IS A LAGERSTÄTTEN BIAS RESPONSIBLE FOR PRESERVATION TRENDS IN THE AVIAN FOSSIL RECORD?

Recent studies in avian paleontology have suggested that a Lagerstätten bias led to smaller bird skeletons being preserved in the fossil record as commonly as larger bird skeletons, yet this hypothesis still remains to be fully tested. To better understand factors affecting avian fossil preservation potential, we used stratified and multivariable regression analyses to investigate associations of well-preserved avian specimens with bird body size, climate, and energy of the environment, with results contrasted to geologic settings’ Lagerstätten status. Using articles from the Paleobiology Database, we abstracted data on 693 avian fossil specimens, spanning the Late Jurassic to the Recent, from 398 publications. We estimated well-preserved status from degree of skeletal articulation, body size from correlations with limb lengths, climate data as reported in the articles or from PALEOMAP Project information, and classified energy based on depositional environment descriptions. Fossils of smaller birds were, overall, less common (18.9%) than those of medium (33.2%) or large/giant (47.9%) size birds. However, more of the smaller birds were well-preserved (28%) compared to medium (18%) or large/giant (10%) birds. Smaller specimens, as compared to larger specimens, were more likely to be well-preserved in low-energy environments (e.g., Lagerstätten; trend p=0.0007); medium-energy environments (trend p=0.037); and high-energy environments (trend p=0.088). Also, well-preserved avian fossils were more common in warm and humid climates (25%) compared to cool or dry climates (4.2%). As bird body size increased in warm and humid climates, fewer specimens were well-preserved (Chi-square trend p<0.0001). Importantly, nearly all (93 of 100) Lagerstätten specimens represented locations and time periods with warm and humid climates. The regression model showed the climate association with preservation quality persisted even when other factors were included. The association of warm and humid climates with higher-quality preservation indicates that a substantial part of the Lagerstätten “bias” may actually be attributable to climate. Therefore, we believe it is premature to accept the hypothesis that the abundance of well-preserved small avian skeletons is attributable only to a Lagerstätten effect.