Tuesday, November 13, 2018

Morrison vs its contemporaries

If I could travel back in time and safari—or even settle—among the dinosaurs, my first pick would be the time and place of the Maastrichtian North Horn Formation, located in what is today Utah.  It's almost like a "greatest hits" of the dinosaurs; large predators (T. rex even) large sauropods (Alamosaurus, which is a rival for "the biggest."  Possibly.)  Large ceratopsians (Torosaurus instead of Triceratops, at least so far.  Close enough.  It's not like I could probably tell the difference in life anyway.)  Some large hadrosaur (either Edmontosaurus or some kritosaurine hadrosaur) appears to be there.  Most likely, many animals from other Lancian locations to either the north or the south of it also lived here, including probably large raptors, pachycephalosaurs, troodonts, ornithomimosaurs, leptoceratopsids, ankylosaurs, large pterosaurs, etc.

But, sadly, we don't know too much about the North Horn formation.  It also appears to be somewhat wetter than I prefer, although maybe that's just the lacustrine and riparian environments where fossils were formed.  A lot of what we suspect is there is based on inference from nearby formations from the same age.  All that we really know for sure is that there was some kind of pterosaur which left prints but no body fossils, T. rex, Torosaurus and Alamosaurus, and some kind of large hadrosaur.

My second favorite option is, of course, the later versions of the Morrison, when all of the biggest species of the genera appear.  It's about as classic as you get, with Allosaurus, Diplodocus, Brontosaurus, Stegosaurus, Camptosaurus, etc. as well as many, many others.  The Morrison in America is followed by a thirty million year stratigraphic gap, after which "early Cretaceous" formations like Cloverly appear.  We don't know for sure what happened to the Morrison dinosaurs.  Maybe, actually, the answer will be found elsewhere.  The Morrison is often compared to the Tendaguru, which in Africa, is actually just a little bit younger.  More recently, it's also been compared to the Lourinha formation in Portugal.  However, the easy and early comparisons have largely turned out to have been just a little bit too facile, and there are significant differences between them after all.

For instance, the Brachiosaurus of Tendaguru is now no longer considered Brachiosaurus, but rather Giraffatitan, and the so far unnamed "Archbishop".  The Allosaurus of Tendaguru is now mostly likely referred to allosaur-sized early and primitive carcharodontosaur Veterupristosaurus.  The Ceratosaurus remains are also now indeterminate.

That doesn't mean, of course, that there weren't a fair number of gross similarities, just that they weren't really as similar as all that.

The Lourinha formation is probably more similar; it does actually have a different species of Allosaurus and Supersaurus and Torvosaurus, for instance.  But now Lourinhasaurus is no longer considered a diplodocid, but rather a likely "cetiosaur-grade" sauropod and the similarly named Lourinhanosaurus is no longer considered an allosaurid, but rather an indeteminate therapod, possibly a megalosaur-grade smaller animal, maybe like a smaller version of Marshosaurus.  Or maybe it's a much more primitive allosauroid, related to the sinraptors.

Although we're bouncing around the globe a bit to do so, one can make a chronological line of some of the Jurassic faunal assemblages and it's likely that close analogs to these faunal assemblages dominated across the globe, which was still a breaking up Pangaea, and therefore probably still relatively connected and relatively similar.  The Lower Shaximiao and the Oxford Clay formations are probably the first that have the "superficial Jurassic aspect" as Lehman called it; cetiosaur grade sauropods, fabrosaur grade ornithischians, megalosaur grade therapods, etc.  The former is probably just a little older than the latter, although they overlap somewhat and seem to feature similar grade animals, if not exactly the same ones.  The Chinese paleontologists call this the Shunosaurus-Omeisaurus Assemblage.  The European assemblage is curiously poorly known, and no comparable American formations are well known at all.

The Upper Shaximiao represents the similar Mamenchisaurus and Chuanjiesaurus assemblages.  Early carnosaurs like the sinraptors start to make major inroads, and there's been a serious turnover in terms of sauropods.  More advanced ornithopods start to appear, as well as coelurosaurs popping in for the first time, and stegosaurs replace scelidosaur-grade thyreophorans.  The Morrison (and Lourinha) would be the next grade; carnosaurs have almost completely replaced megalosaurs, although Torvosaurus and Marshosaurus hang on for the first bit of it as very rarely appearing relicts, early macronarians and diplodocids start taking over in the sauropod realm, early ankylosaurs appear, coelurosaur diversity really takes off (although still lacking large forms at this point), etc.  Tendaguru is maybe the epilogue to this kind of formation; macronarians are common and diplodocids are rare (although that might also have been a regional artifact), carcharodontosaurs start to appear, etc.

After a significant gap, we get a wealth of formations, especially in North America Cloverly, Cedar Mountain, Glen Rose, Burro Canyon, etc.  Here we see the further development hinted at in Tendaguru: carcharodontosaurs like Acrocanthosaurus, large macronarians like Sauroposeidon, iguanodont-grade ornithopods, dromeosaurs, etc.  When we start to get into slightly later South American (in order oldest to younger: La Amarga, Lohan Cura, Candeleros, Huincul, Lisandro, Portezuelo) and African faunal assemblages like Kem Kem, Erlhaz, etc., it almost doesn't look like there's a temporal discontinuity.  Macronarians further develop into titanosaurs (still of the very large variety, not the smaller saltosaur-grade types that appear much later), carcharodontosaurs are large and in charge until neoventaors start to encroach near the end of that run, abelisaurs start to pop up in South America and Africa, although they're not yet dominant.  Spinosaurs run in those regions too, although not in North America, and ornithopods continue their relentless march from primitive to iguanodon-grade to near hadrosaurs, with some bizarre experimentation with back sails, particularly in Africa.  We see the start of the split between northern Laurasian and southern Gondwanan faunas into divergent pathways as well.

I'll note about that last; in many ways, it's easier to draw a line from the Jurassic aspect to the Gondwana faunas, though.  In North America, faunas with a superficially similar aspect and related animals is retained for a time until major discontinuities introduce very different animal families; tyransosaurs, ceratopsians, therizinosaurs, and more.  In Gondwana continents, the discontinuity is that carnosaurs go away, the abelisaurs that were already present in the fauna rise to claim apex predator, and the huge titanosaurs are replaced with the more derived and modestly sized saltasaurs.  Yet even that isn't as clear cut as all that, as gigantic titanosaurs appear in the latest Cretaceous of both northern Asia and North America, for instance, and somehow enough connectedness was retained all along for the iguanodon-grade ornithopods to turn into hadrosaurs across the globe.  The appearances of neovenators, dromeosaurs, and other animals as well seems to defy the regional chimney approach.

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