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959 Non-Dinosaurian Vertebrates Nicholas C. Fraser It is undoubtedly appropriate to refer to the Mesozoic Era as the Age of Dinosaurs , but it was much more than that. Swimming in the Mesozoic seas were giant crocodiles, dolphinlike ichthyosaurs and enormous plesiosaurs. The air was alive with pterosaurs ranging in size from the sparrow-sized Pterodactylus to the truly monstrous Quetzalcoatlus, which was the size of a small airplane. Consequently, the Mesozoic is also sometimes referred to as the Age of Reptiles. But we can go beyond even this basic statement, for it was essentially in the first part of the Mesozoic (the Triassic) that the seeds of today’s terrestrial ecosystems were sown. In addition to dinosaurs, the Triassic period witnessed the first lissamphibians (frogs and salamanders), crocodiles, turtles, sphenodontians (and, by extension, their sister group, the squamates), and mammals. It is not even unreasonable to say that we can also trace the origin of birds back to the Triassic. It is now quite universally accepted among paleontologists that the birds arose from within the theropod dinosaur lineage and that Aves merely represent derived theropod dinosaurs. There are even some who argue that true bird remains have been found in the Triassic. On the basis of fragmentary vertebrate remains from the Dockum, Chatterjee (1990) erected the genus Protoavis which he considered to be a bird. However, this assessment remains highly controversial. Whereas large plesiosaurs, such as Elasmosaurus, or gigantic azhdarchid pterosaurs, such as Quetzalcoatlus, have attracted as much attention as their dinosaur contemporaries, quite a few of the taxa discussed in this chapter have been previously overlooked. Small, 1-cm fragments of the sphenodontian Opisthias jaw bones have seemed insignificant alongside the 2-m femur of a sauropod dinosaur from the same formation, and tiny partial mammal teeth are readily overlooked in a matrix containing 30-cm Tyrannosaurus rex teeth! Yet these smaller vertebrates are tremendously important in their own right, and their study is necessary for a more complete understanding of the Mesozoic world. Indeed, it is typically from the smaller, less conspicuous fossils that we have documentation of the early evolution of modern vertebrate groups. These “modern” groups were present throughout Mesozoic and Tertiary times, although admittedly there were times when they were rare or even absent from the fossil record. This chapter examines most of the major groups of vertebrates that were contemporaries of the dinosaurs. When using the term fish, people can be referring to any of a number of quite different animals. The earliest forms were the agnathan (jawless) Fish 40 Nicholas C. Fraser 960 fishes that extend back to the Cambrian. The first agnathans (the ostracoderms ) are characterized by an extensive exoskeleton comprising a solid carapace or bony shield that covered the head together with large bony plates or scales over the rest of the body. Most of these had died out by the end of the Devonian, but two lineages, the lampreys and the hagfish, survive today, indicating that agnathans lived during the dinosaurs’ reign. The first jawed fish date back to the Silurian. Many of the Paleozoic jawed fish, the placoderms, were, like the jawless ostracoderms, heavily armored and were likely poor swimmers and largely benthonic. They are not closely related to either of today’s major divisions of fishes: the cartilaginous fishes (Chondrichthyes) or bony fishes (Osteichthyes). The Chondrichthyes can be divided into two groups: the sharks, skates, and rays (Elasmobranchii) on the one hand, and the chimaeras or ratfish (Holocephali) on the other. Throughout their history, sharks have been predominantly marine predators. Although the elasmobranchs date back to the latest Silurian, the evolutionary history of modern sharks (Neoselachii) only goes back to the Triassic. Some of the Triassic sharks were actually freshwater forms. The neoselachians radiated in the Jurassic and Cretaceous, and a wide variety of essentially modern-looking sharks and rays were denizens of the Mesozoic seas. Likewise, Mesozoic holocephalians such as Ischyrodus from the Upper Jurassic were very like the modern chimaeras. Almost from their beginnings, sometime in the Devonian or possibly even the Silurian, the bony fish fall neatly into two groups. Those with fleshy lobes forming the bases of their fins are the Sarcopterygians, meaning “lobe fins,” and include the lungfish and coelacanths. The second group is the Actinopterygia, which means “ray fins.” These, naturally enough, have fins supported by series of cartilaginous or bony rods. The sarcopterygians are particularly important to our understanding of the origin of tetrapods sometime in the Devonian. But with...

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