Your Friends The Titanosaurs, part 37: Conclusions

After three years of monthly (and sometimes more frequent) entries, I've finally finished what I set out to do: provide a short description of every titanosaur. That was enough time for eleven new genera and species to be described, and one species covered in the second post to be moved to a new genus (Aeolosaurus maximus to Arrudatitan). To make it easier to navigate the whole shebang, I've created a new page, "Your Friends The Titanosaurs", that collects them all. I've also flipped branches on The Compact Thescelosaurus: macronarians now come after diplodocoids, instead of the other way around.

After all that work, I feel I've earned the right to wave my arms through one last post, to summarize some general considerations that didn't have a place in the other posts.

I think the idea of a monolithic "Titanosauria" (or "Titanosauridae" in the old days) cramps how we think about them. With well over a hundred named species and oodles of undescribed and unnamed specimens spread over something on the order of sixty-five million years and across practically every significant landmass, there's a lot of diversity hiding behind "Titanosauria" as a clade (and if you only look at named material, you're going to be missing some important information). What also cramps our understanding is the vague internal resolution of the group. There are a few well-supported knots, such as aeolosaurs, futalognkosaurs, and saltasaurs, and then a lot of "every-cladogram-for-itself". (All bets are off for middle Cretaceous East Asian sauropods.)

For this post I've updated the figures from "Titanosaurs in time and space". Here are the non-South American titanosaurs, with new names added and ages updated. Click to embiggen. [Figure updated 2021/06/22 to correct typo in spelling of Qingxiusaurus.]

And here are the South American titanosaurs. [Figure updated 2021/06/22 to add Bravasaurus.]

Finally, the non-titanosaurian somphospondylans, with some putative titanosaurs of more questionable affiliations thrown in at the end. [Figure updated 2021/06/22 to add Qinlingosaurus and "Titanosaurus" rahioliensis to lower section.]

Next, their success is underrated. Geographically, the darn things were nigh-on ubiquitous once the Late Cretaceous kicked in. They seem to have their initial appearance in the right place at the right time to spread over the continents just before landmasses had well and truly separated. Appalachia is the only chunk of land where we don't currently have titanosaurs but do have other Late Cretaceous dinosaurs, and I would not be surprised if they turn up there eventually as well. Titanosaurs (and sauropods in general) were also probably quite good at dispersing via swimming, similar to elephants and their relatives, featuring large, buoyant bodies plus the ability to hold their air intake high above the water.

Furthermore, titanosaurs don't seem to have any trouble adapting to the spread of angiosperms. Quite the opposite: they more or less replaced all other sauropod groups during the Late Cretaceous (there are some oddballs of uncertain classification, but they aren't of numeric significance), and appear to have been the *only* kind of large herbivores on a landmass as large as India. People sometimes point to hadrosaurs and ceratopsians as the major dinosaurian beneficiaries of the angiosperm revolution, but they're overlooking a third group—yes, the titanosaurs. (Let us not forget the coprolites from India with grass phytoliths; Prasad et al. 2005.) Incidentally, it's my opinion that without Chicxulub, the hadrosaurs and titanosaurs were well-positioned for Cenozoic success. The hadrosaurs could probably have eaten anything the plants could throw at them, and titanosaurs were ecologically flexible enough to have several species in one depositional basin. I'm less sanguine about the ceratopsians (at least the ceratopsids) due to their specialized jaws and teeth, but you never know.

The historic lack of respect for titanosaurs can be attributed in part to scrappy fossils, but some other, less valid factors come into play: their dominance of Gondwanan landmasses far from the centers of dinosaurs study in Europe and North America; a perception that they were throwbacks in a Cretaceous world of duckbills and horned dinosaurs and tyrannosaurs, in which North America had already dumped its sauropods (making the titanosaurs "dinosaurs among dinosaurs"); and a perception that titanosaurs were more primitive than the fancy Late Jurassic sauropods, especially brachiosaurs and diplodocids. This is all hogwash, as I hope you can tell from the many entries in this series. Titanosaurs were not some quaint backwards Gondwanan phenomenon that would have been easily outcompeted by Laurasian herbivores. In many places they coexisted quite comfortably with Laurasian groups such as hadrosaurs, ankylosaurs, and tyrannosaurs. Alamosaurus is the famous example of plopping a titanosaur into the classic North American LK fauna, and it did quite well for itself. Argentina, central Asia, island Europe, and Mongolia are other places of coexistence. We have very little overlap of ceratopsids and titanosaurs, but that's probably just an accident of biogeography and timing: ceratopsids had a late start and didn't get to spread far. Ankylosaurs seem to have been present in low diversity throughout much of titanosaur space and time, but the low diversity may be more of an ankylosaur thing; they never seem to have packed an area with species. Maybe it's the cost of the armor?

It's simply an accident of history that of the two places where dinosaur paleontology was first studied in detail, one of them doesn't have a lot of Late Cretaceous terrestrial rocks (England) and the other, for reasons that are unclear, is about the only place where they aren't known to have been a major part of the dinosaurian faunas (North America). It's also a historical accident that our perception of titanosaurs up to the 1990s was mostly based on the fragmentary skull of Antarctosaurus wichmannianus (default as the only titanosaur skull, but with a lot of imagination) and the partial skeletons of Titanosaurus australis (=Neuquensaurus) and later Saltasaurus. Neuquensaurus and Saltasaurus, of course, are nearly identical and represent a tiny, out-there branch of titanosaurian diversity. One result of this unintended saltasaur domination was that "short neck bones" was listed as a general feature of titanosaurs in Lambert (1983). If you were going to boil down titanosaurian key features today, you might mention things like the reduced or absent fingers, the flaring hips, and the stepped profile of the lower margin of the upper jaw.

Why were they so successful? Clearly they were great at reproducing (note the nesting grounds in Argentina, India, etc.), but the same thing could be said for any successful clade. The titanosaurian bauplan was flexible enough to take over the roles of other sauropod groups and come up with some new tricks of its own. They could do enormous just fine, but they also seem to have been pretty good at being relatively small, such as the saltasaurs around 9 m long (30 ft) and a number of species around 15 m long (50 ft); this is not counting island species, which were small by ecological necessity. A few features of titanosaurs are noted as making them different from other sauropods: the wide-gauge limbs and broad body, and the presence of osteoderms in many species. The wide stance and tubby torso didn't make them as conventionally graceful as brachiosaurids or diplodocids, but it seemed to work all right for titanosaurs. I can see a couple of potential advantages to the different build: it could have offered different and perhaps more versatile locomotion; and widening the body would have permitted more guts than a narrower-bodied sauropod of the same body length. As for the osteoderms, we had several posts on those. Suffice it to say that for most titanosaurs there weren't enough of these to be very useful for defense, but if the osteoderms were a specialized mineral storage system, they may have provided a reproductive edge. (And who started the current meme of a few spikes at the base of the tail?)

Titanosaurian diversity in one formation can seem excessive, although it's not as strange if you consider the scope of the clade Titanosauria as more like, say, Carnivora than Canidae. It's not uncommon to have at least four species in a formation. Rather inconveniently, titanosaurs also seem to have had a tendency to produce multi-individual or even multi-species bonebeds; apparently they were incapable of dying alone. It's as if they had a perverse instinct for giving future paleontologists a hard time. (Based on his history with the group, if Friedrich von Huene had traveled back to the Late Cretaceous, a titanosaur would have gone out of its way to step on him.)

And this brings our tour to a conclusion, although I have the feeling there will be cause for many updates to come!

References

Lambert, D. 1983. A field guide to dinosaurs. Avon Books, New York, New York.

Prasad, V., C. A. E. Strömberg, H. Alimohammadian, and A. Sahni. 2005. Dinosaur coprolites and the early evolution of grasses and grazers. Science 310(5751):1177–1180.