By the beginning of the Cretaceous period, about 140 million years ago, gigantic, herbivorous dinosaurs like Diplodocus and Brachiosaurus were on the decline. However, this didn't mean that sauropods as a whole were destined for early extinction; an offshoot of these huge, four-footed plant-eaters, known as titanosaurs, continued to prosper right up to the K/T Extinction 65 million years ago. (See a gallery of titanosaur pictures.)
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 found, 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 earlier sauropods, their wide geographic distribution (titanosaur bones have been found on every continent except Australia), and their extensive number of genera (possibly as many as 100) 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 100 feet, though genera like Saltasaurus were considerably smaller). What set titanosaurs apart from the pack were 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 plates covering their bodies.
This last feature raises an interesting question: could it be that earlier sauropods perished at the end of the Jurassic period because their juveniles were preyed on by large theropods like Allosaurus? If so, titanosaur armor (even though it wasn't nearly as ornate or dangerous as the armor found on ankylosaurs) might have been a key evolutionary adaptation that allowed these gentle herbivores to survive tens of millions of years longer than they would have otherwise.
Titanosaur Habitats and Behavior
Despite their limited fossil remains, titanosaurs were clearly some of the most successful dinosaurs ever to walk the earth. In the Cretaceous period, most other types of dinosaurs were restricted to certain geographic areas--the pachycephalosaurs of North America and Asia, for example--but titanosaurs attained worldwide distribution. There may, however, have been stretches of millions of years when titanosaurs were clustered on the southern supercontinent Gondwana (which is where Gondwanatitan gets its name); more titanosaurs have been discovered in South America than on any other continent.
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 nesting grounds (complete with fossilized eggs) hints that females may have laid their eggs in groups, the better to protect their young.
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!).
Here's a list of the most notable titanosaurs; just click on the links for more information.
Adamantisaurus This titanosaur was named 50 years after its discovery.
Aegyptosaurus Guess what country this dinosaur was found in?
Aeolosaurus Could this titanosaur have reared up on its hind legs?
Alamosaurus No, it wasn't named after the Alamo, but it should have been.
Ampelosaurus One of the best-known of the armored titanosaurs.
Andesaurus This titanosaur rivaled Argentinosaurus in size.
Angolatitan The first dinosaur ever to be discovered in Angola.
Antarctosaurus Despite its name, this titanosaur may or may not have lived in Antarctica.
Argyrosaurus A plus-sized titanosaur from South America.
Austrosaurus This titanosaur was discovered near a train station.
Bonitasaura This titanosaur wasn't as beautiful as its name implies.
Chubutisaurus This titanosaur was on Tyrannotitan's lunch menu.
Epachthosaurus This "heavy lizard" was relatively primitive for its time and place.
Erketu This titanosaur had an unusually long neck.
Futalognkosaurus One of the biggest dinosaurs that ever lived.
Gondwanatitan Yet another titanosaur from South America.
Huabeisaurus A titanosaur from northern China.
Hypselosaurus This titanosaur's eggs were a foot in diameter.
Isisaurus Otherwise known as the Indian Statistical Institute Lizard.
Janenschia The earliest titanosaur in the fossil record.
Magyarosaurus This dwarf titanosaur was probably confined to a small island.
Malawisaurus The first titanosaur to be found with an intact skull.
Maxakalisaurus One of the biggest titanosaurs ever found in Brazil.
Mendozasaurus This titanosaur was ancestral to Futalognkosaurus.
Nemegtosaurus This titanosaur has been recreated from a single, incomplete skull.
Neuquensaurus Was this titanosaur really a species of Saltasaurus?
Opisthocoelicaudia A clumsily named titanosaur of the late Cretaceous period.
Paralititan This huge sauropod was discovered recently in Egypt.
Phuwiangosaurus This titanosaur was discovered in modern-day Thailand.
Puertasaurus This titanosaur rivaled Argentinosaurus in size.
Quaesitosaurus This titanosaur may have had unusually sharp hearing.
Rapetosaurus The only sauropod ever to be discovered on modern-day Madagascar.
Saltasaurus The first armored sauropod ever to be discovered.
Tastavinsaurus This titanosaur was discovered in Spain.
Titanosaurus This sauropod may--or may not--have been a unique member of its genus.
Uberabatitan Discovered in the Uberaba region of Brazil.