Thursday, February 02, 2017

Meet the Carnosaurs, Part II: Allosaurids and Neovenatorids

There are three families left within the carnosaurs; the allosaurids (the most famous and well-known group, although it's actually a very small one), the carcharodontosaurs (a large family of animals that have become increasingly famous because some of them are allegedly "bigger than T. rex") and the neovenators (an another interesting family that includes the megaraptors, and which has become increasingly interesting just in the last few years.)

At the next node beyond the sinraptors, the tree splits into the allosaurids and the carcharodontosaurians; the carcharodontosaurians then split into the carcharodontosaurids and the neovenatorids.  For today, partly as a function the number of specimens that need to be described, I'm going to do the small allosaurid family and the medium sized neovenators.  The carcharodontosaurs are such a large group that they need to be treated separately in the next (and last) post in this series.

Allosaurus fragilis.  This is the really famous Allosaurus known from big quarries in Utah and Colorado at Dinosaur National Monument and the Cleveland-Lloyd Dinosaur Quarry—both of which are national monuments.  Allosaurus is well-known from at least 60 excavated skeletons, of all ages and sizes.  The average sized Allosaurus seems to be about 30 feet long (maybe a tiny bit less) and the largest was nearly T. rex sized at over 40 feet.  In fact, this largest specimen is so large that its describers were enchanted by the size and gave it a new name; Epanterias amplexus, but most studies have suggested that at most it's a new species, and probably not even that.

Allosaurus has probably got a number of species and a nearly global distribution, although other than the Morrison Formation allosaurs from the American West they're not well known, and their remains are a bit scrappy, which makes determining exactly how they're all related a bit difficult to determine.  The formal and then informal species A. lucasi and "A. jimmadseni" are both from a bit later than A. fragilis; above it in the later Morrison Formation from the very late Kimmeridgian and Tithonian (A. fragilis is more from earlier in the Kimmeridgian.)  Like the rest of the Morrison fauna, as time passed and one species within a genus was replaced by another, in general they became somewhat larger.  There's also a European species, although there's some doubt that it truly is a different species—in spite of the rather large geographical separation (which was admittedly less at the time) found in Portugal.  There was an allosaur in the Tendaguru Formation of Tanzania, although subsequent analysis suggests that these poor remains might actually be carcharodontosaurian.  And allosaurs have been reported from Siberia and Australia, although there are—again—serious doubts that they've been properly identified.  Allosaurus also has swallowed up a rather large number of names that were applied to animals once thought to be something else but later determined to merely be specimens of Allosaurus: Creosaurus, Antrodemus, Labrosaurus, Epanterias.

Allosaurus is best known from the Morrison, of course, and the environment of the Morrison and the other animals that lived around it are interesting.  While I've pointed out before that the environment in which many of the sinraptors lived was similar, including similar types of animals, this is even more true for the Lourinhã and Tendaguru Formations.  Rather than describe them all, the Morrison can stand in for all of them.  Allosaurus was surrounded by sauropods.  So many varieties, in fact, that scientists are at a bit of a loss trying to explain how they all coexisted without competing head to head.  There are several varieties of rather slim diplodocids (Diplodocus, Barosaurus, Supersaurus and a bewildering variety of many more dubious names), the more stout apatasaurines (Apatasaurus  and now even the name Brontosaurus has been revived) dicraesaurs, haplocanthosaurs, and the future of sauropoda—macronarians come in a few varieties including Brachiosaurus, the largest of the sauropods of the Morrison (unless Amphocoelias turns out someday to be verified) and the more modestly size and apparently by far the most common, Camarasaurus.  Stegosaurus and a few other stegosaurs are also well-known from the Morrison, and there's an also incredibly diverse array of ornithopods; many of them rather small and presumably fleet-flooted hypsilophodonts, but some of them nearing Iguanadon in size and evolutionary grade.  These include (not comprehensively) Dryosaurus, Camptosaurus, Uteodon, Drinker, Othnielosaurus and Othnielia.  There are two early ankylosaurs.  There are many smaller therapods, like Ornitholestes and Coelurus (and others).  There's an early tyrannosauroid.  Curiously, there are also other larger therapods.  Ceratosaurus was roughly half to two thirds the size of Allosaurus but had much more robust teeth and apparently a different hunting style.  There are large megalosaurs—Marshosaurus and Torvosaurus (which includes the nearly T. rex-sized Edmarka rex).  As an aside; both Torvosaurus and Ceratosaurus have also been found in the same formation as the Portuguese Allosaurus.  The lower, thinner bodies of the other two predators has sometimes been interpreted to mean that they preferred hunting in the more overgrown lowlands, while Allosaurus preferred the higher, drier, and certainly more open floodplains.

The Morrison itself is semi-arid, on the shores of the Sundance Sea.  Further to the south it starts to become actually desert-like, but more to the north it's got a strongly riparian flavor, with drier savanas separating the river valleys.  It's often believed that many of the dinosaurs favored the gallery forests along the rivers to the floodplains, but adult sauropods and possibly Allosaurus himself may have preferred the open.

Saurophaganax maximus. Closely related to Allosaurus (there are still a few workers who refuse to recognize it as a valid, separate genus, although most do now, especially after the recent Brusatte, Carrano and Benson phylogeny) but fairly large—again, up to nearly T. rex sized, this guy appears to have appeared fairly late in the Morrison.  Some studies even suggest that it may have replaced the megalosaurs like Torvosaurus as the "predator larger than Allosaurus" role.  It's also better known from not only the latest Brushy Basin member of the Morrison, but also the southern range—finds have been in Oklahoma, New Mexico.  This may suggest that it lived and hunted in a different environment than the other Morrison predators; one that was less conducive to preservation. Or, it may mean that it was simply rather rare and late-appearing compared to Allosaurus.

Datanglong guanxiensis. This is known from only a bit of backbone, hip and ribs from the Xinlong Formation in the Early Cretaceous.  Not much is known about either the formation or the animal, but it appears to be a very basal carcharodontosaurian.  Or, it may be a carcharodontosaurid or neovenatorid. We don't really know.

Chilantaisaurus tashuikouensis.  Another poorly known critter from the Ulansuhai Formation of Inner Mongolia which appears to be from the Turonian period of the Cretaceous—rather late, actually.  It was once thought to be Albian-Aptian due to faunal similarity, which suggests that the "Morrison pattern" of fauna (speaking very broadly—also called the "Jurassic aspect" when referred to some Cretaceous fauna that had broadly similar animals by Thomas Lehman) was more common and lasted longer than many thought.  Chilantaisaurus does appear to be a "good" neovenator, which are the latest appearing family of carnosaurs, for the most part, and which seem to have prolonged the presence of carnosaurs, possibly all the way to the end of the dinosaur era.

Neovenator salerii.  Known from the Isle of Wight, Neovenator was a modest-sized 25 feet or so long and rather gracile in build.  The fossils are very fragmentary, and like many British fossils, exactly what it was and what environment it lived in is hard to determine.  Neovenator is probably more famous for bouncing around a ton in various family trees than it is for anything else.  It was from the Barremian (125 million years ago) and probably lived alongside Iguanadon, Baryonix, and Polacanthus.

Within Neovenatoridae we have the family Megaraptora.  There are a few folks who are captivated by some similarities to the tyrannosauroids and believe Megaraptorans to be related to them, but the mainstream and most likely view is that they are a family of carnosaurs, nested within Carcharodontosauria (but not Carcharodontosauidae) and therefore most closely related to them (the phylogenies by Novas and Porfiri that recovers them as coelurosaurs close to the tyrannosaurs also recovers carcharodontosaurs there as well.  It's not a very trustworthy result.  Wikipedia seems to suggest that nesting with with the tyrants is the last word, but it isn't.)  The interesting thing about them, of course, is also their temporal placement; they seem to start right about the time that the carcharodontosaurs are running out the clock, and the earliest one seems to be a specimen from Australia at about 110 million years ago.  This suggests that that's where the family evolved from somewhere within Neovenatoridae and spread from there.

Fukuiraptor kitadaniensis. One of the few dinosaur fossils from Japan, Fukuiraptor is a modest-sized and slender animal.  Although the type specimen is supposed to not be fully grown, it was found with some other individuals, all of whom were even smaller.  From the Cretaceous Barremian (129-125 million years ago).  It's about 16 feet long, and is known from mostly leg bones, a few arm, jaw and teeth fragments.  Little is known about dinosaurs in Japan in general; needless to say volcanic islands are not super ideal for preservation of fossils for millions of years.

Siats meekorum.  Found in the Mussentuchit Member of the Cedar Mountain Formation (Cenomian Late Cretaceous; about 98 million years ago) Siats is interesting for a number of reasons.  The skeletal remains are a bit on the scanty side, but by using other neovenatorian specimens to scale up what we have, they suggest that Siats was already in the same ballpark as the biggest therapods, in spite of the fact that several features of the skeleton suggest it was not fully grown.  It's also late appearing, at a time when it was thought that carnosaurs had abandoned North America in favor of early tyrannosauroids.  Exactly what a megaraptoran was doing in North America at the time is unsure.  It would have replaced Acrocanthosaurus (a carcharodontosaurid and apex predator of the earlier members of the Cedar Mountain Formation) and preceded the dominance of tyrants.  Indeed; the upper Cedar Mountain is much more "Asian" in character than the earlier Cedar Mountain, which had a more traditional Gondwanan assemblage; or perhaps as Lehman calls it, they retained a "Jurassic aspect" well into the Cretaceous.

Siats lived alongside some early ankylosaurs (Animantarx, Cedarpelta, Peloroplites) some indeterminate neoceratopsian and pachycephalosaur remains, ornithopods like the iguanodont Eolambia, the medium-sized Tenontosaurus might still have survived that long, and the advanced hypsilophodont Zephyrosaurus, a brachiosaur (Abydosaurus), and of course the famous Deinonychus that is the specimen most responsible for launching the Dinosaur Revolution.

The whole Cedar Mountain formation is only recently described; it's only been known since the 90s, really.  There's no doubt still much to be learned about it and the gap between the passing of the torch from the "Jurassic aspect" faunal assemblages and the "Asian Cretaceous aspect" faunal assemblages is poorly understood.  The upper Cedar Mountain, and middle Cedar Mountain assemblages are probably the answer there, but we need to know more about them first.

Aerosteon riocoloradense. "Bird bone" was a medium-sized (about 30 foot) megaraptor from Argentina's Anacleto Formation; Santonian (84 million years ago) of the Late Cretaceous.  Again; the remains are somewhat scrappy and may not represent a fully mature individual; and they were discovered before the existence of the megaraptoran clan was understood, so it was a bit of a mystery for some time, as it didn't fit into any of the known therapod families at the time in South America (spinosaurs, carcharodontosaurs, abelisaurs.)  The faunal assemblage of the Anacleto formation is not super well-known; a small ornithopod, some titanosaurs, and abelisaurs are known, along with a few lizards and crocodylians.

Australovenator wintonensis.  An Australian megaraptor (as you can probably guess by the name) from the latest Aptian (95 million years ago) from a site that tells us little about the faunal assemblage that it occupies other than that a sauropod lived there too.  It was only about 20 feet long and relatively lightweight, and has been informally described as "the cheetah of its time.)  It's very similar to the earlier appearing (Albian 105 million years ago) therapod Rapator and the two may be synonymous, or it may be a descendant of the latter.

Rapator ornitholestoides.  Speaking of which, Rapator is known from very poor remains; only a few bones.  In fact, it took the discover of Australovenator and some of the other megaraptors to make sense of Rapator's remains.  If Rapator were built like Australovenator, then it would probably have been bigger; about Allosaurus sized.  It comes from the Griman Creek Formation, as mentioned above, quite a bit earlier than Australovenator, and it's not really known what was in the Griman Creek formation other than some kind of sauropod, some small ornithopods, and the large iguanodont Muttaburrasauus.

Megaraptor namunhuaiquii.  From the Turonian to Coniacian (somwhere between 93 to 86 million years ago) of Patagonia, Megaraptor is what started the journey of the neovenatorids.  For a long time, it really confused paleontologists.  It's large hand claw was interpreted as a dromaeosaur-like foot claw, then a spinosaur-like feature, and finally it was recognized as a defining feature of the megaraptorans themselves.  A good 26 feet long, it was a decent-sized carnivore.  A juvenile specimen is being used specifically to advance the tyrannosauroid claim.  I'd like to see that go away—unless of course, I'm wrong and it turns out to be the correct interpretation after all.  But I seriously doubt it.  There's no explanation for a radiation of tyrannosauroids in Gondwana that are significantly disparate from what happened in Laurasia.  And if they are tyrannosauroids, they still have some pretty amazing similarities to the carcharodontosaurs and even the spinosaurs, dromaeosaurs and abelisaurs.  I think the real story here is how extraordinarily conservative the bauplan of therapods was that it remains so very difficult to classify them with confidence.

Orkoraptor burkei.  Another South American animal, Orkoraptor was a rather late appearing megaraptoran from the Pari Aike Formation in very southern Patagonia.  Initially believed to be from the Maastrichtian, that would have extended the carnosaurs (unless of course the megaraptors aren't carnosaurs at all) considerably.  Actually, it still does, but it's now believed to be from the Cenomian from about 95 million years ago.  Not much is known about this formation other than a few ornithopods and the giant titanosaur Puertosaurus are found here.

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