The most relevant bit (written by Ken) was:
Actually, the height is 13 feet. The top 1/3 of one neural spine
is obliquely sheared off leaving a U-shaped groove. There is some
remodeling of the bone indicating some healing occured prior to
death. Other neural spines are mangled, with at least two tooth
punctures. These and the missing spine form a nice U-shape that
conforms well with the cast of the T. rex skull we have. Jack has
seen it and agrees that it most likely was done by a T rex,
however, argues that it was a rare attack.
I interpret that to mean that one spine has a small U-shaped groove
that looks like it was formed by a single tooth biting through it, and
the overall pattern of damage forms a larger U-shape that looks like
outline of the Tyrannosaur's jaw. Jerry should comment if I've biffed
that description