A young fossil hunter's discovery

As a child, there weren’t many items I could fit into a plastic lunchbox to take if I ever decided to leave home, but this particular fossil was definitely one of them. It seemed to beckon to me—a four-armed cross peeking through the debris beneath a limestone ledge. It was 1972, and my family was vacationing near Oamaru in southern New Zealand. Bending down for a closer look, I uncovered what resembled a small flying saucer that had somehow settled into the cave floor.

What I had found was a fossilized heart-urchin. Although it was not well-preserved—being merely an internal mold with the outer calcite armor and spines long gone—it was one of my earliest discoveries (I was ten, after all) and remains dear to me. That family trip was a pivotal moment that steered me toward a career in paleontology. Upon returning home, I brought back a treasure trove of fossilized shells, shark teeth, and bones. I meticulously arranged them on makeshift shelves and spent hours poring over fossil books to label them with their Latin names. By then, I had transformed into a dedicated fossil collector, eager to gather specimens from various geological eras—much like a stamp collector striving to complete a collection.

Climbing the Intellectual Ladder

While amateur fossil collecting yields valuable data, it represents the initial step on the intellectual ladder. The real challenge lies in deciphering what these fossils reveal about their historical environments, including past climates. As early as 1944, Charles Fleming utilized fossils to suggest climate change in New Zealand. By the mid-1950s, a variety of scientists were exploring similar questions (e.g., Hornibrook, 1953; Couper and McQueen, 1954; Squires, 1956; Fleming, 1962). By the early 1970s, a consensus emerged—New Zealand's waters had once been significantly warmer, possibly even tropical.

Unbeknownst to me, in 1968, four years prior to that family holiday, a group of scientists in New Zealand convened to share their expertise. Their diverse skills encompassed fossil corals, mollusks, brachiopods, belemnites (squid-like organisms), plankton, plants, and isotopic analysis. By then, the significance of past climate change was becoming widely recognized, leading to a multidisciplinary meeting aimed at addressing key questions: Were the climate change narratives told by various organisms in New Zealand consistent? Did they align with independent estimates based on physical isotopes? Was the consensus regarding climate change in New Zealand comparable to that observed in other regions?

They employed various methods to derive temperature indications from their fossil findings. A logical starting point, given their prevalence and extensive study, was fossilized shells. But how exactly could they interpret this? Alan Beu and Phillip Maxwell (building on their previous work and that of Charles Fleming) compiled data on fossil shell genera that are currently exclusive to tropical regions, yet were once found in New Zealand. Depending on whether they analyzed the total count or just the proportion of warm-water shells, their results varied slightly. However, they concluded that during the Tertiary period, New Zealand sea temperatures peaked three times before declining into the ice ages.

Graham Jenkins and Norton Hornibrook also examined fossil plankton. Jenkins identified temperature peaks early in the Tertiary, followed by a decline. Hornibrook focused on specific plankton types, with one species suggesting that sea surface temperatures in New Zealand during the mid-Tertiary were consistently above 20 °C.

Ian Devereux analyzed oxygen isotope ratios in fossil shells—a distinctive method for estimating sea temperature—producing a curve of highs and lows similar to Beu and Maxwell's findings. Other researchers yielded comparable results. However, one scientist present, German paleontologist Martin Schwarzbach, noted discrepancies between findings in Europe and North America—where temperatures seemed to drop steadily throughout the Cenozoic/Tertiary—contrasted with the mid-Cenozoic temperature peaks observed in Australia and New Zealand. While this issue remained unresolved, it sparked discussions around the movement of landmasses.

The papers produced from that multidisciplinary gathering were published in a special edition of the journal Tuatara, marking a significant advancement from basic fossil collecting to a deeper scientific understanding. New Zealand scientists demonstrated that substantial shifts in latitudinal temperature bands had occurred in their region, allowing them to theorize about underlying causes. Was it due to changes in ocean currents? The movement of continents? These phenomena redistribute energy across the Earth in a zero-sum game: as some areas warm, others cool.

In hindsight, we can identify gaps in that 1968 discussion—specifically, a lack of awareness that the entire planet might have experienced warming, let alone the factors driving this change. Today, the term "global warming" is commonplace, but in 1968, most paleontologists were still unaware that global temperatures could fluctuate. At that time, the primary focus was on tracking the latest developments regarding continental drift. Yet, beneath this research lay a fundamental question that distinguishes mere fossil collecting from genuine paleontology: "What insights can my fossils provide about the past?"

What insights does my fossil urchin provide?

So, what narrative does my fossil urchin convey? It belongs to the genus Pericosmus, most likely the species Pericosmus crawfordi (Henderson, 1975). This urchin hails from the Otekaike Limestone, which dates back to the Oligocene-Miocene boundary (approximately 23–26 million years ago). During this time, much of what is now New Zealand was submerged, and this species was a common resident of the ocean floor. However, as the waters cooled, these creatures seemingly vanished from New Zealand, retreating toward warmer equatorial regions. Across the Tasman Sea, Pericosmus is also commonly found in Oligocene-Miocene rocks along Australia’s southern coast, yet it has since disappeared from that region, now existing solely from southern Queensland northward (McNamara, 1984).

Today, the sea surface temperature in Oamaru is about 11 °C (en.climate-data.org), but Pericosmus is among numerous fossils indicating that the waters were significantly warmer in the past. In his 1992 review of climate data, paleontologist Norton Hornibrook estimated that New Zealand's water temperatures during that era—at the latitude of Blenheim—were around 19 °C. Given that Oamaru is three degrees further south, I surmise it may have been a degree cooler there (noting that due to continental drift, Oamaru likely existed a couple of degrees further south back then).

When I discovered that fossil urchin, Pericosmus was believed to be extinct in New Zealand. Yet, just a few years later, in 1977, fragments of a "recently deceased" individual were found at a depth of 318 m on Rangatira Knoll in the Bay of Plenty (McKnight, 1977). This finding can be interpreted as evidence of a sparse population at its southernmost limit in colder waters, where average sea surface temperatures hover around 17 °C. This strongly supports the notion that the waters my fossil inhabited were considerably warmer than those of today.

For nearly five decades, I had been oblivious to the story my urchin could narrate. However, a little research revealed intriguing insights: every fossil has a tale to tell. That collaborative meeting in 1968 was a turning point in New Zealand science. By piecing together climate data, they contributed to a global understanding—laying foundational work for what we now recognize as climate models.