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When a prehistoric stream that ran near the park would flood, the flotsam such as dead animals and tree branches would be lumped together into a sort of log jam.

“Virtually every kind of dinosaur is represented” in the park’s fossils, Mr. Kranz said.

As the millennia passed, sediment layers shifted, compressed and buried the wide-ranging organic material.

As Mr. Jabo continued to unearth the fossil, there was a palpable feeling of anticipation as the hole got deeper. About two hours after the dig began, the roughly 4-pound fossil was pulled from the ground. It wasn’t all that observers expected, but it was impressive enough for Mr. Hacker to remark that the idea of “bigger is better” tends to be the default opinion for fossils.

“Really, it’s a complete skeleton that is better,” he said.