Paleobiogeography And Evolutionary History Of Ornithopoda

Triassic forms are yet defined by paleontologists. However, some enigmatic dinosaurs have ornithischian features and a few anatomical similarities to ornithopods. These ornithischians made their earliest known appearances in Late Triassic strata of North and South America, as well as in South Africa. Richard Owen first described some of their remains in the nineteenth century, and he wrote of their laterally compressed teeth, which come to a peak but with denticles on their tops and thin enamel on both sides. Unfortunately, Owen interpreted these dinosaurs as lepidosaurs (lizards), and little subsequent research was done on them until the 1970s.

Other primitive ornithischians are in Lower Jurassic strata of South Africa, including Lesothosaurus diagnosticus, the prime example used for comparison to similar skeletal material. Lesothosaurus and other basal ornithischians were originally grouped into a category called "Fabrosauridae," but this classification is now regarded as polyphyletic and has been abandoned. "Fabrosaurids" were categorized partially on the following traits:

1 light, small frames (1- to 2-meters long);

2 fore limb/hind limb ratios of about 40 : 60;

3 tibias longer than their femurs;

4 small skulls; and

5 long tails.

All of these traits indicate bipedalism and the ability to perform rapid cursorial movements. These are evolutionary adaptations that were also echoed by heterodon-tosaurids and "hypsilophodontids" before iguanodontians evolved toward greater sizes. As a result, it is understandable that paleontologists saw these dinosaurs as evolutionary precursors to later clades of ornithopods. Interestingly, one of these former "fabrosaurids," Agilisaurus louderbacki from the Middle Jurassic of China, did turn out to be the most ancestral euornithopod known, so it was recognized as a key species for understanding ornithopod evolution.

Primitive ornithischians described from the Late Triassic are from North America, South America (Argentina), and South Africa. This distribution suggests that land connections between these three continents, represented by the southern continent of Gondwana (Chapters 4 and 6), permitted the dispersal of these dinosaurs prior to ornithopods proper showing up in the Early Jurassic. The heterodontosaurids, the earliest known undoubted ornithopods, also occur in the Early Jurassic of South Africa, which geographically and temporally links them to possible ornithischian relatives. The later ornithopod clades then spread throughout all other continents after the Middle Jurassic, particularly with the ascension of euornithopods from the Late Jurassic through to the Late Cretaceous.

Ornithopods occupied various habitats, their body and trace fossils indicating a former presence in fluvial, lacustrine, swamp, deltaic, and coastal marine (tidal flat and shoreline) environments. Late Cretaceous strata, associated with coal beds of western North America in particular, show abundant hadrosaurid footprints associated with many plant fossils. Some of these tracks are preserved in coal beds and are still frequently encountered by miners. This evidence indicates that at least some hadrosaurid species lived in forested areas adjacent to coal-forming swamps (Fig. 11.8).

\^The paleobiogeography of ornithopods, as noted earlier, was apparently limited to southern Africa during the Early Jurassic. However, adaptive radiation and dispersal of ornithopod lineages had taken place by the Middle Jurassic, and by the end of the Cretaceous they occupied every continent.

FIGURE 11.8 Large track, preserved as positive relief as a natural cast on the bottom of a bed, attributed to a hadrosaurid in the Laramie Formation (Late Cretaceous) near Golden, Colorado. Note that the "heel" portion of the track was obscured by a log that was under the hadrosaurid's foot, providing evidence that the hadrosaurid was walking through a formerly forested area.

One evolutionarily based explanation for changes in ornithopod abundance and diversity in space and time is that the continued splitting of the continents combined with (or contributed to) the creation of new terrestrial ecosystems and the corresponding evolution of new plant species. In accordance with large-scale changes in vegetation, changes in the fauna feeding on the plants would have occurred, particularly during the Early Cretaceous with the onset of flowering plants. The two broadest trends in ornithopod evolution noticeable from the Early Jurassic through to the Late Cretaceous are:

1 their specialized adaptations to feeding, evident through their teeth and jaws; and

2 their increased size, which also may have been partially an adaptation to feeding.

Sauropods (Chapter 10) were also occupying some of the same environments as their ornithopod contemporaries. Nevertheless, the great differences in their sizes, readily apparent by the end of the Jurassic, suggest that they were not competing for the same foodstuffs.

At the same time when sauropods declined in species and numbers, iguano-dontians in particular seemed to increase in both respects. By the end of the Cretaceous, hadrosaurids had replaced sauropods as the largest terrestrial herbivores in many areas of the world, although ceratopsians were new potential competitors (Chapter 13). The relationships between ornithopods, other dinosaurs, vegetation, and their coevolution in continental ecosystems of the Mesozoic will be further explored later.

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