The fossil record of the Cretaceous is critical for understanding the evolution of modern tetrapods. Using a measure of relative completeness of the fossil record—the Simple Completeness Metric (SCM)—quality of the fossil record and diversity during the Cretaceous appear to be closely related, suggesting an artifactual component. The SCM calculations also show that knowledge of the fossil record has improved in the last ten years. Recent proposals that modern orders of birds and mammals originated early in the Cretaceous are rendered unlikely by four arguments: (1) the SCM calculations indicate that the fossil record of Cretaceous birds and mammals is relatively good; (2) it is unlikely that all modern orders, independently, would have remained cryptic throughout the Cretaceous; (3) control samples of exquisitely preserved tiny Cretaceous tetrapods lack any specimens of modern groups of birds and mammals; and (4) the suggestion that the undiscovered ancestors of modern groups are to be found in unsampled parts of the Earth is not supported by cladistic evidence.

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