Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
27. Dinosaurs: Saurischians 1: Therapods
Dinosaurs ("terrible lizards") were first named by the British paleontologist Richard Owen 1843, based on a few Jurassic and Cretaceous taxa such as Iguanadon that had by then been discovered in England (see the section entitled The Jurassic in England for more information on these early finds).
Saurischians vs. Ornithischians (lizard-hipped vs. bird-hipped)
.
Saurischia ("lizard-hipped") is one of two basic divisions of dinosaurs, the other being Ornithischia ("bird-hipped"). This dichotomy, first proposed by Seeley (1888) has been generally retained in present-day classification. Based on Romer's analysis (1966), Saurischians include two major groups of dinosaurs, Theropods and Sauropods, whereas Ornithischians include four dinosaur groups, Stegosauria, Ankylosauria, Ceratopsia, and Ornithopoda.
Saurischian dinosaurs are distinguished from ornithischian dinosaurs by their three-pronged pelvic structure, with the pubis pointed forward (fig.1). This is the ancestral hip anatomy also found in modern lizards and other reptiles. An example of the saurischian hip or pelvis is provided by Tyrannosaurus rex (fig.2).
Fig.1: (upper right): Saurischian or "lizard-hipped" pelvis structure. The acetabulum is the socket-like joint of the femur or upper leg bone.
Fig.2: (lower right): Pelvis of Tyrannosaurus Rex, a saurischian dinosaur.
The ornithischian or bird-like pelvis, by contrast, which evolved from the original reptile pelvis, is arranged with the pubis rotated backward, parallel with the ischium, often also with a forward-pointing process, g
iving a four-pronged structure (fig.3). Although Seeley (1888) did not propose a specific
relationship between ornithischians and birds, as it happened, the "bird-hipped" arrangement was to evolve
several times independently, first in the ornithischians (to be discussed in a later section),
then in the lineage of saurischians which led to birds, and
lastly in the therizinosaurians. Fig.3: Ornithischian or "bird-hipped" pelvis structur
Theropoda
The name Theropoda ("beast feet") was initially used by Marsh (1888) as a suborder to include the family Allosauridae, but later expanded in scope as an order to include a wide array of carnivorous dinosaur families. Romer (1956, 1966) divided the Order Saurischia into two suborders, Theropoda and Sauropoda ("lizard feet"), of which the Therapods will be discussed first.
Ongoing fossil discoveries have led to the creation of a number of therapod infraorders, with several discussed below including Carnosauria, Tyrannosauridae, Coelurosauria, Ceratosauria, and Maniraptora.
Carnosauria
Carnosauria is an infraorder of Therapods, including the Late Jurassic carnivore Allosaurus and close relatives, like Carcharodontosaurus.
Allosaurus
Allosaurus
("different lizard") was a large saurischian theropod dinosaur with a
length of up to 12 meters or 39 feet (fig.4). It was bipedal, with
long, stout legs and much smaller arms with three-digit hands. It had a
a massively built body and a large skull (fig.5) with long, sharp
teeth and a deep, flexible jaw used for chewing and swallowing large
amounts of meat (Romer 1966).Fig.4: Skeleton of Allosaurus jimmadseni (Dinosaur National Monument).
Fossil bones of Allosaurus were first described by F.V. Hayden in 1869 from the area of Granby, Colorado, probably from Morrison Formation rocks. The type species Allosaurus fragilis was described by Marsh (1877), who named it "different lizard" because its vertebrae were different from those of other known dinosaurs, and "fragilis" for light-weight features in the vertebrae. A. fragilis is known from the remains of at least sixty individuals from the Morrison Formation, a vast region of shale, mudstone, and sandstone in the American west dating from the Kimmeridgian–Tithonian stages of the Upper Jurassic (146-138 mya). (Ihe Morrison Formation will be described in more detail in the section on sauropod dinosaurs.) A. fragilis are only found in the higher elevation, Brushy Basi
n Member.A second species, A. jimmadseni, found since the 1990s, has been named for two nearly complete skeletons excavated near Shell, Wyoming and now at Dinosaur National Monument in northeastern Utah. A. jimmadseni fossils are confined to the Salt Wash Member of the Morrison Formation.
Fig.5: Skull of Allosaurus fragilis (after Romer 1966, fig.235E).
Allosaurus is one of the best known of the therapod dinosaurs, given a large sample found at the Cleveland-Lloyd Dinosaur Quarry in Emery County, Utah (within the Morrison Formation). Thousands of dinosaur bones were recovered there between 1960 and 1965, with a predominance of Allosaurus remains, most of Allosaurus fragilis, representing at least 46 individuals out of a minimum of 73 dinosaurs. The bones were disarticulated and well mixed, and the cause of the original deposition is not understood.
Tyrannosauridae
Tyrannosauridae is a family of Therapod dinosaurs, which includes the late Cretaceous carnivore Tyrannosaurus rex and its relatives. The group is divided into two sub-families (fig.6), Albertosaurinae, which includes Albertosaurus and Gorgosaurus (fig.7) (whose timespans are shown in blue in fig.6), and Tyrannosaurinae (shown in brown) which includes the taxa most closely related to Tyrannosaurus (figs.8,9), the last member of the group.
Fig.6: Timeline of the Tyrannosauridae in the Late Cretaceous period. .
Gorgosaurus
Gorgosaurus ("fierce lizard"), closely related to Albertosaurus and more distantly related to the larger Tyrannosaurus, lived in western North America during the Campanian stage of the Late Cretaceous (76.6- 75.1 mya).
G. libratus (fig.7), the only recognized species, is well represented the fossil record, known from dozens of specimens found in Alberta and Montana. The type specimen, the first tyrannosaurid with a complete hand, was discovered in the Dinosaur Park Formation of Alberta in 1913 by C.M. Sternberg (named by Lambe 1914). A number of other G. Libratus fossils including a complete juvenile skeleton have been found there, and along the Red Deer River in Alberta (Matthew and Brown 1923). Several tyrannosaurid skeletons which are porbably Gorgosaurs have also been found in the Two Medicine Formation and Judith River Formation of Montana .
Fig.7: Gorgosaurus libratus (Royal Tyrell Museum, Alberta, Canada) .
Gorgosaurus is very similar to the later Albertosaurus from the Maastrichtian stage, separated only by minor differences in skull bones and teeth. G. libratus was a bipedal carnivore which, as an adult, weighed over two tons and was up to nine meters long. Its skull was massive, and its jaws lined with dozens of long, sharp teeth. Like other tyrannosaurids, it had long lower limbs, and small forelimbs with only two fingers. Gorgosaurus lived in riverine floodplains along the edge of the Late Cretaceous inland sea. It preyed upon plant-eating vertebrates including ceratopsids and hadrosaurs.
Tyrannosaurus
Tyrannosaurus ("tyrant lizard") lived throughout western North America, from Arizona to Montana, on what was then an island continent known as Laramidia in the Late Cretaceous inland sea. Osborn (1905) first named Tyrannosauris rex ("king tyrant lizard") from specimens from Montana and Wyoming found during a 1902 expedition of the American Museum of Na
tural History, led by
the curator Barnum Brown. A partial skeleton found by
Brown from the Hell Creek Formation in Montana with 34 fossilized bones
provided the type specimen for T.rex. Several other T.rex
skeletons have subsequently been found in the Hell Creek Formation,
which during the Maastrichtian was subtropical, with a warm and humid
climate. Triceratops and hadrosaurs were common herbivores in that location. In 1946 paleontologists from the Soviet Union uncovered the first tyrannosauroid remains from Mongolia, named Tarbosaurus ("alarming lizard"; Maleev 1955). Tarbosaurus, represented by dozens of speciomens, lived in riverine floodplains and preyed on herbivores like the hadrosaur Saurolophus.
Fig.8: Skull of Tyrannosaurus rex (after Romer 1966. fig.235F).
Among the largest known terrestial carnivores, Tyrannosaurus (fig.9) reached lengths of 13-15 meters (40- 47) feet, and stood about 5-6 meters (16-19 feet) tall. Its powerfully built hind legs used for walking and standing contrasted with the very small forelimbs, which were too short to reach the mouth, with only two digits (as with Gorgosaurus). The massive skull (fig.8), about 1.5 meters long, was strengthened by fused bones, and lightened by hollowed-out areas. It held powerful jaws lined with large, recurved teeth, some half a foot in length (Romer 1966).
According to a recent study (Hurlburt et al. 2013), Tyrannosaurus had the relatively largest brain of all adult non-avian dinosaurs, with the exception of certain small maniraptoriforms (
Bambiraptor, Troodon and Ornithomimus). T. rex.had
very large olfactory bulbs and olfactory nerves relative to their brain
size, indicating that the sense of smell was highly developed. Research
has shown that T. rex had the most highly developed sense of smell of
21 sampled non-avian dinosaur species. Related to its hearing, T. rex
had a very long cochlea. a trait often related to the importance of
hearing in behavior.Fig.9: Skeleton of Tyrannosaurus rex (Field Museum of Natural History, Chicago).
The reign of Tyrannosaurus as dominant predator was relatively brief, lasting from 68-66 mya in the final, Maastrichtian stage of the Late Cretacous. It ended at the K/T (Cretacous/Tertiary) boundary ca. 66-65 mya (fig.6), when the herbivorous dinosaurs it presumably fed upon, such as Triceratops, became extinct (Romer 1966).
References:
Haydn 1869
Hurlburt, G. S.; Ridgely, R. C.; Witmer, L. M. 2013 Relative size of brain and cerebrum in Tyrannosaurid dinosaurs: an analysis using brain-endocast quantitative relationships in extant alligators . In Parrish, M. J.; Molnar, R. E.; Currie, P. J.; Koppelhus, E. B. (eds.). Tyrannosaurid Paleobiology. Indiana University Press. pp. 134–154
Lambe, L. M. 1914. On a new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of the skull of Stephanosaurus marginatus from the same horizon. Ottawa Naturalist. 27: 129–135.
Leidy, J. 1865 . Memoir on the extinct reptiles of the Cretaceous formations of the United States. Smithsonian Contributions to Knowledge. 14: 1–135.
Maleev, E. A. 1955. " New carnivorous dinosaurs from the Upper Cretaceous of Mongolia. (translated by F. J. Alcock). Doklady Akademii Nauk SSSR. 104 (5): 779–783.
Marsh 1877
Marsh 1888
Matthew and brown 1923
Osborn, H. F. 1905 Tyrannosaurus and other Cretaceous carnivorous dinosaurs.. Bulletin of the AMNH. 21 (14): 259–265.
Osborn, H. F. and B. Brown, B. Tyrannosaurus, Upper Cretaceous carnivorous dinosaur. Bulletin of the AMNH. 22 (16): 281–296
Romer, A.S. 1966. Vertebrate Paleontology. University of Chicago Press.
Seeley, H.G. 1888. On the classification of the fossil animals commonly named Dinosauria. Proceedings of the Royal Society of London. 43 (258–265): 165–171
Glossary