In North America, the hadrosaurs won out over the lambeosaurs over the course of the Eocene; the last North American lambeosaurids died at the end of the Eocene, along with Ceratopsidae, the endemic, ceratopsian clade.  The space left by these large grazers was quickly filled, however, by a group of small hadrosaurids, which radiated into the niche of plains grazer, taking advantage of the grasslands spreading over North America.  These hadrosaurids had evolved as forest-dwellers, in the shadows of such gigantic herbivores as Brontoceratops, but with the primary grazers of the continent extinct, the hadrosaurids could enjoy a long golden age in North America.  These dinosaurs remained dominant until the Pliocene, when protoceratopsids invaded from across the Bering.  This second wave of ceratopsians eventually died off as well, but their invasion, coupled with the subsequent migrations of viriosaurs from the South and therizinosaurs from the North and then the Ice Age have reduced hadrosaurid diversity to a mere fraction of its former glory. At the present time, there exist two groups of hadrosaurids in North America: the hmungos (Megahadrinae) and the singers (Stellosaurinae).

    Megahadrines are all very large (none is smaller than 5000kg), and not particularly diverse (five species in North America, consisting the two genera Megahadrus and Galumphia).  Most likely, these creatures evolved as riverside and swamp herbivores, where they evolved their distinctive short necks and long snouts (Aumala, unpublished notes).  This cranial anatomy soon proved to be a useful adaptation not only for dredging up aquatic plants, but for grazing.  Hmungos, with their long, spatulate bills, and their high-placed eyes, can simultaniously crop grass and watch for approaching danger.

    The two galumph species (both belonging to the genus Galumphia) live in the forests of southeastern North America.  Galumphs are slightly smaller than hmungos and are the principal prey of Phobotyrannus. These megadrines are rather primitive in form, looking rather like the Mesozoic Edmontosaurus, and are primarily browsers, rather than grazers.

    Three of the five megahadrine species (the hmungos, genus Megahadrus) live on the prairies of North America, where they eat grass and small trees.  These elephantine herbivores are highly migrator, and, during the summer hmungo herds habitualy pass only just south of the Arctic Circle.

    Apparently, hmungos once migrated even more extensively.  The east-Asian shambla, and its kin, are unmistakably North American in origion, and not related to the Eurasian/African ungulapeds.

    Hmungos graze by lowering their flat-tipped beaks to the ground, rooting up the grass and breaking its roots.  They then use their flexible tongues to transport the high-silicate grass-stems up their beaks to the batteries of grinding teeth in the backs of their mouths.  These diamond-shaped, often-replaced teeth churn the grass into a vegetable paste, which is then passed into the hmungos' large crops, where gravel, muscular contraction, and a barrage of symbiotic bacteria further break down the meal.  Hmungo digestion is rather more efficient than that of their RL counterparts, the pachyderms, and so hmungo population density tends to greater than that of Africa, even disregarding human intervention.

(Text by Daniel Bensen)
  • Greater hmungo
  • Least hmungo
  • Eastern galumph

        Singers, the stellosaurines, represent the last remnant of the the once-great North American diversity of small hadrosaurids.  Once, these dinosaurs occupied all the small herbivore niches in North America, as well (the smallest known stellosaur, Microhadrus, likely massed less than 10 kg), but these forms have been largely pre-empted by the viriosaurs from the South and the dwarf therizinosaurs from the North.

        Today, the singers occupy most of the small-to-medium (from about 50 to 200 kg) herbivore niches in central North America.  These creatures are most common in the great American praries, but can be found in enclaves across the continent.  Even as far north as the 60th parallel, the domain of the therizinosaurs, the summer sun brings vast herds of migratory herds of grasbucks, Stellosaurinae's most nomadic species.

        Fossil and morphological evidence suggests that the stellosaurines split off the main hadrosaurid line very early in the Cenozoic.  These dinosaurs are easily distinguished from their relatives, the megahadrines by the the bizarre construction of their respiratory passages, which resemble nothing so much as those of a bird.  Stellosaurines, of course, do not possess the air sac system of their distant cousins, but their larynx (voice box) has evolved in startling convergence with the avian syrinx.  With the aid of their modified vocal chords, stellosaurines can produce a wide range of bird-like calls that are easily differentiable from those of other hadrosauroids, which use their nasal passages rather than their voice boxes as resonating chambers.

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