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ofaLateJurassic
Allosaurus
andsay,“Look,it'sthetracefossilofanankylosauror
stegosaur!”Suchleapsoflogicareconnectingtheformidabletailclubsofanankyl-
osaurortailspikesofastegosaurcontemporaryof
Allosaurus
,andonethatsuccess-
fully whacked its tail against this particular theropod marauder's leg. Alternatively,
the
Allosaurus
mighthavetrippedoveralogandbrokenitslegfromthefall.Again,
justlikethe
Edmontosaurus
skininjurydiscussedearlier,paleontologistsareattheir
best as scientists when they consider many possibilities for the origins of dinosaur
wounds. Then, in a Sherlock Holmes sort of way, each possibility is contemplated,
tested, and eliminated until only one, however improbable, remains.
Fortunately,twotypesoftracefossilsrecordedinthebonesofboth
Stegosaur-
us
and
Allosaurus
tell tales of their tails. One is of injured but healed bony tissue in
afewtailspikesof
Stegosaurus
.Thesewoundsareconsistentwithdamageitwould
have suffered from striking solid objects. Another is of a hole in a tail vertebra of
an
Allosaurus
, but one that probably healed around a piece of a
Stegosaurus
tail
spike, an unwanted souvenir.
Stegosaurus
had four pointy spikes on the end of its
tail, held horizontally and paired on each side, which surely became lethal weapons
when whipped by powerful muscles attached to the tail. Paleontologists since the
first half of the 20th century had figured that stegosaur tail spikes were used for
self-defense,butlackedfurtherconfirmationuntilthesetracefossilsweredescribed
in 2001 (broken spikes) and 2005 (a healed hole in the bone of a predator).
Thiswasnottheonlyexampleof
Allosaurus
havinghadabaddayortwo.One
Allosaurus
in particular had its rough-and-tumble life recorded all over its body.
Nicknamed “Big Al” and studied by paleontologist Rebecca Hanna in the 1990s,
this
Allosaurus
wasunusualintwoways:Itsskeletonwasabout95%complete,and
it had 19 instances of bone injuries, including breaks that healed. Among the bone
breaks suffered by “Big Al” were two of its ribs, and its right foot was badly hurt,
meaning that its tracks would have reflected a pronounced limp. These breaks may
betracefossilsfromaninteractionwithanotherdinosaur,whereasotherbreaksmay
have been self-inflicted, which would also qualify as trace fossils. However, where
