I haven't made a post with the PALEONTOLOGY tag in a long time. It's probably overdue! After the incredible disappointment that was the theatrical release of Walking With Dinosaurs, I went back to nurse my bitterness with more documentaries. There's a lot of dinosaur documentaries out these days; enough so that even I'm not really trying to keep up with them all. I don't know how many times I see some new one on Netflix that I hadn't heard of before. Sadly, many of them are kind of difficult to watch, even if they have good CGI (and that's a big "even if" because many of them do not.) Dinosaur Revolution seems like a good one, but we only have half of it available on Netflix, and instead have to deal with the reshuffled and odd little Dinotasia, which is surprisingly difficult to watch, for whatever reason. So, I've retreated again to my favorite of recent dinosaur CGI documentaries, Planet Dinosaur. I've watched it before, and even reviewed it here. The penultimate episode, "New Giants" is my favorite. I've long said that I'm a huge fan of the classic carnosaur vs. big sauropod action, after really becoming aware of the pattern in Tom Holtz's book. This episode also highlights this pattern, referring briefly to the pattern as appearing in various formations all over the globe over a span of many tens of millions of years.
Curiously, though, it's of course not quite so simple. It's interesting, for example, that the "large" sauropods actually come from a variety of not very closely related families. It's also interesting that the "large" sauropods may not have been particularly common relative to the medium sized sauropods in many of the formations in which they appear. The Morrison is the classic example, with modest sized Camarasaurus and Apatasaurus and Diplodocus outnumbering much larger Brachiosaurus or Amphicoelas. Although as the Morrison aged (i.e., got younger--or more recent, in terms of rock layers) the animals got bigger. Earlier, smaller species of Camarasaurus or Apatosaurus are replaced by larger species. The Morrison is famous for its diplodocids, which were not the norm in other formations. Macronarians, including the more highly derived titanosaurs, were much more common in later formations such as Hiuncul or the Kem Kem.
Prior to the arrival of giant allosaurs, the largest predators in the area seem to be megalosaurs: Torvosaurus tanneri and the somewhat dubios Edmarka rex, which may just have been a large individual of the former. But they are also quite rarely appearing, and are not carnosaurs. All in all, the Morrison raises an awful lot of questions about exactly who the apex predators were in faunal assemblages of this type, as well as who preyed on whom. This is probably due to the fact that the formation is quite well known and more dinosaurs have come out of the Morrison than out of most formations. If the Huincul, Kem Kem, Shasximiao, Cedar Mountain, Clovery, Twin Mountains, Tendaguru, etc. formations were as well known as the Morrison, we'd probably see similar challenges to the simplistic view that large allosauroid dinosaurs preyed on large sauropods, and that when the super-sized sauropods went extinct, to be replaced by more modest saltasaurs and others, that the carnosaurs went with them, to be replaced in Laurasia by tyrannosaurs and in Gondwana by abelisaurs.
But... hold the phone! This view is also challenged. Puertasaurus is a very large sauropod; a rival with Argentinasaurus, in fact, for the largest dinosaur ever. And... it's from the quite late Cretaceous--Campanian-Maastrichtian boundary at about 70 million years ago or so. Who preyed on this bad boy? If anyone? Totally unknown. Although, teeth that appear to be carcharodontosaurid in nature also appear at about that same time in Brazil (very far away; Puertasaurus was found in Patagonia). Did the carnosaur-large sauropod relationship continue in Gondwana locations after all, just somewhat invisible to us in the fossil record as of right now? Unknown. And was the relationship really all that pronounced in the first place? What did Allosaurus hunt, for instance? Another episode of this very documentary talks exclusively about Allosaurus (and Saurophaganax) chasing after small-medium sized ornithopod Camptosaurus and Stegosaurus. The Morrison episode of this documentary doesn't even make any reference to sauropods existing in the faunal assemblage at all!
It's also true that throughout the faunal assemblages in which large sauropods and large carnosaurs appear, there would be plenty of other potential prey animals, including fairly large and fairly common ornithopods. While Planet Dinosaur refers to the proposed sauropod-carnosaur relationship in Africa in the "New Giants" episode, it also refers to Carcharodontosaurus in the first episode, where it's chasing Ouranosaurus sail-backed iguanodonts, and interacting (on occasion) with Spinosaurus. Clearly the story as presented is just a bit too tidy.
For that matter; the carnosaurs do have some diversity amongst them, after all. Most of the largest ones are carcharodontosaurids, but not all of them. The largest Morrison allosaurs were not, nor were whatever was going on in China. And very recently, we've had Siats described from the Cedar Mountain formation. As large as Acrocanthosaurus (and thereby a rival for "biggest dinosaur predator ever"--a title that gets lots of attention, but which seems pretty dubious considering our sample size for most of the contenders), but while A. was a carcharodontosaurid, Siats was a neovenatorid--a family not otherwise known for large predators, and not otherwise known from North America at all. Where it came from, how it replaced (which, presumably it did, although this is based on pretty scanty evidence) Acrocanthosaurus, is a big mystery.