WERE THE TRACKMAKERS FOR THE DINOSAUR
ICHNOTAXA GRALLATOR, ANCHISAURIPUS, AND
EUBRONTES REALLY THEROPODS?
SMITH, Joshua B., Department of Geology, University of
240 South 33rd Street, Philadelphia, PA 19104-6316
FARLOW, James O., Department of Geosciences, Indiana
University - Purdue
University at Fort Wayne, Fort Wayne Indiana 46805
As paleoecology and biostratigraphy using footprint taxa require
confident assignments of trackmakers to the tracks, and as such
studies in the Newark Supergroup would involve the common
dinosaur ichnotaxa Grallator, Anchisauripus, and Eubrontes , it is
desirable to determine if trackmakers for these taxa can be assigned.
These taxa have historically been referred to the Theropoda.
Comparisons of the pedal foot skeletons of potential trackmakers of
tridactyl dinosaur tracks show that theropods tend to have long, thin
digits, with digit III relatively longer than digits II and IV. In
addition, theropod feet tend to have relatively short unguals and
relatively long phalanges in the middle portions of the digits
compared with ornithischians and prosauropods.
Using specimens of these taxa from collection and outcrop, we
have made statistical comparisons between the track morphologies
and the morphologies of several types of theropod and ornithischian
dinosaurs. Farlow and Lockley (1993) calculated means and ranges
for variables that distinctly show variation between theropods and
non-theropods for a number of taxa. We have analyzed these
ichnotaxa in the same manner.
The means of digit III/digit II lengths are: for Grallator, 1.47,
Range 1.27-1.87; Anchisauripus, 1.37, R 1.08-1.71; Eubrontes , 1.25,
R 0.9-2.6. Grallator is close to Farlow and Lockley's (1993) mean for
theropods (1.48, R 1.03-1.79), while Anchisauripus is close to
ornithischians (1.38, R 1.19-1.57) and Eubrontes is near
prosauropods (1.21, R 0.99-1.38). The means for digit III/digit IV
lengths are: for Grallator, 1.28, R 1-1.58; Anchisauripus, 1.14, R
0.84-2.30; Eubrontes, 0.96, R 0.81-1.73, in this case all are closest to
the Early Mesozoic theropod mean of 0.94 (R 0.81-1.05) and a
moderate distance from the means of all non-theropods (ca. 0.77).
The means for the ungual II3/phalanx II2 lengths are: for Grallator,
1.26, R 1.04-1.43; Anchisauripus, 1, R 0.66-1.29; Eubrontes , 1.08, R
0.89-1.73. All plot closest to the Early Mesozoic theropods (1.00, R
0.73-1.38), but Grallator is close to Early Mesozoic ornithischians
(1.35). The means for the phalanx III2/phalanx IV1 lengths are: for
Grallator, 1.64, R 1.64; Anchisauripus, 1.43, R 1-1.76; Eubrontes,
1.10, R 0.84-1.37. Grallator and Anchisauripus fall near the Early
(1.38 R 0.78-2.24), but Eubrontes is very close to prosauropods
(0.85, R 0.67-1.20).
Although there is significant variation between these mean
values, the amount of overlap in the ranges dramatically reduces the
usefulness of this data. Many of the proportions fall just as close to
non-theropod groups as they are to theropods or are well within non-
theropod ranges, raising some doubt about how accurate any
trackmaker assignments are likely to be using this method.
However, relationships between the phalanx measurements
and the relationships between the digit measurements are both linear,
and theropods and non-theropods tend to separate nicely if these
proportions are compared. The three ichnotaxa tend to cluster well
within the theropods on a plot of digit III /digit IV lengths vs. digit
III/digit IV lengths. On a plot of phalanx III2/phalanx IV1 lengths vs.
ungual II3/phalanx II2 lengths, the ichnotaxa again fall within the
theropod cluster, with Grallator and Anchisauripus plotting close to
Coelophysis. Interestingly enough, Eubrontes falls much closer to
Herrerasaurus and Liliensternus than to its hypothesized trackmaker
Farlow, J.O., and Lockley, M.G., 1993, An osteometric approach to the
identification of the makers of early Mesozoic tridactyl dinosaur footprints. In
Lucas, S.G., and Morales, M. (eds.), The Nonmarine Triassic: New Mexico
Museum of Natural History and Science Bulletin, No 3, p. 123-131.
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