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SPEC’S EOCENE EPOCH (c.55,000,000 - 37,000,000 b.p.)
The Eocene was a time of plenty. The
continents were altering their outlines and positions as the seafloors
spreads rapidly. The seas had advanced since the Paleocene, severing
land-bridges and facilitating isolation and regional endemism. Global
temperatures were warm, nurturing lush tropical floras as far north as
Britain. In the seas, warm climate and the return of shallow inland seas
put evolution into overdrive. Most modern fish families were now
present, while the mosasaurs underwent a major diversification event,
the archetypal serpentine body radiating into everything from small eel-like
river-dwellers to delphinoid pelagic forms.
On land, one could be forgiven for thinking that they were back in the Cretaceous. In North America, the flat-headed
hadrosaurine duckbills had branched into a number unique of endemic lineages. But these animals lived in the
shadow of the immense Brontoceratops, the most massive ceratopsian to have ever existed. Stalking
the herds were the tyrannosaurs which have obviously hit upon a winning formula, being practically unchanged
since the Cretaceous.
Across the sea in Eurasia, differences between Old and New World fauna were becoming more pronounced
after a long period of uniformity. Hadrosaurines and giant ceratopsids are also present, but were much less
common. Here the dominant herbivores were the hollow-crested lambeosaurs followed by the smaller
cousins of the ceratopsids, the hog-like protoceratopsids. The big predators were also tyrannosaurs, but
they tended to be smaller and a lot more bizarre-looking than their American cousins. Evidence of
other coelurosaurs is sporadic, but it is clear that this epoch was a time of diversification for both the
oviraptorosaurs and the deinonychosaurs, now the only small predators left in the Northern
Hemisphere. Mammals were present as small burrowing and climbing insectivores. In Europe
and North Africa, there was surprise reappearance of the piscivorous spinosaurs after an absence in
the fossil record for 40 million years. Where they had been for all that time is one
of Specworld's many unsolved mysteries.
Africa, now sliced in half by inland seas,
was once again isolated, and its dinosaurs were heading off on their own
evolutionary direction. As with North America and Asia, the land was
dominated by herds of giant duckbills (mostly the descendents of Eurasian
immigrants), but members of a more primitive local clade were also on
the march. Sauropods flourished, as did a host of smaller, poorly
understood herbivores. The main carnivores seemed to be the scaly abelisaurids.
Further south, the Eocene was the final hour for the united Greater Gondwanan dinosaur-fauna,
just before their terminal breakup. The last land-connection to South American had only just been
severed to Antarctica, while Australia was hanging on by a thread. The sauropods reigned supreme;
herds of giant long-necked earthshakers were everywhere. Aside from a few duckbills,
the ornithischians were less conspicuous, but they were diverse in the smaller herbivorous guilds.
Two endemic ornithopod radiations were now recognizable, the medium-sized browsing neodryosaurs and the small, omnivorous antarctornithopods.
Once again, the abelisaurids were on the prowl, but here they played second fiddle to the giant notoraptors,
members of an enigmatic local group of deinonychosaurs.
EXTINCTIONS AT THE EOCENE-OLIGOCENE TRANSITION (c.40,000,000 - 31,000,000 b.p.)
During the close of the Eocene Epoch and the opening phase of the Oligocene, dinosaur communities
worldwide were once again plunged into crisis. Across the globe, countless species were facing extinction,
both on the land and in the oceans. The direction of evolution on the Specworld was to be forever altered.
The extinctions of Eocene-Oligocene transition did not take place as a single discrete event, but rather as
a series of extinction pulses. The first of these die-offs took place close to 40 million years ago, with three or four additional pulses taking place throughout the Eocene-Oligocene transition, the last one occurring 31 million years
ago. A similar phenomenon occured on RL, and the progression of events in both timelines show strong similarities with regards to the marine fossil record. In the oceans during the extinction pulses, those groups most seriously affected were generally tropical and inshore forms. Tropical calcareous nanoplankton were decimated whilst high-latitude floras of
these organisms appeared at the equator.
The evidence points to both a cooling of the world's oceans combined with a severe drop in global
sea-levels. Such conditions at sea would in turn have a direct consequence for life on land. Although there is some
evidence of extraterrestrial impacts during this time, it seems likely that this climatic deterioration was directly
tied in with final breakup of ancient Gondwana and the subsequent shifting of the ocean currents. Towards the
end of the Eocene, Australia rifted away from its last connections with Antarctica. Cold water flowed into
the widening gap, deflecting the warm currents that had previously kept the south polar climates mild.
For the first time in hundreds of millions of years, a permanent ice cap began to form at the South Pole.
With the demise of longitudianl warm-water currents, a great chilling engine
was created in the southern oceans. Warm water was trapped in the south
where it was cooled and forced into the ocean depths before spreading
back towards the equator. Each time the cool temperatures were sent
towards the lower latitudes via wind, oceanic currents or upwellings,
it triggered off a wave of extinctions. In the longer term, this shift marked
the start of a profound transformation of the Earth's climate, which would
ultimately lead to the Ice Ages of the Quaternary.
Overall, the climate on land seems to have gotten colder and drier during the extinction pulses. As seas
retreated, once coastal regions were left far inland and faced increasingly dry conditions.
Areas once blanketed by humid forest gave way to semiarid scrub and, for the first time, grasslands. For
the ceratopsids and the other horned giants, this was the final curtain call. The great evolutionary dynasty
of the Ceratopsidae had come to an end. The duckbills also faced a time of both extreme hardship.
The lambeosaurine duckbills, mostly adapted for moist woodlands and swamps, suffered tremendous losses.
The few Eurasian hadrosaurine duckbills fared only a little better.
In North America, on the other hand, things were looking much brighter for the hadrosaurs. In fact, the
Early Oligocene was a time of incredible diversification for the American duckbills.
By the end of that period, they had produced everything from enormous
graviportal grazers to tiny gazelle-like bipeds.
Africa was also experiencing a duckbill bloom. Although most of the larger, Eurasian-derived species died
out in Africa, the more primitive endemic hadrosaur clade was holding the fort. One lineage began
to develop an unusual hoof-like hand and was destined for greatness later in the Cenozoic. Across the
Northern Hemisphere and Africa, theropod extinctions appeared to have been fairly minor and localized.
Most seem to have adapted to hunting new prey species with little difficulty.
The only significant loss were the spinosaurs, vanishing after their
brief reappearance, this time for good. The herbivorous therizinosaurs,
some of them already adapted to semiarid conditions, became more common.
Being closer to the spreading coldness that was the root of this calamity, life on the fragments of
southern Gondwana were in a complete shambles. The great browsing sauropods were in dire trouble,
being reduced to scattered remnant populations. By the early Oligocene, they had vanished from
Australia and Antarctica while just clinging on to life in Africa. In South America, a lone genus,
the enormous Acrotitan, survived the crises. The South American duckbills also disappeared,
confusingly as they were closely related to forms that did well in North America. Perhaps their
life-cycle was somehow tied in with the doomed sauropods? We simply don't know.
The combination of cooling climate and prey loss lead to the almost total annihilation of the large
Gondwanan predators. In Antarctica and Australia, every theropod species bigger than 100kg
went extinct before the Late Oligocene. Like the sauropods, the abelisaurids found temporary refuge
in South America, surviving there long enough to be obliterated in the Pliocene by North American
invaders.
The only dinosaurs that seemed to be holding their own were the small bipeds. Polar populations
of neodryosaurs and antarctornithopods retreated northwards from the spreading ice and found
large swathes of Australia and South America suitably habitable. With so many of their
predators and competitors missing, they quickly make themselves at home.
A brief look at the oceans showed many changes there as well. Shadowing the loss of the
archaeocetacean whales of RL, the ancestral serpentine mosasaurs were in trouble, vanishing
in the oceans and surviving only as a few small freshwater forms. On the other hand, their
recently evolved torpedo-shaped kin, the saurocetes, were doing fine. Surprisingly, the ancient
ammonites had been given a new lease on life. Reduced to a handful of cool-water species
since the Maastrichtian, the disruption of equatorial marine ecosystems
gave them a crucial window of opportunity to spread and diversify.
Unlike the preceding extinction event at the dawn of the Eocene, this crisis saw the loss of relatively
few family and order sized groups. While a few groups died out, others quickly diversified to fill the
ecological gaps. The Eocene-Oligocene boundary was less a time of catastrophe and more a period
of unusually high species turnover, with new forms quickly stepping in to replace the old.
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