The quality of the fossil record of Mesozoic birds

Abstract

The Mesozoic fossil record has proved critical for understanding the early evolution and subsequent radiation of birds. Little is known, however, about its relative completeness: just how 'good' is the fossil record of birds from the Mesozoic? This question has come to prominence recently in the debate over differences in estimated dates of origin of major clades of birds from molecular and palaeontological data. Using a dataset comprising all known fossil taxa, we present analyses that go some way towards answering this question. Whereas avian diversity remains poorly represented in the Mesozoic, many relatively complete bird specimens have been discovered. New taxa have been added to the phylogenetic tree of basal birds, but its overall shape remains constant, suggesting that the broad outlines of early avian evolution are consistently represented: no stage in the Mesozoic is characterized by an overabundance of scrappy fossils compared with more complete specimens. Examples of Neornithes (modern orders) are known from later stages in the Cretaceous, but their fossils are rarer and scrappier than those of basal bird groups, which we suggest is a biological, rather than a geological, signal.

Figure 1

Summary tree illustrating phylogenetic relationships among Mesozoic birds (modified from Chiappe & Dyke 2002). Neornithes (modern birds) are represented by a duck (Anas). The oldest avians included in our dataset are Archaeopteryx and Wellnhoferia (both ca. 149 Myr ago); numbers of included taxa in each clade and their mininum inferred age are as follows: Archaeopterygidae (four, including Rahonavis for this analysis), 146 Myr ago; Confuciusornithidae (two), 121 Myr ago; Enantiornithes (39), 71 Myr ago; Ornithurae (37, excluding Neornithes), 65 Myr ago; Neornithes (14), 65 Myr ago.

Figure 2

Collector curves for Mesozoic birds: (a) cumulative number of specimens (as a proportion of the total number of specimens for which find-dates are available; n=113) found since Archaeopteryx in 1861; (b) age (Myr ago; oldest estimate) of fossil specimen plotted against year of find. The age of specimen is not correlated with year found (Spearman’s rank correlation: r_s=0.123, n=113, p=0.194).

Figure 3

(a) Total numbers of avian species described per age compared with the number of sedimentary formations yielding avian fossils per age. The number of fossil species described correlates with the number of sedimentary formations (Pearson product moment correlation: r=0.847, n=16, p<0.05). The trend-line is described by the equation number of species=−0.31+1.94 number of sedimentary formations. The relationship holds even if the outlying point is removed, when the number of species=1.97+1.19 number of sedimentary formations (Pearson product moment correlation: r=0.548, n=15, p<0.05). (b) The number of Mesozoic bird species found plotted against number of fossil yielding sites of the same age (estimated oldest age). The number of species found of a given age is significantly correlated with number of sites of the same age. The trend-line is described by number of species=0.73+1.85 number of sites (Pearson product moment correlation: r=0.880, n=16, p<0.05). The relationship holds even if the outlying point is removed, number of species=1.64+1.43 number of sites (Pearson product moment correlation: r=0.574, n=15, p<0.05).

Figure 4

The number of avian species per Mesozoic age described on the basis of one bone, more than one bone, one specimen (i.e. partial skeleton) and more than one specimen. The total number of species found per age is also shown.

Figure 5

The number of species (circles) and sites (plus symbols) found plotted against the estimated age of the formations (Myr ago). Neither number of species described (Pearson product moment correlation: r=0.173, n=16, p>0.05) nor number of sites found (Pearson product moment correlation: r=0.258, n=16, p>0.05) is correlated with age.