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Levin: Reptiles At The Feeder

04/05/12 5:55PM By Ted Levin
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(Host) They may be the same old birds at the feeder, but nature writer, photographer and commentator Ted Levin tells us that the knowledge of their pedigree has changed.

(Levin) Long, long ago, crocodiles and birds sprang from a common ancestor. In fact, modern evolutionary science has shown that an alligator is more closely related to a chickadee then it is to a turtle or a lizard. You might even say that birds are late blooming reptiles or alligators are latent birds. Here's the drill.

When I was an undergraduate studying comparative vertebrate anatomy in the late 60s, I learned that during at least some stage of development all members of the phylum Chordata possess a notochord, a cartilaginous skeletal rod that supports the body. This phylum includes a diverse array of animals, from worm- like creatures called sea squirts to sharks, bony fish, amphibians, reptiles, birds, and mammals. In the more highly evolved chordates - fish through mammals - a column of vertebrae made up of either cartilage or bone, is the body's central joist. In us, it's the spine.

The subphylum Vertebrata or backboned chordates were divided into eight classes, four of which represent "fish." Two of these are obscure and two are eminently recognizable: the cartilaginous sharks and rays; and the pinnacle of fish evolution, the statistically abundant and diverse boney fish, which includes virtually everything else - from guppies to swordfish. The remaining four classes represent the terrestrial vertebrates - amphibians, reptiles, birds, and mammals.

Like tracing Abraham's descendents in the Book of Genesis, the relationship between the classes of vertebrates is easy to plot. Follow all the "begots" and we discover that amphibians emerged from a group of lung-bearing, boney fish called lobefins.

Then, 300 million years ago, during the steamy Carboniferous Period, when the Earth was much hotter than it is today, and all the landmasses of the planet were joined into one huge equatorial belt; amphibians begot reptiles. Eventually, reptiles begot birds and mammals during the Age of Dinosaurs.

My comparative anatomy course subdivided reptiles into sixteen orders. Dinosaurs comprised two of them, and only four still exist today. Of those four, three represent turtles, crocodiles, lizards and snakes. The fourth, a lizard-like group of two living species called tuatara, make their last stand today on islands off the coast of New Zealand.

But like the animals themselves, our classification system has evolved over time. Once, we grouped animals into an ascending pyramid with humans at the apex. Now, through sophisticated molecular research, we're able to track the actual evolutionary relationships between groups of animals based on genetic markers, biogeography, and fossil history.

Terrestrial vertebrates are grouped together as Tetrapoda - that's the four-limbed crowd: amphibians, reptiles, birds, and mammals. Unlike amphibians, however, reptiles, birds, and mammals also have eggs with a shell and embryos protected by three membranes. So they're given a new label that reflects that relationship. Collectively, they're known as Amniota.

So birds and reptiles are very close cousins. In fact, birds are "feathered reptiles" - an idea that's utterly transformed my appreciation of the daily drama at the feeder in my yard.
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