27 October 2006
A PhD student from Johannesburg’s Wits University has uncovered a 360-year-old fossil of a small boneless fish species still found swimming, largely unchanged, in today’s waters – again proving that South Africa is a rich repository of evidence of the evolution of life on earth.
The lamprey, a parasite that sucks the blood of other fish, joins South Africa’s coelacanth as one of the world’s remarkable “living fossils”.
Robert Gess, a student at the Bernard Price Institute for Palaeontological Research, found the fossil fish after slowly chipping away at 300 cubic metres of rock with a penknife. The rock was recovered after blasting for the construction of the N2 bypass around Grahamstown nearly 25 years ago.
The find is described in the 26 October 2006 issue of Nature, the world’s most prestigious scientific journal, in a paper co-authered by Gess and his supervisors, Professor Bruce Rubidge of Wits and Dr Mike Coats of the University of Chicago.
“Apart from being the oldest fossil lamprey yet discovered, this fossil shows that lampreys have been parasitic for at least 360-million years,” Rubidge said. The find is 35-million years older than the next-oldest lamprey fossil so far discovered.
The fossil of Priscomyzon riniensis, a species of lamprey that lived 360-million years ago. Lampreys are boneless, so fossils of the fish are rare (Image: Nature)
Lampreys are long, eel-like parasites that attach themselves to and feed on other fish. Of the 46 000 known species of vertebrates, lampreys and hagfish are the only surviving jawless vertebrates. Lampreys are also the most primitive of the vertebrates, having changest the least from their earliest ancestors. Other than having no jaws, lampreys have no paired pectoral and pelvic fins, and no scales.
A drawing taken from the fossil parasite. The lamprey’s mouth parts are clearly visible (Image: Nature)
“This fossil changes how we look at lampreys today,” said Coates. “They’re very ancient, very primitive animals, yet with highly specialised feeding habits.”
The new fossil shows that the anatomical evolution of lampreys has been more conservative than scientists thought, Coates said. Although they’ve lengthened slightly, they specialised early and successfully in their evolution and so have remained largely unchanged for the past 360 million years.
The find is also remarkable for being a nearly complete soft tissue impression. Without bones or much cartilage, lampreys don’t fossilise easily, so preserved specimens are rare.
“These are pretty insubstantial animals,” Coates said. “Lacking a bony skeleton, they rot down, leaving no hard parts like a skull or ribs. So if a fossil site is discovered that yields impressions of the delicate remains of these animals, then this site needs to be explored thoroughly for other examples of exceptional preservation.”
The scientists will continue to sort through much of the indeterminate material that is emerging from the ongoing dig.
Dedication and passion
“This discovery is a monument to the dedication and passion of [Gess], who has spent many months patiently excavating and unearthing the elusive secrets from the prehistoric past,” Rubidge said.
Gess has named the specimen Priscomyzon riniensis. The first name is Latin, meaning “ancient lamprey”, and Rini is the isXhosa name for Grahamstown. Preserved showing the underside, the fossil is less than five centimetres long and features a set of 14 teeth surrounding the mouth that is proportionately larger than its descendents today.
“The most striking feature of Priscomyzon is its large oral disc, edged with a soft outer lip, supported by an annular cartilage, and surrounding a circular mouth,” the authors wrote in Nature. “This is the first clear evidence of a Palaeozoic lamprey with an oral disc.”
According to the scientists, this find greatly adds to what was a severely limited lamprey fossil record and, for the first time, places the origin of modern lamprey morphology deep within the Palaeozoic period. It adds essential new detail to the emerging and changing picture of early vertebrate evolution.
Until now, the lamprey fossil record included only those that show a side view but reveal little of the gill basket and feeding apparatus.
The new South African fossil shows that these anatomically specialised fish are holdovers from ancient marine ecosystems. Obviously exceptional survivors, these animals predate the advent of modern fish and have survived the world’s four major extinction events.
“There are few representatives of these early branches in vertebrate evolution that are still around today,” Coates said. Although highly specialised in their own right, these primitive animals are used as surrogate ancestors for comparative research on living jawed vertebrates.
“It gives us a calibration point,” Coates said. “We study lampreys because, in many respects, they’re so primitive. They never had jaws, they never had [true] teeth, they never had fins, they never had limbs. Lampreys provide a glimpse of conditions early in vertebrate evolutionary history.”
The lamprey lifecycle
Nearly 50 species of lampreys are found today in temperate rivers and coastal seas. Some live in fresh water for their entire lives, but most are anadromous, hatching in fresh water, migrating to the ocean to grow and mature, and migrating back to fresh water to spawn and reproduce.
When adult lampreys return to fresh water, they stop feeding during winter and spawn the following spring. Eggs hatch after about three weeks and become blind larvae, called ammocoetes. After four to seven years, the ammocoetes metamorphose into juvenile lampreys called macropthalmia, which migrate out to the ocean and become parasitic adult lampreys, living just a year or two and growing up to 60 centimetres long.
Lampreys have a sucker-like mouth with a ring of cartilage that supports the rim of the mouth. It fastens on to a living fish with its teeth, rasps at the host’s soft tissues with its piston-like tongue, produces strands of mucus to trap the food and feeds on the body fluids. A fish attacked by lampreys may be severely injured or even killed.