The fossil record stands as one of the most vital archives for understanding the evolutionary history of life on Earth. Yet, like any archive, it is riddled with gaps, biases, and selective preservation that can distort our comprehension of ancient biodiversity. A groundbreaking study led by Dr. Hank Woolley of the Natural History Museums of Los Angeles County’s Dinosaur Institute now sheds unprecedented light on these challenges within the fossil record of squamates—the diverse reptile group comprising lizards, snakes, mosasaurs, and amphisbaenians. By meticulously analyzing the completeness of squamate fossils spanning over 242 million years, Woolley’s research not only quantifies longstanding biases but also elucidates the underlying reasons why certain skeletal fragments dominate museum collections worldwide.
Squamates have fascinated paleontologists for decades, yet their fossil record remains surprisingly fragmentary. Most fossil species are known primarily from isolated bones—especially lizard jaws and snake vertebrae—that account for less than 20% of their total skeletal morphology. This extreme incompleteness has long obscured the evolutionary narratives of these reptiles, complicating efforts to reconstruct their phylogeny, biogeography, and ecological adaptations. Woolley and colleagues embarked on an extensive meta-analysis, compiling data from nearly 500 scientific papers and extensive fossil databases, to systematically evaluate how various biological and environmental factors influence fossil preservation quality within squamates.
One of the core concepts introduced by the study is that of “megafilters”—large-scale biotic and abiotic processes exerting disproportionate influence on fossil preservation. Through this lens, the research identifies bone density, overall body size, and depositional environment as primary determinants shaping the fossil record’s completeness. Denser and fused bones, such as those found in mosasaurs, tend to resist decay and mechanical breakage far better than more delicate skeletal elements. Consequently, mosasaurs, as marine-adapted squamates, boast one of the most comprehensive fossil records relative to their terrestrial cousins, whose remains are often scattered and degraded.
The study also rigorously examines taphonomic biases related to the environment of deposition. Squamate species buried in aquatic or nearshore settings tend to be represented by more complete fossils. In contrast, terrestrial environments—where weathering, scavenging, and other decay processes predominate—are associated with notably poorer preservation. This environmental signal, the authors emphasize, substantially outweighs human-driven sampling biases that often dominate discussions of fossil record quality in other groups, such as dinosaurs. Unlike the latter, where uneven geographic or taxonomic research efforts skew data, squamate fossil completeness is overwhelmingly dictated by natural decay filters.
Dr. Woolley’s endeavor was fueled, in part, by the digital revolution in paleontology. The consolidation of vast fossil datasets into comprehensive digital repositories like the Paleobiology Database has allowed scientists unprecedented access to global fossil information. However, the challenge remained that these databases often lack the granular detail necessary to assess specimen completeness. To bridge this gap, Woolley undertook a painstaking review of the primary literature, scoring each fossil specimen’s completeness based on the descriptions of skeletal elements. This meticulous process enabled the team to generate a high-resolution, quantitative depiction of global squamate fossil completeness that transcends prior qualitative assessments.
Intriguingly, their analysis reveals that human sampling biases—issues like geographic research intensity, funding availability, or taxonomic focus—play a surprisingly limited role in determining fossil completeness for squamates. This runs counter to many earlier studies where such human factors heavily dictated the fossil record’s fidelity. Instead, biological and geological megafilters dominate, underscoring the inescapable impact of natural processes on what survives into the fossil record. This insight reframes how paleontologists must consider preservation bias, particularly when reconstructing evolutionary patterns across vast temporal and spatial scales.
The implications of this research reverberate beyond methodological clarification. For example, mosasaurs, the iconic marine squamates of the Late Cretaceous, feature a distinctly more complete fossil record due to their robust, dense bones and favorable depositional environments under marine conditions. Yet, where they fit on the squamate evolutionary tree remains hotly debated. Hypotheses range from close kinship with snakes to relationships with monitor lizards or even placements in separate branches altogether. Woolley’s data could pave the way to resolving such phylogenetic uncertainties by highlighting which features are genuinely missing due to preservation bias and which can be interpreted with confidence.
Meanwhile, the fragmentary nature of lizard and snake fossils has traditionally limited insights into their deep-time evolutionary responses, especially through multiple mass extinction events. By quantifying completeness and its controlling factors, the study identifies critical gaps where fossils are missing or underrepresented. These findings offer a roadmap for future excavation and research efforts, guiding paleontologists toward deposits and regions likely to yield more informative squamate specimens. Moreover, it situates squamate fossil biases within larger questions about how vertebrate life survives, adapts, and radiates during and after catastrophic environmental upheavals.
Beyond its evolutionary ramifications, Woolley’s work exemplifies the transformative power of integrating digital databases with traditional scholarship—what some call the second digital revolution in natural history sciences. This synthesis enables not only global-scale analyses but also the fine-grained scrutiny necessary to unearth megabiases that once remained hidden. As digitization efforts accelerate and more fossil collection data become openly accessible, such integrative approaches promise to revolutionize paleontology’s capacity to interrogate its own records.
The study also highlights the pressing need for paleontologists to refine fossil completeness metrics and to prioritize the digitization of detailed morphological data. Current databases provide invaluable occurrence and classification information but often lack comprehensive anatomical coverage. Initiatives aimed at annotating fossils for completeness and contextual environmental data will be crucial in amplifying the utility of digital repositories for evolutionary research.
Dr. Woolley’s exhaustive approach to addressing fossil completeness is a timely reminder of the intricate dance between biological reality and the vagaries of preservation. While the fossil record remains imperfect and fragmentary, these imperfections are not merely obstacles but are informative patterns that can be decoded. Recognizing the megafilters operating across geological epochs allows scientists to interpret fossil data more accurately and to compensate for biases where possible.
In closing, this innovative research invites a more nuanced understanding of squamate evolution and fossilization processes. It challenges researchers to look beyond the mere presence or absence of fossils and to consider the complex interplay of anatomy, environment, and time that governs what we find in the rock record. As paleontology navigates the digital age, studies like Woolley’s serve as beacons, illuminating paths toward more complete and unbiased interpretations of life’s ancient tapestry.
Subject of Research: Animals
Article Title: Taphonomic megabiases constrain phylogenetic information in the squamate fossil record
News Publication Date: 24-Sep-2025
Web References:
http://dx.doi.org/10.1017/pab.2025.10060
Image Credits: Hank Woolley
Keywords: Squamate fossils, fossil completeness, taphonomy, megabiases, paleoenvironment, mosasaurs, squamate evolution, fossil record bias, digital paleontology, meta-analysis
