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THECODONTS AND THE ANCESTORS OF DINOSAURS
Ankles Evolving toward the Dinosaur Condition
In the basal reptilian condition (possessed by many archosaurs as seen in the ankle of a modern crocodile) was that the astragalus and calcaneum could move relative to the lower leg bones above them and also the other tarsal bones beneath them.
|In primitive amphibians, there were a number of small tarsal (foot) bones (gray in the models below) that fused to form two larger bones, the astragalus (light blue) and the calcaneum (pink). In the following photos, the bones of the lower leg are painted gray and blue; the metatarsals (which make up most of the length of the human foot) are green and the phalanges (toe bones) are yellow. In the dinosaur ancestors, these two tarsal bones began to fuse with the bones of the lower leg so that the only movement was between these bones and the tarsal bones beneath them. This fusion was not yet complete in the early dinosaurs (such as Herrerasaurus). Also note that the phalanges (yellow) become elongated (Sereno, 1991).|
|Note that the ankle in the stork below is much higher on the leg (in the region of the mammalian knee).|
Evolving toward the Dinosaurian Pelvis
The first amphibians had a flat, uncurved bottom surface of the pelvis made up of the pubis and ischium known as the puboischiadic plate. In the archosaurs, each of these two bones began to curve downward, approaching the dinosaurian condition. In the following images, the ilium is green, the ischium is yellow, and the pubis is blue.
|The long pubic and ischiadic processes in the dinosaur pelvis would serve as attachments for the muscles which would allow bipedal locomotion.|
The archosaurs known as thecodonts gave rise to the pterosaurs, crocodiles, and dinosaurs. Of the three groups, pterosaurs and dinosaurs are more closely related to each other (and referred to as Ornithodira) than either is to crocodiles (Crurotarsi). The primary anatomical changes which would develop in the dinosaur lineage were those which affected locomotion. The posture would progress from semi-sprawling, to walking erect, to bipedal locomotion. The flat puboischiadic plate (the bottom border of the pelvis) would develop very long processes projecting forward and backward to attach powerful muscles of the leg. The ankle and foot would be modified so that dinosaurs would walk on elongated toe bones while their ankle permitted only flexion and extension as opposed to the twisting movements permitted at crocodile ankles.
The most primitive group of archosaurs, the Proterosuchia, first appeared in the Late Permian. The earliest species measured 1-2 meters in length and held their legs in a sprawling position similar to lizards. The Early Triassic species Erythrosuchus was larger, measuring 5 meters with a 1 meter long skull and weighing an estimated 450 kg (Czerkas, 1990). Prolacerta was a thecodont which was primitive with regards to its leg, hip, and foot anatomy. Its posture was semi-sprawling and it possessed a flat puboischiadic plate (Carroll, 1988).
The Early to Mid-Triassic group known as the Pseudosuchia was ancestral to dinosaurs, crocodiles, pterosaurs, ornithosuchids and rauisuchids. Euparkeria was a Pseudosuchid known from South Africa and China. Euparkeria had a large head with sharp teeth and did not use its arms to capture prey (Czerkas, 1990). It possessed small dermal ossicles along its vertebral column and had lost its pineal foramen. Its rear limbs were 1 ˝ times the size of the forelimbs but the femur and tarsus lacked specializations for bipedal locomotion. The type of ankle was intermediate between that of the primitive archosaurs and the dinosaurs. It had a mesotarsal ankle joint as did dinosaurs while most thecodonts have a crurotarsal joint (as found in modern crocodiles). The foot was more symmetrical with a long 3rd digit (Carroll, 1988; Sereno, 1991; Sereno, 1991a)
Euparkeria would have been capable of bipedal locomotion (as indicated by the shortening of its forelimbs) although it probably was a habitual quadruped (Dingus, 1998).
Longisquama was an enigmatic reptile which seems to have flown, due to feathers/feather-like structures on its back.
Lagerpeton, Lagosuchus, and Lewisuchus are not true dinosaurs, but are classified as dinosauromorpha (given in order of increasing relationship to dinosaurs). Silesaurus and Eucoelophsis are the closest thecodonts relative of dinosaurs with Marasuchus , Lagerpeton and Dromomeron forming its sister group (Irmis, 2007).
The lagosuchids are known from the Middle Triassic. Lagosuchus was similar to dinosaurs in a number of features. Its foot was almost of the dinosaur type although the astragalus and calcaneus bones possessed 2 hinges (proximal and distal) while in dinosaurs there is no proximal hinge since these bones are fused to the bones of the lower leg (tibia and fibula). The forelimbs were shorter and the neck was similar to that of dinosaurs (Carroll, 1988).The legs of Lagosuchus were rotated so that they were close to the body. Although it probably was primarily bipedal, it would have been capable of quadrupedal locomotion as well. Only three toes typically touched the ground (Dingus, 1998).
The rauisuchids and ornithosuchids walked erect. They were the top predators in the Late Triassic but became extinct at the end of the Period. They may have descended from the large Erythrosuchus of the Early Triassic. Saurosuchus measured 6 m while Postosuchus and Ornithosuchus measured 4 meters (Czerkas, 1990). The ornithosuchids were bipedal and their hip socket was slightly open (it was open in dinosaurs). They may be the direct ancestors of the dinosaurs.
THE END TRIASSIC EXTINCTION
There was a mass extinction event at the end of the Triassic. Many archosauromorphs became extinct as did some predatory archosaurs. Synapsids were the dominant tetrapods during the Triassic and achieved considerable diversity but they virtually disappeared after this extinction except for the ancestors of mammals. Many plants became extinct or virtually extinct such as seed ferns, many tree ferns, ginkgoes, cycadophytes, horsetails, and many conifers. These trees were replaced by conifers and cycad-like bennetitaleans.
end-Triassic extinction was the third worst in the Phanerozoic
Eon which caused the extinction of 30% marine organisms (genera), half
the tetrapods, and 95% of plant species. Changes in the carbon cycle
resulting in a 3-4 degree increase seems to have been a factor
(McElwain, 1999). At the end of
the Triassic, the supercontinent Pangaea began to fragment and this was
probably also a factor. The venting of gases from organic deposits such as petroleum as a result of volcanic activity may also have been the cause of the end-Triassic extinction. As Pangaea broke up, significant volcanic activity occurred in the Central Atlantic Magnetic Province between North America, South America, and Africa (Svensen, 2009). In the Triassic Period, North America began to separate from Pangaea. A watery rift separated northern Greenland from northern Norway and much of the East Coast was covered with a shallow sea. The western edge of North America had not yet been formed and was composed of a series of islands in the modern Rocky Mountain region (Russell, 1989).
The venting of gases from organic deposits such as petroleum as a result of volcanic activity may also have been the cause of the end-Triassic extinction. As Pangaea broke up, significant volcanic activity occurred in the Central Atlantic Magnetic Province between North America, South America, and Africa (Svensen, 2009). In the Triassic Period, North America began to separate from Pangaea. A watery rift separated northern Greenland from northern Norway and much of the East Coast was covered with a shallow sea. The western edge of North America had not yet been formed and was composed of a series of islands in the modern Rocky Mountain region (Russell, 1989).
seems that a large meteorite impacted the earth at this time given an
increase in the amount of iridium in rocks, the presence of shocked quartz,
and faunal changes (Olsen, 2002). The Manicouagan Crater in Quebec measures 70 km in diameter (Russell, 1989).
The Manicouagan Crater in Quebec measures 70 km in diameter (Russell, 1989).
One of the potential causes of the end Triassic extinction was the tectonic activity of the Central Atlantic Magnetic Province (CAMP). Volcanic eruptions which occurred here 201.5 million years ago are thought to have lasted more than six thousand years and produced 10 million square kilometers of basalt. This resulted in the splitting of the great supercontinent Pangaea and the beginning of the Atlantic ocean. Great lakes formed, such as one in Connecticut, similar to those formed more recently due to tectonic activity in East Africa (Whiteside, 2010).
Why did dinosaurs thrive after this extinction? Maybe there was an intrinsic reason—perhaps their speed or bipedal posture made them to superior to the other reptiles. Maybe the reasons were extrinsic--they simply took advantage of the new plant life or the niches left vacant by the extinctions of other reptiles.
Sir Richard Owen, who coined the term “dinosaur”, thought that the dinosaurs represented one monophyletic clade. From the 1880s to the 1960s it was thought that dinosaurs were diphyletic; experts thought that the two major groups of dinosaurs, the Saurischians and Ornithischians, had separate origins from thecodont ancestors. In the 1960s and 1970s it became accepted that all dinosaurs share a number of derived features that unite them in a monophyletic clade. Our understanding of dinosaurs continues to develop as more species are discovered. The number of known dinosaur species has actually doubled in the past 30 years (Sereno, 1999).
The dinosaur features which unite all dinosaurs as a clade include an open hip socket, vomer bones which stretch from the snout to the antorbital fenestrae, at least 3 sacral vertebrae (in all except perhaps the most primitive), a rearward-facing glenoid fossa of the shoulder, a humerus with a long deltopectoral crest, no more than 3 phalanges (finger bones) on the 4th finger, an asymmetric hand with 2 small outer digits, a head of the femur which faces medially, a knee aligned below the hip, a tibia far longer than fibula, a cnemial crest on the tibia, a 4th trochanter on femur, an astragalus which is the main ankle bone, an ascending process of the astragalus which rises into the lower leg, and a curved third metatarsal (Fastovsky, 1996).
The ankle of dinosaurs is important for two reasons: it preserves well because of its compactness and the form of locomotion that distinguishes dinosaurs (who walked upright) from the archosaurs (that sprawled) depended on ankle modifications. The dinosaur pelvis and femur also differs from that of thecodonts to allow upright posture.
The dinosaur ankle is classified as an AM (advanced mesotarsal) ankle). The astragalus was larger than the calcaneus and they both attached rigidly to each other and the tibia. The astragalus and calcaneus formed 1 hinge joint with the rest of the foot--the foot swings back and forth under these two bones. Thecodont ankles (like that of crocodiles) allows twisting.
Many dinosaurs had fused vertebrae. This was once thought to be a sign of osteoporosis but it seems that fused vertebrae may have been adaptations. It may be that fused hip vertebrae helped females sauropods bear the weight of males during copulation (broken neural spines in hadrosaurs may be evidence of the difficulty of this). Fused neck vertebrae in ceratopsians probably helped to support the massive head.
There are two great groups of dinosaurs: the Saurischians and the Ornithischians. The saurischians can be divided into two groups: the theropods (which include the largest terrestrial carnivores that ever lived) and the sauropods (which include the largest herbivores that have ever lived). The first dinosaurs were theropods. The ornithischians include a number of diverse herbivores including the horned dinosaurs (ceratopsians), armored dinosaurs (thyrephorans), duck-billed dinosaurs (hadrosaurs), and others.
While dinosaur arms would be adapted for various lifestyles, the arms of early members of the dinosaur clades are fairly similar. In the following drawing, the first two arms are from primitive theropods, the third from a prosauropod, and the fourth from a primitive ornithischian.
|The first dinosaurs had two vertebrae contributing to the sacrum which attached to the pelvis. These first two sacral vertebrae are indicated by the red and purple bones of Herrerasaurus. The number of sacral vertebrae increased in later dinosaurs.|