INCOMPLETE PRESERVATION OF THE SKELETON DOES NOT MISINFORM INFERENCES OF PHYLOGENETIC RELATIONSHIPS IN VERTEBRATE GROUPS

Maximizing the vast amount of evolutionary information available in incomplete fossils within natural history collections necessitates: 1) a quantitative characterization of the biases present in this record, and 2) knowing whether the phylogenetic data preserved in these incomplete samples are reliable and consistent. We compiled a dataset of 19,077 skeletal elements housed in six North American museum collections to characterize the representation of different anatomical regions in the fossil record of the most diverse group of tetrapods on earth: birds. We find that bones from the girdle and limb elements account for 94.6% of sampled skeletal elements, while the rest of the skeleton (e.g., skull bones, vertebrae, ribs) accounts for 5.4% of occurrences, demonstrating an overwhelming bias toward the preservation of limb bones in the fossil record of birds. To determine the effect of this bias on inferences of avian phylogeny, we used metrics of homoplasy (Consistency Index, CI) and retained synapomorphy (Retention Index, RI) in individual morphological characters corresponding to “underrepresented” and “overrepresented” fossil skeletal elements. We applied these metrics to a comprehensive morphological phylogenetic dataset assessing the phylogenetic relationships of waterbirds. Statistical comparisons of character CI and RI values across both datasets reveal that characters corresponding to “overrepresented” skeletal elements in the avian fossil record show similar levels of CIs and RIs to characters from “underrepresented” anatomical elements.These results are similar to previously reported patterns in the second most diverse group of tetrapods: squamates (lizards, snakes, and their relatives). However, the overrepresented regions of the skeleton within the fossil record of birds and squamates differ. While the bird fossil record consists almost entirely of limb bones, the squamate fossil record consists mostly of tooth-bearing bones in the skull, vertebrae, and ribs. Despite these stark differences in representation of the skeleton, multiple measurements of phylogenetic signal suggest that reliable evolutionary information can be found in the fossil record of the two most diverse groups of tetrapods on the planet. In sum, this study proposes a comparative framework for quantifying bias in the fossil record using a novel, phylogenetic approach.