Paleobiogeography And Evolutionary History Of Theropoda

Theropod body fossils are in Upper Triassic to Upper Cretaceous deposits on all seven continents, and so far their trace fossils are only missing from Antarctica.

The small (1 meter long) possible theropod Eoraptor, of the earliest part of the Late Triassic, has been proposed as approximating the characteristics of a theropod ancestor because it shares some traits with theropods but also lacks others that define this clade. As mentioned earlier, herrerasaurids may also approximate the earliest theropods. These are in Upper Triassic strata of both North and South America, although the South American examples are geologically older. Small theropod-like tracks, showing a three-toed and almost bilaterally symmetrical compression shape associated with most theropod feet throughout the Mesozoic Era, are also recorded from Middle Triassic strata. Interestingly, these tracks precede the body fossil record for theropods. However, many paleontologists do not accept such tracks as necessarily belonging to theropods, although some acknowledge that theropods may have originated in the latest part of the Middle Triassic. The overlapping use of skeletal and track data is certainly helpful for narrowing down when theropods first evolved during the Triassic, and such resolution should improve considerably with more discoveries of both forms of evidence.

Theropod track horizons are documented from Alaska, the mid-continental USA, Canada, and Mexico in North America; Argentina, Bolivia, Brazil, and Peru in South America; England, France, Italy, Poland, Portugal, Spain, and Switzerland, in Europe; India, China, Mongolia, Japan, and Korea in Asia; northern Africa, such as Morocco; and Australia, where theropod tracks are much more common than their skeletal remains. Both body and trace fossils of theropods are in facies representing a wide variety of environments: deserts, swamps, river floodplains, upper delta plains, lake shorelines, and seashores. Of course, tracks are the only reliable in-situ records of which environments were actually frequented by theropods, but some other trace fossils (nests and coprolites) and body fossils (eggs, and complete, apparently unmoved skeletons) certainly point paleontologists in the right direction as well (Chapters 7 and 14). The totality of evidence for theropods from such a diversity of terrestrial environments and broad latitudinal range, spanning the entire geologic range of dinosaurs, indicates their biological diversity and adaptations to numerous niches through time. Based on current scientific evidence, theropods represent evolutionary diversification more so than any other major dinosaur clade.

Theropod skeletal remains are uncommon, normally comprising 15-20% of all dinosaur remains in any Upper Triassic-Upper Cretaceous rocks formed in terrestrial environments. Furthermore, they are even more rare (although a few are known) in shallow marine deposits (Chapter 7). A study conducted in 1976 on 171 valid theropod species found that 85% of them were named on the basis of five or fewer specimens, and a single specimen represented about 40% of these species. These percentages have changed little since then, and the recent spate of new theropod species discovered and described during the 1990s and early part of the twenty-first century are primarily based on one or two specimens. The relative scarcity of theropod remains means that numerous "ghost lineages" (Chapter 6) have been proposed for species that have few known close relatives, which constitutes a challenge for phylogenetic classification schemes applied to theropods. Fortunately, a combination of taphonomic factors and perhaps social behavior resulted in some monospecific theropod bone beds, such as those of the Late Triassic Coelophysis, Late Jurassic Allosaurus, and more recently Late Cretaceous tyrannosaurids. With regard to the latter, Tyrannosaurus was originally considered a rare dinosaur, but now its remains are among the most commonly encountered in strata from some areas of the western USA.

As mentioned earlier, theropod species found so far support the hypothesis that they comprised the most diverse of all dinosaur clades. This diversity may reflect

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  • Ronald
    What trace fossils of theropods are missing?
    2 years ago

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