To determine whether the 2-by-3-foot domed rock was a fossil, the three scientists looked for sutures, which are lines where the bones knitted together. "If these are sutures, they would have to be fractured because there are way too many," Hanshumaker said....and some of which is awful:
Concretion is organic matter that decomposed differently from surrounding material, making the dirt harder. DiTorrice said, "This type of fossil is not normal for this rock formation, so it has significance just by its size and shape."From the Wiki entry on concretions- composition (Note that it is not clear in the following passage that they are talking about the cementing mineral; most of the bulk of a concretion is composed of the sediment in which it formed):
They are commonly composed of a carbonate mineral such as calcite; an amorphous or microcrystalline form of silica such as chert, flint, or jasper; or an iron oxide or hydroxide such as goethite and hematite. They can also be composed of other minerals that include dolomite, ankerite, siderite, pyrite, marcasite, barite and gypsum.From the Wiki entry on concretions- appearance:
Although concretions often consist of a single dominant mineral, other minerals can be present depending on the environmental conditions which created them. For example, carbonate concretions, which form in response to the reduction of sulfates by bacteria, often contain minor percentages of pyrite. Other concretions, which formed as a result of microbial sulfate reduction, consist of a mixture of calcite, barite, and pyrite.
Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible to huge bodies three meters in diameter and weighing several thousand pounds. The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost 3 m (10 ft) in diameter. Spheroidal concretions, as large as 9 m (30 ft) in diameter, have been found eroding out of the Qasr El Sagha Formation within the Faiyum depression of Egypt. Concretions are usually similar in color to the rock in which they are found. Concretions occur in a wide variety of shapes, including spheres, disks, tubes, and grape-like or soap bubble-like aggregates.I'm willing to bet at this point that it's a concretion, not a "fossil" per se. It may or may not contain a fossil, and it's of interest, whatever it is, but I think it's unlikely to be a turtle. Nor should the object as a whole be referred to as "a fossil." Previous posts here and here.