THE ORIGIN OF DINOSAURIA |
Edward L. Crisp, Ph.D., Professor of Geology
More than 150 years ago, Sir Richard Owen
coined the term Dinosauria for large, extinct reptiles that he referred
to as belonging to the Secondary Age (The Mesozoic Era). Megalosaurus
(a theropod [carnivorous] dinosaur) (see Figure 1 and check out the link),
Iguanodon
(an ornithopod dinosaur) (see Figure 2 and check out the link), and Hylaeosaurus
(an ankylosaur dinosaur) were the only dinosaurs known at the time.
The belief that dinosaurs were polyphyletic
continued into this century. In fact, it was believed that Ornithischia,
Saurischia, Pterosauria, and Crocodylia arose independently from the "Thecondontia"
(Modern dinosaur paleontologists do not recognize the Thecodontia because
of the group's polyphyletic origins). Many dinosaur paleontologists
believed that dinosaurs were polyphyletic up until the 1960s and 70s.
DINOSAURS ARE MONOPHYLETIC!
The first major challenge to dinosaurs as a polyphyletic group came in 1974 via an article in the British scientific magazine Nature, by Robert T. Bakker (then at Harvard University) and P. M. Galton (at the University of Bridgeport, U. K.). They used a number of skeletal features to unite the Dinosauria as a monophyletic group. They also included the birds in Dinosauria. This concept met with great resistance among the orthodox vertebrate paleontology clan, in some cases open hostility (Fastovsky and Weishampel, 1996).
J. F. Boneparte, an Argentinean paleontologist, also met great resistance when in 1976 he suggested that the Dinosauria was a monophyletic clade.
Beginning in 1984, a variety of cladistic analyses were performed on Archosauria by several dinosaur researchers. This resulted in the following:
The most primitive
dinosaurs are known from Argentina and include Herrerasaurus
(Figure
5), Staurikosaurus,
and Eoraptor (Figures
4). These earliest dinosaurs of Late Triassic age are about 228 million
years old.
Figure 5. Herrerasaurus skull (From: Introduction
to Herrerasaurus http://www.ucmp.berkeley.edu/diapsids/herrerasaurus.html).
At first Staurikosaurus,
Herrerasaurus, and Eoraptor (although meeting the requirements
as dinosaurs) could not be placed in either Saurischia or Ornithischia.
But with further analysis by Paul Sereno of the University of Chicago,
most dinosaur paleontologists now accept the placement of these three genera
in Saurischia as primitive theropods.
ORIGINS
Within the archosaurs,
dinosaurs appear to be most closely related to Pterosauria
(winged + lizard) (Figure 6) and Ornithosuchia
(bird
+ crocodile). Although some paleontologists think the dinosaurs are
more closely related to pterosaurs, a cladistic analysis by Paul Sereno
(University of Chicago) has convinced many that the dinosaurs are most
closely related to a small ornithosuchid archosaur called Lagosuchus(rabbit
+ crocodile) (Figure 7).
Figure 7.
Reconstruction of the skeleton of Lagosuchus, after Paul (1988)
(From Olsen, 1999 Lecture
10 - Triassic: Newark, Chinle ).
Therefore, Sereno would place Lagosuchus
(along
with the related ornithosuchians Lagerpeton and Pseudolagosuchus)
and Dinosauria in a group called Dinosauromorpha. Dinosauromorpha
share several derived characters: a sigmoidal vertebral column in the neck
region, shortening of the forelimbs, and modifications of the ankle bones
and metatarsals (Fastovsky and Weishampel, 1996). The following cladograms
(Figure 8) from Dingus and Rowe (1997) illustrates the relationship of
Lagosuchus
to dinosaurs:
Figure 8. The relationships within archosauriforms (From: Dingus, Lowell and Timothy Rowe; 1997; The Mistaken Extinction - Dinosaur Evolution and the Origin of Birds; W. H. Freeman and Co.; New York; 332 p.
DINOSAURS TAKE OVER - WAS IT LUCK OR WERE THEY SUPERIOR
From the beginning of Triassic time (about 245 million years ago), the Triassic was dominated on land by synapsids (the group that includes mammals). The synapsids of the Triassic were sleek, dog-like predators and fat, beaked, and tusked herbivores.
Medial Triassic to toward the end of the Triassic also contained an abundant fauna of herbivorous archosauromorphs and a few genera of carnivorous crocodile-like archosaurs.
As the Medial Triassic was giving way to Late Triassic (about 225 million years ago), the majority of the synapsids suffered extinction; except for one highly evolved group, the mammals. Of the archosaurs, only the dinosaurs, pterosaurs, crocodiles, and a few other archosaurs survived.
After the early Late Triassic extinction event, the dinosaurs became the most dominant and diverse group of animals on land.
What caused the wave of extinctions of synapsids and nondinosaurian archosaurs? The pattern of waxing and waning dominance (as some groups take over and other groups become extinct) is referred to as the wedge. If all other things are equal, a competitive edge may supply the wedge to allow one group to dominant over another. But were all things equal? Certainly, the fully erect posture of the dinosaurs gave them a competitive edge (an evolutionary advantage). But the carnivorous synapsids (mammals and therapsids) also had a fully erect posture.
Robert T. Bakker in the 1960s proposed that dinosaurs were warm-blooded (endothermic) and thus had a competitive edge over other archosaurs. Bakker argued that this would give the dinosaurs a competitive edge. However, the mammals were warm-blooded, as were the advanced therapsids (a group of synapsids that gave rise to the mammals).
Does the wedge have
to be produced by a competitive edge? No, says M. J. Benton (University
of Bristol). Benton says that edges and wedges can not be documented
in the Middle to Upper Triassic fossil record. Work by Benton and
P. E. Olsen (Columbia University - DINOSAURS
2000
http://rainbow.ldeo.columbia.edu/courses/v1001/dinos.2000.html) and
others, has shown that there were two mass extinctions in the last part
of the Triassic.
The first extinction event of early Late Triassic completely killed-off some archosauromorphs and nearly obliterated the synapsids (the therapsids) and several major groups of predatory archosaurs. There was also a mass extinction of plants at this time.
Dinosaurs appeared as the dominant land vertebrates after the extinction of the large numbers of therapsids and many archosaurs and archosauromorphs. So, at least according to Benton, there was no need for a competitive edge. The dinosaurs were opportunistic and took over from the void left by the extinction of therapsids and other creatures. Thus, the wedge was supplied by opportunism.
Benton suggests that there was a major climatic shift at the beginning of Late Triassic time. The climate may have been becoming hotter and more arid. Thus, changes in plants (the flora) occurred, and this affected the animals (the fauna).
Olsen has suggested asteroid impacts as
the cause of the Triassic extinctions. He has suggested that the
Manicougan Crater (70 km in diameter) of northern Quebec is the smoking
gun (however, Olsen admits at his 1999 web site for his Dinosaur course
at Columbia University, that the age of the crater appears to be 10 to
15 million years too early to have caused even the first major Triassic
extinction; but he says the extinction events still may be due to asteroid
impacts or a combination of volcanic activity and asteroid impacts - see
DINOSAURS
2000
(http://rainbow.ldeo.columbia.edu/courses/v1001/dinos.2000.html).
So, .....................we are still not sure how the dinosaurs took over, whether due to their own biology or external factors (like asteroid impacts or volcanism or something else that caused a wave of extinction of certain other archosaurs and therapsids).
Anyway, during Late Triassic time, the dinosaurs became the dominant land vertebrates.
However, after the approximately
160 million year reign of the dinosaurs, the mammals took over when the
dinosaurs "bit the dust" at the end of the Cretaceous Period (about 65
million years ago).
Last revised: 3/2/04