Scientists in Sweden have come up with a new theory for how evolution produced the ear. The answer, they report in the journal Nature, is that before the middle ear was used for hearing, in its ancient form, it might have been used for breathing.
The researchers, Martin D. Brazeau and Per E. Ahlberg from Uppsala University, base their claim on a study of a fossil of Panderichthys, a 370-million-year-old fish that is an immediate ancestor of the most primitive tetrapods [creatures with four limbs, such as most reptiles and mammals].
The fossil has an enlarged spiracle, a passageway between the jaw and the top of the head, as well as other changes that represent a middle ground between the relatively simple structures of more ancient fish and the complex morphology of tetrapods.
"It's got this combination of fish- and tetrapod-like features," said Mr. Brazeau, who undertook the research as part of his work on a doctorate in evolutionary biology.
Scientists have known since the 19th century that in tetrapods, including humans, the middle ear develops from an embryonic structure called the first gill arch. In fish, the first gill arch forms the support for the jaws including the spiracle.
In fish that are ancestral to Panderichthys, the spiracle is small. In Panderichthys, Mr. Brazeau said, "the first thing that happens is that the spiracle becomes very large." Further alterations follow, he said, including changes to a bone that is the forerunner of the stapes, or stirrup bone, that is part of the middle ear structure.
So the Panderichthys fossil, which was found in Latvia, is a useful snapshot of a moment in evolution. But why do the researchers suspect that the spiracle was part of the fish's breathing system?
Part of the answer can be found in modern bottom-dwelling marine creatures like rays. When they are on the sea floor, rays use spiracles on the top of the head for breathing instead of their mouths, to avoid sucking up sand. Panderichthys, Mr. Brazeau said, may have been a bottom dweller and have had the same need for an alternate respiration route. "It may very well have had its face in the mud," he said.
From the New York Times
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