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I know that Diplodocus is marked by the feature of having a double row of or "double-beamed" chevron bones in the tail, but how diagnostic is this feature if several other sauropods have it? I mean, if there are other genera with the feature, which ones are they, so I know not to confuse them with Diplodocus?

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It's fairly common. It is believed to have evolved two times independently, basically in both groups presumed to use their tails as weapons. Both Diplodocoidea and Euhelopodidae have species with forked chevrons and evolved it independently. It is fairly diagnostic since it is not seen outside these groups.

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Independent half chevrons (two rows of individual elongated splints of bone, one right, one left) pop up in many groups but double beamed chevrons are something different: the ends of the chevrons are joined, creating a single solid structure. In some parts of the tail they are separated into right and left halves (these are not very unique), but the joined ones are taxonomically significant. These are not chevrons dwindling into nothing along the tail but changing structure in a specific structurally significant way.

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  • $\begingroup$ Thank you for your answer, but I am still not clear on something. Did you mean to say that ALL genera in Diplodocoidea and Euhelopodidae had double-beamed chevrons? I knew that at least some did, but I am trying to know which genera specifically so I can have a relatively quick and easy way to distinguish the specific genus Diplodocus from the rest of Diplodocoidea (e.g., Barosaurus and Galeamopus). $\endgroup$ – resplaine Apr 20 at 22:38
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    $\begingroup$ most Diplodocoidea do (it is lost on some of the Dicraeosaurus who re-evolve short tails) and only a few Euhelopus do it appears to be a an independent evolution. It is hard to say definitively because so many sauropods are known from only a few elements. It is not a good distinguishing feature simple because it is one of the smaller more easily lost elements. $\endgroup$ – John Apr 21 at 13:21

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