Evolution of Ear Is Noted in Fossil
Thursday, January 19, 2006
Question: What do you do with half an ear?
Answer: You breathe through it.
That's the conclusion reached by a pair of researchers who say they have found a fossil "snapshot" of the ear partway through its evolution to its current form.
The structure that became the sound-conducting middle ear of land animals began as a tube that permitted ancient shallow-water fish to take an occasional breath of air out of the top of their heads -- at least according to Martin D. Brazeau and Per E. Ahlberg of Uppsala University, in Sweden.
Their conclusion is controversial, as it amounts to a radical reinterpretation of how the ear developed in land-based animals. If it withstands scientific scrutiny, the fossil will be a rare example of an organ glimpsed partway along its evolutionary path, at a point when its function was very different from that of its final form.
Opponents of evolution say such "intermediate forms" should rarely, if ever, exist. They contend that many anatomical structures are too complicated to have evolved step by step. Instead, they had to have been created in their final form.
"This is another nail in the coffin of the creationist view, in my opinion," said Mark W. Westneat, an associate curator of zoology at the Field Museum of natural history in Chicago. "It is a great fill-in-the-gap story that shows a nice transition stage at an important point in evolution."
Brazeau and Ahlberg examined the fossilized skull of Panderichthys , a fish about four feet long that lived in the Upper Devonian period about 380 million years ago. It was an intermediate creature between earlier lobe-finned fishes and true "tetrapods," or four-limbed animals.
The specimen they studied was unearthed in Latvia in the 1970s and resides in a museum in Riga. Previous studies of the skull were done before all the rock encasing it was removed. Looking at it in its less-obstructed form, Brazeau said they noticed two things.
First, a bony channel leading to a hole in the skull called the spiracle was much wider and straighter than in more primitive fish. Second, a bone called the hyomandibula was much shorter and stubbier.
The spiracle is a modified gill slit. Sharks and rays have them on the top of their skulls behind their eyes. They use it for respiration while feeding on the bottom to avoid drawing grit into their gills. Uppsala researchers believe Panderichthys did the same thing.
"This fish probably lived on the bottom, probably in shallow water. It was almost a crocodile-like fish," Brazeau said. "It may have inhaled and exhaled through this passageway. It may, in fact, have breathed air through it occasionally."
Paleontologists have known for a long time that the hyomandibula -- which helps suspend the jaw in fish -- evolved into the stapes, or "stirrup bone," that helps transmit sound vibrations in the middle ear in reptiles, birds and mammals.
When Brazeau and Ahlberg measured this bone in Panderichthys, they discovered it was much shorter than in earlier fish. It also was not directly connected to the jaw joint. It appeared to be in the middle of moving to a different part of the skull where it would become an ear bone -- a migration that would take tens of millions of years to complete.
"This stage sets up the skull and makes it possible for this region to be modified into a middle ear. The interesting thing is that at this point it appears to have nothing to do with hearing," Brazeau said.
Only when the fish's descendants became land-based and breathed through their mouths and nostrils was this passageway "free" to evolve into a sensory organ, he said.
There are serious doubters of this interpretation of the fossil, however.
Michael LaBarbera, a professor of organismal biology and anatomy at the University of Chicago, is an expert in the functional anatomy of extinct animal. He isn't certain that the key structure that Brazeau and Ahlberg say is a spiracle is, in fact, one.
Their theory is "based on the interpretation of a structure that would be completely novel and unprecedented in this lineage," he said. And he's not convinced.