Thursday, January 19, 2006

Ears Evolved from Fish Gills

Tetrapod-like middle ear architecture in a Devonian fish. 2006. Martin D. Brazeau and Per E. Ahlberg. Nature 439: 318-321.

This image shows how the hyomandibula, colored red, receded in fish to create a wider ear opening in early land animals. © Nature (click to enlarge)

Abstract: Few fossils show the incipient stages of complex morphological transformations. For example, the earliest stages in the remodelling of the spiracular tract and suspensorium (jaw suspension) of osteolepiform fishes into the middle ear of tetrapods have remained elusive. The most primitive known tetrapods show a middle ear architecture that is very different from osteolepiforms such as Eusthenopteron, with little indication of how this transformation took place. Here we present an analysis of tetrapod middle ear origins that is based on a detailed study of Panderichthys, the immediate sister taxon of tetrapods. We show that the spiracular region is radically transformed from osteolepiforms and represents the earliest stages in the origin of the tetrapod middle ear architecture. The posterior palatoquadrate of Panderichthys is completely tetrapod-like and defines a similarly tetrapod-like spiracular tract. The hyomandibula has lost its distal portion, representing a previously unrecognized advance towards a stapes-like morphology. This spiracular specialization suggests that the middle ear of early tetrapods evolved initially as part of a spiracular breathing apparatus.
From Bjorn Carey at comes this article:
Humans and other land animals have special bones in their ears that are crucial to hearing. Ancient fish used similar structures to breathe underwater. Scientists had thought the evolutionary change occurred after animals had established themselves on land, but a new look at an old fossil suggests ear development was set into motion before any creatures crawled out of the water.

Researchers examined the ear bones of a close cousin of the first land animals, a 370-million-year-old fossil fish called Panderichthys. They compared these structures to those of another lobe-finned fish and to an early land animal and determined that Panderichthys displays a transitional form.

The fossilized Panderichthys shows the middle step in modern ear development. Credit: Martin Brazeau (click to enlarge)

In the other fish, Eusthenopteron, a small bone called the hyomandibula developed a kink and obstructed the gill opening, called a spiracle. However, in early land animals such as the tetrapod Acanthostega, this bone has receded, creating a larger cavity in what is now part of the middle ear in humans and other animals. The new examination of the Panderichthys fossil provides scientists with a critical "missing link" between fish gill openings and ears.

Read the rest of the article HERE.

Congrats to up and coming Canadian Student, Martin Brazeau, and his advisor (and old Palaeoblogger buddy) Per Ahlberg!