Wednesday, November 30, 2016

Meet the Tyrants, Part I: Proceratosaurs

The tyrant lizards are a rather diverse family, and can be divided roughly into five groups, with a sixth that is somewhat dubious and who's relationship to the rest of the family (if it is indeed one at all) is uncertain.  Due to the strange way in which cladograms are constructed, one of those five groups is kind of "everyone else who doesn't fit into one of the other four"; i.e. "basal" tyrannosauroids.  For this first catalog, I'll be covering the proceratosaurs, which are the earliest appearing and generally the most primitive, showing marked gross morphological similarity to other basal coelurosaurs, like say, Ornithomimus or the Coeluridae.  In fact, the Coeluridae have occasionally been recovered in some studies as very basal tyrannosauroids (I won't do that for my purposes here; I think it most likely that they are basal coelurosaurs, and possibly the sister group to tyrannosauroids at best; which seems to be the most common belief among practicing paleontologists today.)  Although I don't accept that placement, the point is that guys like Tanycolagreus, Ornitholestes, and Coelurus, who are found in the Morrison Formation, are very similar to guys like Stokesosaurus, which is a tyrannosauroid and in fact they have often all been confused for one another at various times without very careful analysis. This highlights the environment, if you will, in which the tyrants got their start and the type of animal that the earliest tyrants really were.

Because different cladograms have different data points, i.e., sometimes basal tyrannosaurs are included in proceratosauridae, which in other cladograms will not be, etc., I've had to settle on one way of doing things.  For convenience, because it breaks up the genera into more or less equally sized groups (more or less) I'm going to be following Hone's "standard" cladogram, more or less, with a few changes from the very recent Brusatte and Carr cladogram that I posted in my last post, and the Loewen cladogram (specifically, the inclusion of Alioramini within Tyrannosaurinae.  So, proceratosaurs with this post, then the rest of the tyrannosauroids who don't manage to belong to Tyrannosauridae specifically.  There really should be a better "paraphyletic" family name for this group in common usage (like we have for iguanadonts, or hypsilophodonts or prosauropods) but there isn't, so rather than type out "basal tyrannosauroids" all the time, I'll probably call them 'roids for short.

Proceratosaurus bradleyi. The earliest (or tied for that position anyway) tyrant is known from an incomplete skull and lower jaw, missing the top/back of the skull, but we have the mandibles, the nares, and a bit of the forward skull roof.  It's from the Bathonian (Middle Jurassic; 167 or so million years ago) in the Cotswold region of southwest England.  It's remains aren't much to go on, in fact until just a very few years ago, it wasn't known what kind of animal this really was.  Because it had a nasal crest, it was presumed to be a small ancestor to the Morrison therapod Ceratosaurus, hence the name.  Later, it was recovered as a generic "coelurosaur" and considered to be similar to Morrison therapod Ornitholestes.  Based on the size of the skull, the entire animal was fairly small; similar to a medium to large sized dog like a collie or labrador and would not have come up to a normal person's waist.  It was recovered quite early (1910) and at the time, little care was made to attempt to link the skeletons to a fossilized ecosystem, if it were even possible, so the environment and surrounding fauna in which it operated is unknown—although nearby fossils from the same age (roughly) include Megalosaurus, primitive sauropods (possibly Cetiosaurus) and some indeterminate ornithopods.  Also from the same general area (although probably a few million years earlier) are the indeterminate Iliosuchus remains.  Nobody knows for sure what these are, because they're too fragmentary to be diagnostic, but they may represent an even earlier (and smaller) earlier tyrannosauroid of some kind.

Kileskus aristotocus. Aside from the possible Iliosuchus, Kileskus is the next proceratosaur, found in Middle Jurassic (probably also Bathonian) strata in southern Siberia.  Not only is it about the same age as Proceratosaurus, curiously it's also about the same size (maybe a little larger, although based on the sample size of one for each, I wouldn't make much of that), and mostly known from very similar fossils; most of the head and a handful of foot and hand bones.  Like many of the proceratosaurids, it had a bony head crest along the nose, although apparently not as dramatic as that of Guanlong.  Little is known of the region in which these fossils were found, so little can be said about its environment or contemporaries as of yet.  That said, its discovery in 2010 was the catalyst that led to the recognition of proceratosauridae as a valid family, and as an early radiation of the tyrant's lineage.

Guanlong wucaii. A little bit later, at 160 million years ago in the Oxfordian of Chinese Turkestan, we find Guanlong.  This guy is known from two relatively complete specimens; a fairly complete adult and an even more complete subadult.  Because of the completion of the specimens, they are useful in reconstructing what more incomplete forms were probably like (like Proceratosaurus and Kileskus.)  At about 10 feet long, much of it tail, Guanlong was about the same size as the former two as an adult, and it shows that tyrants still had fairly long, grasping three-fingered arms.  Its crest is quite tall and delicate, significantly larger in the adult than in the subadult.  In fact, quite a few changes occurred physically between the adult and subadult, including the spreading and growing of the crest, the younger specimen had a comparatively larger orbit, hand, and smaller pubic boot.  Like other rather undifferentiated coelurosaurs, it had a comparatively small head (compared to later tyrannosaurs, certainly), longer neck, slender build and jaws, and overall probably fairly speedy build.  It is from the Shishugou Formation, which is relatively well known with many sauropods, a stegosaur, a tiny, extremely basal ceratopsian (Yinlong) and larger carnosaurs and other primitive tetanurans—all in all, a very similar environment to the Morrison at least in terms of comparable fauna.  It might have competed directly with basal coelurosaur Zuolong which was almost exactly the same size and build, although the stratigraphy isn't completely clear to me; they might not have overlapped.  The environment was a marshy woodland not far from a small, occasionally active volcanic mountain range, and the deposits tend to be fluviatile and lacustrine with a fair bit of volcanic ash.  Because this type of environment is the best for preserving fossils, that isn't necessarily meant to mean too much, though—most dinosaur-bearing fossil beds were similar (with some notable exceptions) but that isn't meant to mean that dinosaurs didn't live in places were erosion was happening, merely that their remains are extremely unlikely to have been preserved.

Stokesosaurus clevelandi.  This Morrison Formation fossil is known from only pretty scanty remains; the holotype is a broken pelvis from the Tithonian of the Brushy Basin member at the Cleveland-Lloyd quarry in central Utah (150 million years ago.)  Another smaller ilium was recovered from South Dakota a few million years earlier, and some other material has been occasionally referred to the genre, but in general, all of that is dubious, and many of those have since been rescinded.  Little is therefore known of Stokesosaurus, but it appears to be, like the three above, a fairly small creature, about 10-13 feet long as an adult, maybe.  For contemporaries, Stokesosaurus has the entire rather famous panoply of Morrison dinosaurs: Allosaurus, Diplodocus, Apatosaurus, Stegosaurus, Ornitholestes, etc.  If you were a kid growing up loving dinosaurs in the 70s and 80s like I was, almost every creature referred to the Jurassic that you've heard of was a contemporary of Stokesosaurus.  The Morrison was a large semi-arid basin with seasonal monsoon-like weather patterns where rivers flowed from the newly rising Rocky Mountains to the epeiric Sundance Sea.  Fern savanas made up much of the environment, and riparian forest valleys, and seasonal flooding and volcanic ash were periodic threats; although aeolian dune-like conditions persisted in the southern end of the formation.

Juratyrant langhami.  Although discovered in 1984, this fella wasn't described until quite a bit later; 2013, specifically.  Although initially believed to be con-generic with Stokesosaurus, it was later determined that many of the features that unite them were in fact quite common among all kinds of Jurassic tyrannosauroids.  Cladistic analysis still shows the two of them as the closest to each other among the proceratosaur family, however, with Sinotyrannus (see below) the next closest relative.  Juratyrant is quite a bit more complete than Stokesosaurus, and combined with the rather large distance between them, it was decided that it was too speculative to assume that they were the same genus, so the name Juratyrant was erected.  It comes from the Kimmeridge Clay formation of Dorset (although not from the Kimmeridgian period of the Jurassic) Tithonian age, about 150 million years ago, like the Morrison, but a more coastline habitat.  A number of dinosaur remains are known from the same formation, but few of them are complete enough to be diagnostic; although indeterminate larger therapods (Megalosaurus?), sauropods (Cetiosaurus?), and ornithopods and the stegosaur Dacentrurus are known from the region.   I've seen size estimates for Juratyrant that range from 10-20 feet, so it would have been (so far) possibly the largest of our specimens; although built lightly and probably rather speedy, it would have weighed maybe as much as the American black bear.

Sinotyrannus kazuoensis. The largest and latest of the proceratosaurs, Sinotyrannus is from the Jiufotang Formation from the Aptian of the Early Cretaceous, 120 million years ago.  This is the formation that is famous for it's lacustrine birds and feathered dinosaurs, including two-winged tiny maniraptorans and what-not that were buried in volcanic ash following eruptions nearby—newly famous critters like Confuciusornis and Microraptor.  Although famous for small fossils, which are normally quite rare, the Jehol biota captured here does not reveal it's secrets with regards to larger fauna very easily.  Psittacosaurus is known from the area, small caudipterids, some scanty ankylosaur remains, etc. No doubt many other animals lived here, and the slightly younger Yixian Formation (which I'll have to refer to later) has a more balanced faunal view.  Sinotyrannus is the largest known predator from the entire formation and one of the largest from the entire 11 million year old Jehol biota spread.  It was initially reported rather sensationally as "as big as late Cretaceous tyrannosaurs like T. rex" but that's not really true.  Although known from reasonably good remains, it's not complete, of course, and estimates for its length range from 20 to over 30 feet, with a weight of up to close to two tons—although probably considerably less (like I said, the early estimates were pretty wild.  I'd think it'd be comparable in weight to a large grizzly most of the time, possibly more in exceptional individuals.)

Note: According to the 2016 Brusatte and Carr cladogram, Stokesosaurus and Juratyrant are 'roids, not proceratosaurs, while Yutyrannus on the other hand is a proceratosaur instead of a 'roid.  The "standard" cladogram prior to that had that reversed, so we'll talk about Yutyrannus next time.  I suspect that over time, the Brusatte and Carr cladogram might gain more steam, but like I said earlier, the one presented by Hone was more convenient for me in terms of breaking the groups up into more or less like-sized chunks.

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