1. SAURISCHIA
• THEROPODA
• PROSAUROPODA
2. ORNITHISCHIA

ANCESTRAL DINOSAURS, MAJOR GROUPS

SAURISCHIA

• subnarial foramen (pmx-mx) present
• jugal forked posteriorly
• axial epipophyses (*)
• axial postzygs lateral to prezygs (*)
• hyposphene-hypantrum present in dorsals (*)
• mtc IV and V lie on palmar surfaces of III and IV (*)
• I-1 longer than mtc I (*)
• ischium rod-like distally
• Evolution of pleurocoels (pneumaticity)
• Gastralia
• No ossified tendons

Saurischia - all dinosaurs closer to birds than to ornithischia
Ornithischia - all dinosaurs closer to Triceratops than to birds

• contact reduced or lost between mx process of premx and nasal
• temporal musculature reaches frontals anteriorly
• posterior cervicals are elongated
• axial postzygs lateral to prezygs (*)
• epipophyses on anterior cervical postzygs (*)
• dorsals with hyposphene-hypantra (*)
• manus more than 45% length of forearm
• second manal digit longest, manus asymmetrical
• metacarpals cupped (*)
• mtc I half length mtc II, digit more robust (*)

• neck makes up more than 40% of presacral series
• posterior cervicals about 35% longer than anterior trunk vertebrae
• ischium enlarged distally
• platform anterior to ascending process of astragalus

THEROPODA:

• unique heterodont dentition, leaf-shaped anteriorly, (sauropodomorph) serrate posteriorly

Theropod distinctions from prosauropods: (Novas 1994, JVP 13: 400-423)
Raking manus:

• humerus less than 50% femur
• elongate manus (>50% length of humerus + radius)
• extensor depressions on distal ends of mtc
• penultimate manal phalanges elongate
• manal unguals of II and III enlarged (but not larger than I)
• extreme reduction of manus digits IV, V

• lacrimal exposed on upper surface of skull
• sliding intramandibular joint (splenial groove, below for angular tongue, above, flexible bite)

• elongate distal caudal prezygapophyses

• distally enlarged pubis
• trochanteric shelf on proximal end of femur

PROSAUROPODA:

• muzzle from skull 20 cm long, body 3 m long
• small heads, coarsely serrated teeth (plesiomorphic)
• gut processing with gastroliths, no occlusal wear on teeth
• short hands with enlarged first ungual
• short feet with enlarged first ungual

• best known Carnian herbivorous dinosaur
• 1.5 m long gracile prosauropod (pointing to relationships to other dinosaur groups)
• small head cf. other Carnian dinosaurs
• teeth higher in front, leaf-like posteriorly (opposite of Eoraptor)
• acetabulum not fully opened
• ascending process of astragalus broad, elongated
• next to last cervical much longer than trunk verts
• third sacral derived from tail
• deltopect crest 50% length of humerus, longer than in other sauropodomorphs
• sigmoid lesser trochanter on femur (primitive)

• fourth trochanter reduced, peculiar in shape
• huge humerus from different site, may be same taxon
• all of material is huge, suggesting an animal 6-7 m in length

ORNITHISCHIA:

Completely known taxa: Lesothosaurus, TEXT, pp. 416-425, see Sereno 1991 (JVP 11: 168-197): (1 m) Hettangian-?Sinemurian (early Jurassic), S Afr
(*) = shared with Padian 1997, below

• skull short (in some prosauropods, effect of herbivory)
• infranarial expansion of premaxilla (also Herrerasarus)
• palpebrals (*)
• cheek fossa
• 5 pmx teeth (also some prosauropods)
• straight quadrate
• quadrato-mandibular articulation low (also some prosauropods)
• predentary (*)(wierd, Hesperornis), rotating mandibles, wear facets on teeth
• ossified tendons (*) (hyposphene in saurischians)
• tetraradiate pelvis (*) (not homologous to that of birds; cf. parallel evolution and origin of birds!)
• long preacetabular process on ilium
• pendant 4th trochanter on femur (linear swing)

• cheek depression well-developed
• small mandibular opening present
• wear facets on dentary teeth do not form smooth occlusal surface
• pubis impression appeara to project anteroventrally, no post-pubic process
• ventral margins of ischia approach each other on midline, deep articulation with pubis (weird pelvis)
• thick ascending process of the astragalus

• teeth with expanded crowns, marginal denticles, wear facets from tooth-to-tooth occulusion, marginal teeth almost always inset medially (oral processing of plant material basic adaptation)
• in Pisanosaurus just enouth of the pelvis is preserve to suggest that rotation of the pubis had not occurred
• in Pisanosaurus ascending process of the astragalus not compressed anteroposteriorly, as in other ornithischians (JVP 11: 168-97)
• Lesothosaurus with predentary nipper, rotating lower jaws (fundamental ornithischian adaptation
• Lesothosaurus with transformed pubis, touching ischium posteriorly
• Ossified tendons along trunk and anterior tail

• leaf-shaped teeth with triangular crowns and constricted roots
• palpebral (*)
• predentary (*)
• edentulous tip pmx, loss of at least one pair of teeth
• reduction of antorbital fossa and fenestra
• five or more sacrals
• posteroventral process of pubis (*)
• pendant 4th trochanter (*)
• mtt V splint wo phalanges
• ossified epaxial tendons (*)

Torpedoes, not dodgers; tail not longitudinally differentiated

• never evolved pleurocoels
• gastralia absent (cf. pachycephalosaurs)
• small, squirrel-like grasping forelimbs

All of the large prosauropods vanished at the Tr-Jr boundary (Olshevsky 1992, p. 106) - radiation after extinctions into larger prosauropod\sauropod groups

The problem of Pisanosaurus: Sereno in the JVP (1991)

• Pelvic impression: pubis projects ant-vent, no postpubic proc
• Ischia approach on midline, broadly contact pubis anteriorly
• Distal tib subquadrate, asc proc of astrag blunt, massive
• Oblique reference to possibility of non-association
• If association correct, support for monophyly

Small scap and other misc. postcran frags too small to belong to type of P. mertii (re. Sereno), but Sues notes scap small in fabrosaurs

Langer, pers. com. 6 September, 2000: only one of his 203 postcranial characters supports an ornithischian affinity: "It's not even a very strong one. Pisanosaurus shares with "other" ornithischians a calcaneum that presents a posterior elevation in the articular surface for the fibula. Whereas other basal dinos have this area nearly flat. It is true, however, that this feature also apears in Tetanurans."

Consider the morphologic steps in deriving saurischians from ornithischians?

Was there a stem dinosaur?

If not, what did a stem ornithischian look like?

The stem dinosaur (Eodinosaurus), would have been a 50 cm long (therefore hidden from sedimentologic record) bipedal carnivore-omnivore of middle Triassic age that in life closely resembled other small bipedal archosaurs (and later small theropods). It would have inhabited rich, lowland (not red bed) environments. The skeleton would show a blend of a few saurischian and ornthischian characters:
-"floating" palpebrals (O)
-infranarial expansion of premaxilla (O)
-subnarial foramen (S)
-jugal forked posteriorly (S)
-partly retroflexed pubis with enlarged ambiens process
-manus digit IV short, with reduced ungual, digit IV without ungual
-short but strongly recurved ("can-opener") preacetabularprocess of ilium
-elongate, saber-like ischia
-blunt, pendant femoral 4th trochanter (O)
-digit V pes reduced, with vestigial ungual(s)

If it never existed, with O and S characters, dinosaurs not a natural group (note theropod and prosauropod characters in Eoraptor)

CHRONOLOGY:

37 - 208 ma - Triassic-Jurasic extinctions, beginning of dinosaurian era
18 - 223 ma - beginning of abundant record of dinosaurs Coelophysis and Sellosaurus
15 - 230 ma - oldest records of dinosaurs (4 taxa)
8 - 237 ma - appearance of stem dinosaurs?
5 - 240 ma - oldest bipedal protodinosaurs
3 - 242 ma - oldest archosaurian records
0 - 245 ma - Permo-Triassic extinctions

Energetics of quadrupedal to bipedal locomotion the same; change from quadrupedality to bipedality would increase stride length without increasing body weight

Lightening of distal limb would decrease inertia

Relatively small initial size would ameliorate physics of transition to bipedal support; with perfection a size increase would follow (50 cm long - eating insects and protein rich early plant growth)

Carnivorous dinosaurs not trophically limited to abundance of herbivorous dinosaurs, as later in Mesozoic; lot of alternate prey animals

Momentous innovation - why only one transition? Good for simple behaviour. Mammals backbone runners, change course rapidly; dinosaurs changed course with tail movements. Bipedalism in dinosaurs left hands free for other functions - mammal carnivores can't have fore-"hoofs"

The stem dinosaur ("sister-group of all dinosaurs") has not been found - what we find are dinosaurs with saurischian and ornithischian specializations

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