EUBRONTES, GIGANDIPUS, AND PLATEOSAURUS: AN EARLY
MESOZOIC "MENAGE A TROIS"
WEEMS, Robert E., 3003 Jonquilla Court, Herndon, VA
Eubrontes and Gigandipus are co-ocuring dinosaur ichnotaxa that
were established from trackways in the Connecticut River Valley by
E. Hitchcock in 1845 and 1855, respectively. Plateosaurus is an
osteological dinosaur taxon, established by Meyer in 1837, that is
known from excellent and abundant skeletal material discovered in
Germany. Although Eubrontes previously has been linked to
Plateosaurus by some workers, this correlation has been disputed by
others who have preferred to interpret Eubrontes as the spoor of a
theropod. Gigandipus similarly has been allied with the Therapoda.
Evidence is presented here suggests that all three taxa represent
essentially the same kind of prosauropod dinosaur.
A foot of Plateosaurus, reconstructed by the author from a
specimen belonging to the Carnegie Museum, has provided new
insight into the pes morphology and pedal mechanics of that dinosaur.
Digits II, III, and IV of the reconstructed foot have proportions that
would readily make a footprint of the Eubrontes type. Digit I is
notably shorter that digits II-IV and has much stouter phalanges that
the other digits. Unlike the other digits, digit I is rotated toward the
midline of the animal and could undergo nearly 180 degrees of flexure
and extension, allowing it to be held is several very different positions.
Flexed up and under the foot, digit I could be carried out of the way
during walking. Extended straight downward, the top of the terminal
phalanx of difit I would barely touch the ground and could aid in
balance. Extended forward, digit I would have served as a powerful
weapon or tool.
It is proposed here that tracks called Eubrontes represent
Plateosaurus walking with digit I held in flexed or fully extended
position, and that tracks called Gigandipus represent Plateosaurus
walking with digit I semi-extended downward either for additional
body support or in readiness to extend digit I for active use. The
normal co-occurrence of a tail drag-mark with Gigandipus footprints,
a very atypical trait in all other dinosaurian ichnotaxa, suggests that
Gigandipus represents an unusual dinosaurian behavior pattern, such
as posturing to achieve an exceptionally high and menacing stature or
stretching to gaze from maximal height while walking. Correlation of
Eubrontes and Gigandipus with different behaviors of Plateosaurus
implies that plateosaurs were behaviorially complex animals that were
more interesting and more empowered than has been assumed
previously. Additionally, assignment of all these taxa to plateosaurs
indicates that plateosaurs were just as abundant in the Newark
Supergroup basins in Rhaetic-Liassic time as they were in western
Europe and China.
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