itwas bipedal. However, the development of a thickenedcollar of bone along the back of the skull has been used tolink the group with the ceratopsians as the margin-ocephalians. The marginocephalians first appeared in theMiddle Jurassic ofAsia, and spread intoNorthAmerica bythe Early Cretaceous. The pachycephalosaurs are charac-terised by a thickened mass of bone on the top of the skull.Pachycephalosaurs were once thought to use the dome forhead butting, but are now thought to be used for show andfor flank-butting over territory or mates. The rib cage isunusually wide for a dinosaur, possibly to protect the vitalorgans from damage by flank-butting (Carpenter, 1997).Pachycephalosaurs appeared in the Early Cretaceous, butremained rather conservative throughout their evolution inthe Late Cretaceous. For example, Prenocephale fromMongolia resembles Stegoceras and PachycephalosaurusfromNorthAmerica.Most pachycephalosaurswere 3morless in length, but the largest, Pachycephalosaurus, mayhave been 4–5m long.Ceratopsians, or the horned dinosaurs, are only char-

acterised by horns in the advanced forms (Figure 9).

Figure 9 The horned dinosaur Styracosaurus shows the various horns that projected from the head, as well as a frill or collar along the back side of the skull.



However, even the most primitive forms are characterisedby a single, triangular rostral bone forming the upper beak,and horn-like projections from the sides of the skull. Thedistinctive facial horns only occur in some of the mostadvanced forms (more below). The oldest ceratopsians arefromChina, and includeYinlong from theMiddle Jurassic,and Chaoyangsaurus from the Late Jurassic. These small(less than 2m) ceratopsians also show an enlarged regionfor the jawmuscles indicating a powerful chewing capacity.In the Early Cretaceous, Psittacosaurus appears and is sowidespread inAsia that it is used to data Lower Cretaceousstrata.

Advanced or derived ceratopsians are collectively calledneoceratopsians. They have an expanded, thin collar ofbone extending along the rear of the skull. The frill is shortin the earliest of the neoceratopsians, but is up to half thelength of a large 2m skull of Pentaceratops. Neocer-atopsians range in size from the 2m long Protoceratopsfrom Asia (known from hundreds of specimens) to the 9mlong Triceratops. Neoceratopsians are in turn subdividedinto the chasmosaurines, which have long, low snouts andelongated rostral bones, and the centrosaurines, whichhave a short, deep snout andapair of largehorns projectingfrom the rear edge of the frill (and sometimes smaller onesas well). Chasmosaurus and Centrosaurus are respectiverepresentatives of the two groups. These hippo-sizeddinosaurs are well known from skulls and skeletons fromCanada.

The horns of ceratopsians might have been used pri-marily in conjunction with the frill to make the individualslook larger and more threatening. Healed puncture in theskulls of several ceratopsian specimens indicates that thehorns were used offensively against others of their kind.They may have also had a secondary function for defenceagainst predators because a specimen of Triceratops hasbeen foundwith partially healedTyrannosaurus bite markson the horns. The ceratopsian head is enormous comparedto the body size (up to 2m in Torosaurus and Pentacera-tops), and the tail relatively short. At the front of the upperbeak is a counterpart to the predentary, called a rostalbone. The beak of ceratopsians is short and deep, resem-bling that of a parrot. The cheek teeth are relatively largeand reinforced on the outer sides by a prominent keel orridge. These teeth are adapted for slicing tough, woodyplant material. Ceratopsians also have inset cheek teeth,suggesting the presence of a fleshy cheek to contain thefood.

Habitats and Abundance

Dinosaurs are found in a variety of rocks deposited in avariety of environments, but, as a general rule, plant fossilsare rarely found where dinosaur bones occur, making itdifficult to reconstruct ancient environments accurately.Plants fossilise best in acidic environments, such as swampsor otherwater-logged soils, where bone is destroyed. In rareinstances, plants and dinosaur bones do occur in the same

formation and theseprovide thebest clues to reconstructingancient environments. See also: Fossils and FossilizationThe most common rock types for dinosaur bone are

sandstones of ancient river channels. In such settings,isolated bones transported from distant areas may bemixed with partial or nearly complete skeletons. Whenseveral individuals co-occur, a bone bedmay result, such asat Dinosaur National Monument (north-western Color-ado and north-eastern Utah, USA). Such sites can give aglimpse into the structure of the dinosaur community ifdifferent species are present, or show the structure of herdsif only a single species is present. Mixed bone beds tend toform by gradual accumulation of individuals dying ofbiological causes (old age, disease, predation, etc.), whereassingle-species bone beds tend to form from external phys-ical processes (drowning in floods, droughts, etc.). Dino-saur bones also occur in mudstones that represent theadjacent flood plain to river channels. Bone beds in suchrocks are rare; instead single skeletons or individual bonestend to occur. Death of such individuals can have bio-logical or physical causes.Other dinosaur bones occur in lithified sand dunes

(found mostly in Mongolia), where intact skeletons areindividuals buried by ancient sand storms. Some of themost spectacular of these are skeletons of the small ther-opodOviraptor found associated with clutches of eggs thatthey were guarding. Perhaps the most peculiar occurrencesof dinosaur bones are those found in marine rocks. Theseprobably originated from drifting carcasses washed to seafrom rivers. Bones would slowly rain down to the sea flooras they were liberated from the encasing tissue by scav-engers and decay, although entire skeletons of the primitiveankylosaur Scelidosaurus have been found in Lower Jur-assic marine rocks of England.

Habits and Lifestyles

The variety of shapes and sizes of dinosaurs implies adiversity of behaviour and biology. The small teeth of theearly theropod Coelophysis imply a diet of small ver-tebrates, and the stomach contents of a few individualsshow that this diet may include young of their own kind.Other theropods, such asTyrannosaurus, havemuch largerteeth, implying larger prey. Tooth marks on dinosaurbones can be matched to some of these teeth and showconclusively that theropods were carnivores. But were theyscavenging from carcasses or actively hunting? In someinstances of partial skeletons bearing tooth marks, theanswer is most assuredly scavenger. But in three otherinstances, the answer is unequivocally predator. In oneexample, a skeleton of the small theropodVelociraptorwasfound with a small ceratopsian, Protoceratops. One fore-arm of the Velociraptor is clenched in the beak of theProtoceratops, and the sickle claw of one foot is extended inthe throat region of the Protoceratops. They were found inanancient sanddune, but it is not known if they died duringor shortly before the sand storm buried them. An




alternative hypothesis is that they were buried when a rain-soaked slope of a sand dune collapsed. In the otherexample, several tail vertebrae of a nearly complete skel-eton of the hadrosaur Edmontosaurus are mangled andshowdamage by large teeth.Regrowth of somebone showsthat the animal survived the attack. Nevertheless, the pat-tern of the tooth damage matches the size of an adultTyrannosaurus. Tyrannosaurus also apparently attackedTriceratops because the pattern of partially healed toothmarks on the horns and facial regionmatch the teeth of thatbig predator.

Sauropods were once thought to have lived in largelakes, where the water would buoy the large body. Butreanalysis of skeletons demonstrated that sauropods werewell adapted for terrestrial life. This interpretation is sup-ported by the absence of sauropods from lake deposits, buttheir presence in river channels and flood plain sediments,where they co-occur with stegosaurs, theropods and orni-thopods. The purpose of the long neck is a puzzle, but mayhave allowed some sauropods to feed over large areas asthey slowly walked, and others to feed from trees. Thisdifference in feeding strategies explains the different toothpatterns seen amongvarious sauropod types.Howeffectivesauropods were at tree feeding is a controversial topicbecause analyses of the neck vertebrae suggest Diplodocusand other long-necked species could not have raised theirnecks vertically. Other research suggests that some short-necked forms, such as Camarasaurus, could. Some speciesof sauropods are thought to have formed herds becauseseveral individuals, sometimes of different sizes, may befound together, or because of numerous overlappingtrackways trending in the same direction. The most classicof these are exposed along the Paluxy River in DinosaurValley State Park, Texas, USA.

Ornithopods also occur as multiple individuals in bonebeds, which, in some instances, may have numbered in thehundreds. These mass accumulations are dominated by asingle species suggest that at least some ornithopods con-gregated into large herds. This interpretation is supportedby fossil trackways that also show numerous individuals ofthe same track maker travelling in the same direction. Theoccurrence of the hadrosaur Edmontosaurus in the NorthSlope of Alaska and in Montana suggests seasonalmigration.At least one ceratopsianmayhave alsomade thesame journey. A skull of Pachyrhinosaurus from Alaska issimilar to those found in a Pachyrhinosaurus bone bedfound in Alberta. Might these individuals of Pachyrhino-saurus in the bone bed have perished together whilemigrating? Bone beds of other ceratopsian species areknown from southern Canada and northern Montana.

Stegosaurs generally do not occur in single-species bonebeds, with the exception of Kentrosaurus from Tanzania.Most other stegosaurs co-occur with other dinosaurs inbone beds. In one instance, about a dozenStegosaurushavebeen found with about an equal number of the ornithopodCamptosaurus, suggesting that the two species congregatedin a mixed herd. The ankylosaur Pinacosaurus occur assolitary individuals as adults, but in small herds or creches

when very young (about a metre long). Other species ofadult ankylosaurs occur in bone beds (e.g. Talarurus fromMongolia), but most others are solitary individuals (e.g.Euoplocephalus, Edmontonia and Ankylosaurus in NorthAmerica). See also: Dinosaur Behaviour

Oddities within the Group

Dinosaurs are full of oddities: gigantism, horns, frills,plates, spikes, knobs, domes, etc. The largest land animalsthat have ever walked the earth are the sauropods,including one with vertebrae that were eight feet tall! Thisgiant, Amphicoelias, is only known from fragmentaryremains that are now lost. From the description, a 24mlong Diplodocus could have just walked beneath Amphi-coelias. Brachiosaurus, or the giraffe sauropod, truly didhave its head in the clouds. The head towered 16mabove the ground. A lizard-like frill is known to have beenpresent along the back of the tail in Diplodocus and itpresumably extended onto the back and possibly the neckas well. The fossilised remains show that the frill consistedof a single row of triangular flaps of skin. The purposeof such a frill is not known or whether similar skin flapswere widespread among other sauropods. Numerous spe-cimens of small theropods with feathers and feather-likestructures are known from China. At least some of thesefeathered individuals, such as Caudipteryx, probablyused the feathers for display, much like peacocks orostriches today.Many of the odd structures are located on or around the

head of dinosaurs, and comparisonwithmodern ungulatesimplies that they were used mostly for display. Forexample, among theropods, Dilophosaurus had two verythin, very delicate crests of bone along the top of the skull.Perhaps these crests were covered by colourful skin duringthe breeding season. Similar structures in other theropodsinclude the single crest or ‘horn’ on the snout of Cer-atosaurus, and the outwardly projecting ‘brow’ horns ofCarnotaurus. Tall processes on the back vertebrae of Spi-nosaurus, Baryonyx and Suchomimusmake the individualslook bigger and more intimidating. It is always better toavoid a fight by bluff, which is probably what happenedfrequently during the mating season. Gigantism alsooccurs in theropods, although not to the extent seen insauropods. Nevertheless, 14–15m Tyrannosaurus, Car-carodontosaurus and Gigantosaurus are the largest everterrestrial carnivores.Display structures on the skull are best developed in

certain ornithopods, especially the hadrosaurs. Thesestructures include tall, hollow, narrow crests in Cory-thosaurus, a backward projecting spike in Sauropolophus, aU-shaped backward projecting crest in Parasaurolophus,and a hatchet-shaped crest in Lambeosaurus (these struc-tures in hadrosaurs have also been suggested as improvingsmell and vocalisation, but the variety of crests is moreanalogous to the variety of horns seen in antelopes). Theiguanodontid Ouranosaurus from Africa had very tall




processes on its back vertebrae, giving the appearance ofhaving a ‘sail’. In profile, Ouranosaurus could look moreintimidating to rivals ormore attractive to a potential mate(bigger sail).Other iguanodontids, such as Iguanodon itself,have a modified thumb that is a spike, possibly used indefence, although there is no agreement among palae-ontologists for this interpretation. Real odd-balls are thepachycephalosaurs, which had greatly thickened bones onthe skull roof. These domes were once thought to act likehelmets in head-to-head butting, but now it is thought thatthese ‘domes’ maximised the mass in butting the flanks ofthe opponent. Some, such as Stygimoloch, had clusters ofspikes at the rear corners of the skull that were probablyused to maximise the pain in this flank-butting.

The largest head of any dinosaur belongs to the cer-atopsiansPentaceratops andTorosauruswith amassive 2mlong skull. Althoughmost ceratopsians have horns on theirface,Pachyrhinosaurushada very roughboss atop thenose.Whether or not this boss was the base of a keratinous hornis not known, but it seemsprobable.Einiosaurushad a largebonyhornon its nose, but it is drooped forward, resemblinga giant can opener. With the tip facing downwards, thishorn could not have been effective as a stabbing weapon.Ceratopsians also have some of the largest nasal cavitiesamong any dinosaurs, the purpose ofwhich is a puzzle. Onesuggestion is that it may have cooled the blood to the brain,but that implies that all other dinosaurs without largenostrils had hot brains. Another idea is that the tissue couldbe inflated much like the throat pouch of frigate birds. Thelarge back plates of Stegosaurus may have had a dualfunction of cooling the body, and also to make the animalslook larger and more intimidating. By increasing bloodflow to the plates, the plates may have blushed, causingthem to stand out against the forest greens. Some stego-saurs, such as Huayangosaurus and Kentrosaurus, hadrearward projecting spikes on their shoulders. What theseodd structures were for is not known because they certainlycould not be used offensively. Among ankylosaurs, theankylosaurids have modified the last half of their tail ver-tebrae into a rigid structure. This structure acts as the‘handle’ to the club-like enlargement of armourplates at theend of the tail. The nodosaurid Edmontonia has a large,forward projecting spike on each shoulder. This spike has ashort tine, much like that seen on a deer antler, which mayhave been used to lock the spike of an opponent in shovingcontests. See also: Dinosaur Physiology: Were DinosaursWarm-blooded?; Dinosaurs and the Origin of Birds

Synopsis

Dinosaurs were a very successful group that survived over165 million years. They underwent rapid evolution,

producing novel adaptive characters. These charactersallow us to divide dinosaurs into seven suborders: Ther-opoda, Sauropodomorpha, Ornithopoda, Stegosauria,Pachycephalosauria, Ankylosauria and Ceratopsia. Six ofthese suborders had appeared within the first 50 millionyears, and the seventh by the last 35 million years. Thesuccess of these adaptive characters is seen in that none ofthe suborders went extinct until the end of the Cretaceous.At that time,whether through the catastrophic impact of anasteroid or the gradual collapse of the ecosystems, alldinosaurs went extinct. Only the birds remain of the sub-order Theropoda.

References

Carpenter K (1997) Agonistic behavior in pachycephalosaurs

(Ornithischia: Dinosauria): a new look at head-butting

behavior. Contributions to Geology, University of Wyoming 32:

19–25.

Carpenter K, Sanders F, McWhinney LA and Wood L (2005)

Evidence for predator–prey relationships: examples for Allo-

saurus and Stegosaurus. In: Carpenter K (ed) The Carnivorous

Dinosaurs, pp. 325–350. Bloomington: Indiana University

Press.

LipkinCandCarpenterK (2008)Looking again at the forelimb of

Tyrannosaurus rex. In: Larson P and Carpenter K (eds) Tyr-

annosaurus rex: The Tyrant King, pp. 166–190. Bloomington:

Indiana University Press.

Norell MA and XuX (2005) Feathered dinosaurs.Annual Review

of Earth and Planetary Sciences 33: 277–299.

Sereno P (1999) The evolution of the dinosaurs. Science 284:

2137–2147.

Further Reading

Ackerman J (1998) Dinosaurs take wing. National Geographic

194: 74–99.

Currie PJ and Padian K (1997) Encyclopedia of Dinosaurs. San

Diego: Academic Press.

Farlow JO and Brett-SurmanMK (1997)The Complete Dinosaur.

Bloomington: Indiana University Press.

Horner JR and Gorman J (1988) Digging Dinosaurs. New York:

Workman Publishing.

Lockley MG, Houck K and Prince N (1986) North America’s

largest dinosaur tracksite: implications for Morrison For-

mation paleoecology.Geological Society of America Bulletin 57:

1163–1176.

Sereno P (1997) The origin and evolution of dinosaurs. Annual

Review of Earth and Planetary Sciences 25: 435–489.

Sloan CP (1999) Feathers for T. rex. National Geographic 196:

98–107.






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