December 30, 2008
When visitors stroll among the remains of ancient beasts in the dinosaur halls of museums, they often focus on how bizarre they were. With the exception of the more bird-like forms, there is nothing like them alive today: immense sauropods with tails and necks that stretched to the horizon, armor-plated ankylosaurs festooned with spikes, stout ceratopsians ornamented with horns and frills, and gargantuan predatory theropods with banana-sized teeth.
What often goes unnoticed is that we share a deep history with these animals. They are, as spectacular as it may sound, our distant relatives. As reviewed in a new paper in the Annual Review of Ecology, Evolution, and Systematics, about 398 million years ago there was a particular group of fish, the lobed-fined or sarcopterygian fish, the organisms that gave rise to our common ancestor with the dinosaurs. The fish lived in freshwater and had a series of bones in their limbs. These and other factors made them different from fish whose fins were supported by a series of spines or fine rays. Within the sarcopterygians was the ancestor of creatures that would appear at about 385 million years ago, the “fishapods” like Panderichthys and Tiktaalik.
Rather than fins, these fishapods had rudimentary limbs which they used to raise their flat bodies up from the muddy bottom. They were equipped with both gills and lungs, and they were among the first creatures to have necks (in their fish ancestors, the shoulder girdle was attached to the skull, prohibiting flexibility). Despite these adaptations, these creatures were not yet walking on land, but their descendants would. It is difficult to pin down precise ancestor-descendant relationships, but it was this sort of creature that gave rise to the first true “limbed” creatures, the earliest tetrapods, which also had fingers and toes. These were animals like Acanthostega that had an amphibious life at the water’s edge.
Even though limbs had evolved in the water, they allowed early tetrapods to haul out onto land, a place inhabited by plants and invertebrates but no other vertebrates. This did not happen until about 330 million years ago, but when it did, it led to an explosion of diversity. Among the diverse forms was the common ancestor of living reptiles and amphibians as well as mammals and dinosaurs.
Lineages diverged and evolved through time, but our common ancestry can still be seen in our skeletons. We and dinosaurs share body plans based upon four limbs. Although our skeletons have been modified in different ways, we have many of the same types of bones (the bones of our limbs and hands are a good example), and this all goes back to our swamp-dwelling common ancestor almost 400 million years ago.
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