Friday, February 03, 2017

Meet the Carnosaurs, Part III: Carcharodontosaurs

I've organized these posts so that each is more or less the same size, but the unfortunate side effect is that other equally (or maybe even more) sensible arrangements are not done.  Ideally, I'd have done basal forms, sinraptors, allosaurids, carcharadontosaurids and neovenatorids in that order, because that's how they appear chronologically in the fossil record, more or less (with some overlap, of course.)

After the basal forms, which are all poorly known and their antecedents are also unclear, we get the sinraptors, mostly in China in the late Middle and earlier part of the Late Jurassic.  The actual allosaurids are next, but seem to be a group lacking in much diversity, from North America and Portugal and (possibly) Africa at the end of the Jurassic, overlapping with later sinraptors.

During the earliest Cretaceous, we have only scanty remains and poorly researched formations that don't tell us much about the apex predators, but by the "middle" Cretaceous, carcharodontosaurs seem to have emerged as the dominant predators globally, with similar forms from Africa, North America, South America and even Asia.  Then near the end of the period of carcharodontosaurid dominance, the neovenators take over for a time, although by now there is no longer a "global fauna" and tyrannosaurs and abelisaurs are starting to become more dominant in most regions.  During the last 15-20 million years of the Cretaceous, there don't seem to be any carnosaurs left, with the possible exception of some poorly dated and controversial remains here and there (Orkoraptor was long held out as a Maastrichtian carnosaur, but it appears to have been earlier in time after all.)

The carnosaurs also appear somewhat ex nihilo; or rather, we don't have much to go on with basal forms, so we have little to no understanding of where they came from exactly.  Although they are part of the avetherapods, and therefore sister group to the coelurosaurs (avetherapods are themselves the sister group to the megalosaurs... which I guess are "cousin" group to the carnosaurs, then?) this lack of early forms is difficult.  In addition to that, as the neovenators replaced the carcharodontosaurs who replaced the allosaurs and sinraptors... well, none of these groups derives from within the other, so there are long ghost lineages somewhere that may yet throw our understanding of these animals and their relationships off considerably.  There is in fact a fair bit of controversy already, although this mostly seems to come from South American paleontologists who make proposals that otherwise seem to be outside of the mainstream, i.e., that maybe carcharodontosaurs are closely related to abelisaurs and/or tyrannosaurs, for instance, or that megaraptors aren't actually neovenators at all, but are instead tyrannosauroids.  The establishment of some specimens that fill in, even if just a bit, this ghost lineage will help to clarify considerably.

In any case, I'm ignoring these non-mainstream opinions for the most part, with the expectation that as we gradually find more specimens and start to close the gaps in some of those long ghost lineages, that the mainstream opinion will be proven correct after all.  In doing so, well—we find that the carcharodontosaurs are a surprisingly successful group of carnosaurs; the earliest ones were found in the late Kimmeridgean (150 or so million years ago) and they stretch easily to the Turonian of the Late Cretaceous (89 million years ago) with their greatest extent of success ramping up in the Barremian (127 million years ago).  Although keep in mind that the earliest Cretaceous periods are not well known and exactly what large predators dominated their ecosystems is unknown at this time.  Some form of carnosaur—allosaurid or early carcharodontosaurid—seem likely for most regions.  As with most carnosaurs, they seem to have dominated in ecosystems in which very large sauropods were the largest herbivore as well, and some thyreophorans, along with ornithopods, usually of the iguanodont grade.  They are nowhere known in areas where ceratopsians and hadrosaurs make up the dominant herbivores, nor do they seem to be common during the ages when more modest sauropods lived among abelisaurs.  Although this may, of course, be a preservational artifact; we never know for sure what we don't know about formations and time periods about which our evidence is poor.  Some isolated teeth from Brazil may suggest that carcharodontosaurs lived on almost to the very end of the Cretaceous; they are found from the Campanian or maybe even Maastrichtian age rocks.  But this evidence is dubious; they might well be something else entirely, or the dating may be off.  As with the tyrannosaurs—even moreso, in fact—there are a lot of questions to be answered.

Another curious fact—not really very important, but one that gets lots of attention nonetheless—is that there are a number of quite large specimens here, including no less than four that are at least as big as T. rex if not larger.  Of course, our sample sizes being what they are, this doesn't really mean much.  But it does mean that the carcharodontosaurs, in spite of their jaw-cracker names and relatively recent vintage, are quite well known to the general public.  Even if all they know of them is their name and the fact that they're so big.

Acrocanthosaurus atokensis.  This quite large early carcharodontosaur lacks a number of features peculiar to the group.  Before the carcharodontosaurs were defined in the later 90s, it was assumed to be a close relative of Allosaurus and some researchers continue to prefer putting it there rather than with the rest of the shark-teeth.  Most phylogenies recover it as a primitive carcharodontosaur, however.  It was found in at least three formations: the Twin Mountains of north Texas, the Antler Formation of southern Oklahoma and the Cloverly Formation of Wyoming's Bighorn Basin.  None of these have been radiometrically dated, but appear to be entirely within the Aptian Stage; 112-125 million years ago.  It is also believed to be present in the Glen Rose Formation of Texas that lies over top of the Twin Mountains rocks, so it may have been relatively long-lived as a taxon and crossed into the Albian stage as well.

Acrocanthosaurus may have also been found in Maryland and Florida, and elsewhere in North America, but this is based on teeth and other rather non-diagnostic remains, so that's not for sure.  For much of its range, the environment was a coastal floodplain next to the epicontinental sea that would later expand and become the Niobrara Sea (or Western Interior Seaway) splitting North America into island continents Laramidia and Appalachia.  All of the formations in which it is found are somewhat poorly known in terms of what the complete faunal assemblage was like, but all of them also feature, in addition to Acrocanthosaurus itself, Deinonychus, Tenontosaurus and either Astrodon or Sauroposeidon or both.  Small hypsilophodonts are known in Cloverly, as are a few early nodosaurs.  A few other small therapods, some titanosaur backbone, and a Psittacosaurus-grade ceratopsian are also known from the Cloverly, while the Glen Rose adds yet another sauropod.  All in all, it would have still maintained a rather Morrison-like aspect, although less dry.
Some significant work has been done on Acrocanthosaurus systematics, including detailed CAT scans of its braincase (determining that it had a crocodile rather than avian style brain with a good sense of smell), fairly light, long and narrow head with huge antorbital fenestrae, and rather limited forearm movement.  And of course, it's very famous for its tall neural spines, which were not nearly as dramatic as that of Spinosaurus, but which would have been unmistakable nonetheless.  It was unlikely that it had a low sail, but it is believed to have been a "buffalo hump" type feature that ran the length of the animal's back.

Concavenator corcovatus. Another early (and basal) member of the shark-teeth is Concavenator from Barremian Spain (130 million years or so ago.)  Much more modestly sized (about 20 feet long, compared to nearly 40 for Acrocanthosaurus) this guy is famous for having what may be quill nodes on its foresarms as well as a very odd structure on its lower back that would have appeared as a very short yet tall triangle over its hips.  No other therapod has a similar structure, so nobody is quite sure what to make of it; although some kind of back adornment supported by neural spines on the vertebrae seems to be a feature of many of these basal carcharodontosaurs.

The La Hoya fossil site that it comes from is an inland lacustrine environment, and most of the fossils are beautifully preserved freshwater fish, worms, arthropods, molluscs and many crocodilians.  Some early birds are also found at the site, and the basal ornithomimosaur Pelecanimimus.

Eocarcharia dinops. The "dawn shark" a reference to it being a relatively early member of the shark-tooth-saurs is not actually all that early appearing.  Eocarcharia is from the Erlhaz Formation of Niger of the Albian-Aptian border (about 112 million years ago) which puts it smack dab in the center of the heyday of the carcharodontosaurs, in spite of its basal position in phylogenies.  It was a more modest sized animal, about 26 feet long, without much in the way of distinguishing physical features.  Its co-Formation inhabitants include Kryptops, an early abelisaur, and Suchimimus, a very large spinosaurid (Spinosaurus itself was from the formation over top of this one, so later in time), a rebbachisaurid sauropod (Nigersaurus), two large iguanodonts, both with unique features—Ouranosaurus with its large back sail and Lurdusaurus with its especially thick and robust skeletal build and extra long neck, and the dryosaur Elrhazosaurus.

Kelmayisaurus petrolicus. A formerly poorly known (well, it still is—some jaw fragments is all we've got) specimen from the Lianmuqin Formation somewhere between the Valangian and Albian stages (140-125 million years ago) of Cretaceous Chinese Turkestan, Kelmayisaurus was recently found to actually be diagnostic, and that it does nest as a basal carcharodontosaur.  Everything in the Lianmuqin Formation is very poorly known, but it seems that Kelmayisaurus may have passed some basal coelurosaurs on its morning commute, as well as Psittacosaurus, some sauropods, and the latest known surviving stegosaur in the world.

Sauroniops pachytholus.  Named to literally be "The Eye of Sauron" this somewhat primitive carcharodontosaurid, and a relatively large one over 30 feet long, was curiously found in the same time and place as Carcharodontosaurus itself; Cenomian Morocco (93-99 million years ago.)  It's only known from a skull roof, but it differs substantially from Carcharodontosaurus.  In fact, the rugose and extremely bumpy skull roof has really captivated the imagination of researchers, some of whom imagine it banging heads like mountain goats.  From the Kem Kem Beds, its neighbors would have been, in addition to Carcharodontosaurus, a wide variety of predators: Deltadromeus, Elaphrosaurus, Majungasaurus and Spinosaurus, the largest and most famous of the spinosaurids.  Lots of crocodylomorphs here as well, and several fishes; the Kem Kem beds are described as representing a former wide river delta environment that was in the process of shrinking and thus bringing together a number of animals that might otherwise have preferred different habitats and wider separation (it's also not clear that everything found in the Kem Kem beds is really from the same time period, and they may have been spread out in time.)  I'm not sure exactly what all these predators would have hunted, as Rebbachisaurus is the only known herbivorous dinosaur, although ornithopod footprints have also been found.

Shaochilong maortuensis. A small (16-20 ft) found in the Turonian (89-93 million years ago) of Mongolia is interesting, in part because it's among the latest found carnosaurs in Laurasia (later even than Siats of the megaraptors—although plenty of late appearing megaraptors are found in Gondwana) suggesting that the dominance of tyrannosaurs in the north was really quite late.  Chilantaisaurus is also found in the same formation, which is poorly known.  It also has an ankylosaur, an ornithopod of some kind, and an ornithomimid.  It's also one of only two Asian carcharodontosaurs known, which is unusual, because Asia is also the cradle of tyrannosaurs (although they also developed in parallel to a great degree in North America.)

Veterupristisaurus milneri.  Another jaw-cracker name, which means "old shark lizard", Veterupristisaurus is the oldest known carcharodontosaurid, from the Tendaguru beds of late Kimmeridgian and early Tithonian (155-150 million years ago) Tanzania.  This was often compared to the Morrison, and many of the species were believed to be the same as the Morrison, but further discoveries and further review have shown that it was not as clean-cut as it was once thought.  Veterupristisaurus was about Allosaurus-sized and probably played its role in the fauna.  It was originally thought to be a large local variant of Ceratosaurus.  The African Brachiosaurus was also renamed as a different animal, Giraffatitan, the African Diplodocus was renamed Tornieria, etc.  It was still a very similar faunal assemblage to that of the Morrison, or the Porto Novo of Portugal, but the differences are just enough to make it interesting.

Carcharodontosaurus saharicus. At this point, Carcharodontosauridae develops the more specialized node, Carcharodontosaurinae, which contains Carcharodontosaurus itself, plus the Giganotosaurini subfamily, which contains the large South American forms.  Although named before WW2 by German paleontologists working in North Africa, the remains were lost and had to be rediscovered in the form of new specimens by Paul Sereno in the 1990s.  When combined with the exciting finds coming out of South America, the Carcharodontosaurid family was finally recognized.  Carcharodontosaurus probably had two species, both of them quite large, rivaling T. rex in size.  I already described the environment in which he lived in the Sauroniops entry, so I won't repeat it here.

Carcharodontosaurus was pretty typical for the family; large sized, narrow but long snout, three-fingered relatively powerful (but somewhat inflexible) forearms, robust design that does not indicate a particularly fast animal (although with its stride length, that probably doesn't really matter too much. Plus, if these guys really did specialize in hunting large sauropods, as believed, then they probably didn't need to be super fast anyway.)  It's also famous for, of course, it's shark-like teeth, which gave the animal and the entire family its name.  Unlike the powerful robust teeth of the late appearing tyrannosaurs, they didn't allow for a particularly powerful bite, and presumably the carcharodontosaurs (and for that matter, most large dinosaurian predators) had teeth designed to slash flesh rather than puncture or crush bone.  Many researchers have even proposed that they didn't have to kill their prey; if they just "snacked" on the flesh of sauropods, they probably could survive and even heal to a great degree from those kinds of injuries, and keep on going to be snacked on later.  Otherwise, they probably dealt massive wounds that removed large chunks of flesh and then let shock and blood loss weaken its prey enough for it to continue attacking at its leisure and with less risk.

There is some evidence, mainly from the South American forms, that it may have hunted in packs as well.

Tyrannotitan chubutensis. This is one of the earlier carcharodontosaurids from the Aptian aged rocks; so about the same as Acrocanthosaurus.  It's much more derived, though—nested well within Giganotosaurini, although at the base of that clade.  Not much is known of it's environment other than it's been interpreted as a return to rather arid conditions after moister floodplain environments which preceded it.  Large titanosaur Chubutisaurus was probable prey for Tyrannotitan.  As the name implies, there were a number of features that somewhat superficially resembled tyrannosaurs, and which captivated some early researchers into proposing a link between carcharodontosaurs and tyrannosaurs.  Sigh.  Everyone wants to tie everyone to the tyrannosaurs.  I prefer, obviously, the carnosaur interpretation as much more robust and well established for both carcharodontosaurs and megaraptors.

Giganotosaurus carolinii. The largest and first discovered of the South American carcharodontosaurs, Giganotosaurus has been rather famous.  It appeared in the rather excellent Chased by Dinosaurs pseudo-documentary with Nigel Marvin, where it's hunting Argentinasaurus, the largest known sauropod.  This isn't exactly accurate, as they are not from the same formation—but Giganotosaurus would no doubt have chased Andesaurus instead; another very large basal titanosaur from South America.  It's curious that the basal titanosaurs are quite large; but later they were replaced in South America by much more modestly sized derived saltasaurs.  This seems to have coincided with the end of the large carnosaurs as well.

Giganotosaurus also had iguanodonts, dromaeosaurs, smaller rebbachisaurid sauropods, and relatively large abelisaurids.  It comes from the Cenomian age (same as Carcharodontosaurus 100-97 million years ago) Candeleros Formation of Neuquen, formerly called the Rio Limay Formation.  It is overlaid by the Huincul Formation (which includes Argentinasaurus and Mapusaurus) which is in turn overlain by the Lisandro Formation which does not (at least so far) have any carnosaurs in it.

Mapusaurus roseae. The last appearing carcharodontosaur of the South American procession, Mapusaurus may also have been the largest, according to some fragmentary remains.  It's also known from a bone-bed with several individuals of various age and size—which is always breathlessly held out by many as evidence of packing behavior, although it isn't really.  At least not necessarily.  It could also be mobbing, which is gregarious, but not cooperative exactly.

Mapusaurus was also found in a riparian depositional environment in what was otherwise determined to be a rather arid climate.  Not terribly unlike the Morrison in that regard, curiously.  Argentinasaurus was the large sauropod; the largest sauropod, or at least a credible contender for that title.  The Huincul is otherwise not a very good depositional environment for fossil bones, actually, so it's rather poorly known.  A study of the various individuals also determined that Mapusaurus displayed heterochrony; a condition in which ancestral conditions are present at one age stage but later go away.  This is significant, because it means that it's not hard to think you're seeing more diversity in the fossil record than you actually are.

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