WEST VIRGINIA UNIVERSITY AT PARKERSBURG
GEOLOGY 307 - PALEOBIOLOGY OF DINOSAURS
 

THE ULTIMATE PREDATORS

THEROPODS

By
Edward L. Crisp, Ph.D., Professor of Geology



 

INTRODUCTION

     The Theropoda (beast + foot) includes the carnivorous dinosaurs known from Upper Triassic to Upper Cretaceous rocks.  If the birds (which are classified as theropods by dinosaur paleontologists) are included, the range of Theropoda is from Late Triassic to Recent.  The name Theropoda was coined by O. C. Marsh (Yale University Paleontologist) in 1881 (Fastovsky and Weishampel, 1996).

     The theropods range in size from Compsognathus (chicken-sized) to the largest land predators of all times (such as the monsters, Tyrannosaurus rex and Gigantosaurus).  They are known on all the continents, even Anarctica.
 

THEROPODA

Characters
 


References

     Padian, K., J. R. Hutchinson, and T. R. Holtz, Jr.  1999.  Phylogenetic definitions and nomenclature of the major
     taxonomic categories of the carnivorous Dinosauria (Theropoda).  Journal of vertebrate Paleontology, 19: 69-80.
(The above is from: theropods http://www2.tamuk.edu/geo/Baskin/dinos/VPTHEROPOD.ht)

THEROPOD FOSSILS AND PHYLOGENY


Figure 1.  General cladogram of the Theropoda (From:  Theropod Dinosaurs http://www.ucmp.berkeley.edu/diapsids/saurischia/theropoda.html).
(GO TO THIS WEB SITE AND READ THE MATERIAL)


     Most of the fossils of theropods are incomplete, although several complete skeletons have been found of the more well known theropods, such as Allosaurus and Tyrannosaurus.  But a large number of theropods have been named from isolated bones, and dinosaur paleontologists have inferred much from these few bones.

     To decipher the phylogeny of theropods (Figure 1) we need complete skulls and skeletons.  The incompleteness of the fossil record makes it difficult to fit them into a coherent theropod phylogeny.  Theropod phylogeny is one of the least agreed upon phylogenies among the dinosaurs.  Different paleontologists place greater emphasis on different features as significant shared derived characters (synapomorphies).  However, in a recent article of the Journal of Vertebrate Paleontology; Padian, Hutchinson, and Holtz (1999) have proposed a revision of the major groups of theropod dinosaurs and have attempted to resolve some of the confusion in theropod phylogeny.

     The old classification scheme divided the theropods into two groups: the Carnosauria (meat + lizards) and Coelurosauria (hollow-tailed + lizards).  This has been modified now to include the basal theropods Eoraptor and Herrerasaurus (and closely related forms), as well as the more advanced basal theropods, the Ceratosauria.  However, there is still much debate (and disagreement) among dinosaur paleontologists as to whether Eoraptor and Herrerasaurus should be placed in the Theropoda (or even within the Dinosauria).  Regardless of the ultimate outcome of this debate, the clade Eutheropoda has recently been proposed to include the the common ancestor of Ceratosauria and all more derived theropods (including birds).
 

THEROPOD CHARACTERISTICS AND GROUPS

     To identify theropods specifically, we must distinguish them from other primitive meat eating archosaurs.  At least 20 shared derived characters unit the clade Theropoda.  Shared derived characters and inherited primitive characters (such as bipedal stance and sharp serrated teeth) from non-theropod ancestors made theropods excellent predatory dinosaurs.

Ceratosauria

     Ceratosaurian theropods of Late Triassic and Early Jurassic are already highly specialized meat eaters.  Much of the specialization as meat eaters is evident from their key derived charaters:


     The ceratosaurs also had marked sexual dimorphism as adults.  There were more robust forms with larger skulls and more gracile forms.

     Some of the best known theropods belong to the Ceratosauria:



The following section is directley quoted from: Olsen, Paul; 1999;Lecture 11 - Triassic: Newark, Chinle http://rainbow.ldeo.columbia.edu/courses/v1001/chinlenewark10.html

"Coelophysis

 Strata of the upper part of the Chinle Group, outcropping on the Ghost Ranch in New Mexico, have produced one of the
world's richest accumulations of a single species of dinosaur.

The so-called Whittiker quarry at Ghost Ranch has produced well over 100 articulated skeletons of a small, lightly built dinosaur identified as Coelophysis bauri by Colbert.

The genus was originally established by Edward Drinker Cope from very scrappy material collected elsewhere in the Chinle. This original material appears somewhat different from the Ghost Ranch specimens and therefore the name Rioarribasaurus was proposed for the much more complete skeletons. This created a huge uproar among some paleontologists because Coelophysis was such a famous name and had been so long associated with the Ghost Ranch skeletons. The matter was surprisingly acrimonious and ended up being submitted to the International Commission on Zoological Nomenclature (ICZN). Recently, the ICZN ruled that the name Coelophysis should indeed apply to the Ghost Ranch specimens and that a new type specimen should be picked. As a consequence someone will probably end up renaming the original specimens described by Cope!
 


Coelophysis skeleton

                                             The skeleton of Coelophysis (gracile form), based on the Ghost Ranch material. After Paul (1988)

      Coelophysis is a small, very delicate dinosaur characterized by a relatively long neck and an elongate skull. In contrast to Herrerasaurus, the foot is narrow with only three prominent toes.
 


Skull of Coelophysis

                                                                                               Skull of Coelophysis. After Paul (1988).
Foot of Coelophysis
 Right pes (foot) of Coelophysis, from  Colbert (1989) and Pelvis of Coelophysis, from Colbert (1989).

The functionally three toed foot of Coelophysis is a shared derived character of a group called the Theropoda. Theropods make up the bulk of all carnivorous dinosaurs and include such familiar dinosaurs as Tyrannosaurus rex and Velociraptor as well as
birds. Coelophysis also has a specialization in the pelvis seen in a number of other theropod dinosaurs. The bones of the pelvis fuse together in the adult, presumably ceasing growth. This is a shared derived character for the Ceratosauria, named after the Late Jurassic genus Ceratosaurus, which we will discuss in a later lecture. Curiously, the best known early ceratosaurs seem to be more specialized than the later ones."


Tetanurae (fused + tails)

     This clade includes the remainder of the theropods more derived than the Ceratosauria.  These theropods are much more bird-like than the ceratosaurs.  Indeed, the fossil evidence indicates that the ancestors of birds lie among theses theropods.

     The tetanuraens have many bird-like derived characters, including the following:


     Until fairly recently, the Tetanurae was divided into two major groups, the Carnosauria (meat + lizards) and the Coelurosauria (hollow tailed + lizards).  Carnosaurs included the large, short necked, and short armed tetanuraens (such as Tyrannosaurus, Allosaurus, Megalosaurus, etc.).  Whereas, the coelurosaurs are small theropods with long necks and long forelimbs (such as Compsognathus [delicate + jaw]).

     In fact, for many years there was no distinction between Ceratosauria and Tentanurae and all theropods were classified as belonging to Carnosauria (the large-bodied theropods) and Coelurosauria (the small-bodied theropods).  These simple classifications are no longer accepted by dinosaur paleontologists because they do not show the true relationships of the theropods (Currie, 1998).

     More recent classifications do not include a monophyletic Carnosauria (or have completely redefined Carnosauria [Padian, et al, 1999] see Figure 2 below) and Coelurosauria has been redefined.  Herrerasaurus is recognized as the most primitive theropod (some classifications accept Eoraptor as a theropod more primitive than Herrerasaurus and refer to these two as basal theropods [Padian, et al, 1999]).  Padian, et al (1999) have proposed the term Neotheropoda for the clade including the common ancestor of Ceratosauria and Tetanurae and all descendents of that common ancestor.  Thus, Ceratosauria would represent the more primitive (least derived) group of the Neotheropoda.  Moving on up the cladogram  we come to the Tetanurae, with genera such as Torvosaurus and Megalosaurus, and the redefined Carnosauria (including the family Allosauridae, Allosaurus, Acrocanthosaurus, and related forms) successively leading to the Coelurosauria.  As we move up the cladogram, the theropods become more bird-like in their features.  Compsognathus (a small chicken-sized theropod - first appearance of the semilunate carpal bone) is at the base of the Coelurosauria clade, with the more bird-like smaller theropods, but also Tyrannosaurus and kin (Tyrannosauridae) and the likes of Velociraptor, Deinonychus, and Utahraptor (Dromaeosaurids) higher up in the clade Coelurosauria.  The birds (Aves) are also placed in the Coelurosauria and are very closely related to the family Dromaeosauridae.


Figure 2.  Cladogram of the Theropoda (Padian, et al, 1999).
 

MAJOR NONCERATOSAURIAN THEROPODS BELOW COELUROSAURIA

     This group of theropods are large of body with big heads, short powerful necks, short forelimbs, massive hind legs, and long tails.  The best known of this group (which is basically the old Carnosauria) belong the family Allosauridae.

Allosauridae

     The allosaruids are known from Lower Jurassic to Lower Cretaceous rocks.  They have the following characteristics:


    The Late Jurassic Allosaurus is the best known genus within this family.  Allosaurus had a lightly built skull (Figure 3) with a roughened ridge just above and in front of the orbit.  This dinosaur is very well known from fairly complete skeletons found in the Morrison Formation of the western U.S.  As pointed out by Dr. Dwayne Stone, Professor Emeritus of Geology, Marietta College, (personal communication, 1999), the bones of 44 individuals of Allosaurus have been recovered from the famous Cleveland-Lolyd Dinosaur Quarry in Emery County, Utah.  In fact, of the approximately 10,000 bones recovered from the quarry, over 90% were of the predator Allosaurus.

     The allosaurids were never very diverse, but were the dominant theropods of the Late Jurassic - Early Cretaceous. Acrocanthosaurus  and Gigantosaurus are other allosaurids known from Lower Cretaceous rocks.  Gigantosaurus was discovered in 1995 in Argentina and is reported to be larger than T. rex.
 


Other Theropods More Primitive Than Coelurosauria

     Most of the other theropods below Coelurosauria are not known from very complete skeletal material.  The best known are Megalosaurus and Spinosaurus.  Megalosaurus is known from the Lower and Middle Jurassic of Great Britain and France.  It is known from skull fragments, lower jaws, teeth, and various post cranial bones.  Of course, Megalosaurus was the first dinosaur to be described in the scientific literature and to receive a scientific name (William Buckland, 1824).

     Spinosaurus is known from Lower Cretaceous deposits of Egypt.  It was a very unsual theropod that is known from jaw fragments, teeth, vertebrae, and part of a hind limb.  Spinosaurus had neural spines on its back vertebrae that had a maximum length of 1.8 meters.  These neural spines would have resulted in a living animal with a "sail-back" (like the primitive pelycosaur Dimetrodon). Also, the teeth of Spinosaurus lacked serrations.
 

COELUROSAURIA

     The coelurosaurs are the most birdlike of the Tetanurae.  The ancestry of birds is within this group of theropods (Lucas, 1997; Fastovsky and Weishampel, 1996).  Key derived characters include the following:
 


     Compsognathus (Figure 6) is the basal coelurosaur and shares a common ancestor with the remaining members of this clade.  Compsoganthus is known from Late Jurassic deposits of Europe and is found in the Solnhofen Limestone (see Figure 7) of Germany with remains of the first bird, Archaeopteryx.  In fact, if the specimens of Archaeopteryx had not shown feather impressions, they would have been identified as small dinosaurs similar to Compsognathus.


     Rather than getting bogged down into the quagmire of the phylogeny of the coelurosaurian theropods, we will take a look at each of the major families of the coelurosaurian theropods.
 

TYRANNOSAURIDAE

      Of course, when most of us think of theropod dinosaurs, Tyrannosaurus rex (Figure 8) (king of the tyrannt lizards) comes to mind.  However, there are several genera of dinosaurs very similar to T. rex and are included in the family Tyrannosauridae.  The best known genera Tyrannosaurus, Albertosaurus, Daspletosaurus, and Nannotyranus from North America; and Tarbosaurus from Asia.  The tyrannosaurids are known only from Upper Cretaceous rocks of western North America and Asia.  A number of characters unit the tyrannosaurids into a monophyletic group, including the following:


      Tyrannosaurus rex and related theropods were ultimate killing machines.  Their eyes are moved more forward than more primitive theropods, such that they probably had steroscopic vision (Lucas, 1997; Fastovsky and Weishampel, 1996).  They had bone crushing banana-shaped, coarsely serrated teeth (with pockets at the base of the serrations that would have acted as havens for colonies of bacteria) (Fastovsky and Weishampel, 1996).  Some propose that T. rex may have used a shark-like attack on its prey, that is, a running surprise attack and bite, then wait until the animal dies.  If the bite didn't kill the animal, the bacterial infection from the teeth may have done the job.  However, John Horner, of the Museum of the Rockies at Montana State University, has long held to the idea that T. rex was a scavenger, feeding on dead carcasses.  Most theropod paleontologists do not accept this view and believe that tyrannosaurids were definetly predators, but certainly would not have turned down a free dead and rotting carcass (as is the case with most predators today).
 

ORNITHOMIMOSAURIDAE

     The ornithomimosaurids are the "Bird mimic lizards'.  They are known only from Upper Cretaceous rocks of Asia and western North America.  They strongly resemble emus and ostriches.  However, they are not in the direct line towards birds, so their resemblence to birds is an example of convergent evolution.  The following derived characters relate this group of theropods:


     Struthiomimus (ostrich + mimic) (see Figure 5.12, Lucas, 2004) and Dromiceiomimus (Dromiceius is the genus name for the emu) of Upper Cretaceous rocks of western Canada, and Gallimimus (chicken + mimic) (of Jurassic Park fame) of Upper Cretaceous rocks of Mongolia, are prime examples of ornithomimosaurid theropods.
 

DROMAEOSAURIDAE

     This group includes theropods that are very closely related to the birds.  They are only known from Upper Cretaceous rocks of western North America and Asia.  Well known members of this group are Velociraptor from Mongolia, Deinonychus (terrible + claw) (see Figure 5.13, Lucas, 2004) from Montana and Wyoming, and the recently discovered large dromaeosaurid, Utahraptor (GO TO THIS SITE  Jonathan's Utahraptor Page ) from Utah.  Dromaeosaurids have the following characteristics:
 


     Dromaeosaurids were probably very active predators and perhaps hunted in packs or in pairs.  When John Ostrom (Yale University) found Deinonychus in Montana (1967), he concluded right away that this small theropod must have been a very active hunter and was probably endothermic.
 

OTHER COELUROSAURS

     Other coelurosaurs are more rare and less completely known than the ones we have already covered.  Of the remaining, there are three or more distinct types.

     The best known are the Oviraptorsauridae, known from the Upper Cretaceous of North America and Asia.  They are close relatives of the ornithomimosaurids.  The oviraptorsaurids have the following unique characteristics:
 


     Oviraptor (the name means egg stealer) (see Figure 5.14, Lucas, 2004)  is a characteristic genus of this group and is known from Upper Cretaceous rocks of Mongolia.  It was long thought to be the dinosaur that ate the eggs of Protoceratops in Mongolia (thus the name) (of course, the eggs found in Mongolia are no longer thought to be the eggs of Protoceratops).  Oviraptor had claws on the hand and slim hind limbs.  The skull and jaws were designed to crush tough food (possibly such as eggs and clams).


      Another group of rare coelurosaurs is the Troodontidae.  The best known genus of the troodontids is Saurornithoides (see Figure 5.15, Lucas, 2004)and Troodon.  Saurornithoides is known from Upper Cretaceous rocks of Asia and Troodon from Upper Cretaceous rocks of western North America.

      The troodontids had long skulls with narrow snout.  They had large eyes and possible steroscopic vision.  They had a large brain case for body size, in fact, troodontids had the largest brain relative to body size of any dinosaur.  The teeth were small, but numerous and were serrated carnivore-type teeth.  The ankle bones were fused to each other and to the tibia, for fast stable running.  There were small sickle claws on the second digit of the hind foot (similar to the dromaeosaurids, except smaller).  The troodontids were probably small, fast running predators that may have hunted in packs like the dromaeosaurids.

Figure 12. Troodon (From: Troodon: A Dinosaur's Life at http://www.dinodon.com/dinosaurs/troodon.html).
 

THEROPOD BEHAVIOR

     Theropods were obligate bipeds that ate meat.  They ran with the body extended forward, nearly horizontal and pivoted at the pelvis, with the tail held out nearly horizontal as a counterbalance.  In the larger theropods, the neck vertebrae form a swam-like curve.  Their sharp, serrated teeth (for most theropods) and skull structure identifiy them as meat eaters.

     Some have suggested that the larger theropods, like T. rex and kin, were scavengers.  They originally said this because specimens were found that did not have worn teeth, however, many now have been found that do have worn teeth (in fact, many are well worn and broke).  But this could be true for even scavengers.  Scavengers also crunch bones and thus could have broken and well worn teeth.  Would there have been enough rotting carcasses around for large theropods to eat.  As stated earlier, the large theropods, including T. rex and kin, probably were keen predators, but would also scavenge when the opportunity presented itself.

     How fast could theropods run?  Estimates of 70 km/hr (44 mi/hr) have been suggested for T. rex by some (Bob Bakker).  Certainly, some of the smaller dinosaurs must have been quick and fast also.

     Where trackways are present, speeds can be estimated.  However, we are not always sure what theropod made the tracks.  Estimates of about 40 mi/hr have been made for what are thought to be trackways of Acrocanthosaurus from the Palaxy River tracksite in Texas.

     How did theropods hunt?  There is a lot of speculation here.  However, many suggest that Allosaurus, as well as many of the smaller theropods (Velicoraptor, Deinonychus, etc.), may have hunted in packs, like wild dogs.  T. rex perhaps preferred the ambush style ("land shark" mode), attacking from behind and running down its prey, then chomping down with a single killing bite behind the neck with huge fang-like teeth - then waiting for the animal to die from loss of blood or from bacterial infection  (a skirming prey might break out many of the teeth of the attacker, so maybe it was better to wait).

     At any rate, the theropods were a very diverse group that were successful predators during the entire reign of the dinosaurs.  They were not habitat specific, but went where the prey was abundant.  It certainly would be a freightening experience for a human to visit (in a time machine, of course) the Late Cretaceous landscape of North America, with the likes of T. rex and Deinonychus on the rampage.
 
 
 

Last updated: 3/9/04