The Tarrant Formation is part of the marine Eagle Ford Group of Texas. This rock bed dates back to the boundary between the Cenomanian and Turonian stages. This is the start of the Late Cretaceous, around 98 to 90 million years ago, when the Western Interior Seaway still flooded the face of North America.
At this time, there was an ocean anoxic event, caused by underwater volcanism. There was a high amount of plate movement due to continental drift, and this had a few disastrous results for animals and plants in general.
There was a small extinction, and several families of animals died out, both on land and in the oceans. It is thought that even the Eagle Ford rocks showcase this event through a lack of fossilized seafloor organisms and a shortage of organic matter during a certain period of time. There are however, body fossils of larger animals within Eagle Ford. The extinction event is limited to a small area of around 94-93 million years ago and we still have the marine diversity of most inland seas. Typical Eagle Ford fare include marine reptiles like plesiosaurs and mosasaurs, fish, sharks, turtles, pterosaurs and other marine animals.