Centrosaurinae
The centrosaurines, as previously described,
are usually recognized by their prominent nasal horns, subordinate brow horns, short
squamosals in a short frill, a tall, deep face relative to the ceratopines, and
a projection into the rear of the nasal fenestra. They experienced their
greatest diversity in the Judithian age of the LK,
tailing off during the Edmontonian before apparently disappearing altogether in
the Lancian. They also have more prominent cranial ornamentation, including a wide variety of
spikes and nodules on the frill. It has been suggested that they wrestled with their nose horns by
pushing against an opponent's horn.
There are two general types of centrosaurines,
centrosaurinids and pachyrhinosaurinids. Derived centrosaurinids have hook-like projections
on the rear of the frill and unreduced nasal horns; derived pachyrhinosaurinids
have rough, thick pads where nose and brow horns should be. It is quite probable
that they could have engaged in pushing and shoving matches with them.
Bonebeds of centrosaurines are known,
particularly for Styracosaurus, Centrosaurus, Pachyrhinosaurus,
and Einiosaurus. It is
believed that these formed when a herd attempted to ford a river, and a number of them
were drowned, like what happens today in Africa when wildebeest attempt a crossing.
From studies of this bonebed material, researchers have discovered that as
centrosaurines aged, their diagnostic facial features changed greatly.
Centrosaurine juveniles as a rule looked alike, no matter the taxon. Their
nasal horns were transversely compressed structures, formed from the paired
nasal bones; this split horn rapidly fused from the tip down as the individual
neared adulthood, and was later subject to further modifications. Their
brow horns were subdued structures that in some cases were lost for uncertain
reasons by the adult individuals. Juvenile centrosaurine frills were thin,
fragile objects, often without the large fenestrae so characteristic of the
adults; the margin was simple and scalloped, and the midline was ridged.
By contrast, the frills of subadults were sometimes as large as those of adults,
with fenestrae, but with varying degrees of full-fledged adult
ornamentation. It appears clear that the major features that separate
adult centrosaurines, like horn and frill configurations, appeared late in life;
young centrosaurines, no matter the genus, all looked pretty much like Brachyceratops. This is an idea that would put the kibosh
to some accepted genera like Brachyceratops and Monoclonius. Each
has a few things to say in their defense, but I'll let that wait until the appropriate
place.
<--Centrosaurinae
|--Albertaceratops
`--+--"Centrosaurini"
|
|--Centrosaurus
| `--Styracosaurus
`--"Pachyrhinosaurini"
|--Einiosaurus
`--+--Achelousaurus
`--Pachyrhinosaurus
Centrosaurinae:
| Taxon or Taxa: | Time\Place: | Comments: |
| Albertaceratops nesmoi Ryan, 2007 | late middle Campanian (LK) of Alberta and Montana | This genus is the first well-supported centrosaurine with long brow horns (no, Avaceratops doesn't count; it might be too basal). It is known from a complete skull from the Oldman Formation of Alberta, and skull fragments from the Judith River Formation of Montana. The skull looks something like an Achelousaurus with long brow horns and a ridge for a nasal pad. In a cladistic analysis, it fell out as the most basal centroaurine. For those of you who collect dissertation names, it traveled under the informal name of "Medusaceratops". |
Centrosaurinae i.s.: There is an unnamed basal centrosaurine or ceratopid from Utah with long brow horns, like Albertaceratops or Zuniceratops.
| Taxon or Taxa: | Time\Place: | Comments: |
| Brachyceratops montanensis (?N.D.) Gilmore, 1914 (?Einiosaurus) | late middle Campanian (LK) of Montana | This animal is known from the remains of five juveniles and a possible subadult or adult skull. With the new understanding of centrosaurine growth, it is clear that the diagnostic value of the juvenile remains is dubious at best; the remains could belong to either an established adult taxon or its own valid species. The larger skull, however, is a different matter. It is not complete, and several lines of evidence indicate it is not full grown; first, the nasal horn is only known from a half, suggesting that it was not completely fused and thus not adult; second, the frill is very large, particularly when compared with the rest of the skull (in particular an unusually short tooth row), suggesting that it belongs to a subadult; third, the frill is thin, as is common with immature centrosaurines; fourth, the frill lacks holes, another character of immature individuals; and fifth, skeletal remains found with the skull are very small for an adult. I am satisfied that there is a good chance that this skull and partial skeleton belong to a subadult individual. Interestingly, it also shows prominent but incomplete nodes on the frill's margin; the position and number of these nodes correspond fairly well to the pattern seen in adult individuals of the proto-pachyrhinosaur Einiosaurus. This evidence hints that the large specimen of Brachyceratops may actually be a subadult of this proto-pachyrhinosaur. |
| Monoclonius crassus (?N.D.) Cope, 1876 | late middle Campanian (LK) of Montana and Alberta | Although it has a good deal of historical significance, Monoclonius has been at the center of a good deal of historical muddling as well. For example, the name, which means "single shoot," (not "one horn"!) refers to the root of a tooth included in the original type material that is actually hadrosaurid. It also has been long confused with Centrosaurus. "True" Monoclonius material, as shown by the partial frill now used as the type, has a simple frill border and is rather thin, unlike other centrosaurines. This is a common pattern for juvenile and subadult individuals of centrosaurines in general, leading to the conclusion that Monoclonius likely is founded on the remains of subadults. Of course, which one is the million dollar question; the type is from the Judith River Formation, which is not well-known in terms of horned dinosaurs, so conceivably that original partial frill, the type of Monoclonius, represents an animal which is otherwise unknown to science and could, with the future discovery of better material, be called Monoclonius. Although it is often illustrated, it is poorly known; many such illustrations are actually of Centrosaurus. |
"Centrosaurini": You know, that "tribe" thing we mid-late
1990s dinosaur website authors loved to use so much never really caught on, did
it? We were all set with our Saurolophinis and Pachyrhinosaurinis and so
on, but professional papers never joined in (well, there is Corythosaurini and
Parasaurolophini). It probably has to do with the fact that nobody in
dinosaur paleo really seemed to want to touch Linnean ranks above genus after
the 1980s, and they didn't want to create any more taxonomic clutter, so they
just stuck with what they already had.
In an SVP abstract from 1999, a new centrosaurinid from the
Oldman Formation of Alberta was mentioned, and described as having at least three procurving
hooks on each side of the parietal midline with associated nodular masses.
Although I am not completely sure, I think that this is an early description of Centrosaurus
brinkmani; what had thrown me the most initially is as issue with the age
(Glut Supplement 2 putting the age as middle Judithian, with Centrosaurus being lower Judithian and Styracosaurus being
upper Judithian).
| Taxon or Taxa: | Time\Place: | Comments: | |
| Centrosaurus: Lambe, 1904 (?Styracosaurus) | C. apertus (type) Lambe, 1904 | late middle Campanian (LK) of Alberta | Centrosaurus has as its most distinctive features a long, variably curved nasal horn, a thick frill, two short hooks at the rear of the frill pointing toward each other, and two long hooks overhanging the holes of the frill. It was long confused with Monoclonius, and for a time in the late 1980s and early 1990s was known as Eucentrosaurus (Chure and McIntosh, 1989) because it was thought its name was preoccupied by that of a lizard's. Eventually, it turned out that the lizard's name had never been published, so Centrosaurus the dinosaur continues along. Occasionally some trouble comes up with Kentrosaurus as well, but the differing pronunciations keep the two separate. It is known from over fifteen skulls, some with skeletons, and some bonebed material as well. It was a common Judithian dinosaur. Dimorphism has been seen in the known specimens; one form has features like a longer frill, a deeper face, and a tall forward-pointing nasal horn, while the other has a shorter frill, a narrow face, and a more subdued nasal horn, variable in orientation. The first form may be male, the second female. Styracosaurus is very closely related, and may well be the same genus. |
| C. brinkmani Ryan and A. Russell, 2005 | late middle Campanian (LK) of Alberta | This new species of Centrosaurus is from older rocks than classic C. apertus (the Oldman Formation, for those of you in the know), and had a few interesting skull features including flattened, laterally-directed orbital horns, prominent bundles of nodes surrounding the projections that arch over the frill from the back, and laterally-projecting spines lateral to the bundle of nodes. Unlike C. apertus, it didn't have the same sort of prominent inward-curving hooks found at the frill indentation. It was found in bonebeds. | |
| Styracosaurus: Lambe, 1913 (?Centrosaurus) | S. albertensis (type) Lambe, 1913 | late middle Campanian (LK) of Alberta | This animal is instantly familiar to anyone interested in
classic dinosaurs, due to its bizarre frill. S. albertensis is
defined by the presence of two large frill spikes on either side of the
midline, with the inner pair longer than the outer pair, but after that
you get quite a bit of variation. Some have a third pair of spikes,
like the type skull, and one or two further spikelets; some have just
rounded nubbins. Some have little tabs where Centrosaurus has
its midine-pointing hooks, others have true hooks. There are two
knobs of varying development where Centrosaurus has its big,
forward-pointing tongue-like processes arching over the parietal
fenestrae. Subadults have pyramidal brow horns, which are reabsorbed
in adults. There may be sexual dimorphism, but it's hard to tell at
this point. The type of this animal is a spectacular skull, missing the lower jaw (recovered at a later date), the right side of the frill, and part of the nasal horn. It has an apparent pathology on the left side of the frill; the next time you see a restoration, note how the smallest of the three spikes is partially overlapped by the next spike. It appears that the frill was fractured between the two spikes and telescoped together, so that the frill should actually be a few cm longer. Additionally, the nasal horn, which is incomplete, has traditionally been assumed to come to a point and be about twice as long as preserved, but remains from other individuals suggests that the tip is blunt, and that not that much is missing. |
| S. ovatus Gilmore, 1930 | late middle Campanian (LK) of Montana | Less well known than the type, this animal differs in having only two large spikes on each side of the frill, instead of three, with the central pair converging instead of diverging. | |
"Pachyrhinosaurini":
| Taxon or Taxa: | Time\Place: | Comments: |
| Einiosaurus procurvicornis Sampson, 1995 (?Brachyceratops) | late middle Campanian (LK) of Montana | Einiosaurus has a very unusual nasal horn which bears a striking resemblance to a bottle opener (although possibly only in males) and has two spines sticking out rearward from the frill's margin. For a time before it was described, it was thought to be a species of Styracosaurus. |
| Achelousaurus horneri Sampson, 1995 | latest middle Campanian (LK) of Montana | Achelousaurus combines the dual frill spines of Einiosaurus with the thick nose and brow pads of Pachyrhinosaurus. Some workers suspect it is actually a species of Pachyrhinosaurus. |
| Pachyrhinosaurus canadensis Sternberg, 1950 | early Maastrichtian (LK) of Alaska and Alberta | Pachyrhinosaurus is known for having thick pads of bone where the nose and brow horns would normally be. It was a fairly common Edmontonian dinosaur; remains from many individuals are known, over a wide geographic range. All appear to have dual side\front pointing spikes at the rear of the frill, but only some have what is described as a "unicorn" horn, which projects from the midline bar of the frill; these individuals may belong in a separate species. |
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