You can blame the Sinclair oil company for this widespread misconception. Sinclair was one of the oldest oil companies in the United States and had a nation-wide chain of gas stations, until 1969, when it merged with Atlantic Richfield and became the ARCO brand. In 1976 ARCO spun off some 2,200 gas stations to a new Sinclair oil company. This new Sinclair operates in the mid-west and mountain time zone, but does not operate on the east coast or west coast.
The geologists at Sinclair oil company were well aware of the fact that the sediments that they drilled for oil were laid down 270 million years ago, long before dinosaurs existed. However, starting in 1930, they began to use twelve different dinosaurs to advertise the great age of their product. They quickly found out that the brontosaurus was the most popular of the dinosaurs they used, and they made that species the trademark for the company. During the 1950s and 1960s, Sinclair used this dinosaur trademark in print ads placed in national magazines, like the National Geographic and Saturday Evening Post.
Source: https://www.sinclairoil.com/history/symbol_01.html