There are a few taxa of interest that haven't yet been covered. They include: 1) "historical titanosaurs", the kind you might stumble across in a 1980s dinosaur dictionary; 2) poorly known species regarded as titanosaurs mostly on the circumstantial evidence of time and place, without the anatomical evidence to back up a classification; 3) species that have turned up in Titanosauria once or twice during the cladistic era (mid-1990s to the present) but are not currently or generally regarded as titanosaurs; and 4) species that appear to have been near Titanosauria and sometimes hop the line in analyses, but usually are found outside. This post is for the first three varieties. We've got 15 in the queue, so as you can image I'm not going to go into a great deal of detail.
Curiously, most of McIntosh's "Sauropoda incertae sedis" from the first
edition of The Dinosauria show up on this page: "Pelorosaurus"
becklesii (=Haestasaurus), Mongolosaurus,
Austrosaurus, and Aepysaurus (=Aepisaurus), plus a shout-out to "Apatosaurus" minimus. The only
ones not here are "Morosaurus" agilis (now described as a
rebbachisaurid [2021/04/02: no, dicraeosaurid; do this long enough and they all run together], Smitanosaurus), and
Campylodoniscus, which was previously featured. This says something about quasi-titanosaurs, but
I'm not sure what. The taxa also skew old, with many of Early Cretaceous or even Late Jurassic age, suggesting it's harder to get a handle on putative early titanosaurs. Unsurprisingly, many of them are dubious, albeit in all kinds of ways: from garden-variety causes like too little material, to "unavailable for study due to being destroyed by monsoons", to "the osteoderms turned out to be ribs", to "the original describer thought a pile of caudals from several sites separated by miles belonged to one individual", to "actually filled mollusk borings".
As a reminder, because the terms come up several times, Titanosauriformes is the clade made up of the most recent common ancestor of Brachiosaurus and Saltasaurus plus all of its descendants, and Somphospondyli is the clade made up of all sauropods more closely related to Saltasaurus than to Brachiosaurus.