Titanosauria

    Titanosauria is composed of a large number of poorly known sauropods, among the last animals of their kind to have existed. The animals listed here are traditionally listed as titanosaurids, but since it's recognized that Titanosaurus itself is based on dubious material that cannot be classified beyond Titanosauria, Titanosauridae is not particularly useful. Titanosaurians commonly are either very large for sauropods or very small, often with armor set in the skin (which when found disassociated earlier was assumed to belong to ankylosaurians). Titanosaurian armor comes in three general forms: mound-like, bulb-like, and disc-like.
    Another characteristic just now becoming appreciated is the wide gauge hips, implying a somewhat different muscle arrangement and gait for these animals compared to other sauropods. Wide gauge sauropod trackways show up in the Jurassic, establishing the presence of titanosaurian-style walking a bit before the actual titanosaurians show up. Their teeth are somewhat peglike, like diplodocid teeth, but this is virtually their only similarity. Here is a chart showing some important differences between the two groups:

    For a time, however, titanosaurians and diplodocids were considered to be very closely related, which was excusable then, because no one was really sure what a "titanosaurid" was. However, it is now obvious that this cannot be. Skulls for titanosaurians were not known until this decade, although Mongolian sauropod skulls from the Seventies and Eighties are now known to belong to probable derived titanosaurians, not to diplodocids as they were first described.
    Titanosauria is divided into several families by some workers, including Saltasauridae and Andesauridae, which supposedly contains certain closely-related "basal titanosaurid"-type sauropods.
    Many titanosaurians come from LK South American formations whose ages have not been pinned down yet. Also, several titanosaurians (Antarctosaurus especially) are based on arbitrary assortments of remains, which may or may not all belong together. Some actually were titanic in size, but there were also "pygmy" titanosaurians, especially in areas thought to have been islands. Titanosaurians are still rather poorly known, and a lot of the taxa aren't especially well defined.
    As an interesting bit of trivia, a taxon named Succinodon was once thought to be a member of Titanosauria, until it was discovered that the holotype "jaw" was actually the filled burrowings of a mollusk.
    Phytoliths (bits of silica that plants grow within themselves to make them less palatable) have been found in LK Indian coprolites attributed to titanosaurians. The interesting part about them is that grass phytoliths are among them. Grass was long thought to have not appeared, or at least not been of any importance, until the early Cenozoic, but these phytoliths indicate diverse grasses formed part of the diet of titanosaurians. These grasses were probably not really common yet. Other finds in South America suggest that grasses first appeared in the southern continents in the Cretaceous, which may explain some of the current blindness about them: Southern Hemisphere continents have not been explored as well as the northern continents when it comes to paleontology.

<--Titanosauria
      |--Andesaurus
      `--+--Argentinosaurus
           `--+--Malarguesaurus
                 `--+--Lognkosauria
                      |     |--Drusilasaura
                      |     |--Futalognkosaurus
                      |     `--Mendozasaurus
                      `-->Lithostrotia

Titanosauria and Lognkosauria: As you can see by scrolling down the page, Titanosauria is one of the largest groups of classic dinosaurs. It will likely continue to grow for a while, as titanosaurian finds seem to be greatly increasing. They also have an unusual tendency to have generic names starting with the letter A. A vast nesting colony of titanosaurians has recently been discovered in Argentina, with some specimens showing skin impressions, and another such possible colony was uncovered in Spain. Numerous undescribed forms are turning up at a frightening pace, some including cranial material. Argentina is particularly rich.
    Now, for a bit of definitions: Technically, Titanosauria is composed of all sauropods closer to Saltasaurus than to Euhelopus or Brachiosaurus. Within Titanosauria, Lithostrotia is composed of Saltasaurus, Malawisaurus, and all descendents of their most recent common ancestor. Similarly, Saltasauridae is Saltasaurus, Opisthocoelicaudia, and all descendents of their most recent common ancestor. Note, though, the effects of the Titanosaur Uncertainty Principle (the more titanosaurs that are observed, the less higher-level classification resolution is achieved; this differs from other situations where the more taxa you have, the clearer things are supposed to be).

Titanosauria i.s.: The taxa here display great diversity for large sauropods. Some of the more basal species are brachiosaurid-like, while other are close to the base of Saltasauridae, with everything in between covered. A titanosaurian braincase from the late Aptian or early Albian (EK) of Texas has recently been described. At the early end, Bathonian (MJ) tracks from England appear to be titanosaurian in origin.