especially relevant to hypotheses of how ceratopsid evolution happened during the Late Cretaceous.
The relatively restrictive distribution for marginocephalians is used as evidence favoring endemism. In this scenario, marginocephalian ancestors developed in certain ecosystems in specific places, then their pachycephalosaur and ceratopsian descendants did likewise with later radiations. For example, the preferred environments for neoceratopsians in the western part of North America, based on occurrences of complete skeletal remains, were lowland areas associated with fluvial floodplains or upper delta plains that were forested. However, these environments differed from those of their smaller ceratopsian predecessors, such as Psittacosaurus and Protoceratops. These dinosaurs multiplied in the semi-arid conditions of the Late Cretaceous in Mongolia. The common ancestry of these neoceratopsians is evident from anatomical data, but their distinctive forms and occurrence in facies representing diverse environments reflect adaptations to their respective environments. Because the smaller Asian ceratopsians have more primitive features relative to the larger North American ceratopsids, the hypothesis is that ceratopsid ancestors migrated from Asia to North America around the mid-Cretaceous Period and later evolved into distinctive forms in their new environments. Once in North America, ceratopsids achieved considerable latitudinal distributions. For example, chasmosaurines have been discovered in Late Cretaceous deposits from Mexico to Alaska.
Neoceratopsian dispersal and evolutionary routes may seem straightforward, but pachycephalosaur evolution is fraught with difficulties. One problem with working out pachycephalosaur paleobiogeography lies in the relative rarity of pachy-cephalosaur material. Additionally, the materials found so far consist mostly of skull fragments, so the majority of pachycephalosaur genera are described on the basis of thick skull fragments and little else. The corresponding absence of postcranial pachycephalosaur material for many of their species thus can be attributed to selective transport and burial. After all, a thick skull was more likely to be preserved than a thin limb bone (Chapter 7). This explanation is supported by the occurrence of pachycephalosaur material in Cretaceous facies representing fluvial environments (floodplains) or arid regions (dunes of deserts). Paleontologists interpret pachycephalosaurs as upland animals that did not normally inhabit lowland coastal areas. This circumstance certainly would have placed them in different
Marginocephalian paleobiogeography is relatively simple, as almost all of its representatives have been found in North America, China, and Mongolia, with only three errant specimens discovered in Thailand, Germany and England (Table 13.1).
Two dinosaur specimens from South America and Australia have been proposed as ceratopsian remains, but these classifications are based on fragmentary material and have not been confirmed. Similarly, a skull fragment from the Cretaceous of Madagascar was initially thought to belong to a pachycephalosaur. It was named Majungatholus, and the paleobiogeographic range of pachycephalosaurs was briefly extended to Africa. However, the later discovery of more complete material from another specimen of Majungatholus revealed that it was actually a theropod (Chapter 9). This subsequently disproved the extension of pachycephalosaurs, and by default, marginocephalians, to Africa.
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