Monday, December 05, 2016

Meet the Tyrants, Part III: Albertosaurines and Alioramines

Today's post combines two small clades for purposes of discussion.  We've finally arrived at the "Tyrannosauridae" proper, within "Tyrannosauroidea"—so in layman's English, we can say the "tyrannosaur family proper" within the extended family of "tyrannosaur-form cousins".  Of course, within the tyrannosaur family proper, there are two subfamilies, the albertosaurs and the tyrannosaurs proper.  And then within the tyrannosaur proper subfamily, there is a radiation of alioramine animals as a separate sub-subfamily.

We'll leave the bigger, more robust "classic" tyrannosaurs for the last post in the series, and talk about the albertosaurs and alioramines today.  Although the albertosaur subfamily is nested outside of the classic tyrannosaur subfamily and the alioramines are nested within it, the two groups yet share a number of superficial similarities.  In fact, the alioramines, which have proven difficult until more recent classifications based on more detailed fossils, are only recently confidently classified at all, and Alioramus has often been considered in all kinds of places all over Tyrannosauroidea.  As one could guess, based on the name, Albertosaurus itself is known from Alberta most especially, and as it happens, the entire subfamily is more confined to the northern part of Laramidia, whereas early classic tyrannosaurs may have been more common in the southern reaches of the continent.  Alioramines are an Asian clade.

Gorgosaurus libratus.  Gorgosaurus is the earliest appearing of both groups, most especially common in the Dinosaur Park Formation of Alberta of the Middle Campanian, which would put it at about 75-77 million years ago.  It also may have been present in the Two Medicine and Judith River formations which would expand its range both geographically and temporally, but the remains are a bit spotty and many belong to an albertosaur instead.  Alberta's Dinosaur Park Formation is the only for sure location for Gorgosaurus.  It is a doozy, though—one of the better known formations, with a real wealth of ecosystem data.  Due a combination of continental placement and other climate factors, the earth was quite a bit warmer than it is today, and the sea levels were quite a bit higher.  The Western Interior Seaway flowed across what are today the Great Plains, connecting the waters of the Arctic Ocean to the Gulf of Mexico, and splitting North America into two island continents; the longer and thinner Laramida in the west and Appalachia in the east.  Gorgosaurus was then found in the northern shores of floodplains coming down from the new Rocky Mountains into this seaway.  The climate was sub-tropical—like the Gulf Coast today in terms of weather, with a heavily forested environment with marked seasonality, including periods of drought and flooding, which seems to have been the source of much of the mass bone-beds that make the formation so famous.  The Dinosaur Park Formation is overlain by a marine sediment, so the sea grew at the end of this period and flooded this particular area, at least.

Like everyone else in this subgroup, Gorgosaurus was large, but smaller than the classic tyrannosaurs; up to maybe 30 feet long at the largest, and rhino sized in weight.  Compared to classic tyrannosaurs, he was relatively small, and especially light-weight, with longer legs, and a longer and less tall snout.  Most likely, it was faster, and may have focused on quicker, more lightweight prey—or at least have been capable of attempting pursuits of such.  One of the curious notations about Gorgosaurus is that it appears in the same time and place as another large tyrannosaur; Daspletosaurus which is an earlier and smaller classic tyrannosaur, with a more powerful build and head.  It is unclear exactly how the two coexisted and what—if anything—they did differently to avoid direct, head-to-head competition.  Suggestions that Gorgosaurus was a more specialized hadrosaur hunter while Daspletosaurus was a more specialized ceratopsian or ankylosaurian hunter seems to be mere speculation, and good hadrosaur remains have been found in the stomach region or more than one Daspletosaurus specimen.  The ecological significance of two similar-sized tyrannosaurs in the same formation is as yet unexplained.

A number of very famous Late Cretaceous fossils were found in the Dinosaur Park Formation.  In fact, I have to admit I'm still just a little bit bitter that almost every classic dinosaur kids book that I read as a kid readily threw Dinosaur Park Formation animals into the Maastricthian and suggested that they were hunted by T. rex itself.  This is not the case.  Famous lambeosaurine (Corythosaurus, Lambeosaurus, Parasaurolophus) and somewhat less famous hadrosaurine duck-bills were common, and many famous ceratospsians (Centrosaurus, Chasmosaurus, Pentaceratops, Styracosaurus, Monoclonius, etc.) animals made up what was probably the most likely prey for both tyrants, and both nodosaurid and ankylosaurid armored dinosaurs are known.  There's also famous guys like Struthiomimus and Ornithomimus, assuming that those are actually two separate genera, dromaeosaurs and troodontids (including Dromeaosaurus and Troodon themselves), therizinosaurs, oviraptorosaurs, pachycephalosaurs, and at least one kind of hypsilophodont.  A curious although unexplained phenomena is the fact that of the four largest-bodied and well-known dinosaur groups from the area, subfamilies were common during the Campanian that later either became uncommon or went extinct altogether to be replaced by representatives from another subfamily within the same family that was previously rare: lambeosaurine hadrosaurs replaced with saurolophine hadrosaurs; the centrosaurine ceratopsians seemingly go extinct completely to be replaced by chasmosaurine varieties, the nodosaurs survive in Appalachia, but fade away in Laramidia to be replaced by ankylosaurs, and—of course—the Albertosaurs do not survive the Campanian, and the classic tyrannosaurs replace them.  The cause of this subfamily faunal replacement is not, to my knowledge, at all adequately explained.

As an aside, there are some small numbers of skin impressions of Gorgosaurus.  Although not meant to convey the entirety of the animals body, of course, it's worth noting that the very small sample that we do have has quite fine, bead-like scales, not terribly unlike what would be found on a Gila monster today.

Dinosaur Park formation
Albertosaurus sarcophagus.  As the Bearpaw sea which flooded Dinosaur Park receded again, Gorgosaurus fossils are replaced by the younger Albertosaurus, which was a very similar animal in many respects (there have been numerous attempts over the years to combine the two into a single genus, which is usually resisted.  But they are still similar enough that the attempt continues to prompt reexamination periodically.)  Although similar in many respects to the environment of the Dinosaur Park formation, it is believed that the Two Medicine Formation, in which most albertosaurs are found, was cooler and drier.  They are found in the boundary of the Campanian and early Maastrichtian, about 70 million years ago or so, up until maybe 68 million years ago at the youngest, at which point the more classic Lance and Hell Creek faunas appear to dominate.  That said, in some ways, the fauna in which Albertosaurus operated is still similar to the one in which Gorgosaurus did; chasmosaurine certatopsians like Achiceratops, Eotriceratops, and Arrhinoceratops have replaced the centrosaurines (although Pachyrrhinosaurus is still common) and the lambeosaurine hadrosaurs have similarly seen a great deal or replacement with saurolophine hadrosaurs, including early species of Edmontosaurus, Saurolophus and Hypacrosaurus.  The faunal replacement isn't as advanced as it is in later formations, but it's also clearly happening.  This is the changeover from the so-called Judithian faunal stage to the Edmontonian.

Another interesting fact about Albertosaurus specifically are the group fossil sites that have almost breathlessly been interpreted quite frequently as evidence of pack hunting.  It may or may not have actually been pack hunting (it could be more similar to Komodo-dragon style carcass mobbing, or shark feeding frenzies than true pack-hunting).  Nonetheless, the intriguing possibility of tyrannosaur hunting packs is a tantalizing idea, to say the least, and it has been popular ever since the discovery of the Dry Island bone bed.

Albertosaurus has also become a bit of a waste-basket taxon, as they call them, for any vague or non-diagnostic tyrannosaurian bone fragment.  It's possible that it extended outside of Alberta, and the name has been used to refer to fossils from the East, Wyoming, New Mexico, and even Asia—in many cases, these may well represent sub-adults of classic tyrannosaurs, or some unknown member of the albertosaur family at most, but probably do not actually represent Albertosaurus sarcophagus at all.  A number of specimens now identified as sub-adult or younger classic tyrannosaurs have also been referred to Albertosaurus in the past, sometimes even raising up new species that fall into ignominy as nomina dubia, or doubtful names.   There may well be more varieties of albertosaurs out there, we just have no good evidence of them today.

Due to the temporal and geographical placement of Albertosaurus relative to Gorgosaurus, it has been proposed that the former may be a literal descendant of the latter by some.

Alioramus remotus, altai.  The alioramine tyrannosaurs are not as well known as the others, and it has often been difficult to place them until some very recent studies.  To further complicate things, few of the taxa are known from fully adult specimens, and there has been vigorous debate about whether some of its unusual features are related to its juvenile age set, or if they represent primitive retained characteristics.  Because of this, the Hone cladogram (and the Loewen cladogram) both place alioramines just barely outside Tyrannosauridae proper, but the newer Brusatte and Carr cladogram reaffirm earlier efforts to place it in a basal position within Tyrannosaurinae itself.  Although basal, however, Alioramus is not early.  There are probably two species, A. remotus and A. altai the latter being known from more complete remains.

The alioramines are relatively lightweight and small with remarkably long, pointed snouts compared to other tyrannosaurines.  This snout is what has so mystified researchers especially, since a longer, less tall snout is both a basal trait in Tyrannosauroidea as well as a juvenile trait known from such classic, highly derived tyrannosaurines are Tarbosaurus and T. rex itself.  This snout seems to have been adorned with a very bumpy nose ridge as well.  The discovery of adult specimen Qianzhousaurus from southern China has probably cleared up the fact that this was merely a retained condition in this otherwise quite derived sub-subfamily.

Not only are the members of this family not particularly well known anatomically, but we also know relatively little about them.  Alioramus—both species—seem to be from the Nogon-Tsav beds in Mongolia.  Not much is known about this formation, although it is presumed to be the same age as the nearby Nemegt Formation, and may have shared a great deal of its fauna.  Of course, the Nemegt has also not been radiometric dated, so its exact age is unknown, but probably Maastrichtian, the final period of the Cretaceous and the end of the Age of Dinosaurs.  Believed to be wetter and more lush than the arid Barun Goyot and Djadochta formations on top of which it lies, forested river valleys and floodplains, subject to periodic drought, seem to be the norm.  Although not described in detail, the discoverer reported informally that Tarbosaurus as well as therizinosaurs and ornithomimosaurs were discovered at the same locality.  If the fauna is indeed similar to the Nemegt, which seems likely, then it probably had troodonts, pachycephalosaurs, very large hadrosaurs, titanosaurs and ankylosaurs as well.

Qianzhousaurus sinensis.  Only described in mid-2014, this is the critter that helps to clear up the relationship of the alioramines with the rest of the tyrants.  Long known by the nickname "Pinocchio rex" due to its extended snout, it shows conclusively that long-snouted tyrants were a real thing; an alternative breed of predators that were fairly wide-spread throughout Asia, and not merely a confusing mess of sub-adult classic tyrannosaurs with juvenile features.  As much as 30 feet long, this one was actually an adult, and it is found pretty far away from the Alioramus specimens, in southern China's Nanxiong Formation, which is believed to be from the very latest Maastrichtian.

A list of the relatively few animals known from the Nanxiong Formation is not a "who's who" of late Cretaceous dinosaurs; it's more of a "who did you say again?" Formation with a very diverse set of oviraptors, but also a hadrosaur, a titanosaur, and a therizinosaur.  It's believed to have been similar in many respects to the Nemegt Formation, which we'll talk a lot more about next time when we discuss Tarbosaurus.

All in all, much of the Asian fauna, with the exception of the longer-known Mongolian beds described by the Soviets, there are a lot of questions and a lot of work to be done to really understand better what the Asian late Cretaceous ecosystems were like.  These are not nearly as well known as the earlier Yixian Formation and other similar northern China formations, and a lot more work needs to be done to better describe the alioramines and the environments in which they lived.

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