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The Completeness of the Fossil Record of Mesozoic Birds: Implications for Early Avian Evolution

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The Completeness of the Fossil Record of Mesozoic Birds: Implications for Early Avian Evolution The Completeness of the Fossil Record of Mesozoic Birds: Implications for Early Avian Evolution Neil Brocklehurst1*, Paul Upchurch2, Philip D. Mannion1,3, Jingmai O’Connor4,5 1 Museum fu¨r Naturkunde, Leibniz-Institut fu¨r Evolutions und Biodiversita¨tsforschung, Berlin, Germany, 2 Department of Earth Sciences, UCL, London, United Kingdom, 3 Department of Earth Science and Engineering, Imperial College London, London, United Kingdom, 4 Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China, 5 Dinosaur Institute, Natural History Museum of LA County, Los Angeles, California, United States of America Abstract Many palaeobiological analyses have concluded that modern birds (Neornithes) radiated no earlier than the Maastrichtian, whereas molecular clock studies have argued for a much earlier origination. Here, we assess the quality of the fossil record of Mesozoic avian species, using a recently proposed character completeness metric which calculates the percentage of phylogenetic characters that can be scored for each taxon. Estimates of fossil record quality are plotted against geological time and compared to estimates of species level diversity, sea level, and depositional environment. Geographical controls on the avian fossil record are investigated by comparing the completeness scores of species in different continental regions and latitudinal bins. Avian fossil record quality varies greatly with peaks during the Tithonian-early Berriasian, Aptian, and Coniacian–Santonian, and troughs during the Albian-Turonian and the Maastrichtian. The completeness metric correlates more strongly with a ‘sampling corrected’ residual diversity curve of avian species than with the raw taxic diversity curve, suggesting that the abundance and diversity of birds might influence the probability of high quality specimens being preserved. There is no correlation between avian completeness and sea level, the number of fluviolacustrine localities or a recently constructed character completeness metric of sauropodomorph dinosaurs. Comparisons between the completeness of Mesozoic birds and sauropodomorphs suggest that small delicate vertebrate skeletons are more easily destroyed by taphonomic processes, but more easily preserved whole. Lagersta¨tten deposits might therefore have a stronger impact on reconstructions of diversity of smaller organisms relative to more robust forms. The relatively poor quality of the avian fossil record in the Late Cretaceous combined with very patchy regional sampling means that it is possible neornithine lineages were present throughout this interval but have not yet been sampled or are difficult to identify because of the fragmentary nature of the specimens. Citation: Brocklehurst N, Upchurch P, Mannion PD, O’Connor J (2012) The Completeness of the Fossil Record of Mesozoic Birds: Implications for Early Avian Evolution. PLoS ONE 7(6): e39056. doi:10.1371/journal.pone.0039056 Editor: Leon Claessens, College of the Holy Cross, United States of America Received November 24, 2011; Accepted May 15, 2012; Published June 25, 2012 Copyright: ß 2012 Brocklehurst et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: [email protected] Introduction forelimbs from the Cretaceous) in a clade containing loons and grebes [6], even though this requires an evolutionary reversal to Birds (Aves) represent one of the most diverse and abundant the plesiomorphic toothed condition [9]. Several studies based on vertebrate groups, with over 10,000 species [1] and an estimated molecular clocks support the traditional view of gradual 300 billion individuals alive today [2]. The avian fossil record neornithine diversification starting in the Early Cretaceous [10– extends back to the Late Jurassic, or possibly further [2] (although 13]. The exact timing of these events varies with each analysis; see [3] and [4] and Materials and Methods below). This fossil Kumar & Hedges [10] and Paton et al. [11] suggested that record, in particular that of the Mesozoic, has recently undergone Neornithes originated during the Aptian (125–112 million years a revolution as a result of an explosion of newly discovered taxa ago [mya]), whereas Cooper & Penny [12] and Brown et al. [13] during the last three decades. At present, over 120 avian species proposed an origin as early as the Valanginian (140–133 mya) are known from the Mesozoic, from all continents except (Figure 1). Biogeographic analysis has also supported the idea of mainland Africa. Despite this new information, controversy a Cretaceous origin: for example, Cracraft [14] found neornithine surrounds several aspects of avian evolution, including the timing evolution to have been heavily influenced by vicariance (the of the origin and diversification of modern birds (Neornithes). isolation of lineages by the splitting of continents), and suggested Much of the debate surrounding neornithine evolution focuses that they diversified with the breakup of Gondwana during the on the apparent discrepancy between the time of their origins Cretaceous [15]. The divergence of the majority of neornithine according to molecular data and their earliest appearance in the clades during the Cretaceous would suggest that the Cretaceous/ fossil record. The ‘traditional’ view of neornithine origins Palaeogene (K/Pg) mass extinction had relatively little effect on envisaged the evolution of modern groups in the Cretaceous [5– this group, although Feduccia [16] considered this unlikely, since 7]. This was based on the assignment of numerous species of birds are often extremely sensitive to environmental perturbations. Mesozoic bird fossils to extant orders [8], such as the placement of the Hesperornithiformes (toothed aquatic birds with reduced PLoS ONE | www.plosone.org 1 June 2012 | Volume 7 | Issue 6 | e39056 The Completeness of Mesozoic Birds PLoS ONE | www.plosone.org 2 June 2012 | Volume 7 | Issue 6 | e39056 The Completeness of Mesozoic Birds Figure 1. The two opinions on the timing of modern bird origins based on molecular clocks. A) an origin 123 million years ago during the Aptian, modified from reference [11]; B) an origin 135 million years ago during the Valanginian modified from reference [13]. doi:10.1371/journal.pone.0039056.g001 In contrast, many recent palaeobiological, and some molecular, One way to reconcile the current fossil record of Mesozoic birds studies have disputed such an early origin of Neornithes. Hope with the molecular clock-based Cretaceous divergence times for [17] documented 50 putative Cretaceous neornithine specimens Neornithes is to propose that members of this clade were present from as early as the Coniacian, but these are all extremely earlier in the Cretaceous but that their fossil record is currently too fragmentary and none were subjected to formal phylogenetic incomplete for them to be unequivocally recognised by palaeo- analysis. The characters used to assign them to neornithine clades biologists. For example, Kumar & Hedges [10] implied that the are often dubious or incorrect (see [18] for a summary), and reason the fossil record has not produced reliable Cretaceous attempts to place these fossils in modern groups have been neornithines is that birds are small animals with delicate skeletons, hindered by poor knowledge of neornithine relationships [19]. and therefore not easily preserved. However, palaeobiologists have There has been relatively little morphological analysis of modern not found this to be a compelling argument. In particular, Benton clades incorporating their fossil representatives, and as such the [20] noted that other animals with delicate skeletons, including relationships between the taxa and the characters that unite them basal birds, lizards, amphibians and small mammals, have been are not well understood. Benton [20] argued that all records of found throughout Cretaceous sediments, so their preservation is Neornithes prior to the K/Pg boundary are either misdiagnosed or certainly possible. However, it is conceivable that a sampling bias are from incorrectly dated localities; for example the Hornerstown or other taphonomic factor might have selectively preserved basal Formation in the USA, originally dated as latest Maastrichtian and birds relative to Neornithes. For example, such a selective containing several putative neornithine species, has been reinter- mechanism might have operated if basal bird species were much preted by some as being from the earliest Palaeocene [21], more abundant than early Neornithes: this is because population although this remains uncertain [17]. A phylogenetically based size might correlate with the probability of individuals making it study of the avifauna of the Hell Creek, Lance and Frenchman into the fossil record. Another such bias might occur if Formations of North America (the only formations containing bird neornithines originated and diversified in a part of the world that specimens that can be reliably dated to the end of the has a particularly poor avian fossil record, such as Africa. Finally, Maastrichtian) [22] found no compelling evidence for a neor- it is possible that neornithines showed a preference for different nithine radiation prior to the
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