Geoscience Reference
In-Depth Information
record. For example, the hypothetical “first dinosaur,” which would have evolved
about 235
mya
and was the common ancestor to both saurischians and ornithischi-
ans,probablywalkedonitsreartwolegs(bipedal)anditsfeetwouldhavehadthree
prominent toes (digits) pointing forward, one toe off to the side, and all toes tipped
withclaws.Itsfrontfeet,haditalsousedtheseforwalking(makingitquadrupedal),
wouldhavehadfivefingers(alsodigits),againallpointingforwardandwithclaws.
All subsequent dinosaur feet were modified from this basic body plan, whether cer-
taindinosaurlineagesstayedbipedal,wenttoquadrupedal,orusedsomemixtureof
the two. As a result, dinosaur tracks made by feet that had more than four digits in
the rear are rare—happening only with sauropod tracks—and more than five digits
in the front would be really weird. If anything, most dinosaur feet reduced or lost
digits throughout their evolutionary histories. Unneeded toes or fingers, which can
be evolutionarily expensive, were weeded out by survival and propagation of spe-
cies, in which it was advantageous to get rid of these over time.
Just as modern trackers might classify mammal tracks by the number of toes,
the same can be done with dinosaurs. For the major evolutionary groups of dino-
saurs, the following modes ofmovement and digit numbers, with only a few excep-
tions, can be applied to help with identifying their tracks:
• Theropods—rear feet only (bipedal), three digits pointing forward, and
sometimes another small one off to one side.
• Prosauropods—rear and front feet (bipedal or quadrupedal), four digits on
the rear, four on the front.
• Sauropods—rear and front feet (quadrupedal), five digits on the rear and
fiveonthefront(althoughdigitsarealmostnevervisibleinsauropodfront-
foot tracks).
• Ornithopods—rear feet only or all four feet (bipedal or quadrupedal); on
the rear feet, three digits pointing forward, sometimes another digit off to
the side, whereas front feet had five or less.
• Stegosaurs—rear and front feet (quadrupedal), rear feet with three digits
and front with five.
