New fossil discovery offers clues to evolutionary change of horseshoe crab legs

Sep 11, 2012 by Eric Gershon
New fossil discovery offers clues to evolution of horseshoe crab

(Phys.org)—Horseshoe crabs, including the iconic Limulus we know today, have existed for more than 450 million years. Over that long history, evolutionary change has particularly affected the nature of their legs.

A new in Britain captures a previously unseen stage in the evolution of these ancient arthropods—the transformation of two-branched legs into nearly identical but separately attached limbs, one of which was destined to disappear.

"This fossil provides remarkable confirmation of the loss of a limb branch during horseshoe crab evolution, a change predicted by the common presence of two branches in the arthropods that appeared earlier, during the ," said Derek E. G. Briggs, director of the Yale Peabody Museum of Natural History and lead author of a paper to be published online the week of Sept. 10 in the journal PNAS.

The fossil dates from the Silurian period, about 425 million years ago.

Modern horseshoe crabs (Limulus) have segmented legs at the front, which they use for walking and feeding. Their rear legs form flattened gills for breathing. In their ancestors, scientists believe, these functions were combined in limbs with two branches—one limb for walking and eating, one for breathing.

Scholars have long assumed that modern horseshoe crabs had lost the extra limb branch as its function was assumed by the limbs at the rear, which form book-like gills. The newly discovered fossil, a 23 mm specimen representing a new genus and species called Dibasterium durgae, shows 4 two-branched limbs, with an important distinction: The extra branch is not attached to the primary limb, but instead originates independently on the body like an extra leg. It appears that the branches first separated before the outer one was lost.

Combined with , the new suggests that repression or loss of might have yielded the modern 's appendage structure. "This discovery should prompt further investigation of the genes that control limb development in living horseshoe crabs, as the process is not fully understood," said Briggs.

The name of the new fossil, Dibasterium durgae, refers to the double limbs and to Durga, the Hindu goddess with many arms. It was reconstructed in three dimensions by stacking digital images of physical surfaces exposed by grinding away layers in tiny increments. The fossil was found in the mid-Siluran Lagerstaette, on the English-Welsh border, a site rich in well preserved, soft-bodied fossils.

Explore further: NOAA team reveals forgotten ghost ships off Golden Gate

Related Stories

Origin of claws seen in 390-million-year-old fossil

Feb 05, 2009

A missing link in the evolution of the front claw of living scorpions and horseshoe crabs was identified with the discovery of a 390 million-year-old fossil by researchers at Yale and the University of Bonn, ...

Climate change affects horseshoe crab numbers

Oct 04, 2010

Having survived for more than 400 million years, the horseshoe crab is now under threat – primarily due to overharvest and habitat destruction. However, climatic changes may also play a role. Researchers ...

Horseshoe crabs are one of nature's great survivors

Jan 24, 2012

It may look like something out of a science fiction movie, but the horseshoe crab is definitely real. In fact it is one of nature’s great survivors, lasting through 3 of the Earth’s major extinction ...

Regulators allow horseshoe crab harvest

Feb 12, 2008

The New Jersey Marine Fisheries Council has declined to extend a moratorium on horseshoe crab harvesting aimed at protecting migrating shore birds.

Fossil find fills in picture of ancient marine life

May 13, 2010

Paleontologists have discovered a rich array of exceptionally preserved fossils of marine animals that lived between 480 million and 472 million years ago, during the early part of a period known as the Ordovician. ...

Recommended for you

King Richard III died painfully on battlefield

1 hour ago

England's King Richard III might well have lost his kingdom for a horse. The reviled king suffered nearly a dozen injuries on the battlefield, but the fatal blows were probably only sustained after he had to abandon his horse, ...

'Hidden Treasure of Rome' project unveiled

5 hours ago

For more than a century, hundreds of thousands of historical artifacts dating back to before the founding of Rome have been stored in crates in the Capitoline Museums of Rome, where they have remained mostly untouched. Now, ...

NOAA team reveals forgotten ghost ships off Golden Gate

9 hours ago

A team of NOAA researchers today confirmed the discovery just outside San Francisco's Golden Gate strait of the 1910 shipwreck SS Selja and an unidentified early steam tugboat wreck tagged the "mystery wreck." ...

Long lost Roman fort discovered in Gernsheim

9 hours ago

In the course of an educational dig in Gernsheim in the Hessian Ried, archaeologists from Frankfurt University have discovered a long lost Roman fort: A troop unit made up out of approximately 500 soldiers ...

User comments : 0