Dinosaurs are accepted to be dominant in the Jurassic and Cretaceous periods, ruled the earth for 135 million years. Many fossils were found that point to many habitats with them existing there. But, one conundrum makes it mysterious, that is when did the first dinosaur exist in the geological period. There is all this information but no definite answers, when did dinosaurs come to be in North America?
Based on fossils found in Argentina, a good estimate when they came to be in the Late Triassic (230 million years ago MYA). It was when Pangaea the supercontinent existed, another school of thought is that fossils North America were until 212 (million years ago MYA) and later in South America. Other ideas are a vast desert that prevented migration across the continent.
Scientists at MIT are no satisfied and have arguments about the subject. Here are some of the ideas that do not connect, based on current ideas of their actual beginning.
a. Some problems like non-dinosaur relatives were living with proto-dinosaurs. It gets more unclear when more advance dinosaurs are living at the same time 12 million years too.
b. There is a 16-million-year gap with no living vertebrate fossils found. It might have been pieces of rocks from that period which eroded from then on?
c. This gap is weird because it should have fossils in there and theories point to South America. From there, they just spread all over the globe, but it seems implausible.
To date, early saurian evolution is best seen in Argentina, with very distinct layers as the dinosaur species evolved and with fossils as evidence. Late Triassic, after the Jurassic, is where proto-dinosaurs began turning to what became "dinosaurs". After this period, the dinosaurs were evolved and became the creatures that ruled the earth. At this point, fossil records were stratified and clear, some overlapping evolutionary processes but still regular.
When the fossil records in North America, it is not clear at all. Triassic fossils are plentiful in the layers of rock, it is the Chinle Formation specifically. This is of particular interest because of these places like Arizona, New Mexico, Utah, Colorado, and the Petrified Forest National Park where most fossils are found. These areas have been dated, the first discoveries are in Mexico, which is as old as 212 million years ago.
One way to confirm the age of the formations where fossils are found, especially the Chinle Formation and taking sample sediments from it. Crushing the rocks to find zircon, which has uranium-containing minerals that form before volcanic activity. This method will measure, the zircon based on uranium, to lead isotopes and see how old the zircon is. Another reason this is done is the spot where it originated in the bedrock. Another wrinkle to add is the Blue Mesa locality of the Petrified Forest National Park which has a lot of late Triassic rocks. One of the oldest strata that are found to have dinosaur fossils.
Analyzing the zircons will map the ages of sedimentary rock, that delineate the Chinle Formation. Based on the dating method, Arizona fossil is older than those in New Mexico by 223 million years ago (MYA). Both evolved, and proto-dinosaur were extant at the same time.
A gap presented by the Chinle Formation is the problem and cannot reconcile the 16-million-year gap when dinosaurs evolved in North America. From co-habiting in a simultaneous time frame that needs more information, the solution is yet to be thought of.
Read: New Questions About the Evolution of Dinosaurs in North America
