It looks like the creature is ready to leap out of the egg. When an egg dies, the living fluid leaks out and sometimes forms things on the back of the egg. This one has what looks like a shell, but I have learned it is petrified cartilage.It also has a sandy, rough feeling. The item "Small flat Dinosaur egg, might be a T-Rex found by Cha in Glen Rose, Texas" is in sale since Wednesday, April 10, 2019. This item is in the category "Collectibles\Rocks, Fossils & Minerals\Fossils\Vertebrates\Amphibian, Reptile & Dinosaur".
The seller is "rnaci_2015" and is located in Sisters, Oregon. This item can be shipped to United States, Canada, United Kingdom, Denmark, Romania, Slovakia, Bulgaria, Czech republic, Finland, Hungary, Latvia, Lithuania, Malta, Estonia, Australia, Greece, Portugal, Cyprus, Slovenia, Japan, Sweden, South Korea, Indonesia, Belgium, France, Hong Kong, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Bahamas, Mexico, New Zealand, Singapore, Norway, Saudi arabia, United arab emirates, Bahrain, Croatia, Malaysia, Chile, Colombia, Costa rica, Panama, Trinidad and tobago, Guatemala, Honduras, Jamaica, Gibraltar, Cambodia, Luxembourg, Paraguay, Uruguay, South africa.