Fossil Untangles Horseshoe Crab Mystery

A little while back, I went on a tear about the phrase “living fossil.” We should scuttle the baggage-ridden aphorism – extant crocodiles, coelacanths, and tuataras are not “unchanged” remnants of an earlier era, but the last branches of amazing animal groups that flourished during Deep Time. I almost wish that I had waited another few weeks before tapping out my rant. That’s because of a new paper by Yale University paleontologist Derek Briggs and colleagues, which puts another misunderstood icon of stasis in context.

I have an abiding love for horseshoe crabs. The unusual arthropods are among evolution’s greatest survivors, but the species that crawl along the ocean bottom today are not exactly like their fossil relatives. This really shouldn’t come as a surprise. Horseshoe crabs, in one form or another, have been around for about 510 million years. All the same, these arthropods are often treated as creatures with an entirely static history. The newly described fossil horseshoe crab should help overturn this unfortunately cherished view of the marine invertebrates.

The new fossil described by Briggs and colleagues records a critical transformation in horseshoe crab history. Discovered at an exceptional site in the 425 million-year-old rock of Herefordshire, England, the new genus is justly called Dibasterium durgae – a tribute to the invertebrate’s mysterious limbs and to Durga, “the Hindu goddess with many arms.” Indeed, it’s the anatomy of those many appendages that makes Dibasterium so important.

Archaic arthropods, which originated during the evolutionary riot known as the Cambrian Explosion, had double-branched limbs. These biramous appendages were multifunctional, with one branch used for feeding and walking, and the other branch for breathing. In time, though, some arthropod lineages developed unbranched – or uniramous – limbs that became dedicated to specific uses. In modern horseshoe crabs, for example, the front legs are unbranched structures used in walking and feeding. What once were the rear limbs have been recast as part of the invertebrate’s book gills.

Dibasterium represents a strange midpoint between modern horseshoe crabs and Cambrian critters. The inch-long arthropod had a set of four double-branched appendages, but each branch derived from a separate origin on the body. This sounds odd, but, as Briggs and colleagues state, these crossed appendage portions are parts of the same limb rather than being distinct legs that alternate between one morphological type and another.

Separate origins for the two leg branches provides a clue about what happened to the now-missing branch among modern horseshoe crabs.

Invertebrate experts had previously thought that horseshoe crabs lost one of their limb branches, the one that supported gills, when the back legs transformed into book gills. As the back legs took over breathing duties, the front legs became dedicated to crawling and feeding.

But Dibasterium indicates a different pattern. By 425 million years ago, the two leg branches had begun to separate. Only after the divide between the leg branches was drawn did the now-missing half start to winnow away, perhaps ushered out by changes in development regulated by Hox genes.

If researchers can combine paleontology and genetics, they might be able to reconstruct the underlying mechanism for the anatomical changes hewn in stone. We’ll have to wait for that. What’s clear is this – despite their ancient appearance, horseshoe crabs have changed quite a bite since their double-branched progenitors scuttled around the Cambrian seafloor.

Reference:

Briggs DE, Siveter DJ, Siveter DJ, Sutton MD, Garwood RJ, & Legg D (2012). Silurian horseshoe crab illuminates the evolution of arthropod limbs. Proceedings of the National Academy of Sciences of the United States of America, 109 (39), 15702-5 PMID: 22967511