Is the Nautilus really a "living fossil?"
Below article from The San Diego Union-Tribune reprinted for fair use purposes only -- copyright acknowledged.
This Megalania page has been visited times since April 24, 1998.
The San Diego Union-Tribune
Copyright 1998 The San Diego Union-Tribune
SECTION: LIFESTYLE Pg. E-3
April 22, 1998, Wednesday
Fathoming secrets of the little-known nautilus
by Peter D. Ward
Every evening across the immense expanse of the tropical western Pacific, millions of white-shelled mollusks the size of dinner plates begin an epic voyage.
They rise from their daytime resting place -- the dark ocean bottom -- and slowly swim upward to shallow coral reefs where they feed for the night.
These animals -- the chambered nautiluses -- look like snails with tentacles. Their closest living relatives are the octopus and squid. But more than that, they look like creatures from a bygone era.
Over the past 500 million years -- before, during and after the age of the dinosaurs -- more than 10,000 related species of nautilus roamed the seas. But in the 65 million years since the dinosaurs died out, the family to which the chambered nautilus belongs has gradually diminished. Today only a few species still exist, and they remain poorly known.
Indeed, only recently have scientists learned some of the key facts about the animal, facts as simple as their extraordinary nocturnal voyages.
These discoveries have prompted paleontologists to ask new questions: Is this nightly behavior a holdover from the age of dinosaurs, when great scaly marine lizards preyed on the shelled denizens of ancient seas?
More important, is the chambered nautilus itself only a recent descendant of an ancient lineage, or a true living fossil?
Many, many tentacles
The nautilus looks like nothing familiar. It swims just above the seafloor, moving like a peculiar fish. It has tentacles and a jet propulsion system somewhat similar those of other cephalopods -- the class of mollusks that includes octopuses, cuttlefish and squid.
But there are great differences, too. The chambered nautilus has about 90 tentacles rather than the more common eight or 10 of other cephalopods, and its weird, primitive eye lacks a true lens.
And no other cephalopod has anything remotely resembling the nautilus shell. The animal itself is housed in the last chamber in the shell's spiral, with only its head and many tentacles visible. As the nautilus grows, it adds a new chamber and moves into it, leaving the old one behind.
During daylight hours, the nautiluses lurk in the depths, out of sight and danger from shell-breaking predators such as turtles and triggerfish. But with the coming of night, they begin to stir.
They follow the slope of the bottom toward more shallow water, swimming just above the mud, sand and coral rubble, eventually reaching the sheer rocky walls that mark the base of the reef itself.
Undeterred by these great coral skyscrapers, whose tops may be 500 feet overhead, the nautiluses continue their voyage, ascending like silent hot-air balloons.
Finally, after journeying for several hours, they arrive in the shallows and spend the night roving the reef, using their sense of smell to find carrion, lobster molts and hermit crabs.
With the first approach of daylight, they swim back over the reef walls and fall down into darkness.
By the mid-1980s, many research projects had amassed a lot of new information about this enigmatic animal, but none had come up with a definitive age.
The genus Nautilus belongs to a family of similar creatures, collectively called nautiloids. Most older textbooks suggest that while the chambered nautilus shell resembles the shells of extinct, dinosaur-era nautiloids, the living species are more recently evolved.
According to this view, the chambered nautilus might date back only a million years or less. If this is the case, the animal's behaviors might be recently evolved as well, making them of little use in interpreting the lives of ancient nautiloids.
As a result, paleontologists have turned to studying the nautiloids' genes and fossils for better clues to their ancestry. DNA sequencing is a now- familiar method for discerning the shape of an organism's family tree, but the process is not easy for the chambered nautilus. To conduct such studies, you need tissue taken from living animals, not shells. Thus each living species of nautilus had to be sampled.
But researchers have long been uncertain exactly how many species of nautilus there are. They tried to identify living nautilus species in the same way that paleontologists identify fossil species, by finding distinguishing structures on their anatomy or distinct, if slight, differences in their shells. The result has been that, by the late 1980s, scientists generally presumed the genus Nautilus contained 11 living species.
But to see how these so-called species were all related, it was necessary to get tissue samples and read their genes. In 1983, paleontologist Bruce Saunders of Bryn Mawr College and myself, a professor of geological sciences at the University of Washington Seattle, set out to discover just how many species did exist, and to discover their age as well.
Over time, we learned that far too many of the alleged species were much too narrowly defined. Only a few living nautiluses seemed truly distinct. One of these standouts, Nautilus scrobiculatus, commonly known as the king nautilus, had never been caught alive. It was known only from its shell.
That changed when Saunders captured a living king nautilus in 1984. Gene sequencing of that specimen, and other nautiluses collected in the South Pacific, revealed something remarkable: There are only two distinct groups of nautiluses. One is composed of the king nautilus, which appears to have descended from the chambered nautilus about 15 million years ago; the other group is composed of everybody else.
If this genetic evidence is accepted -- it's still being debated and analyzed in some circles -- it means that the long-agreed classification of the living species has crumbled. The king nautilus appears to represent a separate genus altogether, while differences in shell morphology of the other "species" seem to be useless in telling them apart.
From 11 living species belonging to one genus, Nautilus, our research now suggests two genera: Nautilus and our newly recognized genus, with only two or three species between them. As a result, we gave the king nautilus a new scientific name, Allonautilus, which is Latin for "other nautilus."
Shell game These results made us wonder if nautiloid fossils might also have new secrets to reveal. Unable to study extinct nautiloid DNA, Saunders and I had to figure out a new way to classify these animals based on their shells alone.
Previous studies of nautiloids had classified them on relatively few features. We hoped we might find more examples with more comparative features. Just such a trove of distinctive new characters, fortunately, was turned up by Neil Landman of the American Museum of Natural History in New York.
The chambered nautilus hatches at a very large size. It emerges from its egg with seven fully formed chambers and a shell diameter of more than an inch, making it the largest invertebrate at hatching in the world.
When a living chambered nautilus emerges from its egg, it stops growing temporarily. This pause leaves a distinct groove in its shell. Since the shell wraps around itself as it grows, these earliest stages are always preserved in the middle.
Landman began dissecting fossils to see if similar marks were found in extinct species as well. He discovered that not only did these marks occur, but many other features also.
Saunders and I combined Landman's new specimens with classical examples, then began to study their occurrence in living and extinct nautiloids. Both of us had been taught that present-day nautiluses are the most recently evolved of the 10,000 nautiloids that have swum through the oceans over the past 500 million years. Thus we expected them to have a lot of features that had evolved relatively recently.
To our surprise, we found that today's chambered nautilus appears to be extremely primitive -- rather than being a descendant of some fairly recently evolved nautiloid, the chambered nautilus evolved much earlier. It may even be the ancestor of most nautiloids present on our planet for the last 75 to 100 million years.
Our fossil analysis also supported earlier DNA results: The king nautilus is much more recently evolved than the chambered nautilus and is probably an evolutionary offshoot.
There was just one problem: If the chambered nautilus is so old, why are there no fossils of it? Until the late 1980s, only a single chambered nautilus fossil had ever been found, in Russia. It was claimed to be 40 million years old, but because no one had found another specimen from the same fossil beds, many paleontologists began to wonder if it had been mislabeled.
Now it turns out that there are many fossils we can now confidently place in the chambered nautilus genus. The first to point this out was Richard Squires at California State University Northridge. Squires had a collection of nautiloid fossils from 50-million-year-old rocks in Washington State, and in 1988 he published a paper describing these as the oldest chambered nautilus fossils ever found.
The living fossil we call nautilus survived the great cosmic collision that killed the dinosaurs, and many other changes in the 65 million years since that catastrophe. Its presence on Earth through all this time gives us one more small peek into the workings of evolution, which we are learning can tick slowly as well as at more staccato rates.
The long voyages of the nautilus, from the depths into the shallows each night, are thus a perfect metaphor for its evolutionary history, which comes up to our world unchanged from the great depths of time.
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