The tracks were first recognized in 1836 by U.S. paleontologist, the reverend Edward Hitchcock, a professor of natural theology and geology at Amherst College. He also described the Connecticut River Valley dinosaur footprints. These tracks are important because they show evidence of the evolution and eventual dominance of the dinosaurs.
What is the Ischigualasto formation and why is it important?
The Triassic rocks of the Ischigualasto formation of Argentina are thought to be some of the richest fossil deposits, and include the earliest known, well-preserved fossils of such dinosaurs as Herrerasaurus ischigualastensis, Eoraptor lunensis, and Pisanosaurus mertii. The drab, gray rocks of this formation were deposited in a humid environment. The area where this formation is found—the Ischigualasto Valley, or the “Valley of the Moon”—is located to the east of a subduction zone. Consequently, it also received deposits of volcanic ash during the Triassic. Dating of this ash has enabled scientists to determine an age of approximately 228 million years for this formation, placing it in the Middle to Late Triassic periods.
Vertebrate land fossils are abundant in the formation, including skeletons of advanced forms of synapsids, and odd beaked reptiles called rhynchosaurs. There are also large carnivores called rauisuchians, and smaller archosaurs, but the most prevalent fossils in the formation are of dinosaurs. In fact, based on these dinosaur fossils and ones found in the Santa Maria formation of Brazil, some scientists suggest that perhaps dinosaurs arose in South America.
TRIASSIC DINOSAURS
Did any Triassic period dinosaurs fly?
No. There were flying and gliding reptiles throughout the entire Mesozoic era—but never any flying dinosaurs.
Did any Triassic period dinosaurs live in the oceans?
No. There were a variety of marine reptiles in the water, but at no time did marine dinosaurs exist. The entire classification of dinosaurs is limited to land-dwelling reptiles with specific characteristics.
Were there many dinosaurs during the Triassic period?
Based on the current findings in fossil records, there were not many dinosaurs living during the Triassic period. Dinosaurs began to evolve from the reptiles toward the end of the Triassic period, but great numbers of the creatures did not flourish until the Jurassic period.
What were the first true dinosaurs like?
Based on current fossil finds, the first true dinosaurs emerged in the Late Triassic period, between about 230 to 225 million years ago. They were small, agile, carnivorous reptiles whose unique characteristics, such as two-legged motion, enabled them to quickly dominate the available ecological niches.
Apparently, by the time dinosaurs evolved, they had already split, even at this early stage, into two major groups: the ornithischians and the saurischians. These two main groups of dinosaurs are based historically on their hipbone structure.
Why did the dinosaurs begin to thrive in the Late Triassic period?
Scientists theorize that there were a number of reasons for the emergence of dinosaurs in the Late Triassic period. One reason was that dinosaurs evolved to become biologically superior. For example, they developed an erect posture, with bipedal (two-footed) motion. This gave them a longer stride and quickness, enabling them to catch and devour other semi-erect reptiles. Another adaptation might have been warm-bloodedness—although this idea is still controversial. If it is true that the reptiles developed a form of warm-bloodedness, it would have allowed them to be more active than their cold-blooded relatives.
Still other scientists suggest that these adaptations were not the reason for the dominance of dinosaurs. Instead, they believe a major extinction of certain therapsids (reptilian ancestors of mammals), rhynchosaurs (lizard-like reptiles), and early archosaurs (reptiles comprising dinosaurs, pterosaurs, and crocodilians) in the middle of the Triassic period opened up ecological niches that the dinosaurs then filled.
What were some of the early dinosaurs?
There were not many early dinosaurs in the Triassic period. Some of the following dinosaurs are represented by only a few fossil discoveries. This list will definitely change in the future as more dinosaur fossils are found and dated.
Why is Herrerasaurus ischigualastensis thought to be one of the earliest dinosaurs known?
Based on the characteristics of its skeleton—a blend of archosaur and dinosaur features—scientists believe the Herrerasaurus ischigualastensis was an early dinosaur. Initially, scientists new this 10-to 18-foot (3-to 6-meter) carnivorous reptile had a skull and foot similar to other archosaurs such as the Euparkeria. Based only on this evidence, it was impossible to determine if the Herrerasaurus was truly a dinosaur. But using the modern cladistic test, the Herrerasaurus skeleton shows all but a few of the characteristics that define dinosaurs. In particular, Herrerasaurus lacks bone in the center of the hip socket, a key characteristic that defines this reptile as a true dinosaur—and one of the earliest known.
Is the Eoraptor lunensis also one of the most primitive dinosaurs?
Yes, as of this writing the Eoraptor lunensis (“dawn hunter”) fossils discovered in the Ischigualasto formation in Argentina, South America, are thought to be those of one of the most primitive dinosaurs known (the other is the Herrerasaurus ischigualastensis described above). This is because the Eoraptor fossils are a mix of primitive and specialized characteristics—a blend of features expected for the “first” dinosaur. The Eoraptor had a three-fingered hand, which connected them to the theropod dinosaurs. But Eoraptor fossils lack the specialized features that would place this reptile in one specific major group of dinosaurs.
Which Triassic period dinosaurs were herbivores?
There were numerous herbivore dinosaurs that evolved at the end of the Triassic period. Fossils of the Thecodontosaurus (“socket-tooth lizard”), a small herbivore, was one of the first Triassic dinosaurs ever found; it was reported in England in 1836. The Plateosaurus (“flat lizard”) was also a plant-eating dinosaur that lived during the Triassic period; the first fossils were found in Germany in 1834, but it took three more years before they were described. The creatures were considered one of the largest known dinosaurs of their time, and had peg-like teeth and huge thumb claws that were perhaps used to gather plants from taller trees.
What is the earliest-known herbivorous dinosaur?
The earliest-known, plant-eating dinosaur was the Pisanosaurus mertii, found in the Ischigualasto formation of Argentina, South America. The dinosaur is dated at approximately 230 million years old (Late Triassic). And like its relatives Herrerasaurus ischigualastensis and Eoraptor lunensis, the Pisanosaurus was relatively small (around 3 feet [1 meter] in length), lightweight, and bipedal.
