Chapter 11 – Overview of the Clade Theropoda
Why Study Theropods
Most intensively studied and
discussed by paleontologists
Popular with public
Large predators – ~8
tons and ~15 meters long, including
Tyrannosaurus,
Carcharodontosaurus, Giganotosaurus
Competition of sorts for discovery of the largest land-dwelling
carnivore
Theropods actually ranged widely in size, from less than a meter to ~15 meters
in length; ~5 kilograms
up to ~7 thousand kilograms
Not all big, but extremely diverse
Speedy
Consequence of bipedalism
(plesiomorphic)
Longer hindlimbs, shorter
forelimbs, modification
of pes, lightening of
skeleton
Lots of tracks
More than other dinosaurs
Greater trace fossil record than for
skeletal remains
Better documentation of other trace
and body fossils giving a better picture of theropod lifestyles
Nests, eggs, embryo and juvenile
skeletons
Toothmarks, coprolites, rare
gastroliths
Spectacular specimens preserving
soft parts
Scipionyx from Italy with partial preservation of
internal organs
Feathered theropods from China: Sinosauropteryx, Caudipteryx, Protarcheopteryx, Sinornithosaurus, Beipiaosaurus, Microraptor
Boundary between dinosaurs and birds is blurring
Definition and Unique Characteristics
of Theropoda
Introduction
Theropoda = beast foot
Created by O. C. Marsh in 1881
Should be Ornithopoda (bird foot)
because theropod feet resemble bird feet more than ornithopod feet do
Ornithopoda created by Marsh in 1882
Characteristics
More than 20
Major characteristics – SEE
Figure 11.2
Bones
Low density, hollow
bones
Pneumatic bones filled with air sacs, including in the skull, vertebrae and ribs
Teeth
Most theropods had large, recurved
and serrated teeth (plesiomorphic) for
eating flesh
But not all, including
Oviraptorians and Ornithomimosaurians,
which may have been omnivorous or insectivorous or herbivorous
Nevertheless, all
carnivorous dinosaurs were theropods
Obligate bipeds
Most theropods were obligate bipeds with the legs positioned proximal
to the midline of their bodies
Some trackways show one foot in front of,
rather than beside, the other
Body
balanced over pelvis
Vertebral
column held nearly horizontally
Walked on
toes
Hands
Most theropods had grasping 3-fingered hands
Stiffened Tails
Tails were stiffened by caudal vertebrae that had long processes
extending caudally
Tails held horizontally off the ground
Encephalization quotient
Some theropods were among the brainiest based on their EQ
EQ is technically ratio of the mass of the cerebral cortex to the mass
of the total brain
EQ more easily measured as brain to body mass or braincase to body
volume
For a modern African elephant
– 0.15% of the body mass is brain
For a human – 2.1%
of the body is brain
Tyrannosaurus rex - 0.0079% of the body is brain
Triceratops - 0.0035% of the body is brain
Allometric line – average
EQ of closely related modern vertebrates on a plot
EQ for modern crocodilians set at 1 on allometric line as a comparison
for fossil reptiles – EQs
less than 1 plot below the line; EQs
more than 1 plot above the line
Theropods EQs are more than 1 and some small theropods, like Troodon, have EQs similar to
modern ostriches
Feathers
Some theropods had feathers; no
species in other dinosaur clades have yet been found to have feathers
Clades and Species of Theropoda
SEE Figure 11.1 and Tables 11.1 and
11.2 (and Table 10.2)
Diversity
More than 90 genera of theropods
Nearly half (more than 40) lived
during the Late Cretaceous
Clade Herrerasauridae
See Figure 10.10 for Herrerasaurus, Staurikosaurus and Eoraptor
Late Triassic primitive dinosaurs from
South America
2-4.5 m long
Some of the largest Late Triassic predators
Because of primitive characteristics,
the herrerasaurids as dinosaurs are somewhat controversial – some
paleontologists consider them to be a clade separate from dinosaurs
Clade Ceratosauria (horned lizard):
Coelophysoidea and Neocratosauria
Mostly Found in Gondwana continents (South America, Africa, India and Madagascar),
only a few genera from U. S., Europe and China
Late Triassic and Jurassic forms from
northern continents, Cretaceous forms from Gondwana continents
~2-8 m long
Coelophysoideans supplanted
herrerasaurids in the Late Triassic
Coelophysis (Late Triassic), Syntarsus and Dilophosaurus (Early Jurassic), and Ceratosaurus (Late Jurassic)
Coelophysis – among the most abundant body fossils of any
dinosaur
Cannibalistic
Dilophosaurus one
of the few dinosaurs in “Jurassic Park” from the Jurassic
Neoceratosaurians Cretaceous forms
from Gondwana continents
Abelisaurus (Late Cretaceous) and Carnotaurus (Early Cretaceous)
Gap between maxilla and premaxilla
Headgear – horns and ridges
About 20 species in at least 15
genera in Ceratosauria
Clade Tetanurae (stiff tail):
Avetheropoda and Its Numerous Clades
At least 80 genera in Tetanurae
less than 1 to ~15 m long
Contains the theropods best known
to the general public
Allosaurus (Late Jurassic), Deinonychus (Early Cretaceous), Tyrannosaurus, Albertosaurus and Velociraptor (Late Cretaceous)
Oviraptor and Troodon (Late Cretaceous) are less well known
Immediate ancestors of birds were
Tetanurans
Beipiaosaurus, Caudipteryx, Microraptor, Protarchaeopteryx, Sinornithosaurus and Sinosauropteryx (Early Cretaceous) are feathered dinosaurs,
all from the same formation from NE China
Tail stiffened by zygopophyses (extensions to the front and aft of the neural
arches)
The stiffened tail gave exceptional balance
Small and highly aggressive theropods,
especially deinonychosaurids like Deinonychus and Velociraptor, used their stiffened tail as a dynamic
counterbalancing device against the motions of the long arms and grasping hands
Megalosauridae and Torvosaurus
Basal (primitive) tetanurans
Avetheropoda
Vast majority of tetanurans, so named
because of the birdlike features of many of its members
Divided into Carnosauria and
Coelurosauria
Carnosauria
Used to include all large theropods;
now only Allosauridae and Sinraptoridae
Allosauridae [Allosaurus (Late Jurassic), Carcharodontosaurus (Early Cretaceous) and Giganotosaurus (Late Cretaceous)]
Carcharodontosaurus
and Giganotosaurus larger than Tyrannosaurus
rex
An example of convergent evolution – distantly related organisms develop similar
morphology
Giganotosaurus replica mounted at Fernbank
Sinraptoridae [Yangchuanosaurus (Late Jurassic) replica mounted in the atrium
at Hartsfield and Sinraptor]
Coelurosauria
Nearly half (at least 40) of Tetanuran
genera are Coelurosaurs, and 75% (at least 30 genera) of Coelurosaurs lived
during the Late Cretaceous
Even more closely related to birds
Used to include all small theropods;
now includes Tyrannosaurids, a clade of mostly huge carnivores
Semilunate carpal is one of the most
distinctive characteristics
Basal forms [including Compsognathus (Late Jurassic)] plus Maniraptoriformes
Maniraptoriformes
3 clades: Arctometatarsalia,
an unnamed clade and Maniraptora
Arctometatarsalia
– Tyrannosauridae [Albertosaurus and Tyrannosaurus
(Late Cretaceous)], Troodontidae (small- to medium sized, large brains and
sickle claws, like Troodon
[Late Cretaceous]), and Ornithomimosauria (small- to medium sized,
“ostrich dinosaurs” similar in size and shape, but with long arms,
to modern ostriches, like Ornithomimus and Struthiomimus
[Late Cretaceous])
Named after the pinched middle metatarsal
Unnamed clade
– Therizinosaurs (really odd theropods, like Therizinosaurus and Segnosaurus
[Late Cretaceous]) and Oviraptorids [Oviraptor (Late Cretaceous)]
Maniraptora -
Deinonychosauria [Deinonychus and
Utahraptor (Early Cretaceous),
and Velociraptor (Late
Cretaceous)] and Avialae (Archaeopteryx [Late Cretaceous], plus all birds, both fossil and
modern)[discussed in Chapter 16]
Deinonychosaurs had a pubis rotated downward or down & back, like birds
Trace Fossils
Tracks assigned to
ornithomimids, Acrocanthosaurus,
and Tyrannosaurus rex
Troodon nests
Troodon and tyrannosaurid toothmarks
Caudipteryx gastroliths
Tyrannosaurus rex coprolites
Paleobiogeography and Evolutionary
History of Theropoda
Main Points
Theropods are among the earliest
dinosaurs
Body fossils appear in the earliest
Late Triassic, ~230 my ago, and include Eoraptor (South America) and the herrerasaurids (South
& North America)
Possible Middle Triassic tracks
Theropod body and trace fossils occur
from the Late Triassic to the Late Cretaceous on all continents (except
Antarctica for trace fossils) and in a wide variety of environments
River channels, levees & floodplains to
lakes to sand dunes
Theropod body fossils normally
15-20% of dinosaur fossil assemblages
(What would you expect it to be?)
1976 study – of 171 valid
theropod species, 85% named on basis of 5 or few specimens and 40% based on a
single specimen (how does this compare for dinosaurs as a whole?)
Hasn’t changed much since
Monospecific theropod bone beds
resulting from taphonomic factors and perhaps social behavior
Late Triassic Coelophysis, Late Jurassic Allosaurus, Late Cretaceous tyrannosaurids
Most theropod skeletons are found in isolation
& disarticulated
Theropods are the most diverse
dinosaur clade
Wide range of sizes,
with trends in particular lineages toward large sizes (Cope’s Law), for
example Allosaurus (Late
Jurassic), Carcharodontosaurus
& Giganotosaurus (Early
Cretaceous), and Albertosaurus
& Tyrannosaurus (Late
Cretaceous), and small sizes, for example, Compsognathus (Late Jurassic) and Sinosauropteryx, Caudipteryx, Protarchaeopteryx and Microraptor (Early Cretaceous)
Carcharodontosaurus
& Giganotosaurus and Tyrannosaurus were the largest land carnivores, and among
the largest dinosaurs, exclusive of Sauropods
Theropods as Living Animals
Reproduction
Sexual Dimorphism
Coelophysis & Syntarsus (Late Triassic) and Tyrannosaurus (Late Cretaceous)
Females probably larger than males
Cranial Processes, Dorsal Sails and
Feathers
Dilophosaurus & Ceratosaurus (Early Jurassic Ceratosaurs), Crylophosaurus (Early Jurassic Tetanuran) and Allosaurus (Late Jurassic Tetanuran)
Spinosaurus, Baryonyx, and Suchomimus
(Early Cretaceous)
Caudipteryx (Early Cretaceous)
Eggs, Embryos and Nests
Known for a few Late Cretaceous
species
Oviraptor eggs mistakenly identified as Protoceratops eggs and Troodon eggs mistakenly identified as Orodromeus eggs until embryos found
Only theropod nests
Have paired, vertically-oriented eggs; 24 eggs in Troodon clutch
Velociraptor embryos and unidentified Therizinosaur embryos
Growth
Growth Sequences Available For Only
a Few Species
Coelophysis bauri & Syntarsus (Late Triassic) and Allosaurus (Late Jurassic)
Coelophysis bonebed indicates a 10- 15-fold increase in
size from hatchling to adult
Bone Histology
Growth Lines result from temporary slowing (annuli) or stoppage (lines of arrested growth [LAGs]) of growth
Annuli – layers
of bone fibers formed parallel to each other without vascularization; contrast with adjacent vascularized
bone
LAGs – thinner
than annuli
Give minimum ages (may be resorbed, especially early in life) and allow calculation of growth rates
Syntarsus lived
at least 7 years and Troodon at least 5 years
Some theropods grew faster than other dinosaurs and modern
crocodilians, but slower
than large, flightless
birds
Locomotion
Based
on abundant trackway data
Theropod tracks outnumber those of all
other dinosaur clades combined
Smaller
theropods were the fastest dinosaurs, probably reaching maximum speeds of 40
kph (25 mph)
Smaller theropods have short femora compared to
the great length of the rest of the hindlimb
Theropod tracks mostly show them simply
walking however
Larger
(>2 tons) theropods probably had maximum speeds of no more than 15 km/hr
Tripping at faster speeds would prove
fatal
Note: Large Late Cretaceous theropods dwarfed
their prey by 35% of body length
May be
due to abundance and rapid turnover of the large herds of rapidly growing,
abundantly reproducing hadrosaurids and ceratopsids
Examples
of theropod locomotion from tracks
Stalking of prey
Running from predators
Pack hunting
Resting
Limping
Abundant
tracks and wide paleobiogeographic distribution (Theropods are found from the paleoequator to high-paleolatitudes like
the Transantarctic Mountains, Antarctica) imply high activity levels
and mobility
Theropods may have been endothermic (“warm-blooded”)
Feeding
Teeth
Mostly Ziphodont for meat-eating
Smaller theropod teeth functioned to “grip-and-rip”
Oviraptorids and most ornithomimids had no teeth
Oviraptorids
had 2 peg-like projections in the middle of the palate
Strong jaw muscles; may have eaten bivalves by
crushing shells
Early
ornithomimids retained a few lower front teeth while later forms had no teeth
May have had beaks like birds for eating
insects, small vertebrates, eggs and fruit
Therizinosaurs (including the Late Cretaceous Therizinosaurus and Segnosaurus)
Therizinosaurus
had huge manus claws for raking tree branches or ripping apart termite mounds
Senses
Stereoscopic vision and large eyes
Troodontids had a pneumatic channel passing through the base of the braincase that connected the ears & the asynchronous perception of a single sound at the two ears would have allowed the direction of the sound to be determined quickly
Well-developed sense of smell to detect prey animals and already-dead animals
Enlarged olfactory lobe in the front of the brain
Probably lots of scavenging
Killing
Teeth
Claws on manus
Most
theropods had powerful forelimbs equipped with strong, grasping hands
Deinochirus mirificus (Late
Cretaceous) of Mongolia
Killing claws on raptor and Troodontid feet
Raised
above the ground during normal walking and running
Could
swing through a large arc of motion
When lowered during a kick at a prey animal’s abdomen, would disembowel the animal in one stroke
“Fighting Dinosaurs”
Deinonychus and Tenontosaurus
Tyrannosaurids
Toothmarks and teeth of Albertosaurus and Tyrannosaurus in bones
Triceratops, Hypacrosaurus, Edmontosaurus and Saurornitholestes
Coprolites
Coprolites attributed to Tyrannosaurus have bone fragments and muscle of juveniles
Stomach contents
Daspletosaurus (Late Cretaceous) skeleton from Two Medicine
Formation of Montana has corroded juvenile hadrosaurid bones in the gut region
Fish-eating
Based on numerous teeth and hook-like claws & long arms of Baryonyx and Suchomimus (Early CRETACEOUS) AND fish scales in association with Baryonyx and the crocodile-like skull of Suchomimus
Tyrannosaurus – Predator or Scavenger?
Couldn’t run fast; Small eyes; Ridiculously small arms
Too large for ambush?
Tyrannosaurid teeth were banana-shaped and
useful for crunching rather than tearing
Pockets at the base of serrations may have
harbored colonies of bacteria, as is the case of the living Komodo dragon,
resulting in bacterial infections from a theropod bite that was not immediately
fatal
Probably like modern predators – grabbed juveniles when it could and chased smaller theropods from their kills
Social Life
Many theropods probably lived in
packs
Deinonychus, Coelophysis, Dilophosaurus, Allosaurus, Albertosaurus
Skeletal material from more than one individual in the same deposit in
a small area
Trackways
Health
Active lifestyle led to more
injuries than observed for other dinosaur clades
Mostly limb injuries
Limping as a consequence of fractures and muscle & tendon pulls
Also evidence of diseases
Bone infection
Cancer
Evidence for bites from other
theropods is ambiguous
Extinction
Nonavian theropods went extinct at
the K-T boundary
Teeth and egg fragments found in
Tertiary strata probably were eroded from older deposits
Theropoda as a clade did not go
extinct at the K-T boundary because birds are nested within Theropoda