Roughly 65 million years ago, the Chixulab boloid, a mountain of rock at least 60 miles across, struck the Earth. The asteroid's origins are unknown, but fate had tied the destiny of our home, Earth, to that of the rock; when Chixulab struck, our Earth was changed, forever.
The Chixulab impact left several legacies: an enormous crater just off the coast of the Yucatan Peninsula, a layer of iridium-rich clay that forms the boundary between Mesozoic and Cenozoic in the Earth's rock strata, and the complete extinction of all of Earth's large animal species.
In short, Chixulab impact marked the end of the Mesozoic with the great Cretaceous-Tertiary (or KT) extinction. The impact was by no means the only calamity to befall the creatures of Earth during the turbulent end of the Mesozoic, but it was the final straw, the coup de grace, ushering in a new era in the evolution of vertebrate life on Earth. Many groups of organisms, the ammonoid cephalopods, the cycadeoidophyte plants, and the enantiornithian birds were completely obliterated by this, the greatest extinction in over 100 million years. Others, like the great dinosaurs, pulled through with only a tiny fraction of their former diversity left intact, never to be regained. The mammals, those furry, lactating therapsids, the creatures that had lost the original battle for ascendancy after the last great extinction to the dinosaurs, suddenly found all the doors open and diversification beckoning.
In the wake of the KT extinction, mammals exploded into dazzling array of giant forms. Within 10 million years, almost all of the modern groups of mammals (and then some) had already appeared. The mammals conquered the land, the water, and even the air (although the last remaining dinosaurs, the birds, are unquestionably dominant in at least the last of these niches). The furry creatures spread to every continent and, only 65 million years after the Chixulab struck, have produced organisms of astonishing variety, from blue whales to shrews, from hyenas to humans. The (non-avian) dinosaurs, once the largest land animals on the planet, have been reduced to mere stone and imagination.
But what if none of this history was true?
What if the rock had missed?
One can speculate.
"Once one dismisses
The rest of all possible worlds,
One finds that this is
The best of all possible worlds."---Leonard Bernstein, CandideIntroduction
The History of the World
The Nature of Spec
- North AmericaThe plains of North America are covered with a thick mat of bamboo-like grass (unrelated to the true bamboos of Asia). This grass can grow to two meters in height and is heavily fortified with silicates, making the vegetation un-digestable to all but the most resilient herbivores. Northern forests are dominated by spiny conifers, while southern forests are composed mostly of deciduous hardwoods (such as oak, sycamore, and walnut), with an underbrush composed mostly of prickly p-Rubus (raspberry) brambles. Few softwood trees can resist the grazing of dinosaurs, and so tend to be small and bushy. In the more arid portions of the continent, spiny cacti are the dominant flora.
North America is quite interesting, biologically, as the continent exists (or existed, until very recently) as a crossroads between two very different ecological domains, South America and Asia. The land bridges linking North America to Asia and South America (the Bering Strait and the Panama ithsmus, respectively), small as they are, have allowed animals (including dinosaurs) to migrate and mix to an impressive degree.
Since North America has been intermittently connected with Asia (and thereby, the Europe/Asia/Africa complex), few novel dinosaur clades are endemic to the region. For the most part, the dinosaurs of North America have changed little since the Mesozoic.
Some groups of North American dinosaurs are:
DEINONYCHOSAURIA=Small, bipedal predators with hyperextendable inner toes.
CORACIIFORMES=Kingfishers, nearcrows, pickpeckers, and a number of other perching birds
AGILIFUGIIFORMES=Swoops, swifts, and minnies
TYRANNOSAUROIDEA=A collection of small and large predators.
HADROSAUROIDAE=Medium to elephantine quadripedal herbivores.
ANKYLOSAURIA = Semi-aquatic, armored herbivores
MULTITUBERCULATA = Arboreal near-mammals
METATHERIA = Native Marsupials in the form of family Didelphidae
- South America
THERIZINOSAURIA=A few mountain-dwelling species that migrated from North America (see above)
DEINONYCHOSAURIA=A diverse group of small and large predators
DINOCERATOPSIDAE=South American survivors of a largely extinct Pliocene radiation of American neoceratopsians
ANKYLOSAURIA = Semi-aquatic, armored herbivores
XENARTHRA = A diverse and strange group of eutherians
CHELONIA = Armored reptiles of an ancient lineage
METATHERIA = Marsupials
Also common throughout the neotropics is the large and agressive (for a termite) genus Mastotermes, which, in RL, is now found only in Northern Australia and adjacent regions - a fraction of their former range.(Text by Drhoz)
PRISCATAURIDAE=A few African migrants into southern Europe (see below)
THERIZINOSAURIA=Bipedal, plumaged herbivores of mountains and tundras
PITHECOSAURIA=Tree-dwelling, herbivorous oviraptorosaurs
DEINONYCHOSAURIA=Small bipedal predators with hyperextendable inner toes
ENANTIORNITHES=A single family of opposite birds
EUROLOPHIDAE=The remnants of a once-diverse group of Eurasian ornithischians
SUCHOTHERIIDAE=Large, semi-aquatic metatherians found in inland aquatic habitats throughout
PRIMATES = A diverse group of omnivorous and carnivorous mammals
XENOTHERIDIA=Small mammals somewhat related to rodents
UNGULATA=Herbivorous Eurasian mammals (A grouping of convieniance, rather than phylogeny)
CEPHALOPODA=Shelled, fresh-water mollusks, with tentacles and central nervous systems
Africa occupies an interesting biological position, consisting of a piece of Gondwana glued onto Eurasia. Thus, Africa (especially south of the Sahara desert) harbors some endemic clades similar to those found in South America and Australia, while northern Africa is almost exclusively populated by Asian migrants.