Ah, ceratopsian dinosaurs. They are quickly becoming my favorite terrible lizards, and in this series of posts (Horns & Spikes), I intend to discuss the various horns, spikes, and generally pointy structures that decorate the typical ceratopsid skull. For convenience's sake, I will here only cover the Ceratopsidae--with one or two invocations of most basal taxa where proper. The series will consist of four parts: Postorbital Horns, Nasal Horns & Bosses, Epoccipitals, and Jugal "Horns." Granted, that last section will be pretty short. I hope you enjoy the series! And here's part one:Postorbital Horns
The postorbital horns are among the most obvious features of the ceratopsid skull: their great length in most chasmosaurines is impressive, and their absence in most centrosaurines is striking. Postorbital horns are basal for Ceratopsidae, occuring in the family's outgroup taxa, Zuniceratops and Turanoceratops, and having been inhereted by both the Chasmosaurinae and Centrosaurinae. Basal members of both groups have elongate postorbital horns. Surprisingly, the basalmost centrosaurines, Albertaceratops and an undescribed Utah taxon, have more exaggerated postorbital horns than basal chasmosaurines! The postorbital horns of Chasmosaurus (C. belli and C. kaiseni) are relatively short--but thick--while those of Albertaceratops are surprisingly long and erect.
As centrosaurines evolved, however, they seem to have lost interest in retaining their postorbital horns. Styracosaurus and Centrosaurus both retain short, rather stubby postorbitals early in life, but these are reabsorbed and shrink away to nothingness during adulthood. A similar process occurs in Einiosaurus. Pachyrhinosaurines, however, make use of their postorbitals differently: the postorbital horns of juvenile animals are small and laterally compressed. With growth, however, the postorbitals grow not upwards, but laterally, forming eventually forming wide bosses that nearly meet in the middle of the skull. Oddly, old adults still reabsorb postorbital tissue, and the surface becomes pitted or, in places, hollow. In some specimens of Pachyrhinosaurus lakustai, both the postorbital and nasal bosses are actually concave due to reabsorption!
A sampling of ceratopsid postorbital horncores. From top to bottom, left to right: "Monoclonius flexus" (AMNH 5239), Styracosaurus albertensis (CMN 344), Albertaceratops nesmoi (TMP 2001.26.1), Achelousaurus horneri (MOR 485), Chasmosaurus kaiseni (AMNH 5401), Pentaceratops sternbergi (OMNH 10165), Pachyrhinosaurus lakustai (TMP 1989.55.1234), Triceratops horridus (SDSM 2760), Anchiceratops ornatus (AMNH 5251), Avaceratops lammersi (MOR 692), Torosaurus latus (MOR 1122). Not to scale: the purpose is to show the diversity of postorbital shapes.
Chasmosaurines retained the ancestral condition and went to town with it. While Chasmosaurus lacks impressive postorbital horns, later chasmosaurines are anything but. Most chasmosaurine postorbital horns are tall, and curve gently forward along their length. A few expections exist, including Agujaceratops, whose horns are nearly erect (and if anything, slightly backswept); Avaceratops, whose horns are surprisingly short; and Arrhinoceratops, whose relatively short horns are curved strongly forward. The most impressive postorbital horncores must belong to a giant specimen of Pentaceratops (OMNH 10165), whose horns are measured at 106 cm along their curve. That's 41.7 inches, or just over three feet!
How did chasmosaurine postorbital horns grow? Centrosaurines reabsorbed their horns, but in Triceratops, at least, the postorbitals changed significantly as the animal matured!
Babies have stubs for horns, and these horns grow straight up before curving back in juvenile animals. This posterior curve is retained in subadults, but the horns then sweep forward strongly, more or less at the base, in adult animals. From this point on, the horns actually begin curving anteriorly until they appear subhorizontal in some individuals. Furthermore, as the animal matures, the interior portion of the postorbital horn is reabsorbed! Surprisingly, this complicated reorientation of the postorbitals may be primitive for Ceratopsidae--the same process can apparently be seen in Zuniceratops! Another important point here is that the great chasm in morphology between juvenile and old adult Triceratops provides evidence against Forster's two-species idea. According to Horner & Goodwin's analysis, T. prorsus is merely a subadult T. horridus.
Do the shape or orientation of the postorbital horns have any phylogenetic significance? Sadly, probably not. Because their shape changes so radically with growth (in all ceratopsids), their taxonomic significance is negligible. What appears to be a unique feature, like the strongly angled horns of Arrhinoceratops, is more likely age-related. Even the mere presence of postorbital horns, which at one point separated chasmosaurines from centrosaurines, is no longer a viable phylogenetic feature, since basal centrosaurines also have long postorbital horns, as did the common ancestor of both groups. Even the presence of a postorbital boss vs. horn may not be significant--juvenile centrosaurines, including pachyrhinosaurs, have small horns while the adults have bosses. This puzzle is perhaps best exemplified by "Brachyceratops," a small juvenile centrosaurine from Montana. Its child-like features include small, slightly recurved postorbital horns and an unfused nasal horn. The structure of its squamosal bone gives it away as a centrosaurine, but given what we now know about postorbital growth, its horns hold little value in determining its classification. Generally, the structure or presence of postorbital horns is helpful only after other factors are considered.
What did ceratopsids use their postorbital horns for? The obvious answer is "defense!" And indeed, I doubt Triceratops turned its horns away from an attacker. However, their primary purpose may have been signaling and/or intraspecific combat. The fact that different age classes in Triceratops look different means that individuals could tell how each other animals were at a glance, or how experienced they are. And look at African antelopes--subtle differences in horn morphology reflects differences in taxonomy. Although between you and me, the ceratopsian differentiator was probably parietal morphology (we'll look at that later). Still, within a group of animals, it would have been very useful to identify individuals' ages and ranks at a glance, which differing postorbital horn morphology can do.
Andrew Farke has argued convincingly that ceratopsids with long postorbital horns may have used them for intraspecific combat. He first played figured out how Triceratops was able to lock horns, as it were, and then went out to the fossils to look for signs of pathology that would validate (or tear down) his idea. Turns out Triceratops skulls show numerous injuries to the squamosal bones, just where the point of the postorbitals would be pressed in a fight. So aside from perhaps defense and intraspecific signaling, ceratopsians with long brow horns used their postorbitals for intraspecific combat, as well! Gotta love practical experiments!
So what have we learned today? Well, large postorbital horns are plesiomorphic for Ceratopsidae, but aside from that, the animals within the two groups (Chasmosaurinae and Centrosaurinae) differ wildly in their expression of those postorbital horn morphology. Not only that, but the postorbitals of all ceratopsids change radically throughout life. Postorbital horns would have had many uses, including defense, intraspecific signaling, and most interestingly, intraspecific combat.
Next up: Nasal horns!
P.S. I'm too tired to write the whole big reference list right now. I'll just copy and paste it from my electronic index later. For now, trust that I'm not making anything up.