Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
31. Ornithischian Dinosaurs 2: Ceratopsia
Ceratopsia
.
Ceratopsia ("horned faces") is a suborder of the dinosaur order Ornithischia ("bird hipped").
Ceratopsia were herbivorous, beaked dinosaurs abundant in northern Asia, North America, and Europe during the Late Jurassic through Late Cretaceous periods (fig.1). The earliest known ceratopsian, Yinlong downsi (fig.2), lived in Asia during the Late Jurassic (161.2 - 155.7 mya). Fragmentary remains found in North America from the Early Cretaceous Albian stage (112-100 mya) show the group had by then dispersed across the Bering Strait.
Early members of the ceratopsian group, such as Psittacosaurus (fig.3), were small bipedal animals. Later taxa, including
ceratopsids like Centrosaurus and Triceratops, became very large
quadrupeds and developed elaborate facial horns and frills extending
over the neck. The last species, Triceratops prorsus, lived until the end of the Cretaceous (66-65 mya) and was contemporary with the therapod carnivore Tyrannosaurus rex.Fig.1: Distribution map of sites with fossil Ceratopsians by period.
The first ceratopsian remains were discovered during an 1855 expedition to Montana led by F.V. Hayden for the U.S. Geological and Geographical Survey, and were at first assigned to hadrosaurids (Hatcher et al. 1907). In a later, 1872 expedition in southwest Wyoming , F.B. Meek found giant bones protruding from a hillside and contacted Edward Drinker Cope, who recovered a partial skeleton which he recognized a distinct type of dinosaur. Soon after, more dinosaurs were found that would eventually come to be recognized as ceratopsid, including Monoclonius who provided first evidence of ceratopsid horns and frills. In 1876 Cope and C.H. Sterberg found several Monoclonius fossils near the Judith River in Montana. In 1888-1890,O.C. Marsh described the first well preserved horned dinosaurs as Triceratops ("three horned face") and grouped them in the family Ceratopsidae (Marsh 1889).
Overall, ceratopsians ranged very widely in size, from 1 meter and 23 kg to over 9 meters and 9,100 kg. Features of the skull include the rostral bone on the tip of the upper jaw, complementing the predentary bone on the lower jaw, with this beak-like formation aiding the chewing of plant matter. The jugal bones below the eye are prominent, flaring out sideways to make the skull appear somewhat triangular. In later ceratopsians a rearwards extension of the parietal and squamosal bones of the skull
roof formed the characteristic neck frill.
Considerable variation exists between ceratopsian species.
Complete growth series from embryo to adult are known for Psittacosaurus and Protoceratops, a series on exhibit at the American Museum of Natural History. Significant sexual dimorphism has been noted in several ceratopsids including Protoceeratops (Lehnman 1990).Fig.2: Cladogram of Cretaceous ceratopsians, showing major traits distinguishing the taxa (after AMNH display).
Yinlong
Yinlong ("hidden dragon") was a small, primarily bipedal herbivore from the Late Jurassic of central Asia (ca. 158 mya). representing the oldest and most primitive ceratopsian so far found. In 2004, a well-preserved skeleton and skull of a sub-adult now classed as Yinlong downsi wa
s found in an upper level of the Shishugou Formation in Xinjiang
province of western China, dating from the Oxfordian stage of the
Late Jurassic (162-155 mya) (Xu et al. 2006). A number of other Yinlong fossils have since been discovered,. Fig.3: Skull of Yinlong downsi, with bones labelled (after Xu et al. 2006).
Yinlong was a relatively small dinosaur, with a total length of about 1.2 meters and a weight of about 15 kilograms Long, robust hindlimbs and shorter, slender forelimbs with three-fingered hands suggests a bipedal lifestyle like those of many small ornithopods. Yinling's skull (fig.3) lacks frills, but contains the diagnostic rostral bone at the front of the upper jaw, identifying it as a ceratopsian. Gastroliths preserved in Yinlong's abdominal cavity, used to grind plant material, have also been found in other ceratopsians such as Psittacosaurus [Xu et al. 2006)
Psittacosaurus
Psittacosaurus ("parrot lizard") is a small and relatively primitive ceratopsian, from the Early Cretaceous in Mongolia, dating from 107 mya. Like the earlier Yinlong, P. mongoliensis lacks skull frills or horns, but has the beak-like rostral bone on the front of the upper jaw which is diagnostic of ceratopsians. It had long hind legs and
short arms, indicating it was mainly bipedal.
Later ceratopsians such as Triceratops became four-legged foragers.The skeletal specimen of P. mongoliensis shown in fig.4 was recovered in Artsa Bogdo Basin in Mongolia in 1922 by J. Wong, during an American Museum of Natural History expedition.
Fig.4: Skeleton of Psittacosaurus mongoliensis, shown in collapsed position (American Museum of Natural History 6254; photo: Athena Review).
Protoceratops
Protoceratops ("first horned face") was a Late Cretaceous ceratopsian who walked on all fours. It had a shield-like frill on its neck, although smaller than that of Triceratops, and some had a small horn on the nose. Its specialized dentition of three-cusped molar cheek teeth is similar to that of Triceratops.
Fig.5: Skull of the type specimen of P. andrewsi; A) side view; B: view from top (after Granger and Gregory 1923, figs. 1 and 2).
The first fossils of P. andrewsi (figs.4,5) were found in 1922 in the Flaming Cliffs formation near Artsa Bogdo in Mongolia, dating from 73 mya (Granger and Gregory 1923). This is the same location as the Psittacosaurus finds described above, but from a formation dating about 35 million years later. The species was named after Roy Chapman Andrews, leader of the American Museum of Natural History expedition, whose original goal was to look for early hominid fossils related to Homo erectus ("Peking Man"), but instead found a wealth of dinosaur fossils. Besides discovering many specimens of the genus Protoceratops, and Psittacosaurus, the AMNH team also found the first fossil skeletons of the theropods Velociraptor and Oviraptor.
The type speciment of P. andrewsi was a relatively small skull (fig.5) measuring 16 by 19 cm, which lacked horns but had remains of the ceratopsian rostral bone, and part of a neck frill consisting of extended parietal and squamosal bones. It had nine teeth on each jaw, each a worn three-cusp molar. This provided much- needed evidence of the ancestors of the considerably larger Triceratops.
Fig.6: Skeleton of Protoceratops andrewsi (American Museum of Natural History; photo: Athena Review).
Based on the large sample obtained in Mongolia, adult Protoceratops (fig.6) were 1.5-2.0 meters long and about 60 cm high at the shoulder, weighing about 80 kg.The large numbers of specimens found in high concentration suggest
that Protoceratops lived in herds. The size
and shape of neck frills varied considerably by individual (fig.7), and such
variation may well have relevance to both differing ages and sexual
dimorphism.Fig.7: Two skulls of P. andrewsi from Mongolia, showing variations in form (American Museum of Natural History; photo: Athena Review)
Styracosaurus
Styracosaurus ("spiked lizard") was a plant-eating ceratopsian from the Campanian stage of the Late Cretaceous (ca. 75.5 -75 mya). It had a unique skull formation, with a crown-like array of four to six large spikes around the edge of the frill, and a large horn over its nose. up to 60 cm long (fig.8).
The first Styracosaurus fossils were found in Alberta, Canada by C.M. Sternberg in the Dinosaur Park Formation. From these finds, the species Styracosaurus. albertensis was named by Lawrence Lambe in 1913. The skeleton of S. albertensis shown in fig. 8 (AMNH 5372) was found at Red Deer River, Alberta in 1915. Originally it was proposed as a new species (Brown and Shlaikjer 1937), but is now considered part of S. albertensis.
Fig.8: Skeleton of Styracosaurus albertensis (American Museum of Natural History; photo: Athena Review
Styracosaurus grew up to 5.5 meters long, standing about 1.6 meters high with short, robust limbs, a massive head, and a relatively short tail, The skull had a pronounced ceratopsian beak, with molar-like cheek teeth for chewing plants. Ceratopsid teeth, including those of Styracosaurus, were arranged in groups called batteries. Older teeth on top were continually replaced by the teeth underneath them. Large bonebeds found in Alberta suggest Styracosaurus was a herd animal, living in large groups, like other ceratopsians, including the smaller Mongolian taxa Protoceratops.
Triceratops
Triceratops ("three-horned face") was a herbivorous ceratopsid in western North America from the late Maastrichtian stage of the late Cretaceous period (69-65 mya). A skull collected in 1888 by J.B. Hatcher from the Lance Formation of Wyoming is the holotype (YPM
1820), first
described by Marsh in 1889. Triceratops remains have
subsequently been found in various terminal Cretaceous deposits in Montana, South
Dakota, Colorado, and in Saskatchewan and Alberta in Canada, including the
Laramie Formation, Lance Formation, and Hell Creek Formation. Two species, T. horridus and T. prorsus, are considered valid today (Lull 1933; Scanella and Horner 2010). Fig.9: Skeleton of Triceratops (American Museum of Natural History; photo: Athena Review).
Triceratops is easily recognizable by the large bony frill and three horns on the skull, whose robust qualities have enabled many examples to be preserved as fossils, allowing variations between species and individuals to be studied. The skull bore a single horn above the nostrils. In the front of the head was a large beak formed by a special rostral bone. Behind the toothless premaxilla, the maxilla bore thirty-six to forty tooth positions, in which three to five teeth per position were vertically stacked. Tooth replacement was continuous throughout the life of the animal. The dentition of Triceratops suggests that they ate large volumes of fibrous plant material (Dodson et al. 2004).
Its large four-legged body was up to nine meters long. with strong limbs, short hands with three hooves each, and short feet with four hooves each. The vertebral column consisted of ten neck, twelve back, ten sacral and about forty-five tail vertebrae.
Triceratops was one of the last ceratopsian genera to appear before the end of the Mesozoic. Formations containing Triceratops fossils date right up to the time of the Cretaceous –Paleocene extinction event (66-65 mya). In the ancient context represented by the Hell Creek Formation of Montana and North Dakota, a wide range of dinosaurs co-existed with a variety of small mammals, including multituberculates, metatherians, and eutherians or placental mammals (Kielan-Jaworowska et al. 2004; Archibald et al. 2011).
References:
Archibald, J. D.; Zhang, Y.; Harper, T.; Cifelli, R. L. 2011 . Protungulatum, Confirmed Cretaceous Occurrence of an Otherwise Paleocene Eutherian (Placental?) Mammal . Journal of Mammalian Evolution. 18 (3): 153–161.
Brown, B. and E.M. Shlaikjer 1937 . The skeleton of Styracosaurus with the description of a new species. American Museum Novitates. no. 955: 12. 1.
Dodson, P., C.A. Forster, and S.D. Sampson 2004. Ceratopsidae. In Weishampel, D. B., P. Dodson, and H. Osmólska . (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 494–513.
Granger, W. and W.K. Gregory, 1923. Protoceratops andrewsi, a pre-ceratopsian Dinosaur from Mongolia. American Museum Novitates 72, pp. 1-9.
Hatcher, J.B., O.C. Marsh, and R.S. Lull, 1907. The Ceratopsia. Government Printing Office, Washington, D.C., 300 pp.
Kielan-Jaworowska, Z, R.L. Cifelli,, and Z-X. Luo, Zhe-Xi 2004 . Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure. New York: Columbia University Press..
Lambe, L.M. 1913. A new genus and species from the Belly River Formation of Alberta. Ottawa Naturalist. 27: 109–116.
Lehman, T.M. 1990. The ceratopsian subfamily Chasmosaurinae: sexual dimorphism and systematics. In: Carpenter, K. & Currie, P.J. (Eds.). Dinosaur Systematics: Approaches and Perspectives. Cambridge: Cambridge University Press. Pp. 211-230.
Lull, R. S. 1933. A revision of the Ceratopsia or horned dinosaurs. Memoirs of the Peabody Museum of Natural History. 3 (3): 1–175.
Marsh, O.C. 1889a. Notice of gigantic horned Dinosauria from the Cretaceous. American Journal of Science. 38 (224): 173–175.
Marsh, O.C. 1889b The skull of the gigantic Ceratopsidae. American Journal of Science. 3rd Series. 38: 501–506.
Scannella, J., and J.R. Horner. 2010. Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny. Journal of Vertebrate Paleontology. 30 (4): 1157–1168.
Xu, X., C.A. Forster, J.M. Clark, and J. Mo, 2006. A basal ceratopsian with transitional features from the Late Jurassic of northwestern China. Proceedings of the Royal Society B: Biological Sciences, 273(1598): 2135-2140.
Glossary