Actually...
"The anaerobic conditions [of a lake, swamp, lagoon, or marsh] prevent many animals from living at the bottom, preventing scavenging and aerobic bacterial decay. Accordingly, this results in a high degree of fossil articulation and, in some cases, soft-tissue preservation." (p. 207)
"Mineralization ... can occur rapidly, anywhere from a few hours to a few years. [...] Dinosaur fossils with preservation of soft tissue exemplify the best evidence for fast rates of mineralization. This mode of preservation required anaerobic conditions and probable bacterial mediation of chemical reactions that fixed certain elements to make a recognizable facsimile of the original organic structure." (p. 209)
"If diagenesis happened rapidly enough, so that phosphatic minerals replaced the original organic material, a pseudomorph of soft tissues (such as muscles) can be preserved. This type of preservation gives more of a three-dimensional character to the parts than mere carbonization." (p. 210)
Enjoy your hand webbing. It's both plausible and possible, especially considering the fact that the creature came from a lagoon.

And by the way, looks terrific!
All quotes above from the textbook "Introduction to the Study of Dinosaurs," by Anthony J. Martin, 2nd edition. Published by Blackwell publishing. I loved this class.