In the absence of any consensus on titanosaur relationships, species are simply alphabetically arranged by continent, the continents also being alphabetically arranged; had there been any Antarctic titanosaurs, they would have received orange bars, completing the rainbow (and then some) of titanosaurs. The Indian Plate (which includes for our purposes the titanosaur-producing portion of Pakistan) and Madagascar both get distinct colors because of their separation from other landmasses. No attempt has been made to make the spans of the stages proportionate to chronological time, for the simple reason that I didn't want to introduce more artificial precision than I already had; there's a lot of "early this" or "middle that", and it isn't always feasible to translate those to the absolute chronology. I do try to keep up with re-datings of formations, though. Some are better-dated than others.
|Paint it black with the South American titanosaurs; fortunately there aren't any more substantial landmasses, because I was running out of colors.|
|A bonus: the unclassified somphospondyls and euhelopodids, because some of them are probably titanosaurians too.|
A few thoughts:
Titanosaur presence seems to closely track... the existence of dinosaur-bearing rocks from about the Aptian to the end of the Cretaceous. This is a statement that is both uninformative and informative. It is uninformative because it doesn't tell us much about titanosaurs except that they were abundant, widespread, and likely to be found wherever dinosaurs can be found in that stretch of time. It is informative for the same reason; just because something is obvious doesn't mean it's not important. The major exception is North America.
Building on that last observation, it's just one of those cosmic chances that North America, long famous for its dinosaurs and the source of many of our historical images of dinosaurs, has almost zip for titanosaurs except Alamosaurus sanjuanensis (which, granted, could easily be more than one species), meaning that one of the most successful and widespread groups of dinosaurs has been seen as some sort of mysterious and exotic thing due to serendipity.
Not to harp on this geographic thing too much, but had dinosaurs first been studied in, say, Argentina or India, the Cretaceous dinosaurs of western North America would have probably been perceived as unusual isolates. There aren't any titanosaurs or abelisaurs, which are otherwise widely distributed, and aside from hadrosaurs, much of the western North American large dinosaur fauna is otherwise only seen in east-central Asia: large tyrannosaurs, derived ankylosaurids, pachycephalosaurids, and ceratopsids. Hadrosaurs get a pass because they are the other great flavor of the Late Cretaceous, even if they did get a later start than titanosaurs and don't appear to have spread as far.
Titanosaurs seemingly showed up worldwide at approximately the beginning of the Aptian, becoming established in Africa, Asia, Europe, and South America without any obvious evidence as to where they came from. That said, if I was going only by the charts I might suspect somewhere in Africa or Europe, because of their central locations and relatively easy access to the other landmasses. Of course, the spread we see in the charts has a lot to do with rocks: the first few stages of the Lower Cretaceous are poorly represented by terrestrial sedimentary rocks compared to the Upper Jurassic and upper Lower Cretaceous. With good sampling of Upper Jurassic rocks from several areas, we can be reasonably confident that if titanosaurs had evolved by then, they weren't major players. Instead, the ancestral titanosaurs probably lived during the Berriasian–Barremian, and the Aptian–Albian forms are part of the initial radiation.
Very cool post, titanosaurs really are a mess aren't they?ReplyDelete
What is the status of Gspsaurus, Nicksaurus, Maojandino, and Saraikimasoom? Are they nomen nuda for now? Obviously not valid enough for you to include them. Three out of four of them are nearly impossible to pronounce so I don't think I'd mind too much if they are invalid!
There's some concern that the four additional names are basically manuscript names. It's hard to tell what's going on, given there's been little independent coverage of the fauna. In terms of the chart and Compact Thescelosaurus tables, they're essentially redundant with the five existing names, so at least we're not missing out on too much.Delete