Diplodocoid sauropods were among the most successful and largest herbivores of the Mesozoic. Here, the systematics and paleoecology of these animals are examined in five parts: a reassessment of the relationships of Australodocus, a phylogenetic analysis of the clade Diplodocoidea, an investigation of feeding behavior in diplodocoid sauropods, an examination of ontogenetic variation in the skull of Diplodocus, and a study of the evolution of tooth shape and replacement rate in Sauropodomorpha. Australodocus, previously considered to be a diplodocid, is re-evaluated. Characters previously considered synapomorphies with Diplodocidae are revealed to be either misinterpreted or more widely distributed within Neosauropoda. Six unambiguous synapomorphies, including pneumatization of the centrum and neural arch, suggest titanosauriform affinities for the taxon instead. A phylogenetic analysis confirms the assessment from the previous chapter, and also identifies several key relationships for the understanding of diplodocoid evolution. The first of these is the resolution of the plesiomorphic taxon Suuwassea as a basal dicraeosaurid. Also presented is a fully-resolved Rebbachisauridae, with two sub-clades recovered (Limaysaurinae, Nigersaurinae). This hypothesis of relatedness was used as a backbone to examine the evolution of feeding strategies in the clade. Some taxa (Apatosaurus, Diplodocus, Nigersaurus) were discovered to have squarer snouts and microwear features more consistent with ground-height nonselective browsing (e.g. subparallel fine scratches and small, round pits) than others (Dicraeosaurus, Suuwassea, Tornieria). The microwear features present on the round-snouted taxa—coarser scratches with no preferred directional orientation and large gouges— suggest selective mid-height browsing behavior instead. Ontogenetic shape variation in at least one of the ground-height nonselective browsers (Diplodocus) suggests the possibility of ontogenetic variation in feeding strategy as well, where subadults may have fed selectively. Although Diplodocoidea is diagnosed in part by narrow-crowned teeth, the inference of at least two distinct feeding strategies for this clade suggests that it is unlikely that the development of narrow crowns in this clade was an adaptation for a particular feeding mode. Histological and proportional data suggest instead that the development of narrow crowns in this clade (and elsewhere) was an adaptation for rapid tooth replacement
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