Ancient Invasions, Modern Warnings: What Fossils Tell Us About Migration in Response to Climate Change Before Human Arrival

From Shell Enthusiast to Scientist

Growing up near the ocean, I was fascinated by the small creatures I found in tide pools—slow-moving snails, tiny bivalves, and the colorful shells used by hermit crabs. I’d walk along the beach collecting shell pairs, wondering if they belonged to the same individual. It amazed me how these animals, which barely move at all, could be found across such a wide area—from warm tropical waters to the colder shores of California.

My curiosity grew deeper when I saw fossil shells trapped in the hills of Palos Verdes, far from the ocean. What were these marine animals doing inland and high above sea level? Why were the same shells I found on the beach also falling out of cliffs at Point Fermin?

These early questions—about where animals live, how far they can spread, and what happens to them over time—sparked my journey from a beach-loving naturalist to a scientist studying ancient oceans.

Why Mollusks Matter

When most people think about the ocean, they picture large open-water animals like sharks, whales, or dolphins. But there’s an entire world of shell-bearing creatures that live along the coast and on the seafloor: mussels, clams, oysters, and more. These animals are part of what’s called the benthos, and they play a crucial role in marine ecosystems. You may even eat some of them in chowder or seafood pasta!

But these animals are more than just food. They act as ecosystem engineers, building habitats, cleaning the water, and serving as an important part of marine food webs, both as prey and as contributors to the structure of benthic communities. Some live attached to rocks, and others burrow in sand, from shallow tidepools to the deep sea. California alone is home to over 2,500 species of these shelly animals!

Despite their importance, we often overlook them—especially the small, slow ones stuck to rocks. But as the climate changes, these slow ones are on the move too, and they may be the first to show us how our oceans are shifting.

What Happens When the Ocean Warms?

Every species has a natural “geographic range,” an area with the right environmental conditions where it can survive and reproduce. But climate change is causing these ranges to shift. Warmer waters are pushing animals toward cooler regions, and we’re seeing this in birds, mammals, and especially marine life.

This process, sometimes called “tropicalization,” is already happening along the California coast. Animals that used to live only in warm southern waters are now showing up farther north. This affects the entire coastal ecosystem, including the native species that already live there.

But here’s the challenge: modern scientists often have to wait and observe these changes in real time, then decide if a species is “invasive” after it has already caused noticeable harm to native species. What if we could see this coming—before it’s too late?

Looking to the Past for Clues

That’s where my research comes in. Instead of waiting to see which species will move as the ocean warms today, I study what happened in the geological past, specifically during the Last Interglacial Period about 120,000 years ago, when the global temperature was around 2 to 4 °C warmer than pre-industrial level. Back then, much of coastal Southern California—including most of what we now call Los Angeles—was covered by the ocean. Believe it or not, scientists have found marine shell beds buried beneath places like the La Brea Tar Pits and even downtown Los Angeles!

Over thousands of years, sea levels dropped and the land slowly rose through geological uplift. That’s how these ancient seafloors were lifted above water, forming many of the flat coastal platforms we live on today. The shells that once sat on the ocean floor are now preserved in hillsides and cliffs and often exposed during the construction of highways, buildings, and schools—a fossil snapshot of marine life from the past.

These fossil beds tell us more than just what lived there. They help us spot “ancient invaders,” species that moved into new areas during past warming events. My goal is to figure out which species migrated and survived, which ones went extinct, and what enabled some to survive while others perished. Were they more adaptable? Did they outcompete native species? And can these ancient patterns help us predict what might happen in today’s warming oceans?

How I Do This Research

My research combines fieldwork, museum studies, and lab analysis to piece together the story of ancient ocean life.

• I study fossil shells collected from Southern California locations (like Palos Verdes Hills), as well as from collections stored in museums. This includes cleaning, collecting, sorting and identifying.
• I identify ancient invaders by comparing their fossil occurrences against their present-day geographic ranges. 
• I analyze the chemistry of the shells, which are made of calcium carbonate (CaCO₃). By measuring oxygen isotopes in the shell, I can estimate the temperature of the seawater when the animal was alive.
• I study the evolutionary relationships of these species to see if the successful “invaders” share certain traits—like being closely related or having similar lifestyles—that may have helped them move into new environments.

During the summer of 2025, I’m working on a project supported by the Wrigley Institute Graduate Fellowship, combining fossil analysis with modern ecological observations. I’m conducting X-ray Diffractometer (XRD) and stable isotope analysis at the Natural History Museum of Los Angeles County and the University of Florida to reconstruct sea surface temperatures during the Last Interglacial period in Southern California.

Alongside this lab work, I’m observing coastal benthic (seafloor) communities to track the current presence of species I’ve identified as ancient invaders in the fossil record. In particular, places like Catalina Island—where warm- and cold-water masses meet—offer useful case studies, especially as some of these species have recently been reported by community scientists (e.g., on iNaturalist). In order to share my work with the broader and local community, I am also training a local high school teacher on how to identify fossils, prepare them for isotope analysis, and how mollusks can be used to study climate change in the past and present.

By comparing fossil data with present-day distributions, my research helps build a clearer picture of how climate change drives species migration over time. This work helps to forecast future marine invasions and understand the long-term impacts of warming oceans on coastal biodiversity.

Priyanka Soni is supported by the USC Dornsife Wrigley Institute Graduate Fellowship.