The classic sauropod as well as
being among the most famous is Brontosaurus, a typical representative
from the Morrison Formation of Wyoming, which is dated to the Late Jurassic (c.
156-147 million years ago). Yet, this thickly-necked, robust relative of the slimmer Diplodocus has had a long, baffling history of
classification.
Figure 1: My digital reconstruction of a solitary
Brontosaurus excelsus wandering around a semi-arid environment. An Allosaurus
fragilis, the apex predator of the Morrison Formation, lurks in the background.
The eminent 19th century American
palaeontologist Othniel Charles Marsh first described Apatosaurus ajax
in 1877 on the basis of a partial juvenile sauropod skeleton, then found
another, better intact skeleton in 1879 that he identified as Brontosaurus
excelsus. In 1903, research carried out by American paleontologist
Elmer Riggs confirmed that Apatosaurus ajax and Brontosaurus excelsus
were so much alike that they ought to be combined as a single genus, and in
accordance with the rules of zoological nomenclature, the name ‘Apatosaurus’
had priority as it was named first. As a result, Brontosaurus was regarded
as an invalid genus for over a century despite its popularity.
Then an extensive in-depth study of
diplodocid phylogeny by Tschopp, Mateus and Benson (2015) using a specimen-based
approach concluded that Brontosaurus was definitely a valid genus of
sauropod distinct from Apatosaurus. This was determined from use of
statistical analysis to more objectively assess the fossil specimens of
different species of Apatosaurus; they discovered that fossils previously
associated with Brontosaurus excelsus
can certainly be classified as being dissimilar enough to bring it back as a
valid genus. They also deduced that the genera Eobrontosaurus
and Elosaurus are in fact synonyms of Brontosaurus. According to
the study, Apatosaurus can be distinguished from Brontosaurus in
that the former has a lower, wider neck. Both genera had greatly bifurcated
cervical vertebrae (meaning that they held paired spines), which is what made
the necks so wide and deep (Wedel & Taylor, 2013).
The massive, unusual and oddly-proportioned
cervical vertebrae of Brontosaurus and other apatosaurine sauropods
has long been subject to controversy, but one interesting recent study
indicates that they may have been adapted for fighting. The cervical
vertebrae show indications of possessing extended areas for muscle attachment,
raised leverage for neck musculature and maybe even anchoring points for horns
or bosses on the ventral surface of each vertebra, which I’ve included in my
reconstruction above. One possible hypothesis that has been proposed is
that male Brontosaurus used their long, powerful necks as armoured organs
to fight each other over dominance or mates (Taylor et al., 2015). Maybe
someday I'll make a reconstruction of a pair of dueling Brontosaurus, as
a pen and ink drawing with a touch of watercolour.
The study by Tschopp et al. (2015)
resuscitating Brontosaurus has helped prove that sauropods have much
greater diversity than previously thought and that the identification of Brontosaurus
as a separate genus from Apatosaurus indicates that even
greater mysteries remain to be fully understood...
References:
Tschopp, E., Mateus, O., Benson. R. B. J. 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3: e857; DOI 10.7717/peerj.857.
Taylor, M. P., Wedel, M. J., Naish, D., Engh, B. 2015. Were the necks of Apatosaurus and Brontosaurus adapted for combat? PeerJ PrePrints 3: e1347v1.
Wedel, M. J., Taylor, M. P. 2013. Neural spine bifurcation in sauropod dinosaurs of the Morrison Formation: ontogenetic and phylogenetic implications. Palarch’s Journal of Vertebrate Palaeontology, 10, 1-34.
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