Was the specimen found in a region where spinosaurs
were already known to exist e.g. Africa, or elsewhere?
Yes. The specimen was found in the Santana Formation of Brazil, where two
spinosaurid taxa, _Irritator challengeri_ and _Angaturama limai_ were
recovered. (The article also says that it is now "generally agreed" that
_A. limai_ is a junior synonym of _I. challengeri_). The tooth embedded in
the pterosaur (ornithocheirid) cervical is "similar in every respect to the
spinosaurid teeth previously described from the Santana Formation".
The article also notes a previous occurrence of predation by theropods on
pterosaurs: a broken troodontid tooth embedded in an azhdarchid tibia from
the Late Cretaceous of Alberta (Currie and Jacobsen, 1995; Can. J. Earth
Sci. 32: 922?925). (As I recall, this tooth was provisionally referred to
_Saurornitholestes_).
In both cases, we don't know whether the pterosaur was attacked by the
theropod (either on the ground or in mid-air) or the theropod was scavenging
the carcass. The authors favor the latter explanation, and regard a capture
of a live pterosaur by a spinosaur as "unlikely". I'm not sure this holds
for troodontids, though. If small maniraptorans are thought to have leaped
into the air against flying insects on the way to evolving flight, why not
pterosaurs too?