Since the publication of the paper by Tschopp et al. (2015), a number of individuals (e.g. Prothero 2019) have had a number of misgivings about the taxonomic scheme for Morrison diplodocids in Tschopp et al. (2015) by arguing that ecosystems in western North America was not suited to support a great many taxa of diplodocids because of Morrison diplodocids being oversplit. However, this opinion overlooks the fact that the Morrison Formation itself spans 10 million years and that the presence of Haplocanthosaurus in the type locality as Supersaurus (Boisvert et al. 2024) demonstrates the co-existence of diplodocids with basal diplodocoids in the lower part of the Brushy Basin Member of the Morrison Formation. Recently, van der Linden et al. (2024) have described a new taxon Morrison diplodocine diplodocid, Ardetosaurus viator, from the Howe-Stephens Quarry in northern Wyoming, increasing the diversity of diplodocines in the Morrison Formation to eight. Therefore, I will take the liberty of the bearing of Ardetosaurus upon the question of the true number of diplodocine diplodocids from the Morrison Formation.
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| Skeletal reconstruction of Ardetosaurus viator (after van der Linden et al. 2024) |
In their paper, van der Linden et al. neglect to provide a detailed cladistic analysis because they note that the description of Ardetosaurus viator is part of a broader collaborate project regarding the systematics of Diplodocoidea. It is understandable that the authors refrain from testing the cladistic position of the new taxon because their planned collaborative phylogeny of Diplodocoidea incorporates new diplodocoid specimens, but exhaustive comparison by the authors of Ardetosaurus with other diplodocines reveals hints about possible cladistic position of Ardetosaurus relative to other members of Diplodocinae. For instance, as noted by van der Linden et al., Ardetosaurus is similar to Barosaurus lentus in having elongated cervical vertebrae, so there is a possibility that Ardetosaurus itself could be more closely related to Barosaurus and Supersaurus than to other Morrison diplodocines. If so, then those three taxa (and possibly Dinheirosaurus) most likely form a distinct clade of diplodocines with elongated necks, and diplodocines from the Morrison Formation might have exhibited a degree of cladistic diversity with respect to evolving a variety of neck lengths in response to the emergence of different types of ferns and cycads in Morrison times. After all, the presence of four different taxa of dicraeosaurids in the Morrison Formation (see Whitlock and Wilson Mantilla 2020) may suggest a similar diversity pattern of diplodocine diplodocids in the Brushy Basin Member of the Morrison Formation as a result of dynamism of the Morrison ecosystems by the late Kimmeridgian.
Given the assignment of Ardetosaurus to Diplodocinae by van der Linden et al. (2024), it is important to note that the interpretation by Maidment and Muxworthy (2019) of the Morrison Formation as being more dynamic, more spatially varied, and temporally changing over the time it was deposited begs the question of the possible upper limit of diplodocine diversity in the Morrison. While the diversity of Morrison diplodocines exceeds the number of valid apatosaurine species, van der Linden et al. note that ontogeny and stratigraphy of the Morrison Formation may affect estimates of diversity of diplodocines throughout the Morrison Formation. Indeed, the type locality of Ardetosaurus viator is low in the upper part of the Salt Wash Member of the Morrison Formation, whereas Supersaurus vivianae has only been found in the lower part of the Brushy Basin Member, and Galeamopus remains occur in both the Salt Wash and lower Brushy Basin Members. The holotype of Ardetosaurus viator is assessed by van der Linden et al. (2024) as mature, in contrast to the holotype of Kaatedocus siberi being a juvenile, so the low stratigraphic position of Howe-Stephens Quarry compared to localities which have yielded known remains of Barosaurus, Diplodocus, Galeamopus, and Supersaurus may indicate that diplodocine diversity in the Salt Wash Member was low compared to a gradual increase over the timespan of the Brushy Basin Member. In other words, the evolution of Morrison ecosystems noted by Maidment and Muxworthy (2019) over the timespan of the Morrison Formation indicates that the relative growth in the diversity of Morrison diplodocines increased over time in response to changes in ecosystems in western North America over the span of time in which the Morrison Formation was deposited. For instance, a growth in Morrison diplodocine diversity during the late Kimmeridgian-Tithonian might have filled ecological niches left by the disappearance of Haplocanthosaurus from the fossil record by the late Kimmeridgian.
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