Gaps in the fossil record are time intervals with no fossils of a taxonomic group after the group first appeared and before it went extinct. A gap might be within a single species, if indistinguishable fossils occur at different times, or within a group of species, such as the gap between the last fossil coelacanth species in the Cretaceous and the two living coelacanths. If the gap is within a group of species (such as a genus or family) then the relationships among the species determine where the gaps are, since some species are more closely related to one another than to the other species in the group.

Nearly all of the gaps in the fossil record of the theropod dinosaur group that includes birds - the Coelurosauria - are between species, because most of these species occur only at a single time in the fossil record. These gaps are implied by the presence of some, but not all, of the groups of coelurosaurians in the Late Jurassic. Definitive specimens of only a few coelurosaurians (Ornitholestes, Coelurus, Compsognathus, and the oldest bird, Archaeopteryx) are known from the Late Jurassic, and the other coelurosaurian groups are known only from the next geologic period, the Cretaceous. The groups of theropods that should also be present in the Late Jurassic include dromaeosaurs like Velociraptor, the gracile troodonts like Saurornithoides, oviraptors such as those found sitting on nests in Mongolia, the enigmatic therizinosaurs that combined long arms and claws with stocky hindlimbs, the strange alvarezsaurs such as Mononykus with short, muscular arms bearing a single large claw, and the ostrich-like ornithomimids. The oldest representatives of several of these groups (dromaeosaurs, therizinosaurs, and oviraptors) are from the Early Cretaceous "feathered dinosaur" beds of Liaoning, in northeastern China, and bear feather-like epidermal structures.

As an example of how the relationships of a group imply when we should expect its fossils to occur, consider the fossil record of humans and chimpanzees. Humans have a fossil record extending back over 5 million years (perhaps over 6), whereas chimpanzees - our closest relatives - have no recorded fossils older than a few thousands of years (gorillas have a similarly poor fossil record). Because the five+ million year old fossil relatives of humans share with us unique features not found in chimpanzees, they already were on the evolutionary lineage leading towards us and away from our common ancestor with chimpanzees. The common ancestor of chimps and humans had to be at least as old as the oldest humans, so the chimp lineage must extend back to that time. At least one species on the chimpanzee lineage must therefore have been present throughout the period spanning the age of the oldest human fossil to the age of the oldest chimp fossil. The fossil humans are presumed not to have been the ancestors of chimpanzees since this would require "de-evolving" the human features they had already evolved, something that is possible but contrary to the evidence at hand.

Applying this logic to the fossil record of coelurosaurians, the presence of Archaeopteryx in the Late Jurassic documents that the bird lineage was present at this time. If birds were related to a group composed of all of the other coelurosaurians then there would be no gaps, since only one of the other coelurosaurians would need to be present in the Late Jurassic and three are known. But studies of their relationships indicate instead that birds are nested well within the coelurosaurians, implying gaps in most of the other coelurosaurian groups. The closest relative of the bird lineage is probably the dromaeosaurs (although there is not yet a consensus among paleontologists on this), so a representative of this group must also have been present in the Late Jurassic. Furthermore, the closest relative of the group that includes both Archaeopteryx and dromaeosaurs (probably the troodonts) must too have been present, et seq.

Fragmentary coelurosaurian remains from earlier in the fossil record hint that the record of this group may extend further back than the Late Jurassic. For example, a lower jaw bone from the Early Jurassic of southern China appears to be from a therizinosaur (Xu, Zhao, and Clark, 2001, Journal of Vertebrate Paleontology 21:477-483). Protoavis from the Late Triassic of Texas, once touted as a true bird, appears to be a basal coelurosaurian, although Shuvosaurus from the same beds, described as an ornithomimid, is probably not a member of this coelurosaurian group (Rauhut, 1997, pp. 17-21 in: Sachs, S., Rauhut, O. W. M. & Weigert, A. (eds.): 1. Treffen der deutschsprachigen Palaeoherpetologen, Dusseldorf).

James M. Clark

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