Six meters long, weighing six tons, the ornithopod dinosaurs walked up-right. When they craned their necks, they stood tall enough to see over the trees and across miles and miles of rolling hillsides. Where they saw that the trail was theirs alone, they bent down to graze – the only times they stood on their shortened forelimbs. Then their hips became their high points.
The smaller juveniles were leaner and more active, which allowed them to keep up with the long strides of the adults. They could all sustain a 30 mile per hour pace if they spied something to run from.
Their extended family group was on migration across South America. Far down the shallow slope, a river bottom ran beside their trail, hidden in tall ferns, cycads, and coniferous browse. These animals avoided that forage – habitats like that could conceal large predators in ambush. The hills on the other side became rocky as they slanted more steeply upward. They were covered with a different growth, a low, woody protea scrub.
One of the differences between the plant growth on the two sides of the trail, unnoticed by the passing pachyderms, was the color. The bottomlands were uniformly deep Jurassic green. But the dry hillsides sported spots of red. Proteas – the first of the flowering plants, were in bloom there. Their pine cone-sized blossoms emerged from the dull chaparral as conical spikes of stacked petals.
Those flowers were noticed by another, much smaller dinosaur, an animal that was one of the first birds. Those avian dinosaurs were feathered, long winged, with beaks that could reach through the whorls of petals and sup up the nectar produced by the proteas. Instead of tail feathers, these early birds had actual tails. They glided across the hillsides, from one red spot to the next, serving as pollinators for the plants. Farther down, the ornithopods had chosen the middle ground, grazing in the moister meadows where the slope flattened out.
The river’s canyon rose up into the hills, and the water spread across the rocks to meet the trail at a shallow, noisy series of rapids. The migrants that had made this trail preferred to cross here – through an opening in the riparian forest. They would not wade or swim through the slow, calmer water down in the flats, out of respect for what could be lurking below the surface.
They picked their way through the rocks, and then followed the trail up the other side of the canyon, leaving South America and coming up into Africa. The landscape looked just the same as the land they left behind – deep conifer forest in the bottomlands, protea scrub on the hillsides.
. * * *
Farther along the trail, at another grazing spot, the ornithopods heard a rustling in the brush. Their heads came up, and they listened – it was just the rodents. These shy creatures lived hidden below the scrub, spending their lives searching for protea seeds. The large grazers went back down to their four-footed stance and ignored the scurrying mammals.
The rodents were more active after dark. They prospected across the hillsides, making their own trails below the undergrowth, busily carrying seeds everywhere – to stock their middens. They carried their seeds across the river-bed during the dry season. They were ultimately responsible for the spread of the scrub all along the trans-continental length of the migration trail.
Farther along, weeks later, the ornithopods’ path descended into another river valley. The hillsides were steeper here, and the scent of a beach hung on the up-canyon breeze. The taller adults could see the sea to the south. A bank of volcanic fog hung on the far ocean horizon. The sea floor was ripping open below those waters, and a crack was propagating northward, lengthening toward the land. Magma rose through the fissure to spill across the bottom. It boiled the sea into an acidic steam that rose to produce the vog.
The canyon the dinosaurs crossed was gradually deepening as it cut inland. This valley grew as an extension of the fissure on the bottom of the sea. As they climbed the trail up the far side, the dinosaurs left Africa and entered Antarctica. There was no ice — the landscape they found grew the same plants and sheltered the same animals – the trailscape was the same on both sides of the rift.
. * * *
Over time, the biggest rivers continued to cut their canyons deeper into the hills. Salt water invaded the bottomlands behind the shore. At the beach, river mouths had grown into fiords. Their opposite shorelines had receded too far across the choppy water to be seen from each other. Successive generations of ornithopods found that their trails bowed farther and farther inland before they reached fords that were not too deep and wide to cross.
The widening rivers finally met in the center of the supercontinent of Gondwanaland to separate South America, Africa, Antarctica, and Australia. The large reptiles continued to follow their trails in circles, around continents that had become islands. The cycad and tree fern forests they marched beside were giving way to the broad-leafed flowering trees of the modern era. Eventually their epic migratory journeys came to their final destination, at the end of the Cretaceous period.
. * * *
The flowering plants, including the proteas, survived the mass extinction at the end of the age of the dinosaurs. Today ornate, prehistoric-looking protea flowers look down from the slopes beside the sea around all the southern continents. The rodents that once sheltered in that scrub evolved to lose their shyness in the absence of the big reptilians. They diversified and refilled the dinosaur niche with migratory herds of large mammalian grazers, complete with the attendant predators.
The long-tailed avian dinosaur pollinators survived along with the proteas into the present. Those animals have become the long-tailed sugarbirds that sup nectar from the protea flowers in modern South Africa. Their scaly feet are unchanged from the talons of their reptilian forebears.
The proteas did not spread to these far-flung continents by moving across the oceans between them. They do not have sea-faring seeds. They colonized the southern continents as a consequence of their earlier spread by land-loving animals across Gondwanaland in advance of the super-continental break-up. As the separate land-masses drifted out of sight over the ocean horizon, members of the Protea family were carried with them. In isolation, each lineage began to diverge.
In modern-day Australia their flowers have grown taller into stacked columns of petals, to become the Banksias. The pollinating birds that have co-evolved with them there include the honey-creepers and the sunbirds. The South American flowers in the Protea family are pollinated by species that evolved after the break-up of Gondwanaland – the New World hummingbirds.
America is now ten thousand kilometers over the eastern oceanic horizon from Australia. Yet the protea flowers on those two distant shores are more similar to each other than they are to those on the other, now more closely situated southern continents. The American and Australian proteas shared a common ancestor that lived beside the trail that once connected the two lands.
That connection is not obvious any more – its central section now lies buried under thousands of feet of ice. The frozen continent is bereft of flowers of any kind, and isolated by the chill Southern Ocean for thousands of kilometers in every direction. Some of the best evidence for the prehistoric existence of that lost connection was the discovery of the close botanical relatedness of the protea species. These floral relatives now live on land masses separated from each other by vast stretches of the Southern Ocean.
. * * *
The Protea family was well established on all the far flung austral continents when mammals were ascending to predominance. Many of its members proceeded to evolve into flowering trees. The Macadamias emerged in Australia as a new branch of the Protea lineage long after the continent became an island. These have resumed the spread of the Protea family, long after their dispersal by continental rafting had run its course.
The elongated flower spikes of the Macadamias are reminiscent of Banksia flowers. They develop seeds rich in oils, which are lighter than water. This seed is encased in an impermeable wooden shell. If a stream-side macadamia tree drops a mature seed into the water, then, unlike the seeds of the proteas and the banksias, this seed will float.
It will float down stream, through the rocks, perhaps reaching the flat water of the lower river. The water may carry it across the shore and out to sea. Macadamia seeds can stay alive and afloat for months. These sea-faring propagules have washed up on the most distant shores. Macadamias have colonized the various continents derived from Gondwanaland, as well as many of the tiny, isolated South Pacific islands that have were never part of the great, lost southern continent.
Gondwanaflora notes. Biogeographic observations provided some of the early evidence in favor of the novel theory of plate tectonics and continental drift in the early 20th century. Similar plants on distant shores might once have been separated only by a river. The Porteaceae is an archetypical example of a far-flung family whose distribution could be explained by division of their original habitat (Barker et al, 2007), though other members of this family apparently did cross oceans (Mast et al, 2008) to populate distant shores. As the continental plates rifted apart, new oceanic basins grew between them, with volcanic sea-floor spreading sites at their centers. Vog (volcanic fog) may rise above areas where this boiling sea-bottom volcanism lies close below the surface.
Barker N. et al (2007) Molecular dating of the Gondwana plant family Proteaceae is only partially congruent with the timing of the break-up of Gondwana. J. Biogeography 34:2012-27
Mast et al (2008) A smaller macadamia from a more vagile tirbe: influence of phylogenetic relationships and divergence times in Macadamia. Am J Bot 95:843-70