In 1939–40, two partial skeletons were found in the Moreno Formation of California.
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A second valid species, S. angustirostris, is represented by numerous specimens from Mongolia, and was described by Anatoly Konstantinovich Rozhdestvensky.
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The 1947–49 Polish-Mongolian Paleontological Expedition recovered the large skeleton that became S. angustirostris as described by Anatoly Rozhdestvensky.
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angustirostris Rozhdestvensky, 1952