Â鶹ÒùÔº

For the more than 242 million years that lizards and snakes appear in the fossil record, they show up mostly as pieces of lizard jaws and snake vertebrae. Exactly why these parts survive as fossils has been a mystery—until now.

In a published in Paleobiology, Dr. Hank Woolley from the Natural History Museums of Los Angeles County's Dinosaur Institute looks at the entire history of squamates (the reptile group that includes lizards, snakes, and mosasaurs, among others) to understand why only certain parts show up—the bias of the record—and to quantify that bias for the first time.

Woolley and his co-authors found that physical characteristics like bone density and body size, along with where the animals lived and died, were the main predictors of how complete a fossil would be. Woolley identifies these patterns as megafilters, which refer to processes that have an outsized effect on the way we understand the fossil record. This unprecedented high-resolution view of the group's biodiversity through multiple mass-extinction events illuminates the mysteries of squamates' evolutionary history, and can help guide future work that examines how animals are preserved as fossils.

Woolley and colleagues examine in detail the relationship between inferred environments in which a fossil squamate species was buried and the completeness of its remains. Environmental and geological processes play a strong role in how complete the fossil record can be. Mosasaurs are almost exclusively found in marine settings and are more complete on average than other groups. Snakes, on the other hand, are found nearly everywhere, but are highly incomplete.

"Squamates are super adaptable, widespread, and incredibly diverse throughout their whole evolutionary history," says Woolley. "However, our new study shows that the majority of fossil squamate species are known from incomplete and scrappy skeletal material: isolated jaw bones and vertebrae. Understanding why this happens on a global scale is a key first step in sorting through the biases that have stood in the way of having a clear picture of the history of this important group of animals."

Many people took up hobbies like baking sourdough bread over the COVID-19 lockdowns, but nobody on Earth took up Dr. Woolley's pandemic project: reading every paper describing a squamate fossil from their approximately 240 million years on the planet.

"I think it's almost 500 papers," says Woolley.

The study leverages the first digital revolution of museum science and newly digitized collections made available to researchers around the world, and is a product of the second digital revolution, where those collections are combined into databases, such as the Paleobiology Database, letting researchers like Woolley get a bigger, clearer picture than ever before of the totality of the fossil record. But to understand how complete each fossil was, he had to dive into each published paper describing the fossils in those digitized collections; hence the pandemic reading project.

"It was the only way to look at each bone that's described and count it towards the completeness of what we know about a squamate species. We just don't have that level of detailed information available yet from the Paleobiology Database, and hopefully, studies like ours can help build toward more and more comprehensive information available to researchers online. For the time being, you still have to go to the papers and museum collections themselves," says Woolley.

The purpose of this deep dive was to explore the concept of megabiases—things like regional/global geologic processes and human-based sampling issues—and how they shape our fossil data. From a human-based sampling perspective, examples of megafilters could include whether resources have been mustered to explore one particular region over others, or asymmetries in the number of scientists researching a group of organisms—dinosaurs get more attention than the smaller creatures living among them, for example.

The key finding of this study is that the chief determinants of how complete a squamate fossil can be are how durable the bones themselves are (for example, bigger bones and fused bones are more resistant to decay and breakage over time), and the environment in which the squamate species died and was buried in (like a river, lake, desert, or open ocean).

Woolley and his co-authors found that human-based sampling issues, which can be the main culprit in fossil record completeness in other groups of animals like dinosaurs, appear to play a secondary role in determining the completeness of a fossil squamate species.

Having such a clear picture of squamates' history through deep time could be key to answering some of the questions around this incredibly diverse group of reptiles. For instance, the extinct group of marine lizards known as mosasaurs has the most complete fossil record of any squamate group. The relative incompleteness of other squamate groups' fossil records may play a role in obscuring mosasaurs' evolutionary history.

"Paleontologists disagree about where mosasaurs fit on the squamate tree of life," says Woolley. "Some workers think they're related to snakes, some think they're related to monitor lizards, and some hypotheses place them in a completely different part of the tree." All this data could make it easier to find a reptile-sized hole in Mosasaur evolutionary history.

Understanding the biases in collections could also help researchers understand how these giants of the Mesozoic oceans radiated across the globe so quickly, and how other squamates, like lizards and snakes, survived mass extinction events and thrive today.

"Between the early evolutionary history of groups and after mass extinction events, this study helps identify key gaps in the fossil record that have the potential to teach us a lot about these lizards and snakes that are around today," says Woolley.

"This study is a great example of how the next generation of paleontologists like Hank are taking advantage of digitized records such as the Paleobiology Database, and combining them with the sweat equity spent combing through the literature and museum collections to help answer big-picture questions about the bias and quality of the fossil record," says co-author and Gretchen Augustyn Director & Curator of the Dinosaur Institute Dr. Nathan Smith.

"Darwin himself devoted a whole chapter of the 'Origin of Species' to the imperfection of the fossil record, but Hank's work is helping to finally quantify those biases and tease out how they impact our understanding of the evolutionary history of major vertebrate groups."

Rather than being discouraged by the inherent "low ceiling" of lizard and snake remains to end up as complete fossils, Woolley points to ground-breaking research and rich data yet to be gleaned from fragmentary fossils.

"If we don't quantify these biases, we could be overlooking some of the very real information that these incomplete fossils can give us," says Woolley. It's a promising case study that shows a better understanding of other collections of fragmentary fossils is possible.

"Continuing to build these detailed fossil datasets can only help our endeavor to understand how our Earth system's natural processes—and how we study fossils—shape the field of paleontology."