The evolution of Ceratopsia

rt TJ

o rt

Figure 6.28. Cladogram of Ceratopsia, emphasizing the monophyly of Psittacosaurus and Neoceratopsia. Derived characters include: at 1, rostral bone, a high external naris separated from the ventral border of the premax-illa by a flat area, enlarged premaxilla, well-developed lateral flaring of the jugal; at 2, short preorbital region of the skull, very elevated naris, loss of antorbital fossa and fenestra, unossified gap in the wall of the lacrimal canal, elongate jugal and squamosal processes of postorbital, dentary crown with bulbous primary ridge, manual digit

IV with only one phalanx, manual digit

V absent; at 3, enlarged head, keeled front end of the rostral bone, much reduced quadratojugal, primary ridge on the maxillary teeth, development of humeral head, gently decurved ischium.

Figure 6.29. Cladogram of basal Neoceratopsia, with the more distantly related Psittacosaurus and Pachycepha-losauria. Derived characters include: at 1, elongated preorbital region of the skull, an oval antorbital fossa, triangular supratemporal fenestra, development of the syncervical (fusion of cervical vertebrae); at 2, greatly enlarged external nares, reduced antorbital fenestra, nasal horn core, frontal eliminated from the orbital margin, supraoccipital excluded from foramen magnum, marginal undulations on frill augmented by epoccipitals, more than two replacement teeth, loss of subsidiary ridges on teeth, teeth with two roots, 10 or more sacral vertebrae, laterally everted shelf on dorsal rim of ilium, femur longer than tibia, hooflike pedal unguals.

Psittacosaurus, a small, Asian biped. All more derived ceratopsians - Neoceratopsia - are quadrupeds. This underlines an important evolutionary event that we can read from the cladogram (Figure 6.28): relatively early in their history, ceratopsians, for whatever their reasons, adopted a quadrupedal stance.

Those early days also brought with them evidence of a major ceratopsian migration. Neoceratopsia (Figure 6.29) consists of a series of small, relatively primitive forms such as: the Asian Protoceratops and Bagaceratops; the somewhat younger, though still primitive North American Montanoceratops; and Leptoceratops; as well as the more derived, exclusively North American family Ceratopsidae, that group of large, familiar ceratopsians such as Triceratops and Centrosaurus (Figure 6.30). When we compare the geographical locations of various neoceratopsians, that is their biogeography, with primitive and advanced ceratopsians on the cladograms shown in Figures 6.28 and 6.29 it becomes clear that, early in neoceratop-sian history, a primitive neoceratopsian - looking perhaps a bit like Protoceratops - migrated to the New World. The route of choice would likely have been briefly exposed land across the Bering Straits (Figure 6.31).

Once in North America, a few lineages retained the comparatively modest morphology of their more primitive forebearers. However, the clade radiated into two spectacular and diverse groups of much larger, flashier ceratopsids: chasmosaurines, after Chasmosaurus; and centrosaurines, after Centrosaurus (see Figure 6.30). Chasmosaurines are generally called "long-frilled," after a tendency in the group to develop large, open frills, while centrosaurines are sometimes called "short-frilled," after a tendency in the group toward shorter frill lengths.

The evolution of behavior. If there is some correspondence between morphology and behavior, then the morphological trends identified by all the ceratopsian cladograms should give us insights into the evolution of neoceratopsian behavior. In those ceratopsians with relatively modest frills and horns - forms such as the Asian Protoceratops, and the North American Leptoceratops and Montanoceratops - display perhaps involved swinging the head from side to side. Should this have failed to impress, these animals may have rammed full tilt into the flanks of their opponent.

Ceratopsian CladogramCladogram For Fossa

Figure 6.30. Cladogram of Ceratopsidae. Derived characters for Ceratopsidae. Derived characters (for chasmosaurines) include: at 1, enlarged rostral, presence of an interpremaxillary fossa, triangular squamosal epoccipitals, rounded ventral sacrum, ischial shaft broadly and continuously decurved. Derived characters (for centrosaurines) include: at 2, premaxillary oral margin that extends below alveolar margin, postorbital horns less than 15% of skull length, jugal infratemporal flange, squamosal much shorter than parietal, six to eight parietal epoccipitals, predentary biting surface inclined steeply laterally.

Figure 6.30. Cladogram of Ceratopsidae. Derived characters for Ceratopsidae. Derived characters (for chasmosaurines) include: at 1, enlarged rostral, presence of an interpremaxillary fossa, triangular squamosal epoccipitals, rounded ventral sacrum, ischial shaft broadly and continuously decurved. Derived characters (for centrosaurines) include: at 2, premaxillary oral margin that extends below alveolar margin, postorbital horns less than 15% of skull length, jugal infratemporal flange, squamosal much shorter than parietal, six to eight parietal epoccipitals, predentary biting surface inclined steeply laterally.

The more derived ceratopsids share more elaborate frills and either nasal or brow horns. Among the long-frilled ceratopsians (for example, Chasmosaurus, Pentaceratops, and Torosaurus), the display function of the frill may have been emphasized (see Figure 6.26). In contrast, most of the short-frilled ceratopsians (such as Centrosaurus, Avaceratops, and possibly Pachyrhinosaurus) were rather rhinoceros-like in their appearance (Figure 6.32), and likely tried to catch each other on their nasal horns, thus reducing to a degree the amount of damage inflicted on the eyes, ears, and snout.

Horns seem almost to drive the evolution of ceratopsian dinosaurs. In this diverse group, we witness a world where display and competition were all important, where - when push came to shove - it may have been better to vigorously nod than to cross horns.

Was this article helpful?

0 0

Post a comment