GEOL 104 Dinosaurs: A Natural History
Fall Semester 2000
Theropoda I: Dinosaurs red in tooth and claw
Theropoda:
Name means "beast foot", after the curved claws
Characterized by extremely hollow limb bones
Most retain the primitive condition of bladelike, serrated teeth,
indicating they were carnivores
All, from smallest to largest, were obligate bipeds
Became the largest terrestrial carnivores of all time, but began at about 1 m long,
and even by end of Late Triassic were dwarfed by rauisuchians
In Ischigualasto Fm., a possible very primitive theropod Eoraptor. However,
is so primitive it might be a non-theropod saurischian, or a non-dinosaurian dinosauromorph.
Another Ischigualasto Fm. dinosaur is Herrerasaurus, member of
Herrerasauridae:
Has longer clutching fingers, shared with neotheropods (see below)
Has an intramandibular joint between the dentary and postdentary bones
- Absorbs the stresses of struggling prey
- Also found in neotheropods
Herrerasaurids were probably true theropods, but have only two sacrals (a
pre-dinosaur feature), have manual digit III longer than manual digit II (a pre-saurischian feature),
and have anterior cervicals as long or longer than posterior cervicals (another pre-saurischian feature)
So, they MIGHT be a non-theropod saurischian group
True, unquestioned theropods form the clade Neotheropoda
Name means "new theropods"
Have many synapomorphies:
- Promaxillary fenestra (an extra opening in the maxilla between the naris and
the antorbital fenestra)
- Five or more sacrals
- Furcula (clavicles fused into wishbone)
- Manual digit V lost: hand has only four fingers
- Pedal digits I and V reduced, so foot is functionally three-toed
Neotheropods are divided into two clades, Ceratosauria and Tetanurae.
Ceratosaurs:
Characterized by fusion of the hip bones and some of the hindlimb bones
in adults
Divided into two clades:
- Coelophysoidea
- Generally more slender
- Characterized by kink between the premaxilla and maxilla, probably for dealing
with small struggling prey
- Appear in Late Triassic (first major neotheropod radiation), dominant predators of Early
Jurassic, rare but present until Late Jurassic
- Best known from 3 m long Coelophysis of the Late Triassic of the American
Southwest and 6 m or more Dilophosaurus of the Early Jurassic of the American
Southwest (and possibly China)
- Neoceratosauria
- Generally stockier, with thicker shorter necks
- Characterized by seven or more sacrals, tall deep skulls, and big
infratemporal fenestrae
- Best known from the Late Jurassic Ceratosaurus of the western U.S. (and
possibly Africa) and the Abelisauridae of the Cretaceous of the southern continents
Ceratosauria may be paraphyletic, with neoceratosaurs closer to tetanurines than to
coelophysoids
The remaining neotheropods form Tetanurae ("stiff tails"). Tetanurines are
characterized by:
- Teeth restricted to front of jaws in front of orbit
- Larger hands for clutching prey
- Interlocking caudal vertebrae for counterbalance while running
Relationships among the basal tetanurines is poorly resolved. Since Megalosaurus
is one of these primitive forms, they can be called "megalosaurs". One specialized group
of "megalosaurs" is Spinosauridae.
Spinosaurids are characterized by:
- Long crocodile-like snouts filled with big, conical teeth
- In some taxa, very tall neural spines making a big sail
- May have been like the modern Nile Crocodile, eating both large fish and big land
animals
- Found in the Cretaceous
The advanced tetanurines form the clade Avetheropoda ("bird theropods").
Avetheropods are characterized by:
- Maxillary fenestra (an extra opening between the antorbital and promaxillary
fenestrae)
- More complex air chambers in their vertebrae, indicating a more advanced air sac
system
- Semilunate carpal block, a specialized half-moon shaped carpal bone structure
which let the hand fold up against the body while running
- Greatly reduced or lost manual digit IV (present in primitive forms, but most
avetheropods have only three fingers, namely I-III
Avetheropoda is divided into two clades, Coelurosauria (subject of the
next lecture) and Carnosauria.
Carnosaurs:
- The dominent predators from the Middle Jurassic until the beginning of the Late Cretaceous
- Most of them were large; in fact, the largest known theropods are carnosaurs
- Characterized by:
- Enlarged nares
- Extra openings in snout
- Best known from Allosaurus of the Late Jurassic of western North America
and Europe (and maybe Africa) and Giganotosaurus (the largest known theropod)
from the earliest Late Cretaceous of Argentina
- In the Late Cretaceous carnosaurs are replaced by abelisaurid ceratosaurs in the
southern continents and Europe, and by tyrannosaurids in Asia and North America
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