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Evolutionary Transitions Among Dinosaurs: Examples from the Jurassic of China

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Evolutionary Transitions Among Dinosaurs: Examples from the Jurassic of China Evo Edu Outreach (2009) 2:236–247 DOI 10.1007/s12052-009-0137-0 ORIGINAL SCIENTIFIC ARTICLE Evolutionary Transitions Among Dinosaurs: Examples from the Jurassic of China James M. Clark & Xing Xu Published online: 9 May 2009 # Springer Science + Business Media, LLC 2009 Abstract Dinosaurs have captured the popular imagination relationships and transitions. Familiar highly specialized more than any other extinct group of organisms and are animals such as Triceratops, Pachycephalosaurus, Stego- therefore a powerful tool in teaching evolutionary biology. saurus, Ankylosaurus, Tyrannosaurus, Brachiosaurus, and Most students are familiar with a wide variety of dinosaurs Parasaurolophus are joined in the fossil record by hundreds and the relative suddenness of their extinction, but few are of their relatives with less-specialized features (Weishampel aware of the tremendous longevity of their time on Earth et al. 2004). Together, they comprise a many-branched and the richness of their fossil record. We first review some evolutionary tree that flourished for over 160 million years. of the best-known groups of dinosaurs and discuss how Dinosaurs are found almost exclusively in sedimentary their less-specialized relatives elucidate the path through rocks formed in terrestrial, rather than marine, environ- which each evolved. We then discuss our recent discovery ments, indicating that they rarely ventured out into the sea. of Yinlong downsi, a distant relative of Triceratops, and These river and lake deposits formed layers, or strata, that other fossils from Jurassic deposits in China to exemplify are overlain by younger rocks and underlain by older ones. how the continuing discovery of fossils is filling out the By studying how these and other layers of rocks and their dinosaur family tree. entombed fossils are stacked, scientists called stratigraphers have been able to recognize the sequence in which Keywords Dinosaurs . Evolution . Transitional fossils . dinosaurs and other fossils occurred during the Earth’s Mesozoic . China . Jurassic history (Prothero and Schwab 2003). This led to the erection in the early nineteenth century of a timescale with Dinosaurs need no introduction, they are ubiquitous in now-familiar names for different time periods, including the popular media as symbols of the terrors of nature and how Mesozoic Era (popularly called the Age of Dinosaurs) and the mighty fall. But the richness and remarkable duration of its three subdivisions, from oldest to youngest, the Triassic, the fossil record of dinosaurs are less widely appreciated, Jurassic, and Cretaceous periods (for a fascinating account and the abundant fossil record of dinosaurs is an excellent of debates in the early nineteenth century over how the time example of how fossils provide evidence for evolutionary periods were recognized, see Rudwick 1985). However, these divisions reflected only the relative ages of these J. M. Clark (*) fossils, and it was not until the last 60 years that ways of Department of Biological Sciences, estimating the age in years of rocks and fossils were George Washington University, developed. These involve the decay of radioactive ele- Washington, DC 20052, USA ments, which are experimentally shown to occur at rates e-mail: [email protected] that take millions or billions of years to complete, depend- X. Xu ing on the element and isotope. Laboratory of Evolutionary Systematics of Vertebrates, Radiometric dating comprises a well-developed set of Institute of Vertebrate Paleontology and Paleoanthropology, techniques that are now capable of very high resolution Chinese Academy of Sciences, 142 Xiwai Street, (Macdougall 2008). Only relatively young fossils Beijing 100044, China (<50,000 years old) can be dated directly, by Carbon-14 Evo Edu Outreach (2009) 2:236–247 237 dating of bone or other carbonaceous fossils, so the dating the Dinosaur Park Formation of Canada (Holmes et al. of dinosaurs millions of years old relies upon dating of the 2001). More to the point, the periodic sampling of fossils rock layers above, below, and rarely within beds containing over millions of years is sufficient to provide documenta- fossils. The radioactive decay of isotopes of several tion of the sequential acquisition of specialized features in different elements (argon, potassium, and uranium) can be all of the major dinosaur lineages. used to date dinosaur-bearing rocks because they all have Fossiliferous rocks are more common near the end of the half-lives (the statistically estimated time it would take half dinosaur’s record, at the close of the Cretaceous Period, of a sample to decay) of the appropriate length, and the use than in the Jurassic and Triassic, so we know much more of different isotopes can provide independent checks on any about the dinosaurs that lived shortly before they went one date. extinct than those from earlier in their evolution. Neverthe- Radiometric dating estimates the age of a rock at the less, the early fossil record of dinosaurs is reasonably well time that it crystallized, by comparing the amount of known and includes many complete or nearly complete “daughter” decay product to the original amount of the skeletons. These early fossils are important because older radioactive “mother” isotope (calculated as the amount of fossils are more likely to include the most basal members of decay product plus what remains of the mother isotope). evolutionary lineages, lacking most of the specializations of Rock layers such as tuffs that formed when volcanic ash, later forms (we use basal here to refer to members of a including recently formed tiny crystals, was ejected from a group that diverged earlier than other members). These volcano and blanketed the countryside, are ideal for dating. early dinosaurs are generally smaller than their later Many tuffs occur in dinosaur-bearing deposits or above or brethren and have only a few of the features distinguishing below them, so we have a good idea of how old most the lineage to which they belong. The search for early dinosaur fossils are (and can estimate all the others). dinosaurs is among the “hottest” areas of dinosaur research The oldest dinosaurs, from the Valley of the Moon in because such dinosaurs are not as common as later ones and Argentina, are dated at 227.8±0.3 million years (Rogers et hold great potential for elucidating the dinosaurs’ early al. 1993; a report of an older dinosaur from Madagascar is diversification into major evolutionary branches. being revised), and the extinction event 65.95±0.04 million A tally of dinosaurs published 3 years ago (Wang and years ago (Kuiper et al. 2008) apparently killed off all of Dodson 2006) recognized 527 genera, the vast majority the dinosaurs alive at that time while leaving their with only a single species. (Dinosaur paleontologists have a descendents, the birds. Dinosaurs were therefore on the partiality to naming new genera even when a new species is Earth for at least 161 million years, and their descendents closely related to an existing genus, something our the birds (reference to Padian contribution) continued on entomological colleagues find mildly disturbing.) For each for another 65.95 million years (and thankfully show few of these genera, a fossil exists with features that, theoret- signs of leaving us). To put this in perspective, the oldest ically, clearly differentiate it from every other known human fossils are six to seven million years old (Brunet et dinosaur genus. For some of these genera, skeletons of al. 2002), so the dinosaur fossil record is about 23 to 27 many individuals are known (e.g., Protoceratops, Psittaco- times longer than that of humans. saurus, Allosaurus, Coelophysis, and Iguanodon), some- As with the fossil record in general (Kidwell and times in the hundreds, and one remarkable bone bed in Holland 2002), the record of dinosaur fossils is not Alberta, Canada, preserves bones from thousands of consistent through time because the deposition of sedimen- individuals of Centrosaurus, a one-horned relative of tary rocks in the environments in which dinosaurs lived Triceratops. Wang and Dodson use the known number varied at different times in the Earth’s history. This of dinosaurs and the rate at which new genera are discovered depended both on local conditions and on global phenom- to estimate that 1,850 genera of dinosaurs had existed, in ena such as plate tectonics and sea level changes, as the which case less than a third have been discovered. mountain building that fueled sedimentation waxed and Evolutionary relationships are evident through the waned. The incomplete sampling of ancient species by the features shared by groups of organisms (Gregory 2008). sedimentary record, coupled with the episodic, greatly Just as a very high percentage of human DNA is identical incomplete record that was left of those sediments, could to that of our closest relatives, the chimpanzees, closely not possibly produce a series of forms showing all stages of related species of all kinds share similar DNA, anatomy, continuous change from one species to another (whether or and physiology. One of the main reasons the theory of not absolutely complete continuity in morphology ever evolution is accepted is that it provides an explanation for existed is an open question). But some examples of small why similar features are found in different organisms. To changes between very similar species over time exist, such cite just one example, the presence in all land-living as the three successive species of the horned dinosaur vertebrates of four limbs with digits (reference to Shubin Chasmosaurus over approximately 2.5 million years within contribution) is explained by the close evolutionary 238 Evo Edu Outreach (2009) 2:236–247 relationships of all tetrapods to each other, as members of Duck-Billed Dinosaurs The duck-billed dinosaurs, or hadro- the same lineages. Supposed alternative theories such as saurs, were abundant and diverse near the end of the Intelligent Design are mute in explaining these rampant Cretaceous and were among those groups that disappeared similarities.
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