The Monkey's Voyage Page 13

  The problem with this argument against the Gondwanan relict idea is that there aren’t a whole lot of vertebrate fossils of any kind on New Zealand from near the time of its separation from Antarctica/Australia: a handful of dinosaurs, a pterosaur, a bird, a turtle, and that’s it.20 No amphibians, lizards, or mammals have been found. Yet it is almost certain that many vertebrates were present, just as they were on every other sizable chunk of Gondwana. In other words, the fossil record of New Zealand vertebrates for that time period is just the tip of what must have been a substantial iceberg. There’s an old cliché in science that applies here—“Absence of evidence is not evidence of absence”—and that is especially true when the nature of the observations makes it highly unlikely that one will see what is there (or what was there, in this case). The absence of moa, kiwi, tuatara, and other vertebrate lineages from the record for early Zealandia doesn’t really mean much; it’s like not finding the needle in the haystack.

  However, the Gondwanan relict idea was not just about vertebrates or animals in general, but about the entire New Zealand biota. For instance, the authors of one early, widely cited paper that used plate tectonics to explain plant biogeography, made the claim that “much of the present lowland flora of New Zealand is similar to that of temperate Gondwanaland 80 million years ago.” Indeed, some of the classic examples of supposed Gondwanan holdovers are plants, including the giant kauri trees and the southern beeches. So how good was the plant fossil record of New Zealand, and what did it have to say?

  The answer to the first question was, “A lot better than the vertebrate record.” That was true even if one just considered so-called macrofossils—leaves, stems, flowers, and seeds—and was more obviously the case if one included the plant microfossil record, the remains of tiny, often microscopic, plant parts, especially fossil pollen.

  Pollen is tough stuff. The outer surface of a pollen grain is made largely out of something called sporopollenin, a substance so resistant to degradation that its chemical composition isn’t completely known, because it has never been fully broken down into its constituent parts. In addition, the sporopollenin covering is made up of two layers, with rods running between them that act as struts, strengthening the entire structure. It’s all apparently “designed” by natural selection to protect what’s inside, namely, the genetic material in the plant’s sperm. Pollen gets out in the environment—on the wind, on the ground, on bees, beetles, and butterflies—and it needs to be constructed in a way that keeps the sperm from getting crushed by other objects or by the pollen walls drying up and shrinking in upon themselves.

  Couple the toughness of pollen with the astronomical quantities of it that plants produce, and the end result is a whole lot of fossilized pollen. Add to this the fact that the distinctive form of the pollen of many plant taxa can be used for identification—one group might have grains with spines and large, round pores, another a scalloped surface with slit-like openings and no spines—and you have an incredibly useful source of information about the past. There are many places in the world where you can take a sediment core and, from the pollen remains alone, get a reasonable idea of changes in the vegetation through time. Of course, there are also unique problems in studying fossil pollen, such as the fact that the stuff can get blown in from far away, and you can’t always be sure about matching up a type of pollen with the plant it came from, especially for ancient samples. Nonetheless, fossil pollen has been critical in the reconstruction of past floras, including those of New Zealand and Zealandia.

  What the plant record—much of it a pollen record—seemed to show was not the dogged persistence of an ancient Gondwanan flora, but a history of taxa in constant flux. Lineages were evolving, of course, as they do all the time everywhere, but new ones were appearing and others disappearing at a rate that, if one believed in the relict story, was somewhat alarming. For instance, a 2001 survey of the fossil records of major flowering plant groups of modern New Zealand (which doesn’t differ much from what was known by about 1980) shows that, going forward in time, three groups (including the southern beeches) first appear in the Late Cretaceous, three in the Paleocene, seventeen in the Eocene, twelve in the Oligocene, nine in the Miocene, three in the Pliocene, and two in the Pleistocene. Meanwhile, many lineages blinked out, never to return (at least until people brought some of them back), including a slew of tropical or subtropical groups such as acacias, Eucalyptus, and some palms. If one takes this record as roughly correct, what it shows is that very few modern flowering plant lineages have a contiguous history on New Zealand dating back to its separation from Antarctica/Australia. Instead, most of the original lineages—the “passengers” on the old continent of Zealandia—seem to have gone extinct, and many others have arrived, presumably by long-distance, oceanic dispersal.

  To get a more personal, tangible sense of the history of the Zealandian flora, I thought back to a place that Tara and I had visited in the Southern Alps, the range that runs most of the length of the South Island of New Zealand. Near Arthur’s Pass, in the heart of the mountains, we stopped to walk a small nature loop by the highway and then hiked up a classic U-shaped valley, carved by Pleistocene glaciers. We were in the subalpine, in an area almost devoid of trees, but with stunted beech forest still above us. Tara is a botanist and has a special affinity for the treeless higher reaches of mountains—her dissertation was on seedlings in the alpine tundra of the Rockies—so she was stopping everywhere on the nature trail, taking close-up photos and looking through A Field Guide to the Alpine Plants of New Zealand. She got slightly more mobile on the hike up the valley, but still only made it about half a mile from the trailhead before the allure of the plants stopped her progress for good. I continued up the valley, taking in the scenery, which was spectacular, but earthier than one might expect in New Zealand—dingy gray scree slopes, crumbling mountain heights dotted with snow, reddish-brown shrubs set against straw-­colored tussock grass, a cold stream churning over dark rock. At some point I tried not to intellectualize what I was seeing, tried to free my mind for a while from plant taxonomy and deep history, but with limited success; in New Zealand, biogeography was always entering my thoughts. I gave up on my Zen interlude. Instead, as I walked back down the trail, I tried to burn the images of certain plants into my brain for later identification.

  4.4 The world’s largest buttercup, Ranunculus lyallii (left), and one of the many species of Celmisia daisies, C. semicordata, from Arthur’s Pass on the South Island of New Zealand. The fossil records of these and all other plant lineages that the author could identify at this site suggest that their ancestors arrived by oceanic dispersal. Photos by the author.

  Much later, from our photos and from memory, I made a partial list of what we had seen that day (see Figure 4.4). The list was short but taxonomically varied: a shrub of the plantain family in the genus Hebe, with scale-like leaves that make it look like a juniper; Celmisia semicordata, a daisy with large, yellow-centered, white flowers (for some reason an inordinate number of New Zealand plants have white flowers) and long, pointed leaves like a lily; a speargrass (genus Aciphylla), another non-lily with lily-like leaves, this one in the carrot family; Ranunculus lyallii, also white-flowered, and botanically famous for being the world’s largest buttercup; the ubiquitous tussock grass (Chionochloa pallens); yet another white-petaled species, a small orchid (possibly Aporostylis bifolia), with its single flower on a tall, hairy stem; and, in a miniature bog, two miniature plants—a carnivorous sundew (Drosera) and a creeping plant in the genus Coprosma, in the coffee family, with long finger-like yellow pistils sticking up out of the female flower cup.

  I worked through this list, finding out from the literature on fossil plants when each of these lineages first appeared in New Zealand. This was neither a large nor a random sample of the flora, but it was still striking what the fossil record of these plants suggested about their origins. None of them had New Zealand histories that stretched back before or
even close to the time of Zealandia’s separation from Antarctica/Australia, 75 million years ago or so: Coprosma was the oldest, appearing in New Zealand in the Eocene, perhaps 50 million years ago, while all the other lineages only turned up within the past 35 million years. Taken at face value, the fossil record indicates that, of this haphazard (but common) set of plants, not a single one is a Gondwanan holdover. They all seem to have arrived by crossing the sea.

  A paleontologist named Charles Fleming had been pushing this primarily dispersalist view of the New Zealand flora (and biota as a whole) since before the validation of continental drift, basing his conclusions in large part on fossils. It was Fleming who had first suggested that the area of New Zealand had been substantially reduced in the Late Oligocene and Early Miocene, leading to the extinction of some plant groups. It was also Fleming whom Gary Nelson was singling out when he wrote of the “bits and pieces of the center of origin/dispersal paradigm” being stranded on the shores of New Zealand.

  Even in the 1980s, at the height of the vicariance movement, there were a few New Zealand scientists, in addition to Fleming, who didn’t buy the Gondwanan relict story for New Zealand. Dallas Mildenhall, a colleague of Fleming’s at the New Zealand Geological Survey, remembers that “there was a disconnect between paleontologists and biologists looking at the modern biota,” with the paleontologists, a fairly small group, seeing a history of changing lineages that required overwater dispersal, while almost everyone else believed the Gondwanan relict story. Mildenhall is a pollen expert and thinks the fossil pollen record, even with its flaws, clearly indicates the late arrival of many New Zealand plant lineages. In 1980, he published a paper making that point and even suggesting that the history of Nothofagus, the southern beeches, might require some long-distance dispersal. By that time, Nothofagus had become one of the prime examples of Gondwanan vicariance, so Mildenhall was potentially striking at the heart of the vicariance movement.

  My sense, however, is that Mildenhall’s paper didn’t have much of an immediate impact on the field of biogeography. Through the 1980s, his paper was cited fairly often, but mostly by other paleobotanists, and even then it was rarely referred to for its general message about the dispersal origins of the New Zealand flora. For New Zealand neontologists—biologists who study living organisms rather than fossils—it was still all about Gondwana.

  “Do Araucarias have double trunks?” That was the kind of question Dallas Mildenhall would get in letters from a New Zealand high-school student named Mike Pole. This was in the mid- to late 1970s. Mildenhall would reply in detail, not wanting to dampen the kid’s botanical and paleontological enthusiasm. There was probably no danger of that though.

  Mike Pole had collected his first plant fossil when he was nine and, as a teenager, he was a paleo nut, swapping specimens with his mates and precociously realizing that there were big knowledge gaps to be filled in the study of New Zealand fossils. While still in high school, he read Charles Fleming’s 1962 paper “New Zealand Biogeography—A Paleontologist’s Approach,” along with papers by other scholars with such titles as “Reconstructing Triassic Vegetation of Eastern Australasia”—not exactly the usual teenage reading material. Even then, Pole knew he wanted to have a career working with plant fossils.

  He also knew that it was the New Zealand flora, in particular, that he wanted to study, so he ended up getting his undergraduate degree at the University of Otago in Dunedin, about 70 miles from his hometown of Alexandra. In Dunedin, he and his adviser, a paleontologist named Doug Campbell, would sometimes walk from campus over to Campbell’s place for lunch and chat about biogeography. A botanist who had recently gotten his PhD at Otago would join them when he was home visiting from New Guinea, where he was teaching at a university. New Zealand is a small academic world, and that botanist was Michael Heads, who would later argue that long-distance dispersal has no importance, that everything is “garden-variety” dispersal followed by vicariance. Pole credits Campbell and Heads for helping him to start thinking critically about biogeography, but Heads’s Croizatian views clearly did not rub off on him.

  Pole continued into the PhD program in Dunedin, and, while preparing an identification key to the modern forest trees of New Zealand, it dawned on him that the fossils that were the subject of his dissertation were neither ancestors nor even close relatives of the living groups. He had been at least slightly brainwashed by the Gondwanan relict idea, but now he was discovering for himself what Fleming and Mildenhall had seen before, namely, that the fossil record suggests a massive turnover of New Zealand plant lineages in the past 60 million years or so.

  In his dissertation, Pole compared the past floras of Australia and New Zealand (using leaf fossils rather than pollen), emphasizing how dynamic they were and how, in both places, the plant lineages that were present reflected the climate at the time. Perhaps most striking was the fact that in the Early to Middle Miocene (roughly 10 to 20 million years ago), the New Zealand flora in some places came to appear very “Australian,” with flowering plants, such as acacias, Eucalyptus, and the conifer-like Allocasuarina, that are adapted to forests that regularly burn. Pole suggested that the similarity between the Australian and New Zealand floras at this and other times was a result of relatively easy movement of plants across the Tasman Sea; the sea was obviously a barrier, but it was a barrier that many plants could overcome. Plants such as Eucalyptus had presumably crossed the Tasman at all times, but it was only when the climate of New Zealand became sufficiently “Australian” that they were able to survive there. And when New Zealand’s climate shifted again, making the forests less prone to burn, the fire-adapted Australian plants disappeared. In short, the character of New Zealand’s flora was dictated not by the set of original, Gondwanan inhabitants, but by long-distance dispersal and climate.

  In a subsequent paper, published in 1994, Pole fleshed out the case for the importance of the long-distance colonization of New Zealand. He spent a good deal of time developing an argument reminiscent of Darwin, noting that several oceanic islands have native plant lineages that on New Zealand are considered to be Gondwanan relicts rather than overseas colonists. A conspicuous case is the Norfolk Island pine, Araucaria heterophylla, a tree with an attractively symmetrical, triangular shape and dark green foliage, well known as a house plant. A. heterophylla is in the conifer family Araucariaceae, a name that for anyone with much knowledge of worldwide biogeography immediately conjures up thoughts of Gondwana. Araucariaceae species have no special means for dispersing their seeds and so are thought to be poor long-distance colonists; thus, their presence in areas as far-flung as southern South America, Australia, New Zealand, and New Caledonia has typically been explained by Gondwanan fragmentation. Pole, however, pointed out that Norfolk Island is a volcano that emerged from the sea within the past 3 million years. He also noted that the island is 435 miles from New Caledonia, the closest likely source for the tree’s ancestors. Theoretically, the Norfolk Island pine shouldn’t be on Norfolk Island, but there it is, and its ancestors must have arrived by oceanic dispersal.

  Pole listed many other examples of this kind—plants that had been categorized as “non-oceanic” because they supposedly couldn’t get to oceanic islands, and yet were found on Norfolk, Lord Howe, Campbell, and other islands in the New Zealand region. These islands are all situated on ocean rises or plateaus that had once been part of Zealandia, but there is no geological evidence to suggest they have long histories as land; they may be perched on continental crust, but they are all apparently recent islands, most of them having emerged from the sea as volcanoes within the past few million years. Pole’s point was that, if “non-oceanic” plants had reached these oceanic islands, then they and other such lineages could have made it over water to New Zealand as well. For instance, who was to say that the ancestors of New Zealand’s single living species of Araucariaceae, the giant kauri tree, which was almost universally considered a Gondwanan holdover, couldn’t have arr
ived there as seeds or trees on a natural raft? In essence, Pole was repeating Darwin’s argument about the effectiveness of long-distance oceanic dispersal, but using it to shoot down the Gondwanan relict idea, rather than to argue against the idea of separate creations on different landmasses, as Darwin had done.

  Like Mildenhall, Pole also questioned the ancient persistence story for that archetypal Gondwanan lineage, the southern beeches. In particular, he observed that there are three groups of Nothofagus, distinguishable by their pollen, but that only one of them, the fusca group, has a fossil record in New Zealand going back to the time of the isolation of Zealandia in the Late Cretaceous. The other two, the menziesii and brassii groups, are known from fossil pollen at that time, but from South America and Australia, not New Zealand.21 The implication was that these latter two, although living on Gondwana, had missed the Zealandian boat as it left the harbor and had colonized the area only after its separation from Antarctica/Australia. Pole argued that, although the menziesii and brassii groups, along with many other plant taxa, might be Gondwanan in the sense of having come from some landmass that had once been part of the southern supercontinent, they were not relicts with an uninterrupted presence in New Zealand dating from the time of continental breakup. They were not Gondwanan in the sense implied by the vicariance worldview. In pushing his dispersalist view, Pole went farther than even Fleming or Mildenhall had: he suggested that perhaps the entire New Zealand flora was descended from overwater colonists.