Geoscience Reference
In-Depth Information
Isuppose,then,thatwemustonceagainresorttodinosaurtracefossilstolearn
more about how they rested. Sure enough, we do know of some so-called “resting”
or “crouching” traces—made when a dinosaur sat down—although these are quite
rare.Asof2013,onlytenhadbeenfoundintheworld,withthreeinMassachusetts,
three in Utah, two in China, one in Poland, and one in Italy. For some unknown
reason, these trace fossils are time-restricted, as they are only preserved in Early
Jurassic rocks. Of the three from Massachusetts, two are from medium-sized thero-
pods and one from a small ornithopod, whereas all the rest are from medium-sized
theropods. So far, we do not know of any resting traces made by quadrupedal dino-
saurs such as sauropods, ankylosaurs, stegosaurs, or ceratopsians.
Identifying a dinosaur resting trace is relatively straightforward. First of all,
look for a side-by-side pairing of the rear feet. Most bipedal dinosaur trackways
show a diagonal-walking pattern, meaning that any pattern deviating from that
catches our attention and is examined more carefully. Hence, where dinosaurs
stopped, they would have pulled up their trailing leg next to their lead leg, meaning
their footprints will be parallel and adjacent, or slightly offset. Upon sitting down,
the dinosaur will have lowered the long parts of its legs just above its feet, which
consist of its metatarsals (equivalent to our heels). This pressing of its metatarsals
onto the ground would have given its tracks elongated extensions. Once seated, the
dinosaur didn't stop moving and may have shifted its position as it settled in, caus-
ing multiple prints in a small area. Additional parts of the body might have made
contactandlefttheirmarkstoo,suchastherearpartofitsanatomy—whichisprop-
erlycalledan
ischial callosity
,andnotthemoreappealingterm“dinosaurbutt”—as
well as its tail and front-foot (hand) impressions. Of these, the ischial callosity is
most likely to be preserved, although tail marks and hand imprints have been recor-
ded in a few, too. Incidentally, dinosaur tail impressions are quite rare, with fewer
than forty reported from the entire geologic record, and many of these are associ-
ated with resting traces.
