By the beginning of the Cretaceous period, about 145 million years ago, gigantic, plant-eating dinosaurs like Diplodocus and Brachiosaurus were on the evolutionary decline. However, this didn't mean that sauropods as a whole were destined for early extinction; an evolutionary offshoot of these huge, four-footed plant-eaters, known as titanosaurs, continued to prosper right up until the K/T Extinction 65 million years ago.
The problem with titanosaurs--from a paleontologist's point of view--is that their fossils tend to be scattered and incomplete, much more so than for any other family of dinosaurs. Very few articulated skeletons of titanosaurs have been discovered, and virtually no intact skulls, so reconstructing what these beasts looked like has necessitated a lot of guesswork. Fortunately, the close similarity of titanosaurs to their sauropod predecessors, their wide geographic distribution (titanosaur fossils have been discovered on every continent on earth, including Australia), and their huge diversity (as many as 100 separate genera) has made it possible to hazard some reasonable guesses.
As stated above, titanosaurs were very similar in build to the sauropods of the late Jurassic period: quadrupedal, long-necked and long-tailed, and tending toward enormous sizes (one of the biggest titanosaurs, Argentinosaurus, may have reached lengths of over 100 feet, though more typical genera like Saltasaurus were considerably smaller). What set titanosaurs apart from sauropods were some subtle anatomical differences involving their skulls and bones, and, most famously, their rudimentary armor: it's believed that most, if not all, titanosaurs had tough, bony, but not very thick plates covering at least parts of their bodies.
This last feature raises an interesting question: could it be that the sauropod predecessors of the titanosaurs perished at the end of the Jurassic period because their hatchlings and juveniles were preyed on by large theropods like Allosaurus? If so, the light armor of titanosaurs (even though it wasn't nearly as ornate or dangerous as the thick, knobby armor found on contemporaneous ankylosaurs) might have been the key evolutionary adaptation that allowed these gentle herbivores to survive tens of millions of years longer than they would have otherwise; on the other hand, some other factor may have been involved of which we are not yet aware.
Titanosaur Habitats and Behavior
Despite their limited fossil remains, titanosaurs were clearly some of the most successful dinosaurs ever to thunder across the earth. During the Cretaceous period, most other families of dinosaurs were restricted to certain geographic areas--the bone-headed pachycephalosaurs of North America and Asia, for example--but titanosaurs attained a worldwide distribution. There may, however, have been stretches of millions of years when titanosaurs were clustered on the southern supercontinent of Gondwana (which is where Gondwanatitan gets its name); more titanosaurs have been discovered in South America than on any other continent, including huge members of the breed like Bruhathkayosaurus and Futalognkosaurus.
Paleontologists know as much about the everyday behavior of titanosaurs as they do about the everyday behavior of sauropods in general--which is to say, not a whole lot. There's evidence that some titanosaurs may have roamed in herds of dozens or hundreds of adults and juveniles, and the discovery of scattered nesting grounds (complete with fossilized eggs) hints that females may have laid their 10 or 15 eggs at a time in groups, the better to protect their young. There's still a lot that's being worked out, though, such as how quickly these dinosaurs grew and how, given their extreme sizes, they managed to mate with one another.
More so than with other types of dinosaurs, the classification of titanosaurs is a matter of ongoing dispute: some paleontologists think "titanosaur" isn't a very useful designation, and prefer to refer to smaller, anatomically similar, and more manageable groups like "saltasauridae" or "nemegtosauridae." The doubtful status of the titanosaurs is best exemplified by their eponymous representative, Titanosaurus: over the years, Titanosaurus has become a kind of "wastebasket genus" to which poorly understood fossil remains have been assigned (meaning that many of the species attributed to this genus may not actually belong there).
One final note about titanosaurs: whenever you read a headline claiming that the "biggest ever dinosaur" has been discovered in South America, take the news with a big grain of salt. The media tends to be especially credulous when it comes to the size and weight of dinosaurs, and the figures touted are often at the extreme end of the probability spectrum (if they're not completely made up out of thin air). Practically every year witnesses the announcement of a new "biggest titanosaur," and the claims usually don't match up with the evidence; sometimes the "new titanosaur" that has been announced turns out to be a specimen of an already-named genus!