Red Hill includes a wall of rock along the shoulder of a highway in Pennsylvania’s Catskill Mountains. The discovery site, Red Hill, holds a series of ancient stream deposits that preserve the birth, life and death of tropical to subtropical ecosystems from the Late Devonian period of Earth’s history. The scientists discovered the fossil in 1993, but they only recently exposed the entire bone from its red sandstone encasement and realized the significance of the specimen. The evolution of limbs from fins, one of the key transitions in the history of life, took place gradually and largely separate from the water-to-land transition.Īfter comparing this fossil arm bone to those from other tetrapods and closely related fish, the scientists conclude that many of the limb and muscle developments needed for walking on land developed before fish and limbed animals split.įor example, some modern fish can hold the same spot or lift up their trunks in moving water, thanks to bone and muscle changes that occurred before fish and limbed animals went their separate ways. Streams full of toothy carnivorous fish - some with joints in their skulls to increase the size of their bites - may have also inspired tetrapods to take their head-propping, muck-stomping skills to higher ground. This oxygen scenario, combined with the newfound ability to stick their heads out of the water, may have encouraged the evolution of lungs in fish. Plants contributed to dropping oxygen levels in water and rising oxygen concentrations in the air. In addition to altering land and water habitats in ways that probably encouraged limb development, the rise of more advanced plants also changed the oxygen balance. This shoulder and arm orientation would have made the front limbs more useful as weight-bearing limbs than paddles. With shoulders facing sideways, the tetrapod’s arms projected out from its trunk at a right angle, similar to the limb arrangement of a crocodile. The fossil also shows that tetrapod shoulders changed the direction they face in order to adapt to life in a plant-tangled, aquatic environment. Plant-clogged waterways made weight-bearing fins, and eventually limbs, useful for getting around, Shubin suggests. Plants grew thick in streams and rivers, their roots stabilized ecosystems, and their decomposing biomass generated organic muck. “This humerus is quite unique,” said Jennifer Clack, the author of an accompanying article that provides some background information on this fossil.Ĭlack said the fossil’s “bizarre shape” does not fit the patterns of other early tetrapods.She believes that there were a variety of limb shapes, sizes and strengths among early tetrapods experimenting with adaptations for life on land.īefore vertebrates took their first steps - in or out of the water - plants with stems, roots and leaves colonized shallow rivers and changed the nature of many aquatic environments and nearby shores. Without Devonian photo albums full of fish trying to put weight on their pectoral fins, paleontologists must rely on the fossil record to understand how new fossil finds fit into the story of life. The fossil evidence suggests that the first limbed animals tried all kinds of walking strategies. It helps clarify when, where and how animals first learned to walk. The new fossil, like a photo taken with high-speed film, catches limb evolution in action. Shubin and his colleagues compared the arm fossil to the bones of both ancient and modern fish, and arrived at a new understanding of the beginnings of limb evolution. This posture reflects an intermediate stage in the evolution of amphibian limbs from fish fins. The humerus of a Devonian tetrapod, found in Pennsylvania, shows that it could support the front end of its body in a low, wide stance. Reconstructing the posture of an extinct animal from isolated bones. It’s a mosaic of fish features, tetrapod features and unique features,” Shubin explained. These carnivores were some of the first tetrapods, members of a group that includes all creatures with limbs, including reptiles, amphibians, birds and mammals. The upper arm bone, or humerus, that was discovered by the authors belongs to a new variety of four-limbed carnivore that lived long before the dinosaurs. “We are trying to untangle a very complicated and important evolutionary event - the shift from water to land,” said study author Neil Shubin from the University of Chicago.
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