The three stages of mammalian backbone evolution are far clearer now, thanks to work by a team of researchers that examined fossilized backbones of primitive mammal ancestors and applied novel statistical analyses. Their efforts included analyzing cabinet drawers' full of fossil vertebrae stored at museums (see related photos). Among mammals' unique characteristics, including in making us distinct from reptiles, is a vertebral column with anatomically distinct regions - those that facilitate everything from speedy running to up-right walking. But, when and how variability across the spine evolved in mammals remains unknown. Here, to better inform this process, researchers led by Katrina Jones studied synapsids -- existing mammals and their extinct forerunners - both by measuring fossils and reconstructing vertebral regions, in some cases. The team's evaluations included that of a fossil Edaphosaurus, also known as the "pavement lizard," which lived between 300 and 280 million years ago and exhibited incredibly long spines that formed a sail along its back. The team also closely measured a fossil Thrinaxodon, which lived later in time, during the Early Triassic, and was part of the first group to display an extra 4th region in the backbone called the pectoral region. Absent from it and other mammal ancestors is the 5th and characteristic region in the backbone of modern mammals: the lumbar. The researchers' reconstructions and analyses of these species show that as synapsids evolved, forelimb reorganization was important; it drove regionalization across the spine - even began before the organism supported by the spine began to change. Following development of distinct spinal regions, the related synapsids adapted further, leading to the highly varied forms of mammals we see today.
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Journal
Science