The Mystery of Hyracotherium

Perhaps I reminisce too much, but when I was a child, dinosaurs and prehistoric mammals were much simpler. Take, for example, the evolution of the horse: what once seemed a straightforward line from hyracotherium to Equuus caballus has now become a complex cluster of related species from a number of eras, more resembling a jumbled cloud of LiveJournal interests than a simple family tree. While scientists have accepted the many-armed tree of equine evolution for quite a long time, it has taken time to trickle down into the popular consciousness. More recent discoveries have shifted some relatives further from the central trunk of this complex family tree, and revealed others as the ancestors of non-equine species as well as of modern horses.

We can thank computers for that. The science of paleontology has evolved in leaps and bounds over the past twenty-odd years thanks to improvements in computer technology. Paleozoologists can now access medical advancements to closely scrutinize the cellular structure of fossilized bones and take ultrasounds of skulls, which gives them a clearer picture of how prehistoric animals are related and how they might have evolved into their modern counterparts.

We know now that while hyracotherium (or Eohippus, meaning 'dawn horse') is the forerunner of modern horses, its place in the family tree is not so straightforward. This nine-inch-high herbivorous mammal also spawned the species that would eventually evolve into rhinoceroses and tapirs, which makes it a valuable link among Perissodactyla, or odd-toed hoofed mammals. Its classification has changed, as well, and it is now a member of the Paleotheriidae family rather than the horse family. Paleotheriidae includes a number of species, many of which took a divergent path from hyracotherium: the genus paleotherium, for example, is more closely related to tapirs than horses.

This demonstrates paleozoologists' expanding knowledge on prehistoric evolution and the many divergent paths that led to our modern biodiversity. The hyracotherium is most certainly the ancestor of our modern horse, but it also plays an important part in the development of a number of diverse species.

Did Dinosaurs Breathe Like Birds?

When I was growing up, the relationship between dinosaurs and birds was tenuous at best. The concept of swift and warm-blooded dinosaurs was hotly contested, and most illustrations depicted Tyrannosaurus Rex as a slow, sluggish, tail-dragging reptile chasing after equally ponderous herbivores. Scientists now theorize that dinosaurs were much faster than previously thought; that Tyrannosaurus Rex carried its tail horizontally to counterbalance its body as it chased after prey; and that many dinosaurs, especially dromaeosaurs (raptors) might have had primitive feathers. The evidence linking birds to dinosaurs is now so overwhelming that some researchers have taken to classifying birds as living members of the dinosaur family.

A recent excavation in Argentina adds yet more weight to the dinosauria-avis connection. Paleontologists from the USA, Canada, and Argentina recently exhumed the fossilized remains of an Aerosteon riocoloradensis, a newly-discovered carnivorous dinosaur from the Cretaceous period. This particular fossil displays a trait that paleontologists in support of the dinosaurian birds theory have been hoping for: a network of air-filled cavities inside the animal's bones.

Modern-day birds stay aloft in part thanks to a specialized system of air sacks distributed throughout their bodies. These sacks act like a bellows, pumping air in and out of a rigid lung rather than relying on a diaphragm to expand and contract the chest cavity. Air cycles through pneumatic chambers inside of bone, giving the animal more loft and keeping them lightweight. The hollow chambers inside of Aerosteon's bones suggest that it, too, might have had a system of air sacks throughout its body.

In addition, several ribs bear the imprint of what could be air sacks. Because of the air sacks' structure, finding the imprints of them on fossilized bone is a welcome stroke of luck for the international team responsible for Aerosteon's discovery.

If the theories regarding this dinosaur's pneumatic bones and imprinted ribs prove true, it will lend yet more weight to the idea that birds are simply highly evolved dinosaurs. The one drawback to this is that it will only further the exploits of researchers attempting to reverse-engineer raptors from emus.

Classifying birds as modern-day dinosaurs is fine, but does the world really need an emu with teeth?